Wilkinson microwave anisotropy probe

Age of the universe: Refers to the age of the observable universe. Cosmologists use the rate of the expansion of space (the Hubble constant) to determine the age of the universe. Knowing the rate at which galaxies are separating, and the distances among them, one can work backwards to determine how much time has passed when, in theory, there was no separation at all. Most cosmologists expect the expansion rate to change with time. They had thought that the rate was faster in the past and that it slowed down to its current rate as gravity from all the mass in the universe gradually overcame the momentum from the Big Bang. Recently, however, astronomers observed very bright exploding stars called supernova to measure the universal expansion, and the results of these observations indicate that the universal expansion is speeding up, or accelerating. This implies that the expansion rate was less in the past, so it took more time for galaxies to reach their present separations than if they had always been receding at the more rapid current rate. This indicates that the universe is actually older than it appears from a simple calculation using the current Hubble constant.