The Age of the Universe
The universe is 13.8 billion years old. This number is not a guess. It is a measurement — one of the most carefully determined in all of science, with an uncertainty of about 20 million years, which is roughly 0.2%. The age of the universe is known from multiple independent methods, and they all converge on the same value. That convergence is what makes it reliable.
From the cosmic microwave background
The most precise measurement comes from the cosmic microwave background — the afterglow of the Big Bang. The Planck satellite mapped the CMB across the entire sky, measuring tiny temperature fluctuations that encode the physical conditions of the early universe. These fluctuations are like a snapshot of the universe at 380,000 years old — a baby picture, if the universe were old enough to have a face.
By fitting a cosmological model to the CMB power spectrum — the statistical distribution of fluctuation sizes at different angular scales — cosmologists can determine the parameters of the universe: the density of ordinary matter, dark matter, dark energy, the curvature of space, and the Hubble constant. From these parameters, you calculate the age. Planck's 2018 results give an age of 13.787 ± 0.020 billion years. That is the most precise single measurement available.
The CMB age does not measure time directly. It infers it from a model. But the model is the standard cosmological model — Lambda CDM — which has survived decades of independent tests. The age derived from the CMB matches the ages of the oldest stars in the universe. It is consistent with the expansion rate measured locally. And it predicts structures we can observe at high redshift.
From the oldest stars
An independent check comes from the oldest objects we can date. Globular clusters — dense balls of ancient stars orbiting the Milky Way — contain the oldest known stars. By modeling stellar evolution and comparing the observed properties of these clusters with theoretical predictions, astronomers estimate their ages. The oldest globular clusters are about 13 billion years old, with uncertainties of order 1 billion years. This is consistent with the CMB age, because the universe must be older than the stars it contains. The first stars formed a few hundred million years after the Big Bang, leaving plenty of room between their formation and today.
White dwarf cooling ages — another stellar chronometer — provide an independent line of evidence. White dwarfs are the remnants of low-mass stars. They cool slowly over billions of years. The coolest white dwarfs in the Milky Way give an age for the galactic disk that is consistent with 13.8 billion years for the universe.
From the expansion rate
The third path to the age is through the expansion rate itself. If you know how fast the universe is expanding today — the Hubble constant — and you know how that expansion has changed over time (which depends on the matter and energy content of the universe), you can integrate backward to find when the scale factor was zero — the moment of the Big Bang.
This gives a different value for the age than the CMB method, because it depends on the locally measured Hubble constant. Using the Cepheid-supernova value of $H_0 \approx 73$ km/s/Mpc, the age comes out closer to 12.7 billion years. Using the Planck CMB value of $H_0 \approx 67$ km/s/Mpc, you get about 13.8 billion years. This discrepancy is another manifestation of the Hubble tension. If the local $H_0$ is correct, the universe is younger than the oldest stars — a physical impossibility. Either our measurement of $H_0$ is wrong, or our model of the universe is incomplete.
Why 13.8 billion years matters
The age of the universe is more than a number. It is a constraint on every other measurement in cosmology. It tells us how much time gravity had to form galaxies and clusters. It tells us how many generations of stars could have lived and died, enriching the cosmos with heavy elements. It tells us how much time life had to emerge, evolve, and create observers who could measure the age of the universe.
13.8 billion years is the timescale on which everything in the observable universe happened. The first stars burned for a few million years before dying as supernovae, seeding the galaxy with carbon and oxygen. New stars formed from this enriched gas, and around one of them — a middle-aged G-type star in the galactic suburbs — a planet formed 4.5 billion years ago. The universe had time for complexity to arise from simplicity.
The age is also a boundary condition. Nothing in the universe can be older than 13.8 billion years. The oldest stars, the oldest galaxies, the oldest black holes — they must all be younger. And they are. This consistency between independent age estimates is one of the great achievements of modern cosmology.