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 confirms a prediction of inflation but of course was one of the motivations for the idea of inflation in the first place, so it’s not really an independent test of the idea. Second, the fluctuations of the cosmic microwave background radiation have a particular random character and fit a distribution in the amplitudes that follows a power law. This could not have been the case, and it is in fact a close prediction of a quantum process to produce such a distribution of fluctuations. So the microwave background radiation is consistent in detail with the inflationary idea. As for the nature of the force that drove this exponential expansion, that’s still poorly understood, and in fact the energy scale involved is beyond that that is reachable with ground based accelerators or even any accelerator on the drawing board. The only way we can test the inflationary idea is with observations of the early universe itself.