About the CMBR being uniform temperature

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lgw
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About the CMBR being uniform temperature

Postby lgw » Mon Jan 06, 2014 11:12 pm UTC

One of the early surprises from the study of the cosmic microwave background radiation was how uniform the temperature was (once adjusted for red/blueshift). More uniform than the universe would have had time to equalize, leading to theories about inflation that have been a big part of new work in the field.

But I don't get it - could someone please explain why this result was surprising?

The thing about the CMBR is that it's a snapshot taken at the point where the universe had cooled enough to become transparent. Wouldn't we naturally expect the temperature to be the same everywhere at the moment when we can see it. Different parts of the universe could have had significantly different temperatures, and cooled to the magic temperature thousands of years apart, but we'd see every part at that highest-transparent-temperature because that's when it becomes visible, no?

I feel like I'm missing the obvious here, but I just don't see it (someone cool it down for me so I can :P).
"In no set of physics laws do you get two cats." - doogly

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Tass
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Re: About the CMBR being uniform temperature

Postby Tass » Tue Jan 07, 2014 8:56 am UTC

If a part cooled down later, then it would have had less time to redshift, and therefore look hotter. So it ends up the same.

lgw
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Re: About the CMBR being uniform temperature

Postby lgw » Tue Jan 07, 2014 6:58 pm UTC

Less time to redshift? That doesn't make sense to me. AFAIK, the patches of red/blushift we see in the CMBR after normalizing for the Earth's motion represent the expansion/contraction of large areas of slightly-more-dense and slightly-less-dense regions of the universe, results of the sound waves that filled the universe before "re"combination. If anything, I'd expect the time to cool down to be directly tied to those differences in density (temperature is an odd concept here - but it means energy density as far as I can gather).
"In no set of physics laws do you get two cats." - doogly

andyisagod
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Re: About the CMBR being uniform temperature

Postby andyisagod » Tue Jan 07, 2014 7:56 pm UTC

The point is that the red shift of the photons is a function of the time that they have been travelling which is equivalent to the distance that they have travelled. A colder patch in the CMB means it will have cooled earlier and thus have travelled for a longer time before reaching earth than a hotter patch. The difference in temperature is actually a difference in redshift which is the same as a difference in distance to the last scattering surface which equates to a difference in time of cooling.

lgw
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Re: About the CMBR being uniform temperature

Postby lgw » Tue Jan 07, 2014 9:44 pm UTC

Ah, so what you're saying is that we're sampling the same temperature everywhere (at the time of emission), precisely because opaqueness is temperature-based, so any difference in observed temperature must be from redshift? And the only way the light reaching us isn't from a uniform distance (and thus uniform redshift) is because different pockets of space cooled down to the magic temperature at a different times?
"In no set of physics laws do you get two cats." - doogly

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Tass
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Re: About the CMBR being uniform temperature

Postby Tass » Wed Jan 08, 2014 8:00 pm UTC

lgw wrote:Ah, so what you're saying is that we're sampling the same temperature everywhere (at the time of emission), precisely because opaqueness is temperature-based, so any difference in observed temperature must be from redshift? And the only way the light reaching us isn't from a uniform distance (and thus uniform redshift) is because different pockets of space cooled down to the magic temperature at a different times?


Yes. Of course some motion along our line of sight (due to local faster/slower expansion, could also contribute.

lgw
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Re: About the CMBR being uniform temperature

Postby lgw » Thu Jan 09, 2014 12:23 am UTC

Thanks Tass and andyisagod, that actually makes some sense now.
"In no set of physics laws do you get two cats." - doogly


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