The universe's clock: 13.787 billion years and counting
Everything has a start date. Here is how the universe's age is measured, what happened when, and a calculator that turns any redshift into a date in cosmic history.
13.787 billion years
± 20 million years (Planck satellite, 2018)
Since you opened this page, every megaparsec of space has stretched by0 km
A megaparsec is 3.26 million light-years; space grows by about 67.7 km/s for each one.
How old is everything? A redshift calculator
The farther away a galaxy is, the longer its light has been travelling and the more the expanding universe has stretched it towards the red. That stretch, the redshift z, is the universe's own clock. Enter any redshift to see when the light left and how old the universe was then, using a flat ΛCDM model with Planck 2018 values (H₀ = 67.66 km/s/Mpc, Ωm = 0.311).
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Lookback time is the light's travel time. The "distance today" is where that galaxy is now, after space kept expanding while the light was in flight — which is why we can see objects that are now more than 30 billion light-years away in a universe 13.8 billion years old.
The universe's timeline
| Time after the Big Bang | Epoch | What happened |
|---|---|---|
| 10⁻⁴³ s | Planck era | Physics as we know it breaks down; gravity and quantum effects can't be separated. |
| ≈ 10⁻³⁶ – 10⁻³² s | Inflation | Space expands by a factor of at least 10²⁶, smoothing it out and seeding the structures of today. |
| ≈ 1 µs | Protons and neutrons form | Quarks bind together as the universe cools below about a trillion degrees. |
| 1 s | Neutrinos break free | A cosmic neutrino background, still unseen, fills space from this moment. |
| 3 – 20 minutes | First nuclei | Hydrogen and helium form in about a 3:1 ratio by mass, plus a trace of lithium. |
| ≈ 50,000 years | Matter takes over | Matter's gravity begins to outweigh radiation. |
| 380,000 years | First light | Atoms form and light travels freely — the cosmic microwave background we see today, then at about 3,000 K. |
| ≈ 100–250 million years | First stars | The cosmic dark ages end as the first stars ignite. |
| ≈ 1 billion years | Reionization ends | Starlight has re-ionised the gas between galaxies. |
| ≈ 9.8 billion years | Dark energy dominates | Expansion begins to accelerate — about 4 billion years ago. |
| 9.2 billion years | The Sun forms | 4.6 billion years ago. |
| 13.786 billion years | Now | The cosmic background has cooled to 2.7255 K. |
How do we know the age?
The most precise figure comes from the cosmic microwave background, the light released 380,000 years after the Big Bang. The European Space Agency's Planck satellite mapped its tiny temperature ripples; fitting them with the standard cosmological model gives an age of 13.787 ± 0.020 billion years. Independent checks agree: the oldest stars in globular clusters come out at around 12–13 billion years, and the decay of radioactive elements in very old stars gives similar numbers.
There is one loose thread. Measuring how fast the universe expands today by stepping out through Cepheid stars and supernovae gives about 73 km/s/Mpc, while the early-universe method gives about 67–68. This "Hubble tension" has survived checks with the James Webb Space Telescope. If the higher number wins without other changes, the universe would come out roughly a billion years younger — which is why it is one of the biggest open questions in cosmology.
Clocks the universe keeps
The universe has no calendar, but it has several clocks, and astronomers use whichever fits:
- Redshift — how much the universe has expanded since light left a source. z = 1 means space has doubled in size since then.
- Temperature — the cosmic background cools as the universe grows. It is 2.7255 K now; at z = 1 it was 5.45 K, and when it was released it was about 3,000 K.
- Cosmic time — time measured by an observer moving with the expansion, the "age" in this page's numbers.
- Element abundances — the fixed mix of hydrogen, helium and lithium recorded the first 20 minutes, like a timestamp no later event could change.
- Observable universe radius≈ 46 billion light-years
- Background temperature2.7255 K
- Hubble constant (Planck)67.7 km/s/Mpc
- Hubble constant (distance ladder)≈ 73 km/s/Mpc
- Ordinary matter≈ 5%
- Dark matter / dark energy≈ 27% / ≈ 68%
Frequently asked questions
How old is the universe?
13.787 billion years, with an uncertainty of about 20 million years, according to the Planck satellite's 2018 results.
How big is the observable universe?
About 46 billion light-years in radius (93 billion across), larger than 13.8 billion light-years because space has kept expanding while the light was travelling.
What is redshift?
The stretching of light to longer, redder wavelengths as space expands while the light travels. A redshift of z means the universe has grown by a factor of 1 + z since the light was emitted.
What is the Hubble tension?
A disagreement between two ways of measuring the universe's expansion rate: about 67–68 km/s/Mpc from the early universe versus about 73 km/s/Mpc from nearby stars and supernovae. It has not been explained yet.