Triumph of the Big Bang

The big bang model is now almost universally accepted by astronomers, but this was not always so.
The Steady State Model
The big bang model had an early challenger that was called the steady state model. The steady state model did the cosmological principle one better by invoking what has been termed the perfect cosmological principle: Not only is the Universe the same at all places and in all directions when averaged over a large enough volume; it is the same for all time too. Since the Universe was known to be expanding, the steady state model had to postulate continuous creation of matter in the space between the stars and galaxies to maintain the same density over time and thus satisfy the perfect cosmological principle of a universe unchanging in time on large scales.
Deciding Factors
For a time, the steady state theory and the big bang theory competed with each other, but beginning in the 1960s new observations eventually all but ruled out the steady state theory while providing strong support for the big bang. Probably the three most important developments were

  • Deep space observations (which therefore peered far back in time because of the finite speed of light) indicating that the early Universe looked very different from the present Universe. For example, there appear to be more quasars at great distances, implying that there were more quasars in the early Universe than the present one. This contradicted the steady state hypothesis that the Universe was unchanging over time on large scales.
  • The discovery of the cosmic microwave background that appeared to be coming from all directions in space (see the next module). This was an expected consequence of the big bang model, but was very difficult to explain in any simple way in the steady state theory.
  • The detailed accounting of light-element nucleosynthesis in the big bang and its agreement with the observed abundances of light elements in the present Universe. It is difficult to arrange this agreement in the steady state theory, which must form both light and heavy elements in stars.

    • As a consequence of these and other findings, the steady state theory is no longer considered viable by most astronomers.