In cosmology, the age of the universe is the amount of time since the Big Bang. The best measurement of the age of the Universe is 13.8 billion years (specifically, 13.798±0.037 billion years). The most recent and most accurate measurements were made by the Planck spacecraft. These measurements were made using the cosmic microwave background radiation and the expansion of the Universe.
In the 1800s, evidence from geology showed that the Earth was at least millions of years old. Later, it was thought that the universe was unchanging. However, in the 1920s, Georges Lemaître and Edwin Hubble determined that the universe was expanding (see Hubble's law). Although the estimates made using the initial data were low, radioactive dating on Earth could be used to give measurements of 11–20 billion years to 13–15 billion years. Eventually, scientists were able to make much better measurements using spacecraft.