tag:blogger.com,1999:blog-1513704378254120283.post8712840850380795096..comments2023-10-14T14:48:57.183-07:00Comments on The Trenches of Discovery: The universe as seen by Planck - Days Three and Four IShaun Hotchkisshttp://www.blogger.com/profile/04832423210563130467noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-1513704378254120283.post-40210770158171152972013-04-23T13:04:40.117-07:002013-04-23T13:04:40.117-07:00Hi Rick,
It's an unfortunate use of language ...Hi Rick,<br /><br />It's an unfortunate use of language from Planck. The non-Gaussianities being described in the two papers are different.<br /><br />In the paper specifically on non-Gaussianities, they are dealing with effects motivated by various models of inflation. They find no evidence for those specific types of non-Gaussianity and favour the simpler models of inflation.<br /><br />In the isotropy and statistics paper they are looking instead for less well motivated types of effects. Essentially they're just looking to see if the data is anomalous in <i>any</i> unexpected way. When you find something like this it is very difficult to quantify how anomalous it is. When you have enough noisy data, you expect to see some areas where the data doesn't match the model precisely (that's essentially the definition of noise). This is what Planck sees, i.e. some areas where the data doesn't fit the model precisely, but nowhere where it is so significantly wrong that it couldn't possibly be noise.<br /><br />They have quantified how unlikely these specific deviations are, but what is harder to quantify is the over-all probability that <i>any</i> type of deviation would be seen, which is definitely larger than the probability of the specific anomalies that were observed.<br /><br />What this means is that these unexpected anomalies might point towards something new, but they might just be noise. Anything new that is constructed to explain them would of course need to explain both the anomaly <i>and</i> everything else that the original model predicted. This is very difficult to do for these anomalies.<br /><br />I hope that helped in some way, sorry again for the delay.Shaun Hotchkisshttps://www.blogger.com/profile/04832423210563130467noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-82321738356345901172013-04-14T17:32:19.953-07:002013-04-14T17:32:19.953-07:00Perhaps even more relevantly:
Why are directly co...Perhaps even more relevantly:<br /><br />Why are directly contradictory assertions published by the same team on the same question at the same time?Rick DeLanohttps://www.blogger.com/profile/06675522207482535734noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-25285966070470855642013-04-14T17:30:03.687-07:002013-04-14T17:30:03.687-07:00"Planck showed that WMAP's evidence was o..."Planck showed that WMAP's evidence was only a statistical fluctuation and that, to Planck's accuracy, there is no evidence for primordial non-Gaussianity."<br /><br />From the Overview abstract:<br /><br /><br />"Planck finds no evidence for non-Gaussian statistics of the CMB anisotropies"<br /><br />From Planck paper #23 "Isotropy and statistics":<br /><br />"We detect pronounced signatures for both non- Gaussianities and anisotropies........a highly significant detection of both non-Gaussianities and anisotropies in the Planck data, consistent with those obtained previously with WMAP data"<br /><br />So which is it?<br /><br /><br />Rick DeLanohttps://www.blogger.com/profile/06675522207482535734noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-41190330621586271792013-04-11T04:00:29.463-07:002013-04-11T04:00:29.463-07:00Maybe, kind of.
The biggest problem would be qua...Maybe, kind of. <br /><br />The biggest problem would be quantifying what the initial conditions before inflation were like. We can't really "predict" them and without knowing them we can't really know what they would look like if we were to start seeing them.<br /><br />Based on the fact that the largest scale fluctuations are ~\(10^{-5}\), I think it would be relatively safe to say that the "inflated patch" would be at least \(10^4\) times bigger than our horizon (because fluctuations shrink proportionally to the scale factor during inflation). <br /><br />I'm not 100% confident with that, but it's my best guess. This is also all based on knowledge we've had since COBE's results. The precision from Planck and WMAP doesn't really help if we don't have a good theory for what the pre-inflationary universe would look like.Shaun Hotchkisshttps://www.blogger.com/profile/04832423210563130467noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-44048735775861699962013-04-09T14:17:10.028-07:002013-04-09T14:17:10.028-07:00Thanks, Shaun, that was extremely helpful.
I just...Thanks, Shaun, that was extremely helpful. <br />I just have a small followup question:<br />Based on Planck's observations, can we put a lower bound on the size of our inflationary patch today in comparison to the horizon ? For eg, can we say the patch is at least 1000 times bigger than the observable universe, perhaps even more ?Cosmonutnoreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-7814107384005241772013-04-09T12:06:10.729-07:002013-04-09T12:06:10.729-07:00Hi Cosmonut, sorry that it took me a while to answ...Hi Cosmonut, sorry that it took me a while to answer this.<br /><br />Your understanding is right. In principle the curvature could have any value today. However, the amount of inflation required to make the curvature of order 1 today is basically the same as the amount of inflation required to make sure the current horizon was in causal contact before inflation. Therefore, if the curvature was close to 1 today, our current horizon would also be close to the edge of our inflationary patch.<br /><br />The consequences of that would be that the largest scale fluctuations would not have been damped out significantly by inflation and we would not expect things like the CMB to be the same temperature in opposite directions.<br /><br />When inflation was first being constructed we already knew that the CMB was uniform to a certain degree (at least one part in 10^3); therefore (ordinary) inflation would also predict that the curvature was equivalently small.<br /><br />Basically, because of the isotropy of the universe on the largest scales we know that inflation persisted for at least a little bit longer than it would need to just to generate our horizon. Therefore, the maximum allowed curvature is smaller than 1, by a factor of 1/e^2N, where N is the number of additional efolds. It doesn't take many efolds to make the curvature very small.<br /><br />Also, when many people think "inflation" they think "eternal inflation" (because it can arguably overcome the problem of initial conditions in inflation). Eternal inflation would have many more efolds of inflation than just 60 and a prediction of a perfectly flat universe (with the normal inflationary spectrum of small perturbations).<br /><br />But of course there are exceptions. And that was what the industry was in the 90's. If the homogeneity and isotropy was set down in a false vacuum period of inflation, which then tunnelled into a slow-roll type of inflation, you could evade the problems I mentioned at the beginning of this comment. The false vacuum inflation would create an open, isotropic, universe and then if you had just enough slow-roll inflation you could get whatever (open) curvature you wanted today as well as an isotropic universe on large scales.<br /><br />I hope that helped.Shaun Hotchkisshttps://www.blogger.com/profile/04832423210563130467noreply@blogger.comtag:blogger.com,1999:blog-1513704378254120283.post-91636168748685871632013-04-09T00:05:26.283-07:002013-04-09T00:05:26.283-07:00I had a question - Why is it claimed that inflatio...I had a question - Why is it claimed that inflation predicts a flat universe ? <br />If I understand right, just after inflation the curvature is very close to zero - because the universe becomes much bigger than the visible horizon.<br />But afterwards the curvature gets pushed away from zero, so today it can have any value, right ?Cosmonutnoreply@blogger.com