Cosmology and the Origin of Life

Remember the basic problem:

• One billion photons per baryon
• Matter/Anti-Matter Assymetry has produced the observable universe
• Average energy per photon is very high

After t = 15 minutes:

• Temperature = one million degrees
• Fusion stops
• Universe consists of:
  • H,2(H),3(He),4(He),7(Li)
  • Still a long way to go to reach The Periodic Table
  • Note; nuclei with 5 or 6 nucleons are very unstable and can't exist very long

• The Universe is completely ionized at this point; no electrons are bound to atoms
• Universe is an expanding sea of mostly photons in which are mixed small amounts of atomic nuclei and free electrons

So How do we make GALAXIES ????

The First Problem:

• Matter and Radiation are coupled. This is similar to your coupling to the distribution of kangaroos.

• Coupling mechanism is photon scattering off of free electrons

• Scattering is the random change of direction of a photon. Watch the Blue photon

• The problem in the early universe now is that there is no place for the photons to go - they can't escape the matter as long as it remains ionized.

• Radiation and matter temperature are the same due to momentum transfer between the photons and matter Therefore the matter is also at a temperature of one million degrees.
• This momentum transfer between the radiation and the matter will not allow the matter to clump together imagine trying to meet your friend in the renegade roo room.

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

• Clusters of Galaxies

  to

• Huge Structures

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:

1. The ionization energy of hydrogen (13.6 ev)
2. The ratio of photons to protons

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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