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.