Transcript: Throughout most of the history of the universe structures have been forming under the action of gravity. Remember that the visible parts of galaxies just represent the tip of an iceberg of mostly dark matter. The way in which structure forms depends on the detailed properties of...

Transcript: As we trace events in the universe back toward the big bang, the first important epoch reached is a time about three hundred thousand years after the big bang. The universe by this time has cooled to a temperature of about three thousand Kelvin like the photosphere of a cool star. ...

Transcript: Inflation is an extraordinary idea. What is the evidence in favor of it, and is it even possible to test something that happened so long ago? First, space is observed to be close to flat, and recent observations show that it’s extremely close to flat, within a few percent. This...