There are several methods that scientists use to calculate the age of the universe. One common method is to measure the expansion rate of the universe and then extrapolate back in time to estimate when it began. Another method is to measure the abundance of certain radioactive isotopes, such as uranium and thorium, which have known decay rates and can be used to date the oldest stars in the universe.

Here is a more detailed explanation of each method:

* Expansion rate of the universe: The universe is constantly expanding, and the rate of expansion is known as the Hubble constant. By measuring the Hubble constant, scientists can calculate how long it would have taken for the universe to expand to its current size, which gives an estimate of its age. The current estimate of the Hubble constant is 70 km/s/Mpc, which means that for every million parsecs (3.26 million light-years) away from us, galaxies are moving away from us at a speed of 70 kilometers per second. Using this value, the age of the universe is calculated to be 13.8 billion years.

* Abundance of radioactive isotopes: Certain radioactive isotopes, such as uranium and thorium, have known decay rates. This means that scientists can measure the amount of these isotopes in rocks or other objects to determine how old they are. The oldest stars in the universe are estimated to be about 13.6 billion years old, which provides a lower limit on the age of the universe.

* Cosmic microwave background: The cosmic microwave background (CMB) is the leftover radiation from the Big Bang, the event that is thought to have created the universe. By measuring the temperature and other properties of the CMB, scientists can estimate the age of the universe. The current estimate of the age of the universe based on the CMB is 13.77 billion years.

These are just some of the methods that scientists use to calculate the age of the universe. As new data is collected and analyzed, our understanding of the age of the universe may continue to evolve.