- Inflation to explain key features of the universe
- Olbers' paradox


[SLIDE 1]

A key feature of the Big Bang Theory is inflation -- the idea that in a fraction of a second, possibly at the end of the GUT era, the universe underwent a dramatic expansion.

What key features of the universe are explained by inflation? While the Big Bang Theory gained wide acceptance because of the strong evidence of the cosmic microwave background and the abundance of helium in the universe, it left three questions unanswered that can be answered by inflation. They are:

Where did the density enhancements that led to galaxies come from? Quantum mechanics states that in energy fields, the distribution of energy is slightly irregular. Inflation would have dramatically blown up those tiny quantum ripples in the early universe to enormous sizes, making them become the density enhancements around which galaxies would form.

Why is the large scale universe nearly uniform? The inflation hypothesis explains why the universe in general is so uniform in temperature and density by saying that distant regions of our observable universe today were once close enough to exchange radiation.

Why is the geometry of the universe flat? Einstein's theory of relativity says that matter and energy produce curvature of spacetime, but all studies seem to indicate that the large-scale geometry of the universe is flat. Inflation explains this flatness - just like a balloon seems flatter the larger it is blown up, the universe expanded so much that it appears flat.

Given that the concept of inflation offers convenient explanations for many features of the universe, how do we know if inflation really happened? The idea of inflation makes specific predictions about what we should see in the cosmic microwave background --  patterns in the temperature that have been confirmed.


[SLIDE 2]

Olbers' paradox states that if the universe were infinite, unchanging, and everywhere the same, then the entire night sky would be bright, because it would be completely covered with stars. If stars are farther away, they will be less bright but more numerous.

The Big Bang Theory explains why the sky is dark - the universe began at a particular moment, so it is not old enough for the light from some stars to have arrived.