Since the concept of an expanding universe was first introduced, there have been many estimates of the age of the universe that have evolved with scientific discoveries and developments. Many early estimates used the recessional velocities of distant galaxies to calculate the age of the universe, often getting results of about 2 Gyr (2 billion years) or less. Because this was less than calculations of Earth's age at the time, which were considered more accurate, doubt was cast upon the theory of expansion. This problem was resolved in 1952 when Walter Baade discovered that distances to neighboring galaxies, which were being used in the calculation of the age of the universe, were larger than previously thought; this brought the estimated age of the universe to about 4 Gyr. As time went on and observations improved, estimates for the age of the universe rose, with most estimates in the 1970s through 1990s in the range of 8–20 Gyr. Many of the higher estimates were due to the age of the oldest globular clusters, which had been estimated to be as high as 16 Gyr; these ages were later revised lower after new data revealed that they were further than previously thought. The launch of the Wilkinson Microwave Anisotropy Probe (WMAP) in 2001 and the Planck spacecraft in 2009, both measuring anisotropies in the cosmic microwave background, allowed astronomers to make more precise measurements of the age of the universe. The first year of WMAP data resulted in an estimated age of 13.7 ± 0.2 Gyr by two independent research groups. As of 2026, the accepted age of the universe is 13.787 ± 0.020 Gyr, based on data from the Planck spacecraft analyzed in 2018.
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See also Age of the universe List of measurements of the Hubble constant Cosmic age problem Lambda-CDM model FLRW metric BOOMERanG experiment
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