96 6. THE PAST
Afterglow Light Pattern
375,000 yrs.
Dark Ages
Present
Inflation
Quantum
Fluctuaations
Development of
Galaxies, Planets, etc.
Big Bang Expansion
13.77 Billion yrs.
Dark Energy
Accelerated Expansion
1st Stars
about 400 Million yrs.
Figure 6.2: A graphical illustration of the history of the universe. e time scale is on neither a
linear nor a logarithmic scale. (Graphic credit: NASA/WMAP Science Team.)
Normal gravity was the dominant factor in the early universe, and this made the universe
expand at a slower and slower rate. But as normal gravity became weaker, dark energy became
stronger. About 4 billion years ago—9.8 billion years from the Big Bang, this dark energy effect
became dominant. And so now we see a universe that is expanding more rapidly as time goes by.
6.4 A GRAPHICAL SUMMARY
Figure 6.2 shows a graphical depiction of the history of the universe. e illustration shows,
through its overall shape, the evolution of the scale factor—the changes in the distance between
two arbitrarily-chosen co-moving points in the expanding universe.
Notice that the scale factor initially expands at an accelerating rate, and so is concave-
outward in the diagram. is is meant to represent the period of cosmic inflation. Afterward,
the shape of the diagram is ever-so-gently curved concave inward, representing the decelerating
effect of ordinary gravity, slowing the rate of expansion as time passes. But near the end—that
is to say, the present—the diagram begins to again expand at an accelerating rate, signifying the
onset of the domination of dark energy over ordinary gravity.
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