The cosmic microwave background (CMB) is the electromagnetic radiation that is thought to be the remnant thermal radiation of the Big Bang, the event that is believed to have created the universe. The CMB is a faint glow of light that permeates the entire universe, and its temperature has been measured to be 2.725 Kelvin.

Scientists can calculate the age of the universe by measuring the temperature of the CMB. The CMB is thought to have been much hotter in the early universe, and it has been cooling ever since. By measuring the current temperature of the CMB, scientists can calculate how long it has been cooling, and thus estimate the age of the universe.

2. Radioactive Dating

Radioactive dating is a method for determining the age of an object by measuring the decay of radioactive isotopes. Radioactive isotopes are atoms that have unstable nuclei, and they decay into other elements over time. The rate of decay is known for each radioactive isotope, so by measuring the amount of a radioactive isotope in an object, scientists can calculate how long it has been since the object was formed.

Scientists have used radioactive dating to measure the age of various objects in the universe, including stars, planets, and meteorites. The oldest objects that have been dated using radioactive dating are about 13.8 billion years old, which is the estimated age of the universe.