How Scientists Know The Universe Is 13.8 Billion Years Old
In 1964, two radio astronomers at Bell Telephone Laboratories in Holmdel, New Jersey, could not get rid of a faint hiss in their horn antenna, even after evicting the pigeons nesting inside it and cleaning out their droppings. The hiss turned out to be the afterglow of the Big Bang, and it is the main reason scientists can say the universe is about 13.8 billion years old.
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| Representational Image Via: @Nereide on X |
An Expanding Universe
The story starts with the discovery that the universe is growing. In 1927, the Belgian priest and physicist Georges Lemaître published calculations showing that galaxies should be moving away from one another, faster the farther apart they are. Two years later, the American astronomer Edwin Hubble confirmed it with observations of distant galaxies. In 2018, the International Astronomical Union recommended renaming the relationship the Hubble-Lemaître law in recognition of Lemaître’s priority.
If the universe is expanding, it was once smaller, denser and hotter. Run the expansion backwards far enough and everything converges on a single moment: the Big Bang. The time since then is the age of the universe.
Hubble’s own measurement of the expansion rate was about seven times too high, because his distances to galaxies were wrong. Getting the age right meant measuring the expansion far more precisely.
The Afterglow Of The Big Bang
That is where the hiss came in. Penzias and Wilson were using a 6-metre horn antenna on Crawford Hill, built to track early communications satellites. They checked every component and found nothing wrong. The noise came equally from every direction in the sky.
A colleague pointed them to work by physicists at nearby Princeton University, where Robert Dicke and Jim Peebles had predicted that radiation left over from a hot early universe should still fill space. Penzias called Dicke. The two teams published their papers side by side in 1965, one reporting the signal and the other explaining it.
The signal, now called the cosmic microwave background, is light released about 380,000 years after the Big Bang, when the universe had cooled enough for light to travel freely. It has since been stretched by expansion into faint microwaves, at a temperature of about 2.7 degrees above absolute zero. Penzias and Wilson shared the 1978 Nobel Prize in Physics for its discovery.
Measuring 13.8 Billion Years
The afterglow is not perfectly smooth. Tiny variations in its temperature, one part in about 100,000, record the seeds from which galaxies later grew. Their sizes and patterns depend on what the universe contains and how fast it expands.
The European Space Agency’s Planck satellite, launched in 2009, mapped those variations across the whole sky in greater detail than any mission before it. Planck’s final data in 2018 were fed into the standard model of cosmology, which includes ordinary matter, invisible dark matter and the dark energy that is speeding up expansion. They gave an age of about 13.8 billion years, with an uncertainty of a fraction of a per cent.
The Hubble Tension
The same Planck data predict how fast the universe should be expanding today: about 67.4 km per second for every megaparsec of distance, a megaparsec being about 3.26 million light years.
Measured directly, by gauging the distances to nearby galaxies through pulsating stars and exploding stars, the rate comes out at about 73. Each method is now precise enough that the gap cannot easily be put down to chance. Astronomers call it the Hubble tension.
If both measurements are right, something is missing from the standard model, perhaps a change in dark energy over time or an unknown ingredient of the early universe. If one is wrong, an error is hiding in a measurement that many teams have checked. Either answer would matter. A faster expansion rate, taken at face value, would imply a somewhat younger universe.
What The Webb Telescope Has Seen
The James Webb Space Telescope looks back towards the beginning directly. In 2025, astronomers confirmed a galaxy called MoM-z14, seen as it was about 280 million years after the Big Bang, the most distant galaxy yet confirmed. Webb has found more bright galaxies this early than many astronomers expected. That is a puzzle about how galaxies formed so quickly, but not one that changes the age of the universe.
India’s Part
Indian astronomers study the early universe through radio waves. The Giant Metrewave Radio Telescope near Pune, run by the National Centre for Radio Astrophysics, observes hydrogen gas in distant galaxies. In February 2024, the government approved about ₹1,250 crore (₹12.5 billion) for India’s part in building the Square Kilometre Array, a radio telescope under construction in South Africa and Australia. The National Centre for Radio Astrophysics is the lead Indian institute.
Europe’s Euclid space telescope, launched in July 2023, is mapping billions of galaxies to trace how expansion has changed over time. Its results, and those of the radio arrays to come, will show whether 13.8 billion years holds. The figure is precise to a fraction of a per cent, on one condition: that the model behind it is right.
Key facts
- The universe is about 13.8 billion years old, based on Planck satellite data released in 2018.
- The cosmic microwave background, the Big Bang’s afterglow, was discovered in 1964 by Arno Penzias and Robert Wilson.
- The Hubble tension is a disagreement between two measurements of the expansion rate: about 67.4 and about 73 km per second per megaparsec.
- The most distant confirmed galaxy, MoM-z14, is seen as it was about 280 million years after the Big Bang.