Home Table of Contents About the Authors Glossary Buy This Book Joseph Henry Press


COSMOLOGICAL TEMPERATURE AND DENSITY



frontiers: TOC for Knowledge Concepts, Exercises, and Solutions



  (1+z)
  ----- = constant            (T = the cosmic background temperature)
    T


             3 * H^2
  (rho) =  ------------       (the critical density of the universe)
            8 * pi * G         here, H is the Hubble constant.


(Omega) = (actual density of the universe)/(critical density of the universe)

(Omega-Total) = (Omega-Matter) + (Omega-Lambda)

 
The Big Bang theory predicts that the universe should glow with the remnant
energy of the huge explosion that began the expansion of space and the passage
of time.  This background glow was indeed proven to exist, and is 2.7 degrees K
at the present epoch.  Objects with that temperature produce primarily microwave
radiation, so we call that glow the "cosmic microwave background."  In the past,
depending on redshift, the background temperature was warmer than it is today.

The average density of the universe determines the ultimate fate of the universe,
Whether it will eventually recollapse or expand forever.  The boundary between
these two scenarios will occur if the average density is a particular value,
called the "critical density."  

Astronomers often express the density of the universe in terms of a fraction
of the critical density.  The capital Greek letter (Omega) is the symbol for
this fraction.  

  So if (Omega) = 1, then the universe is "flat" and has critical density.
     If (Omega) < 1, then the universe is "open" and will expand forever.
     If (Omega) > 1, then the universe is "closed" and will eventually recollapse.

The total value of (Omega) has two main components:

   (Omega-Matter)   which is produced by matter and its gravity (attraction), and
   (Omega-Lambda)   which is produced by space and its springiness (repulsion).

For most of the 20th century, (Omega-Lambda) was believed to be zero.  However,
recent observations now strongly suggest that (Omega-Lambda) is not zero.  The
accurate measurement of (Omega-Matter) and (Omega-Lambda) is one of the frontiers
of astronomical research.  When we know these numbers, we will know the ultimate
fate of the cosmos!