How old is the universe?

That makes the universe about three times as old as the Earth, which formed 4.54 billion years ago. Squeeze all 13.8 billion years into one calendar year, and each day stands for about 38 million years. The Big Bang is the first moment of 1 January. The Sun and Earth form around 1 and 2 September. The dinosaurs die out early on 30 December, and our species, about 300,000 years old, turns up about 11 minutes before midnight on 31 December.

What is the age of the universe?

The age of the universe is the time since the Big Bang. The Big Bang wasn't an explosion of matter into empty space. It was the time when everything we can see was packed into a tiny, extremely hot and dense state, and space itself began expanding. As space expanded, the universe cooled, which let atoms, then stars and galaxies, form.

So "13.8 billion years old" means: run the expansion backwards with the laws of physics we know, and that is how long ago everything reaches that hot, dense state. What happened at the very first instant, or whether "before" even means anything there, is still an open question.

How do scientists know how old the universe is?

No one method settles it. Three very different lines of evidence agree, which is why scientists are confident in the answer.

1. Rewind the expansion

In the 1920s, Georges Lemaître and Edwin Hubble found that distant galaxies are moving away from us, and the farther away a galaxy is, the faster it recedes. This is Hubble's law, which the International Astronomical Union recommended in 2018 calling the Hubble-Lemaître law:

H₀, the Hubble constant, is the expansion rate today. It is measured in kilometres per second per megaparsec (km/s/Mpc), where a megaparsec is about 3.26 million light years. Planck's value of 67.4 km/s/Mpc means a galaxy 1 megaparsec away moves away at 67.4 km/s, and one 100 megaparsecs away at 6,740 km/s.

Now run the film backwards. A galaxy twice as far away is moving twice as fast, so every galaxy arrives back at the same moment. If the speeds had never changed, that moment would be distance ÷ speed ago, and since speed = H₀ × distance, the distance cancels:

This is called the Hubble time. A faster expansion rate means a younger universe, as the graph shows.

Here x is the Hubble constant in km/s/Mpc and y is 1/H₀ in billions of years. The 977.8 comes from converting the units: it is the number of kilometres in a megaparsec divided by the number of seconds in a billion years. Planck's 67.4 gives 14.5 billion years, and the 73.04 measured from nearby galaxies gives 13.4.

2. The oldest light: the cosmic microwave background

For its first 380,000 years, the universe was a glowing fog of charged particles that light couldn't travel through. When it cooled enough for atoms to form, the light escaped, and it still fills all of space. Stretched by 13.8 billion years of expansion, it now reaches us as microwaves at about 2.7 degrees above absolute zero. This is the cosmic microwave background, or CMB.

The CMB has tiny hot and cold spots, and their sizes and pattern depend on what the universe is made of and how fast it has expanded. The European Space Agency's Planck satellite mapped them across the whole sky. Fitting the standard model of cosmology to its map, combined with galaxy surveys, gives an age of 13.787 ± 0.020 billion years (Planck 2018). The Atacama Cosmology Telescope, measuring the same light from the ground with a different instrument, found 13.77 ± 0.04 billion years.

3. The oldest stars

The universe must be older than anything in it, so the oldest stars set a minimum age. This check doesn't use the expansion at all. Globular clusters are tight balls of hundreds of thousands of very old stars. Stars of different masses age at known rates, so the mix of stars left in a cluster acts as a clock.

• A 2020 study of 68 globular clusters in our galaxy found the oldest are about 13.3 billion years old, give or take about half a billion years, which points to a universe of about 13.5 billion years.
• Dating the cooling white dwarf stars in a globular cluster with the Hubble Space Telescope gave 12 to 13 billion years.

Why isn't the age exactly 1/H₀?

The Hubble time assumes galaxies have always moved at today's speeds. They haven't. For most of history, the gravity of matter slowed the expansion, so it used to be faster than today. More recently, dark energy has been speeding it up. To get the real age, cosmologists add up the expansion through every era. For a universe of matter and dark energy, that gives:

Ω_m is the share of matter (ordinary and dark) and Ω_Λ is the share of dark energy. With Planck's values, Ω_m = 0.315 and Ω_Λ = 0.685, the number in front of 1/H₀ comes to 0.951. So the age is 0.951 × 14.51 = 13.80 billion years, matching Planck's full answer. The early slowing and the later speeding up almost cancel out, which is why the simple Hubble time is so close.

Worked examples

Example 1: how fast is a galaxy moving away?

A galaxy is 150 megaparsecs away, about 489 million light years. Using H₀ = 67.4 km/s/Mpc, how fast is it receding?

It is moving away at about 10,100 km/s, roughly 3% of the speed of light.

Example 2: the Hubble time from H₀

Find the Hubble time for H₀ = 70 km/s/Mpc, in years. The trick is the units. A megaparsec is about 3.086 × 10¹⁹ km, so H₀ = 70 km/s per 3.086 × 10¹⁹ km.

A year is about 3.156 × 10⁷ seconds, so 4.409 × 10¹⁷ ÷ 3.156 × 10⁷ = 1.397 × 10¹⁰ years. The Hubble time is about 14.0 billion years.

Example 3: the age that was younger than the Earth

Hubble's first estimate of the expansion rate, in 1929, was about 500 km/s/Mpc. What Hubble time does that give, and why was it a problem?

977.8 ÷ 500 = 1.96, so about 2 billion years. Geologists already knew the Earth was older than that, and a universe can't be younger than its planets. The fault was in the distances: Hubble's galaxies were much farther away than he thought, so his H₀ was far too big. Better distance measurements over the next decades brought the value down to the 67 to 73 range used today.

Try one yourself. This question asks you to reason about the age from the expansion rate and the oldest stars. Answer here, then press Mark to check it in the course.

The Hubble tension

There is one loose end. The CMB, read through the standard model, says H₀ = 67.4 ± 0.5 km/s/Mpc. Measuring it directly, using exploding stars and pulsating stars in nearby galaxies as distance markers, gives 73.04 ± 1.04 km/s/Mpc. The error bars don't overlap, and checks by many teams haven't made the gap go away. This is called the Hubble tension.

Both sides agree the universe is roughly 13 to 14 billion years old. The argument is about the details. Either one of the measurements has a hidden error, or the standard model is missing something, such as dark energy that changes over time. Our How the Universe Ends course looks at the newest dark energy results and what they could mean for the future.

A timeline of the universe

Light takes time to reach us, so looking far away means looking back in time. In 2025, the James Webb Space Telescope confirmed a galaxy called MoM-z14 as it was about 280 million years after the Big Bang, the most distant galaxy NASA had reported by early 2026. Our Big Bang Theory course goes through each stage of this story with interactive models.

If the universe is 13.8 billion years old, how big is it?

You might expect the farthest we can see to be 13.8 billion light years. It is much more. Light from the most distant regions set off 13.8 billion years ago, but space kept expanding while it travelled, so those regions are now about 46 billion light years away. That makes the observable universe about 93 billion light years across. The whole universe is at least that big, and may be infinite: we can only see the part whose light has had time to reach us. Our guide to how big the universe is counts its galaxies, stars and atoms.

Common mistakes

• Picturing the Big Bang as an explosion at one point in space. Space itself expanded everywhere at once. There is no centre that everything flew out from.
• Taking the age to be exactly 1/H₀. The Hubble time is a first estimate. The real age allows for the expansion slowing down and then speeding up.
• Thinking the observable universe is 13.8 billion light years across. Expansion stretches the distances, so its radius is about 46 billion light years.
• Thinking a bigger H₀ means an older universe. It is the other way round: a faster expansion gets back to the start sooner, so the universe is younger.
• Saying we know what happened at time zero. Physics describes the universe from a tiny fraction of a second onwards. The very first instant is still unknown.

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