1. ## Re: Universe size questions

hey nFA,

"So for a photon T ~ 1/a while for non relativistic particles T ~ (1/a)^2."

I may be way out in left field here (as in 101% wrong), but,

we know the CMB went from 3000K to 3K.
its scale factor went from .001 to 1.

did non relativistic matter go from 3,000,000K to 3K?
(its scale factor went from .000001 to 1.)

2. ## Re: Universe size questions

Originally Posted by chas53
hey nFA,

"So for a photon T ~ 1/a while for non relativistic particles T ~ (1/a)^2."

I may be way out in left field here (as in 101% wrong), but,

we know the CMB went from 3000K to 3K.
its scale factor went from .001 to 1.

did non relativistic matter go from 3,000,000K to 3K?
(its scale factor went from .000001 to 1.)
No. It's a bit more tricky. At the time that the CMB was formed the photon temperature was the same as the matter temperature. They were in thermal equilibrium with each other. Since the entropy of the universe (photon to baryon ratio) is high the photons act as a thermal reservoir and maintain the temperature of the white which baryons they are in thermal contact. That's another implication of that epoch being "radiation dominated". The photons not only determine the pressure but also the temperature of all the charged particles with which they are in contact.

It is only at the moment that the CMB forms that matter is no longer in thermal contact with photons. This is because this is the same moment that the ionized hydrogen becomes neutral. The temperature of the hydrogen evolves independently from that point.

The difficulty with simple scaling arguments is that they can lead you astray if there is some physics that you are ignoring that doesn't simply scale.

I've made a short search for a handy link but everything looks very technical.

3. ## The Following 2 Users Say Thank You to not_Fritz_Argelander For This Useful Post:

chas53 (04-21-2019),helicon64 (04-21-2019)

4. ## Re: Universe size questions

Oh, what the hey.... Here is a link just to demonstrate how truly headache inducing the thermal history of the early universe can be.....

https://www.astro.up.pt/investigacao...ures/kunz2.pdf

5. ## The Following 2 Users Say Thank You to not_Fritz_Argelander For This Useful Post:

chas53 (04-21-2019),helicon64 (04-21-2019)

6. ## Re: Universe size questions

thank you very much nFA!!

is there a typo here:

"Since the entropy of the universe (photon to baryon ratio) is high the photons act as a thermal reservoir and maintain the temperature of the white which baryons they are in thermal contact."

7. ## Re: Universe size questions

Originally Posted by chas53
thank you very much nFA!!

is there a typo here:

"Since the entropy of the universe (photon to baryon ratio) is high the photons act as a thermal reservoir and maintain the temperature of the white which baryons they are in thermal contact."

"Since the entropy of the universe (photon to baryon ratio) is high the photons act as a thermal reservoir and maintain the temperature of the baryons with which they are in thermal contact."

8. ## The Following 2 Users Say Thank You to not_Fritz_Argelander For This Useful Post:

chas53 (04-21-2019),helicon64 (04-21-2019)

9. ## Re: Universe size questions

ah thank you nFA.
and yes the link is very humbling to my knowledge level.

10. ## Re: Universe size questions

Originally Posted by chas53
ah thank you nFA.
and yes the link is very humbling to my knowledge level.
Well the intention was not to humble. The intention was to illustrate the complexity of the problem, that scaling ideas have a very limited role to play....

11. ## The Following 2 Users Say Thank You to not_Fritz_Argelander For This Useful Post:

chas53 (04-21-2019),helicon64 (04-21-2019)

12. ## Re: Universe size questions

awesome. thank you very much nFA.
I super appreciate you helping me understand soooo much!!

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