tag:blogger.com,1999:blog-6976071487922527618.post7098737802651362041..comments2024-03-27T05:38:30.610+01:00Comments on Blank On The Map: Seeing neutrinos in the CMBSesh Nadathurhttp://www.blogger.com/profile/07155102110438904961noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-6976071487922527618.post-22023124107497895082013-02-14T11:28:20.186+01:002013-02-14T11:28:20.186+01:00Good question Simon. Actually what is done is to p...Good question Simon. Actually what is done is to parameterize the effect of any extra weakly-interacting, relativistic species present when the CMB was formed as though they were neutrinos, whether or not they actually are. So if in some model the extra species actually is a neutrino, which decoupled from the photon-baryon plasma at the same temperature as the regular neutrinos, then you would have $N_{\rm eff}=4$ (or 5 if there were two extra generations, and so on). But if it is a relativistic species that decouples at a different temperature, $N_{\rm eff}$ takes a non-integer value.<br /><br />Actually, in parameterizing these other species by $N_{\rm eff}$ you also implicitly assume that they have the same sound speed and anisotropic stress as neutrinos ... basically if you are trying to construct a non-standard model that still fits the data there are plenty of knobs to turn. Actually ruling out $N_{\rm eff}=3$ would have been more exciting, but doesn't seem to be happening.Sesh Nadathurhttps://www.blogger.com/profile/07155102110438904961noreply@blogger.comtag:blogger.com,1999:blog-6976071487922527618.post-48156185371472032452013-02-13T22:15:34.172+01:002013-02-13T22:15:34.172+01:00Hey Sesh, out of interest, how does Neff scale wit...Hey Sesh, out of interest, how does Neff scale with the number of neutrinos? does it go to something near 4 for 4 types or is 3 just a coincidence?Anonymoushttps://www.blogger.com/profile/00599349668793867470noreply@blogger.com