tag:blogger.com,1999:blog-6976071487922527618.post1812570560550356315..comments2024-03-27T05:38:30.610+01:00Comments on Blank On The Map: What Planck has seenSesh Nadathurhttp://www.blogger.com/profile/07155102110438904961noreply@blogger.comBlogger9125tag:blogger.com,1999:blog-6976071487922527618.post-65440400939741726832013-03-24T21:16:23.432+01:002013-03-24T21:16:23.432+01:00It looks like you've misunderstood. Which is f...It looks like you've misunderstood. Which is fair enough, because my comments were probably a little cryptic. So let me clarify.<br /><br />First up, there's a distinction to be made between the inflaton field value, $\phi$, and the energy scale of inflation, $V(\phi)$, which is proportional to $\phi^4$. "Inflation taking place above the Planck scale" would mean $V(\phi)>M_P^4$; there are no models in which this happens. Indeed, the fact that gravitational waves have not already been seen places an upper limit on the scale of inflation quite a bit lower.<br /><br />However, there are plenty of models of models in which $\phi$ itself has to take values larger than the Planck mass. Such models are viewed with suspicion by many field theorists; there are many theoretical consistency issues that I won't go into here. Other models ("small-field models") don't have these problems but may have others.<br /><br />Planck's design sensitivity means that it could detect gravitational waves if the tensor-to-scalar ratio $r$ is $>0.05$ or so. A well-known result, called the Lyth bound, shows that in order to get such large values of $r$, the inflaton must take values $\phi>M_P$ (except there is actually a tiny bit of wiggle room here too). Small-field models – which many consider more natural candidates for the correct model of inflation – give values of $r$ orders of magnitude smaller, such that they can never be detectable, not by Planck or any future successors. All inflation models Planck constrains are large-field ones.<br /><br />I don't know enough about the experiments you mention to make a detailed comment about their capabilities compared to Planck. However, the ultimate limitation facing attempts to detect gravitational wave backgrounds is that our galaxy is a source of B-mode polarisation which drowns small enough primordial B-mode signals out.Sesh Nadathurhttps://www.blogger.com/profile/07155102110438904961noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-28684125371089070652013-03-23T22:01:55.884+01:002013-03-23T22:01:55.884+01:00Sesh,
I would've expected that Planck was des...Sesh,<br /><br />I would've expected that Planck was designed<br />(< 2009)with the sensitivity to detect B-modes based upon existing inflation models, most of which place the inflaton mass between GUT & reheating scales ~ 10^15 Gev. This is 4 decades below the Planck mass. I find it hard to believe that inflation takes place above the Planck scale.<br />Also, if B-modes are so hard to detect, why are there ongoing terrestrial efforts to detect them, e.g. BICEP2, & QUIET @ the S.Pole ? Surely their detection schemes have a higher noise level to contend with than Planck ?<br />Unknownhttps://www.blogger.com/profile/01117245295485526985noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-87176756199115765552013-03-22T17:04:43.588+01:002013-03-22T17:04:43.588+01:00Gosh, tough question. Almost nothing.
There are ...Gosh, tough question. Almost nothing. <br /><br />There are many things whose existence, or lack thereof, could rule out some <i>class</i> of inflation models, while favouring other classes of models. These include gravitational waves (the B-mode signal), non-Gaussianity, deviation from a power-law power spectrum, and so on. None of these are seen in Planck. Certain other things, like relatively large curvature, might be hard to explain, though you can get models for that too.<br /><br />But inflation as a paradigm? I'm struggling to think of anything that we don't already know doesn't exist. (I also don't know of any fully self-consistent, theoretically sound, believable model of inflation, but that's a different story.)<br /> Sesh Nadathurhttps://www.blogger.com/profile/07155102110438904961noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-59229391623129223822013-03-22T15:00:45.505+01:002013-03-22T15:00:45.505+01:00"Incidentally, a non-detection of a B-mode si...<i>"Incidentally, a non-detection of a B-mode signal by no means rules out inflation."</i><br /><br />What would definitely rule out inflation?<br />Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-52871092815163839692013-03-22T10:04:09.495+01:002013-03-22T10:04:09.495+01:00Searching for the B-mode signal is hard. It's ...Searching for the B-mode signal is hard. It's very small and the galactic foreground is much more difficult to remove. It will probably take them another year of working on the data before they can announce any results there, and even then I would advise you not to bet on a positive detection. Meanwhile there is lots of very interesting science that can be done more easily with the temperature power spectrum, lensing, ISW etc.<br /><br />Incidentally, a non-detection of a B-mode signal by no means rules out inflation. "Small-field" models of inflation (in which the inflaton field value does not exceed the Planck scale) generally do not predict any detectable gravitational waves (though some exceptions can be constructed).Sesh Nadathurhttps://www.blogger.com/profile/07155102110438904961noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-22938445134162436342013-03-21T22:39:20.003+01:002013-03-21T22:39:20.003+01:00Thanks for the insights. I have quoted part of yo...Thanks for the insights. I have quoted part of your post at length <a href="http://dispatchesfromturtleisland.blogspot.com/2013/03/precision-cosmic-background-radiation.html" rel="nofollow">at my blog</a>.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-2158717351073240632013-03-21T21:30:33.881+01:002013-03-21T21:30:33.881+01:00So does anyone know Why they did not search for B-...So does anyone know Why they did not search for B-modes first,<br />as a top priority ? If found, this would be a great victory for inflation, and if not, a truly revolutionary result empowering the cyclic universe to center stage.Unknownhttps://www.blogger.com/profile/01117245295485526985noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-44244729414859759582013-03-21T12:22:28.799+01:002013-03-21T12:22:28.799+01:00There was no mention of anything about the dark fl...There was no mention of anything about the dark flow. Presumably Planck can say something about this because of their SZ measurements, but I don't know what. The papers are now online <a href="http://www.sciops.esa.int/index.php?project=PLANCK&page=Planck_Published_Papers" rel="nofollow">here</a>, but there are 29 of them, so I can't make any more precise statements just yet!Sesh Nadathurhttps://www.blogger.com/profile/07155102110438904961noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-58833657915418284492013-03-21T11:57:44.457+01:002013-03-21T11:57:44.457+01:00what about 'dark flow?what about 'dark flow?Anonymousnoreply@blogger.com