# Cosmic Microwave Background Origin

## Main Question or Discussion Point

If the light from a distant galaxy was red shifted far enough wouldn't it appear as microwave? Is it possible that the CMB is just more galaxies beyond what we consider to be the "observeable universe?"

nicksauce
Homework Helper
If that was the case, there would be no way to explain why the CMB is (almost) a perfect black body.

Chalnoth
If that was the case, there would be no way to explain why the CMB is (almost) a perfect black body.
That and its extreme uniformity: the CMB is uniform to one part in 100,000.

If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?

If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
But no one really expects the universe to be infinitely large anymore.

Chalnoth
If the universe is infinitely large and the microwave background is actually light from extremely distant galaxies then wouldn't we expect it to be pretty uniform?
Not remotely. Basically, if you were to propose a steady-state model where this sort of thing sort of makes sense, then the far-away universe would be every bit as inhomogeneous as the nearby universe. So you wouldn't be looking out to the same distance in every direction: every place you look on the sky, you'd see a galaxy at a different distance. The temperatures would vary hugely.

Nicksauce also has a very good point about the spectrum: galaxies are very poor black bodies. Their spectra are chock full of all sorts of interesting features. By contrast, the CMB is almost a perfect black-body. So the CMB cannot simply be made up of lots of redshifted galaxies anyway.

If you start stacking multiple layers of random features, what do you get? I bet it starts to look pretty uniform at some point. If we pick one point in space at our best resolution, in an infinite (or vastly larger than currently projected) universe, that point could still have trillions of galaxies. Even if they all have random and interesting features, they would blend together and look like static to us. If we get better resolution maybe we start to see through the static but no matter how good our instruments are at some point it will just look like static again.

mgb_phys
Homework Helper
Nicksauce also has a very good point about the spectrum: galaxies are very poor black bodies. Their spectra are chock full of all sorts of interesting features. By contrast, the CMB is almost a perfect black-body. So the CMB cannot simply be made up of lots of redshifted galaxies anyway.
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.

Although if the universe was infinite, you wouldn't have 3K temperature for the dust.

What temperature would you expect?

mgb_phys
Homework Helper
What temperature would you expect?
Either 0 or the average temperature of the surface of a star!
That's the problem with infinity, it causes a few problems!

If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.

Although if the universe was infinite, you wouldn't have 3K temperature for the dust.
No it would not. No time for that.

Chalnoth
If the universe was infinite the light from those galaxies would have been thermalised by an infinite number of collisions with an infinite amount of gas/dust.
Well, no, because it would still have to interact with one last galaxy along the line of sight, which would always produce some combination of emission and absorption lines depending on said galaxy's chemistry.

mgb_phys
Homework Helper
That's also true of the CMB - you have to remove the effects of foreground galaxies.

I'm not convinced that you can't produce a uniform thermal spectrum by putting any source through an infinite column on absorbing material.

Chalnoth
That's also true of the CMB - you have to remove the effects of foreground galaxies.
That effect is extremely small at the frequencies where the CMB is strongest. Even our own galaxy isn't that bright at around 90-100GHz.

I'm not convinced that you can't produce a uniform thermal spectrum by putting any source through an infinite column on absorbing material.
If the column were uniform, sure, that would work. But the problem is that the universe is expanding and sparsely-populated, which means that the radiation won't get a chance to thermalize before it redshifts away on its trip between galaxies.

Chronos
Gold Member
See Olber's paradox. If the universe was infintely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.

Chalnoth
See Olber's paradox. If the universe was infintely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
Expansion counters this. I was assuming an expanding universe, along the lines of a steady state universe idea, where you have an exponential expansion where hydrogen is continually produced out of the vacuum (completely unrealistic, but at least it isn't quite as obviously false as a non-expanding eternal universe).

See Olber's paradox. If the universe was infintely large, infinitely old, and populated by an infinite number of galaxies, the night sky would be as bright as the surface of an average star.
How would it be as bright as the surface of a star if the light redshifts out of the visible spectrum?

Chalnoth
How would it be as bright as the surface of a star if the light redshifts out of the visible spectrum?
The Olbers' Paradox assumes a static universe, with no expansion.

The Olbers' Paradox assumes a static universe, with no expansion.
So do I, a static universe with redshift caused by something other than expansion.

Chronos
Gold Member
In an infinitely spacious, ancient and populated universe, how does expansion prevent the night sky from being lit up like a birthday cake? Bear in mind every photon ever emitted in the universe had an infinite amount of time to reach us [and energize every molecule blocking its path]. Something is very wrong with this picture.

Chalnoth
So do I, a static universe with redshift caused by something other than expansion.
That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.

Furthermore, a static universe is unstable: any slight perturbation will cause either collapse or eternal expansion.

That's not really possible. The only way to get redshifts of atomic emission/absorption lines is to have some sort of doppler or gravitational redshift. When applied to the universe as a whole, that means expansion.

Furthermore, a static universe is unstable: any slight perturbation will cause either collapse or eternal expansion.
Photonic clocks? Time running slower in the past. I know its been refuted by gravity probe B, but nevertheless it is interesting. At least it is in the good agreement with pioneer anomaly.

Chalnoth