How do scientists figure out how old the universe is?

Scientists determine the age of the universe primarily by looking at the expansion rate of space and the light from the earliest stages of the cosmos. Here is how they piece it together:

1. Measuring the Expansion (Hubble’s Constant)

If you know how fast the universe is expanding today and you know how it has expanded in the past, you can essentially "rewind the clock" to see when everything was condensed into a single point. Scientists use the Hubble constant, which measures the rate of this expansion, to calculate the time elapsed since that initial state.

2. The Cosmic Microwave Background (CMB)

The most precise tool we have is the study of the Cosmic Microwave Background. This is the "afterglow" radiation left over from the Big Bang. Satellites like the Planck observatory have mapped this radiation in incredible detail. By analyzing the patterns and fluctuations in this light, scientists can build a detailed model of the universe's composition—including how much dark matter and dark energy it contains—which allows them to calculate its age with high accuracy.

3. Studying the Oldest Objects

Scientists also look for "age markers" within the universe to act as a sanity check. They study the oldest known objects, such as globular clusters (tight groups of ancient stars). By calculating how long it takes for stars of certain masses to burn their fuel and reach the end of their lives, astronomers can estimate a minimum age for these clusters. Because the universe must be older than the objects within it, this confirms that the calculated age (roughly 13.8 billion years) is consistent with the physical evidence.

By combining the data from the expansion rate and the patterns in the CMB, scientists have arrived at the widely accepted age of approximately 13.8 billion years, with a very small margin of error.