• A. Where is the matter? What is a typical building block?
  Edwin Hubble found that a "typical" building block of the universe was a large space containing a cluster of galaxies. Although clusters have different numbers of members they have roughly the same percentages of elliptical and spiral galaxies (fuzzy groupings of hundreds of billions of stars and their planets).

  These building blocks appear to be distributed in space in long filaments and sheets, surrounding enormous voids (no visible matter). This distribution is observed in all directions from the Milky Way and out to very large distances from us.

• B. How is the matter moving?
  Edwin Hubble observed the spectra of individual galaxies and noticed the frequency (or wavelength) of light absorbed by specific chemical elements. The Doppler effect interprets this as indicating a velocity of approach or recession from us. Careful measurement of the frequency shifts implied that the galaxies within a cluster have similar velocities, and all are receding from us except for the galaxies within our own cluster. Also, the faster receding galaxies are found farther from us. This is a straight line correlation.

II. Cosmological Models to explain the basic measurements (1930-1950)

• A. Big Bang Model of the Universe:
  A small very hot very dense blob of matter exploded violently about 13.7 billion years ago. The fireball expanded, cooled, became transparent when atoms "froze" (about .3 million years afterwards), then cooled more quickly. This allowed gravity to pull the outrushing matter into clumps which became moving clusters of galaxies with lots of stars and planets in each. Eventually the stable clusters of galaxies will be very far apart, their stars will die, and cold cinder clusters will diverge from each other for ever. (A semi-eternal open universe)

• B. Oscillating Universe Model of the Universe:
  Same as the Big Bang Model up until now. However, this model has enough mass to gravitationally stop the expansion eventually (after hundreds of billions of years) and so begin an equally long contraction. Another big bang type of explosion might result in many cycles of expansion and then contraction. (An eternal closed universe)

III. Evidence for choosing and refining the models (1965-present)

• A. In 1965 Penzias and Wilson detected microwave static (T=3 K) from the flash when the universe became transparent. COBE satellite says that it is extremely uniform in all directions. This says that there was an explosion.
• B. Abundances of the light elements (helium, lithium) say that the initial event was very violent. Of the helium we have today, 80% formed within the first 15 minutes after the big bang. This favors the more violent Big Bang Model.
• C. The mass needed to close the universe (reverse the present expansion phase) is greater than the observed mass of all the galaxies. However, there is disagreement as to how much "dark matter" really exists. This favors the Big Bang model.
• D. Measurements of light from very distant type Ia supernovae and of microwaves by the WMAP (Wilkinson Microwave Anisotropy Probe) found that the universe is now expanding at an accelerating rate. This favors the Big Bang Model also.

This page is http://www.csam.montclair.edu/~west/cosmology.html. Last updated Dec, 2003