View Full Version : Absence of systematic shadows on CMB from galaxies, still an issue?

2015-Dec-14, 06:08 PM
Hello, everyone!
In 2006, some researchers reported that not all nearby galaxy clusters cast a shadow on the CMB, contradicting one of the predicitions of the Big Bang model.

Has that issue been solved since then?

Ken G
2015-Dec-14, 08:57 PM
I'd say this paper http://mnras.oxfordjournals.org/content/402/2/1179.full.pdf makes the case that there is indeed something going on there, but it doesn't seem like modifications to the Big Bang model are the most plausible explanation. Rather, they focus on the possibility of point sources of microwave and/or x-rays that are in some sense helping to "fill in" the shadows. It is even possible the shadows are just fine-- there is some model dependence to what the shadows "ought to" look like. Maybe a bit of a tempest in a teacup?

2015-Dec-17, 04:41 PM
As is often the case, the search for 'shadows' relied upon assumptions about near-galaxy environments. So rather than being firm evidence 'against' the Big Bang, it is evidence that this evidentiary trail supporting the Big Bang is infertile. Baryonic Acoustic Oscillation trails face similar challenges.

A larger issue is found here: The small scatter of the baryonic Tully-Fisher relation http://arxiv.org/abs/1512.04543 ; charting a quantifiable disparity between the theoretical Dark Matter / baryonic matter distribution and the resulting core rotation rates. Again, it may be just a matter of tweaking parametric assumptions; but that is awful fluffy science: There are limits to how many times you can pound the dents out of a theory and still expected it to drive.

2015-Dec-17, 09:00 PM
Can someone explain why there might be a shadow in the first place? Since a galaxy is mostly emptiness, the only CMB photons which would be effected are primarily those which land on a star, and secondarily those which strike random particles. To the observer who would have otherwise seen them, those which land on a star are effectively replaced by the entire bright spectrum of the star. If I consider gravitational lensing, then I'd expect no change at all, since just as many photons should be redirected toward an observer as are directed away. So, why a shadow?

Reality Check
2015-Dec-17, 09:40 PM
The paper is Lieu, Mittaz and Shuang-Nan Zhang, UAH, "The Sunyaev-Zel'dovich effect in a sample of 31 clusters: A comparison between the X-ray predicted and WMAP observed decrement," Astrophysical Journal, Sept. 1, 2006, Vol. 648, No. 1, p. 176 (http://arxiv.org/abs/astro-ph/0510160)

Sunyaev-Zel'dovich effect (https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zel%27dovich_effect)

The Sunyaev–Zel'dovich effect (often abbreviated as the SZ effect) is the result of high energy electrons distorting the cosmic microwave background radiation (CMB) through inverse Compton scattering, in which the low energy CMB photons receive an average energy boost during collision with the high energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe. Using the Sunyaev–Zel'dovich effect, dense clusters of galaxies have been observed.
This is an interaction with electrons in the intra-cluster medium, not stars.

This effect has been observed to exist even in the paper. Lieu et. al. note that using the X-rays from the high energy electrons to predict the SZE gives a result that is 4 times the observed SZE.
In Big Bang's Afterglow Fails an Intergalactic Shadow Test (http://phys.org/news/2006-09-big-afterglow-intergalactic-shadow.html) there is

"If you see a shadow, however, it means the radiation comes from behind the cluster. If you don't see a shadow, then you have something of a problem. Among the 31 clusters that we studied, some show a shadow effect and others do not."

But this is not obviously stated in the paper. Figure 2 is " WMAP Q, V, and W band radial profiles of the CMB temperature deviation ..." for the 31 clusters. Most of the profiles are close to the predicted SZE profile but there are a few that I would say show little or no "shadow".