Professor, Department of Astronomy and Astrophysics
University of Chicago

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Cosmic Expansion

Key Concepts

The big bang or hot expanding model of cosmology finds strong support from the recession of the galaxies, that is, when we look out in the night sky, distant objects appear to be flying away from us. And also the cosmic microwave background - the fact that we are bathed in primordial light that bears evidence from and earlier, hotter, and denser period in the universe. As we shall see, this evidence also supports the gravitational instability paradigm - the picture that gravity can make wrinkles into galaxies.

The relation between recession and expansion is easy to understand.

Given that light behaves this way, it's easy to see how the sea of primordial light we observe in microwaves supports the hot expanding (big bang) model. Run these pictures backwards in time. The wavelength of light becomes shorter and shorter and matter becomes denser and denser implying that the universe began in a hot dense state.