and scientists attempt to determine its age by calculating when it had no mass at all. The calculation is not simple, however, because it’s impossible to know whether the universe has always expanded at the same rate it does now.

Only 13 years ago, astronomers assumed that the universe expands slower as time goes by, because the gravitational effect between galaxies slows down the expansion. If that hypothesis were correct, the universe would be around 10 billion years old, but because the oldest known stars are 12 to 13 billion years old, stars cannot be older than the universe.

The problem was solved in 1998 when astronomers discovered that the universe is actually expanding ever faster, due to so-called dark energy, which can counteract the gravitational effect between galaxies over long distances. Consequently, the universe started out expanding slowly, and much time passed before it reached its present size. Allowing for the slower expansion in the past, astronomers have concluded that the universe is 13.7 billion years old.

The agreement between the age of the oldest stars and the universe is a good indication that scientists have a relatively precise idea of the universe’s age.