tag:blogger.com,1999:blog-1748430270390313082024-03-12T16:38:29.797-07:00Statistics, Space, Strings and StuffMiscellaneal musings on statistics, astrophysics, cosmology and string theory.Unknownnoreply@blogger.comBlogger13125tag:blogger.com,1999:blog-174843027039031308.post-78190600099469052552016-02-11T13:24:00.003-08:002016-02-11T13:41:25.929-08:00Today my heart is rippling tooI cried today. About science.<br />
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This morning, the<a href="http://www.ligo.org/multimedia.php" target="_blank"> LIGO consortium</a> announced that they had detected the gravitational wave signature of a pair of inspiralling black holes. Large ones, at that.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://1.bp.blogspot.com/-RHn1d8xhJmI/Vrz7ZswePgI/AAAAAAAACMI/83yX-ZGZ03w/s1600/Screen%2BShot%2B2016-02-11%2Bat%2B10.37.47.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="238" src="https://1.bp.blogspot.com/-RHn1d8xhJmI/Vrz7ZswePgI/AAAAAAAACMI/83yX-ZGZ03w/s320/Screen%2BShot%2B2016-02-11%2Bat%2B10.37.47.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Check out the "chirp" signal of the inpiralling black holes as measured both in Louisiana and Washington. The right hand panel also shows the Washington signal superimposed (and flipped), which shows how synchronous they are.</td></tr>
</tbody></table>
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You might ask, so what? We already have a lot of evidence that supports General Relativity on a wide range of scales. It isn't as if we were waiting to 'prove Einstein right' (as many of the news headlines would have you believe). In fact, it would have been more of an issue if we <b>hadn't </b>seen gravitational waves.<br />
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So, more than it being an unexpected discovery, I'm emotional because it highlights a few of my favourite things about science:<br />
<b><br /></b>
<br />
<ul>
<li><b>We make predictions for what we should see. </b></li>
</ul>
The theory of GR predicts that gravitational radiation should be generated under these extreme conditions -- two black holes orbiting each other and spiralling in towards each other in the final moments before they merge into a lower mass black hole. But more than just simple analytical equations, scientists (many of which are in the LIGO consortium) numerically solve the relativistic equations that govern these scenarios, using incredible supercomputing resources (big shout out to my local supercomputer, SciNet, here in Canada!) and simulate what these signals should look like. Seeing the data match these simulations took my breath away.<br />
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<iframe allowfullscreen="" class="YOUTUBE-iframe-video" data-thumbnail-src="https://i.ytimg.com/vi/p647WrQd684/0.jpg" frameborder="0" height="266" src="https://www.youtube.com/embed/p647WrQd684?feature=player_embedded" width="320"></iframe></div>
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<ul>
<li><b>We persevere when the going gets tough.</b></li>
</ul>
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Many of the scientists working spent years petitioning their respective national science foundations for money and painstakingly improving the detector efficiency, detection algorithms, methodology etc. - even in the face of non-detections! Pushing those sensitivity curves into regimes we'd expect to see a signal can be thankless work - and seeing today the joy on the faces of the men and women who have been working so hard was increadible!<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://4.bp.blogspot.com/-0HviGCpZsU0/Vrz66TRf3UI/AAAAAAAACME/_st3MRMPDCQ/s1600/Screen%2BShot%2B2016-02-11%2Bat%2B16.19.15.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="241" src="https://4.bp.blogspot.com/-0HviGCpZsU0/Vrz66TRf3UI/AAAAAAAACME/_st3MRMPDCQ/s320/Screen%2BShot%2B2016-02-11%2Bat%2B16.19.15.png" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">The LIGO team kept pushing down on their sensitivity curves!</td></tr>
</tbody></table>
<br />
<ul>
<li><b>We work together.</b></li>
</ul>
<br />
LIGO, like the LHC and Planck, are huge collaborations.<br />
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I mean <a href="https://www.instagram.com/p/BBqFbNMrwXj/" target="_blank">check out this institutional logo slide!</a><br />
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My friends were teasing me about the author list taking up the first few pages of the papers today. But you know what, folks? That's what happens sometimes when you want to take on these incredible challenges. And true, collaborations aren't always that large, but a major part of science is that we work together. We check each other's results, we find bugs, we test, we push - we disagree strongly! And yet at the end of the day we all do this because we feel like we are pushing back the boundaries of our own, and the world's ignorance. And illuminating (be it optically or now GRAVITATIONALLY) the cosmos.<br />
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<br />
Today I'm thankful to be a scientist.<br />
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<div>
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Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com5tag:blogger.com,1999:blog-174843027039031308.post-1587116902193797002015-06-13T15:51:00.002-07:002015-06-13T15:51:28.753-07:00The scientific and historical gem that was CMB@50! I'm still on a high from the recent CMB@50 conference held at Princeton to celebrate fifty years since the discovery of the CMB.<br />
<br />
The conference was incredible in that it had so many leaders in the field in attendance, but also because the program highlighted not only the science but also the history of the field. This is useful for younger scientists like me who were academically born "post-WMAP".<br />
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I live-tweeted the conference with the hashtag #CMBat50, with some other colleagues joining in (for what it is worth, it is a great way to take notes if you can do it fast enough - and it encourages you to think critically in all the talks and to attend all the sessions!)! Given that not everyone wants to wade through all the tweets, I decided to collate them all in a few storify storyboards.<br />
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Storify is a great way to collate and annotate tweets from an event, and allows you to create a narrative (and also to catch those tweets that maybe don't use a hashtag or are in reply to a thread etc.)<br />
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It's my first time using the software, so it may be a bit rusty initially, but I hope you enjoy them!<br />
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<a href="https://storify.com/reneehlozek/cmb-50-day-one" target="_blank">CMB@50 Day One</a><br />
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<a href="https://storify.com/reneehlozek/cmb-50-day-one-557a08be718c422a739526c7" target="_blank">CMB@50 Day Two</a><br />
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<br />
<a href="https://storify.com/reneehlozek/cmb-50-day-three" target="_blank">CMB@50 Day Three</a><br />
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<br />Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com3tag:blogger.com,1999:blog-174843027039031308.post-77671930576945577212015-01-28T13:54:00.001-08:002015-01-28T13:54:16.232-08:00Statistics, Space, Skateboarding and StuffOver the summer I spent some time working on a video project and article about skateboarding and physics with some of the awesome folks at Perimeter Institute (PI), where I work.<br />
<br />
You can find the video <a href="https://www.youtube.com/watch?v=OL7erd8yVaE&feature=youtu.be">here</a>, and the article <a href="http://perimeterinstitute.ca/news/defining-gravity-defying-gravity?utm_source=TWITTER&utm_medium=BODY&utm_term=TPOST1&utm_content=DODDY&utm_campaign=SLICEOFPI">here</a>. The photos of me throughout this post were also taken while we were putting the article together.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-LKicz2c7SfM/VJWt_3VN56I/AAAAAAAAAK4/NUvQIugd9Ho/s1600/sweeper_1.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/-LKicz2c7SfM/VJWt_3VN56I/AAAAAAAAAK4/NUvQIugd9Ho/s1600/sweeper_1.JPG" height="320" width="300" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Sweeper, for Duane, Waterloo. Photo: Gabriela Secara</td></tr>
</tbody></table>
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I'm not sure where the original motivation for the article came from, but it began for me some time in June (I think) when I was approached by <a href="http://perimeterinstitute.ca/people/Colin-Hunter">Colin Hunter</a>, the senior scientific writer at PI, who was interested in profiling physicists and finding out about our passions outside of work, and what makes us tick. Over the next few months we spent a number of mornings and afternoons working on the video and interviews and exchanging lots of emails. One of the most important things for me was getting across the right sense of what skateboarding is to someone from outside of it. Skateboarding has a really strong sense of itself and it's cultural identity. It's nuanced, and generally those of us on the inside find that people on the outside just don't get it. But what made me really stoked on this project and gave me confidence in the whole outcome was that from very early on Colin and I got along, and I could see he got it, and that we both wanted to make something rad that hopefully everyone, including other skaters, physicists and the general public, could take something away from.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-hnQW6Dtn-nY/VJWw8hjtRcI/AAAAAAAAALE/KyzDP2lcQys/s1600/boneless_1.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://2.bp.blogspot.com/-hnQW6Dtn-nY/VJWw8hjtRcI/AAAAAAAAALE/KyzDP2lcQys/s1600/boneless_1.JPG" height="291" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">(muted) Boneless, tranny to bank, Waterloo. I loved that crazy rainbow bandana. I lost it after (?) burning man. Photo: Gabriela Secara</td></tr>
</tbody></table>
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/-g1GziX6F9Xw/VJW4TdNEVoI/AAAAAAAAALs/8RAXVSzXkTE/s1600/stand_up_cam_3.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://3.bp.blogspot.com/-g1GziX6F9Xw/VJW4TdNEVoI/AAAAAAAAALs/8RAXVSzXkTE/s1600/stand_up_cam_3.JPG" height="212" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Stand up, with additional finger pointing action, Cambridge. Photo: Gabriela Secara</td></tr>
</tbody></table>
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I've left the writing of this blog post a bit too long, so I can't remember what I was going to say. Apologies for the rambling.<br />
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One of the best things that came out of working on this piece for me was getting feedback from some of my skateboarding idols. The first person I wrote to was <a href="https://www.youtube.com/watch?v=kJVr8SksrYI">John</a> <a href="https://www.youtube.com/watch?v=KZ16mqspyps">Rattray</a>. John (look at me, pretending we're on first name terms) is a Scottish skateboarder who studied astrophysics at university. I studied for my undergraduate degree in Scotland too, which is already a great connection. The first section I've linked to there is from when John was pro on a (sadly now defunct) British skateboarding company called Blueprint. Blueprint defined what it was to be a skateboarder on these drizzle-covered isles in the early 2000's, and everything they put out was hugely influential on me growing up. Most of it wasn't my style, per se, me being a bowl skater and Blueprint being pretty heavy street, but it didn't stop me loving what they did. John's section from Waiting for the World was inspirational beyond words, and still one of my favourite sections of all time. He had fun (and skated bowls), and skated to an awesome song by the Seahorses (I bought their album on the strength of it, but the album didn't have that song on and generally sucked. You win some...). John then "made it" and went pro for Zero, Jamie Thomas' power company from the USA. The second section I linked to is (I think) his first for them. Again, John kind of broke the mould of the other skaters on the video. They were mostly hammers and rails, but John came through with no complys (before they were cool again) and that same fun energy. Top boy.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-WkcS3S0QXdM/VMlSMbdXVII/AAAAAAAAAMY/pWFTCwSQvTc/s1600/IMG_2053.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/-WkcS3S0QXdM/VMlSMbdXVII/AAAAAAAAAMY/pWFTCwSQvTc/s1600/IMG_2053.JPG" height="320" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Early grab in the slightly over vert pocket, Cambridge. You can see the PI filmers, Max and Craig, in this insta-filtered photo by Renée Hlozek.</td></tr>
</tbody></table>
<br />
<br />
I got in touch with John through a mutual Scottish friend (thanks Russ). I was so stoked that he replied to my email. It was another thing that gave me confidence we were doing something that mattered. John responded to some questions that Colin wrote for him. His replies were short, but they really helped me keep my feet on the ground. For example: "Studying physics is an intellectual pursuit and although the act of learning to do something physically complex and demanding can have intellectual benefits they are utterly different. Not to say they're not connected, I mean, they exist in the same universe, as far as we can tell." John's reply there made sure I didn't go ahead on my high horse and say anything stupid about connections between physics and skateboarding. I just stuck with my story, and how I feel about both.<br />
<br />
The next person I managed to get in touch with, and who along with John completes the "skateboarders with a link to science dream team", was <a href="https://www.youtube.com/watch?v=DBbmNAZWq-E">Rodney</a> <a href="https://www.youtube.com/watch?v=gN0GDRWvRig">Mullen</a>. I mean, holy shit, Rodney Mullen! Renée (the other author of this very blog) met Rodney at TED a few years back. He was really friendly, and came up to her wanting to talk about cosmology after her talk. We exchanged quite a few emails back and forth about this project, and there were just so many encouraging words of inspiration from Rodney that upped my stoke levels through the roof. If you're not a skateboarder, you may not have heard of Rodney Mullen, but then again, you may have. He's one of a few "breakthough" skateboarders that are almost household names. You'll see that from one of the links: the man's given TED talks. But that's not why he's important to skateboarding. Rodney invented pretty much every trick that modern skateboarding relies on: ollies, kickflips, 360 flips, and literally dozens more (there's a section about this on his wikipedia entry!). He invented these tricks in the context of "freestyle," which is a kind of skateboard dancing, in the early 80's. At that time, everyone else was skating vert and doing big airs, but Rodney was skating alone in his barn and changing the world. It took another generation to adapt his style and take these tricks to the streets and to become modern skateboarding as we know it. But no Mullen, no dice. He had to break those barriers and show what could be done, and he had to do it his own way, outside of what was happening at the time.<br />
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Why did I think of him as a scientist? I can't tell you really. He's well known in the skateboarding world as being very intelligent, and also being super nice. I took a chance on contacting him, and it worked, and he had lots of awesome things to say. Again, something to make me confident that it was a good thing to be working on this project.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/-5QObDWjlSgs/VJW4XG7BkpI/AAAAAAAAAL4/DMkqdl3BfHE/s1600/fs_rock_cam_1.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/-5QObDWjlSgs/VJW4XG7BkpI/AAAAAAAAAL4/DMkqdl3BfHE/s1600/fs_rock_cam_1.JPG" height="212" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">FS rock (to the hilt), Cambridge. If you haven't seen a dozen pictures of me doing this trick, we obviously aren't friends on facebook. Photo: Gabriela Secara.</td></tr>
</tbody></table>
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Now, where was I?<br />
<br />
Oh yeah, the filming and photos. It was super fun going on the filming missions from work. First of all, what red-blooded skateboarder wouldn't be hyped to have fully sanctioned days to bunk work and go skate in the sun? Well, as a postdoc I can pretty much do that whenever I want anyway, and I do, and I guess that was kind of one of the points of the video, but still: sanctioned skate-bunk! Me, Colin, and some selection of other PI video/photo types would jump in the car of an afternoon, put on some music, and head to a skatepark. Unless it was the Waterloo park, in which case we'd just get some drinks and walk over from the office. Having a park that close to the office is great: after work skates, lunchtime skates, all easy. Having work that close to the park is great too: somewhere to refill water and have a shower after a Saturday session before heading to the bars.<br />
<br />
Taking these non-skateboarders into my world was a bit daunting at times. What if they didn't get it? Filming and photos could have been a show-stopper too. They know how to do their jobs, but what if they didn't get the shots that looked right from a skateboarders perspective? In the end, though, it all turned out really well. There was a good back and forth, with me suggesting angles and tricks, and then compromising when the light was wrong or a different background worked better. Max was also kind enough to let me go through his rough cut and suggest changes and different clips, and Ela let me select the best photos from her. Again, this gave the right balance of something professional that could relate to non-skaters and look good, and something I could be proud of as a skater.<br />
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It's hard enough bringing close friends who don't skate to skateparks, and then it's normally lubricated with a good number of street beers. This was work people, and could have been really awkward or weird. I let skateboarding work its magic for me, and just cruised around. I wanted them to see the relaxed side, and the natural side, nothing like "extreme" tricks or anything like that. Skateboarding, like physics, is playful. But, it was hard work shooting sometimes. Doing things over and over for the right angle and light. And it's only me with my shitty bag of tricks. I can't imagine how much more hard work it is for guys chucking themselves down stairs being asked to do it again. Mad respect. The hardest work was the 5-0 right below you. If you know me well, you know I suck at street, and I had to do this 5-0 at least a dozen times before we were all happy.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-4WUcAmqNXyE/VJW34P8XvVI/AAAAAAAAALg/zt0IHzkGYbk/s1600/5-0_1.JPG" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/-4WUcAmqNXyE/VJW34P8XvVI/AAAAAAAAALg/zt0IHzkGYbk/s1600/5-0_1.JPG" height="213" width="320" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Street for the kids! FS 5-0, Waterloo. Photo: Gabriela Secara.</td></tr>
</tbody></table>
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I'd better wrap this up. But we haven't even talked about physics yet. Watch this space for some more. Renée and I have been talking about roping in a bottle of Gin for this...<br />
<br />
Thanks to everyone who has been involved in this. PI for paying me and everyone else, and publishing and promoting. Max, Craig and Ela for the patience and good work filming and shooting. Colin for being the gaffer and writing an awesome article, and Renée for coming along on a Cambridge morning for moral support (I needed it after that heavy slam).<br />
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<a href="https://www.youtube.com/watch?v=PqNBqv6cOfo">Zig it up</a><br />
<br />Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-90664158012794988012014-12-01T11:27:00.000-08:002014-12-01T13:43:44.177-08:00Why we shouldn't try to have our (Planck) cake and eat it too...Like many cosmologists, I am eagerly awaiting the next update from the Planck mission.<br />
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The previous release of Planck result had me all <a href="http://statsandstrings.blogspot.co.uk/2013/03/twas-week-before-planckmas.html">aflutter</a>, and the release of results from BICEP had the whole community talking/discussing for ages.<br />
<br />
But we were told recently that the papers will be released on the 22nd of December 2014. While a conference devoted to the announcement of results (rather than the release of data and papers) is being held this week in <a href="http://www.cieffeerre.it/Eventi/eventi-in-programmazione-nel-2014/planck-2014-the-microwave-sky-in-temperature-and-polarization">Ferrara</a>.<br />
<br />
So this morning, with no official press conference and a press release from the team (in French) the only meat to go on was bits of information on Twitter from those at the conference.<br />
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So what do we know*?<br />
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*from Twitter/discussions and from the <a href="http://public.planck.fr/resultats-1">press releases here</a><br />
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Well, first of all there are the beautiful spectra in both temperature and polarisation on small scales (the larger scale measurement is in flux at the moment as the team work hard on systematics).<br />
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<a href="http://www.insu.cnrs.fr/files/imagecache/largeur-622/france_120114.005.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://www.insu.cnrs.fr/files/imagecache/largeur-622/france_120114.005.png" height="180" width="320" /></a></div>
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And some spectacular other images of (parts of) the maps in 353 GHz polarisation</div>
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<a href="http://www.insu.cnrs.fr/files/imagecache/largeur-622/lic_3_page_7.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://www.insu.cnrs.fr/files/imagecache/largeur-622/lic_3_page_7.jpg" height="320" width="318" /></a></div>
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with the colours indicating the dust and the relief showing the galactic magnetic field. Pretty, isn't it?</div>
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There are still some interesting tensions with the amplitude of clustering from Planck relative to other measurements of galaxy lensing (from the CFHTLens collaboration). The matter density <span style="font-family: Times, Times New Roman, serif;">is <span style="background-color: white; color: #222222;">Omega_m=0.316 +/- 0.009.</span></span></div>
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Some important parameters like the scalar spectral index, n_s shift around by around 1 sigma. The optical depth, tau, shifts downwards - which is important for how we understand how the universe reionised (tau i<span style="font-family: Times, Times New Roman, serif;">s in the range of <span style="background-color: white; color: #222222;">0.71-0.79 from Planck lensing and low-multipole LFI measurements.) </span></span>The errorbar is still converging, so I look forward to the results in December for the final number on this.</div>
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The level of non-Gaussianity detected by Planck including polarisation is consistent with zero.</div>
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The number of effective degrees of freedom (Neff) is consistent with the simple picture, Neff = 3 +- 0.2, a number consistent with the WMAP9+ACT numbers we presented <a href="http://arxiv.org/abs/1302.1841">a while back before Planck</a>.</div>
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<a href="http://www.insu.cnrs.fr/files/imagecache/largeur-622/dm_et_neutrino2.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://www.insu.cnrs.fr/files/imagecache/largeur-622/dm_et_neutrino2.png" height="182" width="320" /></a></div>
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Planck have a very robust detection of a non-zero lensing deflection power spectrum (I don't think the term 40sigma really means much when we get to such high confidence!) and detected polarisation B-modes from lensing too.</div>
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And Planck have improved the constraints (over WMAP) on dark matter models significantly, ruling out previous results (look for the grey rectangle between the yellow WMAP exclusion limit and the blue exclusion limit).</div>
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Also, very importantly, the Planck team have also gotten a great handle on their calibration, which brings the calibration into alignment with WMAP at the 0.3% level.</div>
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So this is good news! Some have been commenting that the results aren't 'exciting' enough - but I actually think this is great news. The Planck team should be commended for waiting/checking/testing/understanding their data for as long as possible before releasing the papers: they are being cautious. Also, we as a community are taking small, tough and important steps to narrowing parameter space. Being in the large-data limit makes things hard, and they've been under tremendous pressure too to release the data, but I think we should take our hats off for a moment and congratulate the team on their (amazingly) hard work. The scientific method is gruelling, and we can't always get the great upsets that make for great tweets.</div>
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I would have loved more of a press release, live stream or presentation of the results publicly. But I'd rather have a cautious late-December gift of papers than a early rush that they aren't happy with.</div>
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Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-49740854859118320132014-11-12T10:06:00.000-08:002014-11-12T12:05:54.561-08:00Hey grrrl... the reasons why I'm furious about ESA's #shirtgate The dust hasn't even settled yet on the amazing, incredible feat of human achievement - we have landed on Comet 67P.<br />
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HOORAH! Let me take the moment (ok many, many moments) to reiterate how wonderful this is.<br />
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And yet it was marred a little bit for me by the ESA #shirtgate incident (note that the hashtag has been reused from a previous incident on the internet, sorry about that folks).<br />
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When I opened up my social media this morning to get ready for the pre-announcements and hype (because these moments are what I live for as a scientist) I was shocked by something I saw in a colleague's post. She mentioned that the Rosetta Project Scientist Matt Taylor (@mggtTaylor) was on multiple media sources (an official BBC video, his own, and ESA's twitter feed etc.) wearing a crazy shirt. And sure enough, when I looked it up, this is what I saw:<br />
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Ok, wooaah.<br />
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There were also articles about the fact that Matt wants to challenge stereotypes of scientists and openly wear his tattoos - and this is something I whole-heartedly support. This is something I've blogged about <a href="http://womeninastronomy.blogspot.com/2013/12/is-science-is-in-eye-of-beholder-hint-no.html">before</a>. It is extremely important to me that we concentrate on the science that someone has to offer rather on their appearance, because scientists come in all sizes and shapes and we should let them be just like everyone else.<br />
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So isn't this a double standard? I spend time writing about how I should be able to wear what I want as a scientist and here I am really upset by his shirt?<br />
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This is the really important reason why it is different, in case it wasn't immediately obvious to you right away. It objectifies women.<br />
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Matt's shirt portrayed several images of a naked woman, allegedly as a tribute to a sci-fi character.<br />
He also allegedly said on air (and this is something I'll admit I didn't hear myself - it was relayed to me): "She's sexy, but I didn't say she was easy." [Edit: I've since been shown the <a href="http://www.dailymail.co.uk/video/sciencetech/video-1135292/Dr-Matt-Taylor-wears-fun-shirt-calling-Rosetta-mission-sexy.html">link</a> where Matt gives the "sexy" quote. He's talking about Rosetta, not the woman on his shirt! Thanks to Dave for reminding me to get the facts straight.]<br />
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Now - we have a huge problem getting women and girls into STEM fields. And spend lots of energy talking about how women aren't in <a href="http://news.nationalgeographic.com/news/2014/11/141107-gender-studies-women-scientific-research-feminist/">science and should be</a> (note: a Google search will yield many articles, that is just a recent one!).<br />
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And yet, here is a male scientist at a predominantly male science press conference from a male-dominated field - <b>that is going to be broadcast to schools around the world </b>- wearing a shirt objectifying women.<br />
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So, obviously the internet exploded. I, and many other people tweeted about it and were very angry, and later Matt changed his shirt (thank goodness before the most watched part of the landing).<br />
But this begs the question, why did Matt choose to wear the shirt? Or rather, did he think about the message it would send? Did he care? Did anyone at the press conference even look at the shirt?<br />
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I hope that in the coming days we will hear more from Matt and/or ESA, but I feel like now I need to redouble my efforts to remind young women interested in science that yes, your mind is important. That yes, you are capable of being taken seriously in STEM fields. That yes, we do want you here (come and join me). And that no, your body isn't what defines you.<br />
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Until then, I'm going to look at pictures of the glorious mission and hope my anger subsides. It is a great day for science. It is not a super day for getting women into science.<br />
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[Edit: Thanks to Summer and Emily at @startorialist for <a href="http://startorialist.tumblr.com/post/102460563537/in-light-of-the-wardrobe-malfunction-that-some">some happier space shirt designs</a> to brighten my day - and more <a href="http://startorialist.tumblr.com/post/102110951617/todays-saturday-diy-features-rachael-livermore-a">here</a>]<br />
<br />Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com22tag:blogger.com,1999:blog-174843027039031308.post-4869175959711635072014-09-29T14:10:00.002-07:002014-09-29T19:06:39.201-07:00"Black Holes don't exist:" giving context to sensational science newsA friend of mine recently pointed me to <a href="http://www.fromquarkstoquasars.com/new-research-mathematically-proves-quantum-effects-stop-formation-black-holes/">this</a> article about how "Black Holes don't exist." The article concerns two recent papers<br />
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<a href="http://arxiv.org/abs/1406.1525">http://arxiv.org/abs/1406.1525</a><br />
<a href="http://arxiv.org/abs/arXiv:1409.1837">http://arxiv.org/abs/arXiv:1409.1837</a><br />
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Since it came out the following edit has been added to the article:<br />
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"Due to some confusion, we feel it is important to clarify. The notable word in the title is “mathematically.” In science, there are conflicting predictions that come from different theories, assumptions, and equations–different equations result in different outcomes and different proofs. In short, one set of assumptions leads down one path and give us new (potentially important) things to consider. But there are many paths. It seems that many people were not sure how to situate or read these findings. Hopefully, this clarifies things. We’d like to apologize to anyone who took this out of context or who was confused by the implications. In the coming days and weeks, more physicists will weigh in with their findings. Things will update as they develop. Science on."<br />
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And this edit is actually exactly why I wanted to discuss the article. What does a claim like this mean, and how can a non-expert interpret it? The following is basically word-for-word what I wrote in reply to my friend who sent me the article.<br />
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I skipped reading the article at first [I have since read it, and you don't learn all that much], as lots of articles like it miss the point with physics. People love to say "Einstein was wrong!" or basically "[New sensational thing about physics hopefully with the word "Quantum" somewhere!]" while not appreciating what is really going on. And it annoys me. A lot. Anyway, onwards with constructive things...<br />
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I'm no expert, so I'll be arguing from authority, basically. First things first, Mersini-Houghton, the author, appears a totally respectable physicist who has published highly cited work on a variety of topics in the past, and works at a respectable institution. I'm in no way trying to slander her or her work. It's completely within the confines of work on these topics (black holes, the information paradox), as far as I can tell. Her recent work on these topics isn't highly cited (so far: and that is only a few months, but physics moves fast these days), but citations aren't a perfect gauge of a work's relevance. The important point to take away is: the author is certainly no two-bit crank posting on vixra (I hope I don't get bombarded by any more cranks than usual, but just look at the <a any="" at="" bombarded="" but="" by="" cranks="" get="" goes="" href="http://vixra.org/abs/1406.0178" just="" kind="" look="" more="" of="" on="" stuff="" t="" than="" that="" the="" there="" usual="">kind</a> <a href="http://vixra.org/abs/1408.0025">of</a> <a href="http://vixra.org/abs/1409.0146">stuff</a> that goes on there! If you've never lost a few hours on vixra, I highly recommend it.)<br />
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What I want to discuss here is how the paper was reported, and more importantly how non-physicists should read reporting on physics in general. It's all about context, and what we mean by "exist" I think...<br />
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The paper referred to in the article was published in a respectable journal, so it went through peer review and someone with more knowledge in the field than me thought it was correct. However, let's put it in context. There has been a whole lot of interest in issues related to black holes recently, thanks to a <a href="http://arxiv.org/abs/1207.3123">famous paper</a> on "firewalls." (<a href="http://en.wikipedia.org/wiki/Firewall_(physics)">http://en.wikipedia.org/wiki/Firewall_(physics)</a>) There seems to be something we don't understand about black holes in that they lead to a paradox that requires giving up one of three major pillars of physics (three according to wikipedia, I thought it was just two: unitarity and locality, but there ya go). The name "firewall" comes from the easy way out: there is a wall in the way that prevents anyone seeing the paradox. I'm not sure how seriously the firewall itself is taken. I think of it as a last resort to sweep things under the rug. But like I said, I'm no expert.<br />
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This debate over what to do with firewalls has led to a huge number of papers proposing different resolutions to the problem. To give that a number, the original firewalls paper has gotten 249 citations in the 2 years since it was published. Experimental papers, and confirmed theory papers, get more than that. But for a "pure thought" paper, that is astounding.<br />
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(Although, the <a href="http://arxiv.org/abs/hep-th/9711200">original AdS/CFT paper</a>, a paper on pure thought depending on your take on AdS/CFT applications, just became INSPIRE's most cited paper. It took over the <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.19.1264">model of leptons</a>, which is manifestly about the real world, but there ya go, that's physics!)<br />
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Many people have said we may need to do away with black holes in one way or another. For example, Hawking <a href="http://arxiv.org/abs/1401.5761">argued</a> that there is no paradox if black holes aren't "eternal." Another solution is that black holes are really <a href="http://en.wikipedia.org/wiki/Fuzzball_(string_theory)">fuzzy quantum</a> or turbulent things, and that makes the calculation that led to the paradox incorrect.<br />
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Okay, enough context. Mersini-Houghton's idea is one of these many that say black holes never form. The idea is that when a star collapses on the way to forming a black hole the "<a href="http://en.wikipedia.org/wiki/Hawking_radiation">Hawking radiation</a>" kicks in causing a pressure that stops collapse before a true black hole forms. If it never forms, there is a never a paradox. In her second paper out this month she works numerically with collaborator to show this happens in realistic situations (for example breaking the assumption of spherical symmetry). <br />
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So, is Mersini-Houghton correct? Her first paper came out in June and hasn't really been picked up in the community. The excitement over firewalls has died down, so maybe it's that. But if she was correct, it would be enough to set people off. It hasn't, so I judge there must be something about it that isn't compelling. Maybe she made some simplifying assumptions that people believe would make her argument wrong in a realistic situation? I'd like to ask an expert. The new numerical work makes me think she is correct, within the parameters she's set herself at least. It's whether those parameters are right.<br />
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But do I think it means anything? I'm inclined to say "no" for the following reasons:<br />
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1) It appears that we do see black holes out there in space. I'm not familiar with the observations, but as I understand it, it is an established fact. Maybe Mersini-Houghton can get around it, and her system still forms these things, but they aren't "formally" black holes because they miss the little paradoxy bit. Then, for all practical purposes (i.e. in astrophysics), it is a case of walking and quacking like a duck, but without the particular nuanced existential consequences. Practically, then, "black holes" still exist. This has to be true of all the other firewall solutions. The black holes are still there, because we see them, but some little bit in the middle is subtly different. <br />
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[edit: the <a href="http://www.eventhorizontelescope.org/">Event Horizon Telescope</a> hopes to directly image black hole event horizons in the coming years. Things like "<a href="http://en.wikipedia.org/wiki/Sagittarius_A*">Saggitarius A*</a>" are pretty good candidates for them. Any other comments about direct evidence for black holes are very welcome!]<br />
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Now you have to go out and find an observation that can confirm that. People are trying, but it ain't easy (see <a href="http://arxiv.org/abs/1312.4017">this</a> interesting proposal to tell proposed firewall solutions apart observationally using gravitational lensing!). An <a href="http://arxiv.org/abs/1409.4031">example </a>discussing Mersini-Houghton's work is that the "bounce" of the star instead of forming a black hole that she predicts could be the source of some high-energy cosmic-ray type things (fast radio bursts in this case). We see stuff like that, and by looking at them in detail maybe you can see what they came from. But this is messy astrophysics, and the number of explanations for these things is often very wide.</div>
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I'd like to see whether (i) Mersini-Houghton and co can still make real things that look enough like black holes that they are consistent with what we have seen already, and (ii) if they can make any novel observational prediction to test their theory. If the answer to either of these questions is "no" then they are dead in the water.<br />
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2) On a purely philosophical level, Mersini-Houghton's solution doesn't solve the firewall paradox in my opinion. It may solve it "in real life" if there are no collapsing stars that form tricky black holes. But it doesn't solve the problem for theory. In theoretical physics you can still set up a "thought experiment" and if that makes your theory inconsistent then you are in trouble. The whole firewall thing began with a thought experiment. Black holes are solutions to Einstein's theory, as long as they are you can always imagine one just sitting there. Conjure it out of nowhere. It doesn't have to be made by a collapsing star (because in the thought experiment you also conjured the star from nowhere too). </div>
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Mersini-Houghton's solution doesn't alter Einstein's theory or quantum mechanics, and so you can still do that thought experiment, and the firewall problem is still there.<br />
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So, Mersini-Houghton hasn't solved that problem in my opinion. And indeed, black holes still "exist" in the theory, so for many theorists they are still just as relevant as thought experiment probes of whether we really understand the universe. And many theorists are platonists anyway, so black holes, even of the thought experiment kind, do "exist."<br />
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3) Outside of thought experiment, as long as black holes are still this "formal" solution of Einstein's equations (and they are in Mersini-Houghton's theory as far as I can tell) then they do still *really* exist. This is thanks to quantum mechanics, where things are allowed to "pop in and out of the vacuum" (<a href="http://en.wikipedia.org/wiki/Virtual_particle">http://en.wikipedia.org/wiki/Virtual_particle</a>). Black holes do the same thing in quantum gravity, at least in string theory they do as far I know (I've heard people argue that they needn't in other theories, but I don't find these arguments compelling: in quantum theory you need a very good reason for things not to happen, or else they do). So, black holes still exist at the quantum level even in her theory.<br />
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Does that have any relevance to real life? Well, virtual particles do. We have very strong evidence that virtual particles are important. They have observable effects on particle collisions. They are the reason the electron has the charge and magnetic properties it does, so probably phones and microchips and such wouldn't work without them. (Does anyone have a better example of the reality of virtual particles? I mean, an accessible one, not just "electroweak precision observables, duh")<br />
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What about virtual black holes? (i) I don't know if they suffer form the firewall paradox, because of the popping quantum business, so maybe they aren't a problem. (ii) They are much harder to see. They are intrinsically quantum gravity things, and that is, as far as we know, not relevant to anything we have a hope of measuring on earth. But there is hope, and this is exactly why I study cosmology: the early universe is a lab for very high energy things, and we can hope for signals of quantum gravity in the sky (in fact this is a lot of what Mersini-Houghton did in her earlier career, too).<br />
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So, in summary:<br />
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* There are lots of theories like this. Maybe this one's right, maybe it isn't.<br />
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* There are at least "black hole-like" things out there in space that we have seen.<br />
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* Philosophically, black holes-proper still exist in this theory thanks to quantum mechanics.<br />
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* <b>We need to come up with observational and experimental tests and consequences of all this. </b></div>
Unknownnoreply@blogger.com3tag:blogger.com,1999:blog-174843027039031308.post-62081813025978062082014-09-11T19:50:00.000-07:002014-09-11T19:50:42.631-07:00The "Yes! And?" of science.I personally believe that the academic "brand" of Impostor Syndrome (IS) is particularly tricky to deal with because underlying it is a certain type of arrogance. It took me quite a lot of time with a coach (thanks to the wonderful SupporTED program I participate in through the TED Fellowship) to realise that I really was arrogant in my Impostor Syndrome: anyone could say what they like about me being talented, but I was holding onto the belief that I the only person qualified to make judgements about myself. So with a slight of hand, I can disregard your positive statement. Easy Peasy. My coach had to bring out the Logical Data Big Guns to deal with me, but she did so, wonderfully. She showed me this internally arrogant attitude was seriously flawed. My data analysis software, my ability to process external feedback, was broken. I realised that I was rejecting data points based on my faulty Bayesian prior, and then refusing to quote the prior when making statistical inference. I know! I know! Bayes would be rolling in his grave! I was shocked, and chose to rename the problem Self-Data Malfunction.<br />
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So, if you know that this part of yourself is faulty and you want to repair it, what can you do? Well this is all happening subconsciously to some degree, so it isn't a case of just hearing and accepting the opinions of others. If it were that easy, I would have done it already!<br />
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When talking about the issue with people, I often heard a phrase that I realise was intended to be helpful, but to me expressed exactly the wrong idea: "fake it ‘til you make it."<br />
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The idea is that even if you don't feel worthy to be in your job, position of authority or degree program, you just "fake it" and act like you are worthy until some time later you realise, hey - you are in fact the woman who deserves to be there! And there are lots of strategies online and in books to help you build up the skills to "fake it". <br />
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But this just hit right to the core of my Self-Data Malfunction. If I was "faking it" at all, surely there must be some truth in my "you don't belong here" Bayesian prior? So then maybe my self-data analysis software was right after all! Cue the spiral of non-productive thinking.<br />
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And then I remembered a wonderful thing I've learned from doing improvisational comedy (which, by the way, I highly recommend - it's like emotional version of walking in traffic: all the excitement, none of the physical harm). The improve technique is the principle of "<b>Yes! And?</b>"<br />
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Here is how it goes.<br />
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Say you’re doing an improv scene with someone on stage and they suggest something, like they are your long-lost sister, or the floor you are walking on just happens to be made of fire, rather than rejecting it outright for being crazy (as these improvised suggestions often are), you imagine and accept the universe they've just created. You say "<b>Yes!</b>" to the idea. <b>And?</b> Then you run with it!<br />
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The "And?" part means that you build on it and immerse yourself in it. That often involves justifying the suggestion they just made - making it work within the context of the scene and your established characters. And then, ‘hey presto!’, you're doing improv.<br />
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When I was thinking about the Self-Data Malfunction, I realised that rather than faking it 'til I make it, I can "<b>Yes! And?</b>" my life in academia. It is incredible what that subtle change in emphasis did for my outlook on academic life.<br />
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So, what happens when you find yourself on the shortlist for a job you didn't think you could possibly get? You say <b>yes! And?</b> Go give a great talk/interview! You now live in the reality where you are a viable and attractive candidate for the position. <b>Yes! And?</b><br />
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What about when you think you aren't good enough at writing this code, doing this derivation, finishing this paper? You remember that <b>yes</b>, you already got here, and you have skills that will enable you to tackle the task. <b>And</b>... then you go and smash it!<br />
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What happens when you are invited to submit that review paper or chapter and you feel like they may have asked the wrong person by mistake? You remember that <b>yes</b>, you have interesting things to contribute. <b>And</b> you now live in the universe where people want to hear/read them.<br />
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And what happens when someone like me wants to write about impostor syndrome, but there have already been great posts by incredibly smart, talented and accomplished men and women (for example <a href="http://womeninastronomy.blogspot.ca/2012/10/guest-post-by-john-johson-impostor.html">John John’s post,</a> <a href="http://www.ted.com/talks/amy_cuddy_your_body_language_shapes_who_you_are?language=en">Amy Cuddy's post on body language and how it can change your life,</a> <a href="http://womeninastronomy.blogspot.ca/2011/12/impostors-welcome.html">Ed Bertschinger's post on his own struggle</a>) on the subject? What if I don’t yet have a faculty post, and the authority that comes with that to be able to write about impostor syndrome without fear of the affect it may have on people’s perception of me?<br />
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I remember that <b>yes</b>, I think I have something new to add to the mix, and then I remember that as a graduate student and postdoc I would love to hear from someone who wasn’t so accomplished or high up the academic ladder to tell me about things they’ve learned and are dealing with. <b>And</b> so I write this here blog post!<br />
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Does it mean you will always then succeed at things? Definitely not. I imagine your rate of success may be exactly the same as before. But your rate of<i> trying new things</i>, and putting yourself out there and taking risks will definitely improve, and with more opportunities come more chances to do an awesome job and succeed. And as we all know, it's all about statistics really.<br />
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It isn't easy to do all the time. The "No, but" voices are much more skilled and generally shriek banshee-like in my head, but this feels to me like a much more holistic way of enabling me to live and grow into my career and my life. The change is slow, but what I find happens is that I start to really enjoy new challenges and scary things, not because I’m trying to prove myself, but because I enjoy taking that journey to the “and” part of myself and find that it isn’t so crazy a world in the first place.<br />
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So…. <b>Yes! And?</b><br />
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Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com3tag:blogger.com,1999:blog-174843027039031308.post-54934511877538435332014-05-21T08:50:00.000-07:002014-05-21T08:52:29.805-07:00BICEP2 and Axions. A few comments.<span class="">After our paper on Axions and BICEP2 came out (<a href="http://arxiv.org/abs/arXiv:1403.4216">here</a>) we were contacted quite a bit by various media outlets for comment. One article appeared in <a href="http://www.nature.com/news/gravitational-wave-finding-causes-spring-cleaning-in-physics-1.14910">Nature News</a>. There is another due to appear in <a href="http://www.quebecscience.qc.ca/accueil">Quebec Science</a> tomorrow. All the answering of interview questions made me think quite hard about explaining this business in a manner understandable by the lay person, and I think I got quite good at it. So I've decided to reproduce for you here the transcript of the interview I gave for Quebec Science. Their article only used a few quotes of mine in the end, but this here is the whole shebang!</span><br />
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<span class="">(P.S. Sorry for the weird formatting: I'm not really sure what happened)</span><br />
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<b><span lang="FR">On March 17th, astronomers announced they’ve found some evidence of gravitational waves. Could you tell us quickly what they </span>are and how/why they confirm the theory of cosmic inflation ?</b></div>
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What was actually detected is large angle so-called "B-mode polarisation" in the cosmic microwave background (CMB) (the other "E-mode polarisation" was detected years ago by WMAP). If this polarisation is not due to some foreground, for example dust or magnetic fields within the galaxy, then it is what we call "primordial", or at least "cosmological in origin". A leading theory for what could have caused this large angle B-polarisation is gravitational waves produced during inflation. There are other possibilities to produce large angle B, but the production via inflationary gravitational waves was a key prediction of inflation, worked out by a number of theorists in the 80's and 90's. It is therefore seen as the "simplest" explanation for B, and so seen as a strong confirmation of inflation, if the B is truly primordial/cosmological. Confirmation of this is necessary, and will be provided fairly soon by other experiments measuring the polarisation at different frequencies than BICEP; for example, the European Planck satellite should confirm this in their next data release scheduled for some time in late 2014.</div>
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<span lang="FR"><b>What do these results, if they are confirmed, reveal about inflation ? Is it exactly what the theory had predicted ? Or does it help to define more precisely how powerful inflation was, for instance?</b></span></div>
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In the simplest models of inflation the gravitational wave production is a direct probe of the energy scale at which inflation operated. The predictions of inflation give the amount of fluctuations in "curvature" or "scalar" modes: fluctuations we see as temperature fluctuations in the CMB, compared to the fluctuations in gravitational waves, as a function of the energy scale inflation operated at. We already knew the amplitude of the scalar fluctuations from as early as COBE in the 1990's, so the inflationary prediction boiled down to saying "if you measure B-modes with a certain magnitude, this implies inflation operated at a certain energy scale". BICEP measured a large amount of B-modes with an amplitude that implies the inflationary energy scale is very high, up near what particle physicists call the "grand unified scale".</div>
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<span lang="FR"><b>If I get it right, you used their measurements to rule out some models about inflation and dark matter. Could you first try to explain what is the link between inflation and dark matter ?</b></span></div>
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What we did was to work out what this high energy scale of inflation implies for certain theories of dark matter (DM), in particular axion DM. Inflation imprints the seed fluctuations of structure in the universe. If the inflationary field, the "inflaton" is the only game in town during inflation, then these fluctuations have a characteristic shape, called "adiabatic", and this shape is very close to what we observe in the CMB. However, if there is already DM around during inflation, then the inflaton imprints fluctuations in the DM which are non-adiabatic (so-called "isocurvature"). Given that the universe is seen to be largely adiabatic we are left with just a few options: 1) The DM was not around during inflation 2) There was not too much DM around during inflation 3) Inflation did not happen at too high energy (so that the non-adiabatic fluctuations were small). The B-modes imply a high energy for inflation, so we are left with option 1) or option 2).</div>
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<span lang="FR"><b>You work on the axion. What is it ?</b></span></div>
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Axions are a candidate for the DM in the universe. They come in many flavours. The most minimal scenario posits just one axion. It was put forward in the late 70's by Peccei, Quinn, Weinberg and Wilczek, not as a DM particle, but as a solution to a problem within the standard model of particle physics. Later it was realised the axion could be DM, and so it is seen as a strong candidate, since its existence was predicted for other reasons. Unfortunately, axions interact very weakly (as all DM has to, or it wouldn't be dark!) and we haven't seen any in the lab yet, but many experiments are trying to close in on them. There are also many other possible flavours of axion, in particular they are predicted to be very abundant in string theory. In string theory the axions come from having extra dimensions of space-time, and the number of them comes from the huge complexity of shapes possible for the extra dimensions. This has come to be called the "String Axiverse". Any or all of these string theory axions can contribute to DM, so in this scenario the DM is not just one particle, but possibly many different ones.</div>
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<span lang="FR"><b>What do these measurements about gravitational waves reveal about axions (that they could not have been created before inflation, is that right ?) ? Why ?</b></span></div>
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In the axion DM scenario, there is an energy scale, f, that controls whether the DM was around during inflation. What we worked out is that if the axions are around during inflation exactly how little of the DM must they be. That is, we worked out the answer to 2) above. For almost all possible axion masses we showed that if this energy scale f is large then the axions cannot be the DM. On the other hand, if f is small then the axions are produced after inflation, that is option 1).</div>
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To reiterate, if axions are produced before inflation then they screw up the adiabaticity of the universe as measured by the CMB. This means that any axions produced before inflation have to be just a small amount of the DM. Axions produced after inflation can still be the DM. This "before or after inflation" tells us about the axion energy scale, f. For the "after inflation" scenario, f has to be small and axions are squeezed into quite a small window.</div>
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<span lang="FR"><b>What does it mean for dark matter research ? Is this exciting ?</b></span></div>
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This is very exciting for many reasons: it is a very strong constraint on axion DM. Firstly, if axions are made after inflation then we now have a really good idea what energy scale they can be at, and experiments can target this. But more exciting for me is how we can now use axions to check on what we think about inflation by looking for them in exactly the opposite place, where they are made during inflation at high f. That is, if we can find high f axions (which are very light, the mass is inversely proportional to f) that should have been made before inflation, then this challenges the simple model where the B-modes were caused by gravitational waves from high energy inflation. </div>
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There are quite a lot of ways to look for these high f axions, and actually some reason to believe they may exist. Firstly, in string theory many models have high f, so if we think these models of string theory describe the world, then we had better find these DM axions, and there had better be something more complicated happening with inflation. There are new experiments that have been put forward that can look for high f axions in the lab. From my own research perspective we can go looking for high f axions out in the cosmos, by looking for the imprint they leave on galaxy formation. There are a few avenues where it is even possible that high f axions "fix" galaxy formation and make it more like what we observe, forming galaxies in a nicer way than other models of DM do. It's possible that in the near future, if high f string theory axions exist, that we will see their imprint in galaxy formation with surveys like Euclid. If we find any evidence for axions formed before inflation in galaxy formation, or in the lab, then it will be back to the drawing board with the simplest inflation models.</div>
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<b>And the summary</b></div>
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The clear cut answer is the following, I think. If we assume axions are all of the dark matter, then *the simplest interpretation* of BICEP2 rules out axions that were made before inflation, as the other blog says. On the other hand we can, as you quote from my blog, use this new constraint to limit the fraction of the dark matter that can be made up of axions. The other key point is that there are other ways to detect axions that are independent of inflation, for example in the lab or in galaxy clustering. If any of these searches actually detect them, then that means they are a large fraction of the dark matter, and the simplest interpretation of BICEP2 would have to be wrong.</div>
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So I think saying, as the <a href="http://atlas.ch/blog/?tag=inflation">CERN blog</a> does, <span class="">that "BICEP2 points out where to look for Axions" is too simplistic. </span><span class="">It only does that in a way that depends on the model of inflation. </span>We Should still look everywhere for them until That model of inflation can be pinned down.</div>
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Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-39820652201616904712014-03-17T20:10:00.004-07:002014-03-17T20:10:53.870-07:00B-eautiful tensorsThat's what BB said.<br />
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Yes, I've been waiting my whole life to make a post title like this.<br />
But seriously, if you haven't been hiding under a rock this morning you will have noticed the internet go crazy for the detection by BICEP2 of tensor modes, the 'smoking gun' of inflation. Even the <a href="http://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html?smid=fb-share">NYTimes</a> got in on the action.<br />
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The detection is parameterised by the tensor-to-scalar ratio "r", the ratio of tensor modes to the usual scalar modes whose spectrum we have characterised well with experiments like WMAP, Planck and the ground based experiments like ACT. This detection is r = 0.2 + 0.07 - 0.05 (the two numbers give the upper and lower 68% confidence intervals). This means that the detection is significantly non-zero. Why hello, tensor modes.<br />
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The B-mode polarisation spectrum is shown here below, where all the other limits are just that, upper limits. This is pretty awesome if you think about how this fits in with all the efforts of so many.<br />
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<a href="http://4.bp.blogspot.com/-ZFbZKR2YVTs/Uyckas7IBjI/AAAAAAAABLk/p7JfE8VSqdY/s1600/Screen+Shot+2014-03-17+at+09.35.37.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://4.bp.blogspot.com/-ZFbZKR2YVTs/Uyckas7IBjI/AAAAAAAABLk/p7JfE8VSqdY/s1600/Screen+Shot+2014-03-17+at+09.35.37.png" height="223" width="320" /></a></div>
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Figure 1. The BB-mode spectrum from BICEP2 with previous data.</div>
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This detection is really exciting, and has implications not only for the specific theory of inflation and the kinds of models it supports - it also allows us to place constraints on other physics. For example, my colleagues David Marsh, Dan Grin, Pedro Ferreira and I wrote a paper investigating what the detection would mean for axion-like particle dark matter. Such a large value of r places a constraint on the energy scale of inflation, H_I, which in the axion model place constraints on the initial misalignment angle - leaving a model that has a high level of fine tuning (fine tuning in physics is generally considered a bad thing, you don't want to have to tweak your model to give you something reasonable, you want that reasonable thing to emerge organically). If we consider very light axions, then this constraint on r tells you about the fraction of the total dark matter that can be made up of these axion-like particles (as a function of their mass). </div>
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We show that this new constraint (indicated by the red curve) limits the fraction of the dark matter that can be made up of axions... which helps us rule out parameter space (which is a good thing!) You can read all about it <a href="http://arxiv.org/abs/1403.4216">here</a>.<br />
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While the claimed detection of B-modes from BICEP2 is awesome and very exciting, it is also important to remain skeptical about possible systematics and issues with the detection. It is a very tough game, and such an important result that we need to make sure we pass all the possible tests we can throw at it. I for one am a little worried about leakage between temperature and polarisation in the spectrum. If you look at the cross-correlation between this measurement and the BICEP1 data, it seems that there is excess power on small scales (large multipoles).</div>
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<a href="http://3.bp.blogspot.com/-iJmDTMCrquY/Uycmha3bj9I/AAAAAAAABLw/aj5taOw1Dco/s1600/Screen+Shot+2014-03-17+at+09.44.41.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="http://3.bp.blogspot.com/-iJmDTMCrquY/Uycmha3bj9I/AAAAAAAABLw/aj5taOw1Dco/s1600/Screen+Shot+2014-03-17+at+09.44.41.png" height="320" width="317" /></a></div>
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Now it bears repeating that the BICEP2 result on r is only based on the scales between 30< ell<150, but these high ell issues to need to be addressed, as leakage could bias your signal high (make the evidence for tensor modes stronger).</div>
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Another thing to worry about are foregrounds. The team have presented reasons why they think foregrounds are not an issue for a signal so large, and it looks like they've done their homework, but I'll spend the next few days digesting the paper in more detail.</div>
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Also, this is such a large signal that we need to think about why other experiments have not seen it. In fact, if you consider the figure below from their paper:</div>
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you might be worried about a conflict with the results presented by the Planck team last March. First of all this plot is made by marginalising over running of the spectral index, so it is beyond the "vanilla" model + tensor modes (it has another parameter in it, the running of the spectral index, which gives it two free parameters relative to the base LCDM model without tensor modes at all).</div>
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So, the bottom line: I am excited by this (and so should you BB!) but there is more to understand and this result needs to be battle tested and confirmed. Long life the scientific process!! </div>
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To BB or not to BB.</div>
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Ok, I'm done. Happy BB-day all.</div>
Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-70562647116414498732014-02-17T08:46:00.002-08:002014-02-17T08:46:38.716-08:00Belief in Quantum Gravity (and Cosmology)Recently a few discussions have alerted me to the role of belief when it comes to theories of quantum gravity. This comes about essentially because of the huge energy scales involved in quantum gravity: because we have no (direct) experimental access to the Planck scale.<br />
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Firstly, what is the Planck scale? We need to a short lesson on units to get there. The Planck scale is what we assume to be the natural scale in gravity. As a mass scale it is approximately the square root of 1/G, where G is Newton's constant (I normally prefer to include a factor of 8 pi and call this "reduced Planck scale" simply "the" Planck scale, but that is a matter of preference, although as we are discussing, preference is a driving factor here...). Planck noticed that "natural" units for physics can be established based on a few fundamental constants, that is, we measure things in units of those constants. The first is Planck's constant itself, h (or "h-bar" if you divide it by 2 pi), which measures units of angular momentum (Joules per second in SI), and is the fundamental constant associated to quantum mechanics. Next is the speed of light, c, which measures units of speed (duh!) (metres per second in SI), and is the fundamental constant associated to relativity. Finally, then, comes Newton's constant, G, which measures the force of the gravitational field of body of fixed mass (per unit distance squared from that body, per unit mass of that body, per unit mass of the test particle feeling the force, which all follows from Newton's famous law of gravitation). Newton's constant also appears in Einstein's theory of general relativity, and so is associated to all gravitational physics (it is inserted by hand into general relativity to fix the units and the weak limit, but by consistency carries through the rest, and in all that spectacularly verified glory).<br />
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Here's where the fun starts: we can measure *all* dimensionful quantities in physics in terms of these three constants. Let's focus on the Planck mass. First of all, notice it involves masses, in particular, two masses, and so mass squared (hence why we took the square root above). All the other things it involves can be expressed as appropriate powers of c and h. We can get acceleration from using the units of c and part of h (the seconds bit), and we can also use c (via E=mc^2) to turn energies, i.e. the Joules part of h, into masses. That leaves G just a measure of 1/mass^2, and the mass it measures is the Planck mass.<br />
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Now, gravity is a very weak force. What does that mean? It means that for all the fundamental particles we know if you consider the force between any two of them then the gravitational force is far weaker than any of the other forces (yes, even the Weak force). But, if there were a particle that weighed a Planck mass (which is about 10^18 times the mass of the proton, or the same mass as about one ten thousandth of a gram, judging roughly from a mole of hydrogen which contains 10^23 protons) then the strength of the force of gravity between those particles would be equal to the strength of all the other forces.<br />
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There is also that sneaky "per unit distance squared form that body", which means if you bring the particles closer together, gravity gets stronger. When you compute that change in force taking account of the appropriate quantum mechanics (the renormalisation group flow) then we find that all the forces not only change in this simple high-school physics way, but also fundamentally, as we go to short distances. The constants of nature "flow" with energy scale (though h and c, and debatably G, do not). This means that in addition gravity becomes of comparable strength to other forces on very short distance scales, in fact at the Planck length (using our units we can change mass into length too). (If you want to read more about all of this, go and read Frank Wilczek's great book "The Lightness of Being")<br />
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Normally in computing quantum effects we can ignore gravity because it is so weak, but at the very high energies of the Planck scale, gravity becomes so strong that we cannot ignore it, and this is therefore the scale at which a theory of quantum gravity is needed. At all the energies below the Planck scale gravity was so weak that we could treat it as a "classical background". (It is a common misconception that physicists "cannot treat quantum mechanics and relativity at the same time". We're actually very good at it: we can do so-called "quantum field theory in curved space-time", but to do this we always treat both halves separately, that is we have "classical space-time")<br />
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Okay, so now we are finally there and we can discuss why quantum gravity involves belief. It involves belief because the Planck scale is so very big. It is 10^18 GeV in particle physics units. The rest energy of a proton is about 1 GeV. The LHC runs at about 10^4 GeV. The biggest machine physicists can even think of making in the foreseeable future is about 10^5 GeV, which is still a very long way from the Planck scale. (I read somewhere that a particle accelerator capable of reaching the Planck scale would have to be the size of the solar system and use a large fraction of the sun's total output. I don't know where I read that, or how the maths was done) At these comparatively low energies we can don't need to specify our theory of quantum gravity in order to do calculations in normal theories. As long as the quantum theory reduces to general relativity in the right limits, pretty much anything goes (although some things may not, they may "resist embedding", as recently and elegantly discussed in <a href="http://arxiv.org/abs/1402.2287">this paper</a>).<br />
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The enormity of the Planck scale means we cannot do experiments to test quantum gravity directly. And this means that for the most part whether you think string theory is a better theory than loop quantum gravity, or vice versa, is based on your aesthetic opinion about those theories. The role of aesthetics in physics *is* important, and helps guide us towards new laws (for more on this read/watch Feynman's "Character of Physical Law", or read Weinberg's "Dreams of a Final Theory"). It is precisely that aesthetics that has even got us as far as being able to contemplate quantum gravity, but beliefs about aesthetics diverge at the edges of our knowledge.<br />
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I came to think about this recently during a conversation with colleague. We were discussing what kind of indirect evidence could possibly be considered as for or against a given theory of quantum gravity, where by indirect I mean evidence discovered well below the Planck scale, either in cosmology or in a spectrum of new particles that could be found at foreseeable collider. I was primarily thinking of whether this evidence could support a complex theory of quantum gravity with many possible solutions, in particular, the "string landscape". Certain solutions and low energy physics scenarios appear "more likely" (in quotes because of the notorious measure problem: there is a *lot* to discuss here) in the landscape, and I argued that seeing such signals could be indirect evidence for the landscape (I do argue this a lot, and was particularly inspired by Paul Langacker's recent colloquium at PI on this subject, which you can see <a href="http://pirsa.org/displayFlash.php?id=13010116">here</a>). My colleague replied:<br />
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<b><i>"In [theory of quantum gravity] which I believe in, the situation is..."</i></b><br />
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and we went on to try and interpret (unsatisfactorily in my opinion) all such results in light of said theory. And so, it has become abundantly clear to me how important our beliefs are in interpreting indirect evidence. I guess this is obvious, but it does get a little worse. Earlier the same day I had discussed during a mini-conference this exact topic of indirect evidence pointing to string theory and the landscape. I asked the audience, "if we discovered ultra-light axions in cosmology would you consider this a good pointer towards string theory and the landscape?". An audience member replied:<br />
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<b><i>"No, I would try and interpret it in light of [theory of cosmology]"</i></b><br />
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I found this very honest, but depressing. The role of belief is so strong in the far and esoteric reaches of cosmology and quantum gravity that even when faced with a nominal prediction and hypothetical evidence for that prediction, someone cannot be convinced away from their beliefs. I'm not trying to be above all of this. I admit to being in a similar situation myself. I *believe* that the landscape is unavoidable, and that this behooves us to interpret the world in light of this. Why? Because, following Gell-Mann "anything that isn't forbidden is mandatory" (quoted from that same elegant paper linked to above) the landscape has a much wider space of what is possible, and thus not forbidden, and is therefore an interesting playground that forces us to question all possible assumptions. As a phenomenologist this is daunting, but I love the challenge of trying to find tell-tale needles in this haystack.<br />
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I wonder, even if we could do experiments up at the Planck scale, if all parties could ever be convinced? If scattering carried a uniquely stringy character (there are some, but I don't know them) could this still be "interpreted in light of [theory]"? On the flip-side, and this is more important to me, what types of evidence would I consider as being counter to my own beliefs that might force me to revise them?Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-44113144792323210452013-03-18T20:03:00.000-07:002013-03-18T20:03:00.478-07:00'Twas the week before Planckmas...This week will see cosmologists excitedly waiting for, and celebrating, the upcoming results from ESA's <a href="http://www.esa.int/Our_Activities/Space_Science/Planck">Planck</a> satellite. We've been waiting for this day since the <a href="http://www.youtube.com/watch?v=AQ8XVEHeh_g">launch of Planck in 2009</a> (in fact, most people having been waiting for this day since the late 1990s, when the satellite was proposed, initially called COBRAS/SAMBA). This multi-national collaboration has already released some data and results a year ago (on subjects such as point sources and clusters detected through their Sunyaev-Zel'dovich signature), but the first large suite of cosmology results will be announced on Thursday the 21st of March 2013, at a <a href="http://www.esa.int/Our_Activities/Space_Science/Planck/Call_for_Media_First_cosmology_results_from_ESA_s_Planck_mission">large press event</a>.<br />
Here at Princeton Astrophysics, we are having our own Planck Party at 5 am, and event which will no doubt have as much excitement as the <a href="http://www.ias.edu/about/publications/ias-letter/articles/2012-summer/higgs-celebration">pre-dawn Higgs</a> party we had at the Institute for Advanced Study last summer.<br />
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So what is all the excitement about?<br />
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Until the Planck release, the tightest constraints at multipoles less than 1000 have come from NASA's <a href="http://map.gsfc.nasa.gov/">WMAP</a> satellite, which was recently awarded the Gruber Foundation Cosmology Prize. WMAP operated for nine years and really helped to pin down the cosmological model on the largest scales.<br />
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<a href="http://lambda.gsfc.nasa.gov/product/map/current/pub_papers/nineyear/cosmology/images/med/gh9_f06_M.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="320" src="http://lambda.gsfc.nasa.gov/product/map/current/pub_papers/nineyear/cosmology/images/med/gh9_f06_M.png" width="400" /></a></div>
The plot above shows the power on the y-axis as a function of multipole (x-axis). Multipoles are inversely related to angle, that is, large angles correspond to small values of the multipole, while small scales are large values of l.<br />
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On smaller scales (i.e. to the right of this graph) two experiments have dominated the game recently, The Atacama Cosmology Telescope (based in the Chilean desert, and the collaboration I'm a part of) and the South Pole Telescope (no prizes for guessing where this telescope is!)<br />
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<a href="http://2.bp.blogspot.com/-4OKPvg6VbQ4/UUc5o76rjJI/AAAAAAAAAuM/qpOYd6Ec3a8/s1600/wmap9+spt+act.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="228" src="http://2.bp.blogspot.com/-4OKPvg6VbQ4/UUc5o76rjJI/AAAAAAAAAuM/qpOYd6Ec3a8/s400/wmap9+spt+act.png" width="400" /></a></div>
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The gold points are the same as the black points in the top plot, but with a logarithmic scale on the y-axis. From this plot, it is clear to see how ACT and SPT provide all the signal at small scales - the WMAP data points end around l=1000. Combining the data from WMAP with these experiments helps us put tight limits on our cosmological model and on non-standard physics in the early universe.</div>
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Planck will improve on this picture by making the error bars much smaller on all scales. On large scales we are looking to see if any of the <a href="http://arxiv.org/abs/1001.4758">WMAP anomolies</a> are present, and on intermediate scales Planck will also greatly reduce the error bars (on multipoles of 800 - 2000), where the WMAP error bars are large or unconstrained (see the linear scale plot at the top of the page). </div>
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This is particularly interesting for a parameter of recent interest, namely the effective number of relativistic species, or Neff. If we had three neutrino species (which is the standard picture) - Neff would be 3.046 (this number is not exactly three due to electron-positron annihilations in the early universe). It helps to think of the number in terms of extra neutrinos, but what Neff actually measures is if there was any extra (or less) energy from such a relativistic species. It doesn't specify what that species should be, and many authors have proposed some interesting candidates, from sterile neutrinos to `dark radiation'. If there was more relativistic energy when the CMB was formed, this would lead to a few interesting effects, the most obvious being the decrease in amplitude of the small scale Silk damping tail - the intrinsic CMB spectrum which drops in power as l increases. Of course, there are many degeneracies between Neff and other parameters, which is why better data (and independent data) help us tease apart the degeneracy.<br />
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<a href="http://1.bp.blogspot.com/-Bmc6yQY8O-Q/UUdcIDVeuhI/AAAAAAAAAuc/U8Gi4w3B96g/s1600/wmap+spt+act_neff.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="192" src="http://1.bp.blogspot.com/-Bmc6yQY8O-Q/UUdcIDVeuhI/AAAAAAAAAuc/U8Gi4w3B96g/s640/wmap+spt+act_neff.png" width="640" /></a>All three experiments (WMAP, ACT and SPT) recently released their constraints on cosmological parameters including Neff (they are <a href="http://arxiv.org/abs/1212.5226">here</a>, <a href="http://arxiv.org/abs/1301.0824">here</a> and <a href="http://arxiv.org/abs/1212.6267">here</a>).<br />
The three experiments have some mild tension the best-fit values of Neff (we discuss the consistency between them in <a href="http://arxiv.org/abs/1302.1841">a recent paper</a>) - the plot above shows this. In both cases the ACT and SPT data are combined with the latest WMAP9 results. The left-most panel shows the one-dimensional contours for Neff, while the two right panels show an error ellipse. Dark ellipses shows models which are consistent with the data at 68% confidence, while the lighter ellipses show models consistent at 95% confidence. Any model outside of the ellipses is less than 5% likely to fit the data. The red lines/curves are for WMAP9 and ACT, the green for WMAP9 and SPT and the black curves/contours show the combination of all three experiments together. While SPT sees a higher value of Neff than 3.046 at Neff = 3.74 +/ 0.47, and ACT a slightly lower value with Neff = 2.90 +/ 0.53, the combined data are completely consistent with the standard picture: Neff = 3.37 +/ 0.42 (which may dismay or delight you, depending on your camp of interest!).<br />
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By improving the constraints on the power at intermediate scales, Planck should tell us more in a few days. This is particularly interesting because while ACT and SPT look at different regions of the sky (on smaller patches), Planck will release results based on the full sky - another independent measurement of the same underlying physics.<br />
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<i>[There is a great post by Jester on <a href="http://resonaances.blogspot.co.uk/2013/01/how-many-neutrinos-in-sky.html">Réesonances</a> about Neff (posted just before the ACT constraints were released) written for those with a particle physics interest.]</i><br />
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Planck will also measure the weak lensing of the CMB by gravitational structures - an extremely subtle effect which moves power around on the maps of the CMB temperature on arcminute scales, but coherently over degrees. <a href="http://arxiv.org/abs/1301.1037">ACT </a> and <a href="http://arxiv.org/abs/1202.0546">SPT</a> have measured this deflection - and Planck will improve the errors on this measurement by a great deal on all scales. The deflection power spectrum is a strong probe of structure, and things which would wash out that structure, such as a massive neutrino.<br />
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Another key constraint that will come from Planck is one on the non-Gaussianity of the initial conditions of the universe, which is a strong test of the various inflationary models out there today.<br />
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<i>[There is an awesome TEDx talk by Ed Copeland on <a href="https://www.youtube.com/watch?feature=player_embedded&v=uGbWIVzjays#!">CMB physics and inflation</a> which provides a nice summary of the link between the CMB and the early universe.]</i><br />
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One way to think of non-Gaussianity is by imagining a distribution with some level of skewness and kurtosis (so, a normal distribution that has been distorted). A simple picture for how to produce a two-dimensional temperature map from the power spectrum above, is to generate a Gaussian realisation of the power spectrum - at each angular scale (defined by the multipole), use the power to define the variance in temperature on that scale. However, if the temperature field is non-Gaussian, then the full map is not described by the two-point function, or power spectrum: we need to use higher order statistics to characterise the initial conditions if they are non-Gaussian! That is typically why we use the bispectrum (the three point function) and higher order statistical correlation functions to measure non-Gaussianity.<br />
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The <a href="http://arxiv.org/abs/1212.5225">WMAP bound</a> is consistent with zero fNL (the parameter describing the level of non-Gaussianity, a quantity we expect to be vanishingly small in the simplest single field models of inflation) with -3< fNL < 77 at 95% confidence. However, the expected errors on fNL from Planck should go from the errors on fNL of about 20 to errors of a few! If the central value of fNL = 37.2 found by WMAP remains while the errors decrease we will put some serious pressure on many inflationary models - it is always a theoretical treat to find you aren't living in a `vanilla' universe.<br />
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These are only a few of the presents we are expecting on Thursday. Make sure to tune in to hear the results, and enjoy the flurry of papers on the latest cosmological bounds using the temperature of the CMB. For the polarisation measurements, you will still have a little wait before Planck (and ACTPol and SPTPol) entice you with more results - as it is an even more delicate procedure to tease out polarisation from these sensitive instruments.<br />
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Until then, we wait to boldy constrain where only a few experiments have constrained before...<br />
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<br />Reneehttp://www.blogger.com/profile/14619548071779937248noreply@blogger.com0tag:blogger.com,1999:blog-174843027039031308.post-31350226022703506972012-12-17T08:54:00.002-08:002012-12-17T08:54:37.842-08:00The Folklore of the Untestability of String Theory<br />
I recently received an email from an undergraduate after agreeing to give a talk to their society about string cosmology and testing string theory. The undergraduate expressed amazement about being able to test string theory, given the "folklore that it is untestable". I thought this warranted some explanation, obvious to any sensible Bayesian. Here is an extended version of my reply to this student.<br />
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I apologise for overstating Bayesian, but I wanted to ram it down the student's throat: you, my discerning readers, may not need so much ramming. I also apologise to any better Bayesians than I for perhaps liberal use of terminology and butchering of concepts.<br />
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Firstly, the folklore applies to string theory "as a whole", rather than individual models. I will ignore the obvious point to be made that string theory "as a whole" is a beautiful mathematical framework and testing it is not the point. I will instead approach things as a cosmologist and a Bayesian.<br />
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The folklore is too simplistic, in the sense that one can always assume a model and verify its parameters. This is what one always does in a Bayesian philosophy of science, which is manifestly what practicing science actually is. However one is able to construct other models that may have similar consequences. This is often the case in fundamental theory: think of the plethora of models being tested at the LHC! (Although silly particle theorists aren't Bayesians and use silly concepts like the "look elsewhere effect". Tut tut...)<br />
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The selection for the models to test, outwith unexpected and contradictory results (lack of concordance, which we all hope and pray for), comes down to a selection based on priors (also Bayesian, except Bayesians choose to recognise them!). These priors are either (arbitrary) prejudice based on intuition, unification etc, or (less arbitrary) priors based on fine tuning and the ability to perform meaningful calculations. String theory and other theories fall into both camps on priors depending on your taste. In my opinion string theory falls into both at once positively and negatively. A Bayesian picks one model and tests it, then compares models to one another.<br />
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String theory comes up trumps in (practical) cosmology because it is complete enough to actually give meaningfully testable cosmological models, although many of them. More standard particle theories also give such many models, as can explicitly non-fundamental models. Currently the data cannot tell them apart, therefore a Bayesian accepts both as equally likely modulo the priors. By this I mean that I accept modified gravity is equally as likely as a cosmological constant based on the evidence, but my prior is a hard prejudice that it is not the truth.<br />
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An aside on this point: there many theories of modified gravity and field theory Lagrangians. As many as low-energy models based on string theory? More? I don't know: what is the measure on theory space? Clearly all these low-energy theories (paradigms?) fall foul of being "untestable" based on the folklore, which I now hope you are starting to agree is fallacious.<br />
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The folklore refers to "complete" theories, so the testable other models referred to above in particle and non-fundamental theory (by which I mean low-energy modifications of gravity) are manifestly not "complete". But then, "completeness" is still an arbitrary prior.<br />
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The folklore is applied in an ideal world that may not exist. In this ideal world the many additional parameters needed to turn string theory into a model make it unpalatable (although, as I've said we do measure some of them in the *context* of cosmological models. For example I can construct a stringy model of inflation and use the scalar amplitude to bound some property of the compactification.). In this ideal world one can do experiments at arbitrarily high energy and across all of space time and "test" whatever you like. But this is not the world we live in, certainly in practice, and in fact maybe not even in theory. In theory I mean we cannot do all the experiments required to gain complete knowledge of cosmology. In simple terms this is due to the special relativity fact that events outside your light cone are inaccessible to you. In more refined terms it is boiled up in complex arguments about the existence of observables in quantum gravity in de-Sitter space and eternal inflation.<br />
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Anyhoo, practically we are always limited by our finitude and fallibility to not have access to a perfect world of infinite data at the Planck scale, and so we are Bayesians. Comparing models we have and measuring their parameters. Sometimes these models come from string theory, and we may prefer these models to low-energy field theory models for their being part of a larger framework. Or we may not. Model selection with insufficient data is prior driven. But you do need a theory that gives you models to test, and string theory certainly does that in cosmology!<br />
Unknownnoreply@blogger.com3tag:blogger.com,1999:blog-174843027039031308.post-37728617435550638182012-06-27T15:27:00.000-07:002012-06-27T15:27:00.195-07:00Scalar FieldsBeginning this blog on the "s" theme of its title, I want to talk about scalar fields, or scalars. A scalar field is something that has a value at every point in space: the canonical example is temperature. There is a value, but no direction.<br />
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A magnetic field on the other hand, points from north to south, it has a direction and is known as a vector field. If I look at the Earth the "normal" way up, I see this as going down on the map, but I can look the Earth the other way up, because there is nothing special about the choice of map makers, and I see the magnetic field going up. Vectors care about how you look at them. They care about the frame of reference. Scalars don't care what direction you look at them or how fast you are moving: they are Lorentz invariant.<br />
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For this reason, scalars are odd things, as they don't transform under the Lorentz symmetry of special relativity. This allows them to have a vacuum expectation value, or vev. A vev means that the scalar field has a value that is the same everywhere in space. This is not something a vector field could have without picking out a preferred direction in space, and violating a form of the cosmological principle. The vev has an energy, and it contributes to the cosmological constant, which effects the expansion rate of our universe.<br />
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Interestingly, this is contrary to the point of this <a href="http://blogs.discovermagazine.com/cosmicvariance/2012/05/10/higgs-ripples-in-the-koi-pond/">video</a> explaining the Higgs (which I found on Cosmic Variance). The video explains quantum fields as things whose ripples are what we observe: they are everywhere but we only see them when they ripple. However, the Higgs is a scalar, and its vev also contributes to the cosmological constant, which changes when the electroweak symmetry is broken in the early universe. With scalars, we see more than just their ripples.<br />
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I mentioned above that temperature is the canonical example to explain what a scalar field is to the non-specialist. However, in relativity, we express the temperature as the energy density of the electromagnetic field, but the energy density changes when we move to different frames of reference. Temperature is the fourth root of the energy density, which forms part of a four vector and transforms under Lorentz transformations. Temperature is NOT a scalar in relativity! In Cosmology a major number is the temperature of the CMB. How can we speak of this in a way that does not violate the cosmological principle? The truth is that we only measure the temperature of the CMB in the preferred frame of reference that is at rest with respect to the CMB. The CMB defines the preferred frame. Models where we mess with this are called "tilted universes" (see e.g. Liddle and Lyth's book on Inflation).<br />
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When we write down fundamental theories we don't want them to care about such arbitrary choices like the maps of the Earth. Scalars fit this bill. If we want our fundamental theories to contain vector fields or other more complicated objects, however, we must "contract" them up to make scalars. Being a scalar is very important and is related to fundamental concepts in physics about symmetries and the action principle.<br />
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In group theory, scalars are the singlets of any group. A representation every group shares. They normally don't feel whatever force is associated with a group, for example neutrinos are singlets under the U(1) of electromagnetism (EM), so they are not charged. but neutrinos are not scalar fields, we name things "scalars" in the sense of "scalar field" by the way the field behaves under the symmetries of General Relativity (GR), the diffeomorphisms. However, because GR sees all energy density, even scalars of diffeomorphisms source gravity, in a way that scalars of EM do not source EM.<br />
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Fundamental scalars are very odd things. By which I mean things that are scalars without us having to "make" them that way, in the way we build scalars in EM to make the Maxwell Lagrangian by contracting the vector fields. The only one that we think exists in the standard model is the Higgs field, and an aversion to fundamental scalars is one philosophical motivation for theories like technicolor that are alternatives to the Higgs. The scalars of technicolor are not fundamental, they are "composite" scalars. Like the pion of the strong force, they are built by adding two fields of opposite spin.<br />
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However, one theory that is full of scalar fields is string theory. These scalars are describing things to do with the internal space that we lowly 4d mortals can't see (see for example <a href="http://arxiv.org/abs/1204.2795">http://arxiv.org/abs/1204.2795</a>). These fields are called moduli. These apparently fundamental scalars in 4d are just vestiges of higher dimensional gravity, and appear to us after the famous phenomenon of Kaluza-Klein compactification. String theory scalars are very important in making string theory reproduce the physics we know, give us lots to play with in the physics we don't, but are infamous as the source of the string landscape and the charge of loss of productivity often brought against the theory. (string theory also furnishes us with many other similar fields called axions, which are "pseudo-scalars" and behave differently under spatial reflections, but they are the subject of a whole other post…)<br />
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Another theory that contains lots of scalars is supersymmetry (SUSY). SUSY builds in scalars in what are called chiral multiplets. The chirality (or handedness) of the weak nuclear force is one of the most important facts about our universe and is very constraining for model builders. Incorporating SUSY into a theory that is chiral gives us all the superpartners as scalars or "sparticles" - and at least as many as there are fermions (quarks and leptons) in the standard model. We know the weak force is chiral: if the world is also supersymmetric, then scalars play a key role.<br />
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But why, apart from the possible imminent detection of our first genuine scalar in CERN next week, am I telling you about scalars? I'm a cosmologist, and scalars are everywhere in cosmology theory. Why? Because they are easy to work with. One can often do away with the awkward phase space descriptions one needs to properly describe for example photons and neutrinos (the analogous distributions to the Maxwell-Boltzmann distribution in ordinary statistical mechanics). All modified gravity theories are GR plus a scalar or more. (the exception are the odd (and oddly named) "Galileons", also the subject of future post)<br />
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What do we use scalars for in cosmology? Well, scalars can have potentials. Because they are Lorentz invariant, any function of them is, and this is their great utility. They serve us as inflatons, dark energy, and dark matter. However, our greatest theoretical problem, the cosmological constant problem, is intimately related to these very potentials, and the vevs that allow scalar fields to perform the Higgs mechanism. (if you've never read it, I highly recommend Weinberg's 1989 <a href="http://rmp.aps.org/abstract/RMP/v61/i1/p1_1">review</a>)<br />
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Do true scalars exist? Because of the need for a change in, or constant, vacuum energy in any theories of electroweak symmetry breaking (what the Higgs does), inflation, or Dark Energy/modified gravity, one always requires at least an approximate, or composite, scalar degree of freedom: technibaryons, or massive gravitons are all described by effective scalar degrees of freedom. One could even argue that string theory, in its true 10 or 11 dimensional form doesn't have scalars: the effective ones are degrees of freedom of a string. The question of whether we have fundamental scalars is still open, but could come one step closer to being resolved on Wednesday at <a href="http://press.web.cern.ch/press/pressreleases/Releases2012/PR16.12E.html">CERN</a>. Even if particle physicists find the Higgs, knowing whether it is really fundamental will be a whole other game. Although the LHC should be able to distinguish it from many popular technicolor theories, we will probably have to wait longer to find out. (I'm sure <a href="http://resonaances.blogspot.co.uk/">Jester</a> will have plenty to say about this)<br />
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Finally, even in a strongly coupled theory like technicolor, compositeness is not a definite concept thanks to the hot topic of holography and dualities. In dual theories the "fundamental" fields swap roles, so composites on one side of the duality are fundamental from the other point of view.Unknownnoreply@blogger.com1