Radiation Dominance in the Early Universe


Cosmology and the Origin of Life


Remember the basic problem:

After t = 15 minutes:

So How do we make GALAXIES ????

The First Problem:

To build a galaxy we need to grow a density fluctuation ! How is this going to happen?

The tight coupling between the radiation and matter means that the matter should be distributed like the radiation and we know, from observations of the MWB, that the distribution of radiation is very smooth.

So How come there are galaxies? How come there is all sorts of large scale structure from

The variety of structure which is seen is very difficulty to characterize but one sees hierarchical clustering on a variety of size scales. A similar examples is represented by the distribution of population in the United States

The coupling between matter and radiation will continue as long as the Universe remains ionized.

When does the radiation dominated era end? Remember it must end Because the radiation energy density falls off with time faster than the matter energy density

When the universe cools to the point where the electrons can recombine with the protons, then the radiation will decouple because the scattering network is now gone

Thus there is some surface of last scattering in the evolution of the Universe from which the photons are finally no longer scattered but free stream to fill the Universe.

This is the Microwave Background that we now see, greatly redshifted from its initial characteristic energy defined by a temperature of 3000 K (see below). Prior to this time, no external observer (e.g. us) could get any signal because the photons were still being scattered around.

So, as in the animation, the detector (us) will never see the photon source (the cannon) as long as the scattering network is in place. For the early Universe this scattering network was the existence of free electrons.

The time it takes for the electrons to recombine depends on two things:

For the conditions of our universe, this time is approximately 100,000 years.

Recombination occurs at a Temperature of 3000 degrees. The photons now fill the universe and this is what we presently observe as the 3 K microwave background. This radiation has been redshifted by a factor of 1000 by the time the Universe has aged to 10 billion years and we detect it.

This also means that we can not observe the Universe when it was younger than 100,000 years.

After recombination has occurred, radiation is no longer an influence on the distribution of matter. Hence, matter will clump around any surviving density enhancements.

The question now becomes, how did those density enhancements survive the radiation dominated era, and what is their nature? Ultimately these density ehancements have to grow to produce the structure we observe today and their signature is imprinted on the Microwave Background.

This signature is what COBE detected

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