How old is the universe, and how do we know?

How does the oldest light tell us the age?

About 380,000 years after the Big Bang, the universe cooled enough for atoms to form, and light could finally travel freely, NASA explains. That light still reaches us from every direction as a faint microwave glow, the cosmic microwave background. It has tiny warm and cool spots, the seeds of later galaxies.

The pattern of those spots depends on what the universe contains and how it has grown. ESA says that by analysing them, scientists can work out the universe’s makeup and history from its birth to today. In practice, they fit a model to the map. The standard model, called Lambda-CDM, describes a universe of ordinary matter, dark matter (unseen matter known from its gravity) and dark energy (whatever is speeding up the expansion), using just six basic numbers. Once those numbers are pinned down, the model can be run forward to the present. The time that has passed is the age.

Planck’s final results were published in 2020. Using Planck’s own data, the age comes out at 13.797 billion years. Adding maps of how galaxies are spread across space gives 13.787 billion years, plus or minus 0.020 billion. Earlier, NASA’s WMAP satellite had found 13.77 billion years, to within half a percent.

This is an inference, not a reading from a cosmic stopwatch. Planck’s measurement of the spot pattern itself depends only weakly on the model. But the paper calls the figures it infers for today’s universe, such as the expansion rate, model-dependent. The star clocks below check the age another way.

Do the oldest stars agree?

They do. Stars are a completely different kind of clock.

Old star clusters. Globular clusters are tight balls of hundreds of thousands to a million stars, among the oldest objects in our galaxy. Stars burn their fuel at rates physics can calculate, so the mix of stars in a cluster reveals its age. In 2020, a team led by David Valcin studied 68 globular clusters. The oldest averaged about 13.3 billion years, give or take about half a billion. After allowing time for the clusters to form, the team estimated the universe’s age at about 13.5 billion years, compatible with Planck.

Cooling embers. A white dwarf is the hot, dense core left behind when a star dies. It no longer burns fuel. It just cools, at a predictable rate. NASA compared this to judging how long ago a campfire burned by measuring the temperature of its coals. The analogy breaks down in one way: nobody can touch these coals, so astronomers rely on models of how white dwarfs cool. In 2002, Hubble found the faintest white dwarfs in the nearby cluster M4 and dated them at 12 to 13 billion years. NASA called this a completely independent reading.

A radioactive clock. In 2001, astronomers using the European Southern Observatory’s Very Large Telescope measured uranium in an ancient star called CS 31082-001. Uranium decays at a known rate, much like the carbon-14 used in archaeology. The uranium clock read 12.5 billion years, with an uncertainty of about 3 billion.

These clocks have caught problems before. NASA recalls that in the late 1970s, age estimates from the expansion ranged from 8 to 18 billion years, and the ages of old stars clashed with the lower values. Better measurements, and the discovery that the expansion is speeding up, settled that clash.

Why do two expansion measurements disagree?

The Hubble constant is how fast space is stretching today. It is given in kilometers per second per megaparsec, where a megaparsec is about 3.26 million light-years, as NASA explains. A value of 70 means a galaxy one megaparsec away moves away at about 70 kilometers per second, and one twice as far moves away twice as fast.

There are two main ways to get it. One predicts today’s rate from the early universe: take Planck’s map, apply the standard model, and calculate. That gives 67.4, plus or minus 0.5. The other measures the nearby universe directly, with a “distance ladder.” Astronomers find distances to pulsing stars called Cepheids, whose rhythm reveals how bright they truly are. They use those to calibrate a kind of exploding star, then measure galaxies farther out. In 2022, the SH0ES team led by Adam Riess reported 73.04, plus or minus 1.04. The team called the gap with Planck a 5-sigma difference, far too large to be a likely statistical fluke, and said its source is still unknown.

Why does this matter for the age? NASA notes that a faster expansion rate leads to a younger age for the universe, and a slower one to an older age. So the tension is really a question about the model that links the early universe to today. ESA offers an analogy: the distance ladder sets an anchor on one shore of a river, Planck sets one on the other, and the stretch in between has not been directly observed. The analogy has a limit. Both anchors are measurements with their own error bars, not fixed posts.

What did the Webb telescope add?

A simple explanation would be a mistake in Hubble’s measurements. Cepheids sit in crowded fields, and their light can blend with neighboring stars. The James Webb Space Telescope sees them more sharply in infrared light. In 2023, Webb checked more than 320 Cepheids and confirmed Hubble’s results. In a 2024 follow-up, it measured about 1,000 Cepheids in five galaxies that hosted eight supernovae, out to 130 million light-years. The team ruled out crowding as the cause at 8.2 sigma.

Not everyone reads the Webb data the same way. Wendy Freedman’s team at the University of Chicago uses different stars, red giants and carbon-rich stars, as distance markers. In 2025, their result was 70.4, plus or minus 3%, which agrees with Planck within the uncertainties. “This new evidence is suggesting that our Standard Model of the universe is holding up,” Freedman said. Riess’s group has replied that the small Webb samples differ from the full Hubble set mainly because of which supernovae they happen to include, and that all Webb methods together give 72.6, plus or minus 2.0.

In April 2026, a large collaboration combined decades of distance measurements into one network. It reported 73.50, plus or minus 0.81, and said this effectively rules out any single overlooked error in local distances. So most local measurements land near 73, one leading team lands near 70, and the cause of the gap is still argued. We label it Supported, still debated.

What do people often get wrong?

“Astronomers found a star older than the universe.” In 2013, a Hubble study dated the nearby star HD 140283, nicknamed the Methuselah star, at 14.5 billion years, plus or minus 0.8 billion. That range reaches down to 13.7 billion, so it overlaps the universe’s age. Earlier estimates had run as high as 16 billion. A better distance measurement shrank the error.

“The Webb telescope broke the Big Bang.” Webb found early galaxies that looked too massive for the standard model. A 2024 study found that growing black holes made some of them look bigger than they are. NASA reports the researchers found no crisis for the standard model. Webb still sees more massive early galaxies than expected, and NASA says one possible reason is that stars formed faster in the early universe.

“The universe is 13.8 billion light-years across.” Space kept stretching while the light traveled. In the current best-fit model, the most distant places we can see are now about 47 billion light-years away, as the UCLA cosmologist Edward L. Wright explains. See How Big Is the Universe, and Does It Have an Edge?

How can you check this yourself?

• Find the number at its source. Planck’s final paper on cosmological parameters is free on arXiv (1807.06209). Look for the row labeled “Age” in its Table 2.
• Read the plus-or-minus. A result is a range, not a point. The Methuselah star only looked older than the universe if you ignored its error bars.
• Do a rough check. The University of Chicago explains that dividing distance by speed, using the Hubble constant, gives a first guess of about 14 billion years. That is close to 13.8, as it should be.
• Look at the date and the team. The expansion results on this page run from 2022 to 2026 and come from rival groups. A headline saying the tension is “solved” is usually one team’s view.
• Start at the beginning. For what happened in the universe’s first moments, read What Happened in the First Second After the Big Bang?

After all this detail, the answer is unchanged. The universe is about 13.8 billion years old, and independent clocks agree. The open question is how fast it is expanding today, and why two careful methods disagree.