Microwave Background Radiation???

[Cosmo_Ken]

There are things about cosmology that I just don't understand!  Microwave Background Radiation is one of them.  We are told that this is an extremely uniform residual from the Big Bang and that it indicates that the early universe (before stars formed) was extremely uniform.  How can it be as uniform as reported, don’t stars, galaxies and even planets (like Jupiter) emit radiation in the microwave band?  Wouldn’t microwave radiation emitted by the Big Bang at the time traveled far beyond us ages ago???  If they are talking about a “current” (more or less) afterglow from the universe, i.e. thinking of the entire universe as a Black Body Radiator as it has cooled since the start of the BB, the same question above still applies and also what exactly is the source of this “current” microwave radiation???

[swansont]

Radiation emitted by stars is not background.

The background literally filled the universe, going in all directions. So it has to continue filling the universe. 

[Strange]

One of the technical challenges of measuring the background is accurately removing the foreground (stars, galaxies, dust, etc).

[Cosmo_Ken]

But was this background microwave radiation emitted at the time of the start of the big bang or is it from the universe itself "currently" radiating in the microwave region of the electromagnetic spectrum because of the universe's "current" temperature???

[Mordred]

No its a bit more complex than that. After the first 10^-43 sec. Inflation occurs, this caused a rapid supercooling via the ideal gas laws. However inflation didn't instantly stop but slow rolls to a stop. This event caused a rapid reheating stage. Now the volume has increased exponentially so particles will become stable, ie drop out of thermal equilibrium. The temperature is low enough that atoms can now form.

 This removes a large percentage of free ptotons neutrons and electrons. Thus reducing the opacity of the CMB, this is the surface of last scattering.

 The uniformity we observe is due to the rapid expansion supercooling and slow roll reheating in such a short time frame, that temp/mass density anistropies didn't have time to redeveloped after being evenly redistributed.

[Strange]

To add to that, it took about 360,000 years for the universe to cool enough so that it became transparent to light and the CMB radiation was "released". At that point, the temperature was about 3000K.

[Mordred]

By released, the density was such that prior to the "release". That the mean free path of photons was approximately [latex]10^{-32} [/latex] metres if I recall correctly. epoch commonly called dark ages under Cosmology. We cannot get messurements of nor past this epoch.

 Once atoms started forming the average density decreased enough that photons could travel far enough to reach us today. We can see the haze, via a combination of Hubble and strong gravitational lenses.

Edited October 22, 2017 by Mordred