Cosmology and the Origin of Life

Important Simplification in the Early Universe: Thermal Equilibrium:

• Only the temperature of the universe is important
• The condition of the Universe at any epoch does not depend on what happened at earlier times

MATH ALERT!!

Unfortunately we now have to do this:

• The Universe is expanding
• The density in the universe is its mass divided by its volume
• since the volume is increasing then the density goes down
• the volume depends on the radius so we have:
  

• The density of photons decreases just like the matter density so it goes as
  

• Now here is the important part: Not only does the photon density of the Universe decrease with time, the average energy per photon also decreases because the Universe is expanding and cooling: Therefore we have the following
  

When the radiation energy density exceeds the matter density the universe is said to be RADIATION DOMINATED

When the matter energy density exceeds the radiation density the universe is said to be MATTER DOMINATED

As we will see later, when the universe is radiation dominated gravity is effectively unimportant (or at least its effects are greatly diminished)

MOST IMPORTANT POINT:

• because the radiation energy density decreases with the expansion of the universe more rapidly than the matter energy density --> the universe will always end up in a matter dominated phase. This is is illustrated here:
  

Next Page ___________________________________________________________________ ___________________________________________________________________

The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu