© NASA, WMAP Science Team.
The diagram above outlines the history of our universe, from the Big Bang up to the present, taking into account many different types of cosmological observations. We have observed relic photons from the time atoms first formed, the distribution of galaxies and galaxy clusters that formed as overdense regions in the early universe collapsed, and the shift to the red in the spectra of distant supernovae that indicates that the universe is expanding. All of these observations, and many others, are consistently described by general relativity, which gives us confidence that Einstein's theory holds true at distance scales up to 1028 cm. There is also strong evidence to support the period of rapid expansion (called "inflation") on the left side of the diagram, which string theory may actually be able to explain. (Unit: 4)