Anisotropic universe

Factual Questions
1

This has to do with a Bad Astronomer column: Are some parts of the Universe expanding faster than others? Maaaaaaybe. I’d discuss this in the comments, but you have to have a facebook or twitter account and I don’t have either.

A recent paper suggests that the universe is not expanding isotropically which Phil discusses in that column. The paper depends on two ways of measuring the luminosity of hot gas within galactic clusters: one that does not depend on redshift and one that does. The one that doesn’t is based on the measurement of X-rays given off by that hot gas:

But wait a minute. By ‘kinds’ does he mean the wavelength/frequency of those X-rays? I can’t think of anything else it could be. Well, maybe polarization, but I’m pretty sure that can’t be used to find the temperature of a gas. At any rate, to know the true wavelength of the X-rays, we’d have to know the redshift. Admittedly, it’s just using the pure redshift, and not the distance calculated from the redshift, so that may be why it’s OK.

So I guess I have to ask a question: Is my objection valid or am I way off base?

2

It is the frequency of the X-rays, but it’s not just a matter of hotter things producing more of the higher frequencies. That could, as you surmise, be fooled by not knowing the precise redshift. The catch is that what frequencies of X-rays are produced also depends on whether atoms are ionized (or more precisely, what atoms are ionized and by how much), and that’s a purely local phenomenon, and so wouldn’t depend on redshift.

3

Actually, I was thinking that the frequencies of the ionization X-rays are all going to be redshifted, so they’d have to adjust by the redshift of the cluster to figure out which atoms are emitting them. But after reading your answer, I realized they could use relative locations of various lines, so they wouldn’t have to do that adjustment.