Is the CMB unique to our galaxy or present in all galaxies?

[wolram]

How do we know that the cmb is not just related to just our galaxy, and that all galaxies have there own cmb?

 

[bapowell]

A few thoughts off the top of my head. 1) the uniformity of the CMB, and the fact that it is a near perfect blackbody cannot be explained through any known galactic process. We would need dramatic new physics to account for it, whereas the big bang model predicts it just as we see it. 2) The CMB encodes the history and contents of the universe in its power spectrum, so it tells us something globally about the observable universe, rather than reflecting local galactic processes. 3) As the CMB travels through the universe, it interacts with matter (ionized gas, hot electrons, etc), and these interactions leave imprints on the CMB. These imprints are found to correlate well with large scale structures, like galaxy clusters, in the universe. That ain't going to happen by chance.

 

[rootone]

Also because we can determine the amount of redshift, we can make reasonable estimates of how old it is and how far away it is.
Since it is older than our galaxy and more distant than any galaxies we can see, then it can't be anything to do with our own galaxy.

 

[wolram]

Thanks for clearing that up Bapowlell

 

[bapowell]

Also because we can determine the amount of redshift, we can make reasonable estimates of how old it is and how far away it is.
Since it is older than our galaxy and more distant than any galaxies we can see, then it can't be anything to do with our own galaxy.

But I think the OP is asking: suppose you measure a 2.7K microwave. How do you know it didn't come from our galaxy? Extrapolating via redshift presumes that it is a CMB photon.

 

[rootone]

OK, I see your point, though I can't think of any galactic scale process which might produce a highly red shifted signature of Helium.

 

[Chalnoth]

There's also the related question, "Which photons in this frequency range came from the CMB?"

That question is answered by observing the sky at many frequencies (WMAP uses five, Planck uses nine). The CMB itself has a very specific frequency dependence, while the gas and dust in or own galaxy, as well as other objects that are bright in the relevant frequency range such as some quasars, have very different frequency profiles.