How Far Light Travels: Measuring The Size Of The Universe
The sunlight falling on Earth left the Sun about eight minutes and 20 seconds earlier. The oldest light astronomers can detect has been travelling for about 13.8 billion years, and the matter that released it now lies about 46 billion light years away. That makes the observable universe about 93 billion light years across.
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| Representational Image Via: @AMAZlNGNATURE on X |
What A Light Year Measures
A light year is a distance, not a time: how far light travels in a vacuum in one year. At almost 300,000 km a second, that comes to about 9.46 trillion km.
Kilometres stop being useful almost as soon as you leave the solar system. The nearest star after the Sun, Proxima Centauri, is about 4.24 light years away, close to 40 trillion km. Its light reaching Earth tonight set out more than four years ago. Every time astronomers look into space, they look into the past.
How Astronomers Measure Distance
No tape measure reaches the stars, so astronomers use a ladder of methods, each rung calibrated by the one below.
The first rung is parallax. As Earth circles the Sun, a nearby star appears to shift slightly against more distant ones, and the size of the shift gives its distance by simple geometry. The European Space Agency’s Gaia spacecraft, launched on 19 December 2013 and retired on 27 March 2025, measured positions and parallaxes for about 1.3 billion stars, the most detailed map of the Milky Way yet made.
The second rung came from Henrietta Swan Leavitt, who worked at Harvard College Observatory as a “computer”, measuring stars on photographic plates. In 1912, she showed that a type of pulsating star, the Cepheid variable, brightens and dims at a rate tied to its true brightness. Compare how bright such a star truly is with how bright it looks, and its distance follows. Edwin Hubble used Leavitt’s discovery to show that Andromeda and other spiral nebulae lie far outside the Milky Way, separate galaxies in their own right.
The farthest rungs use exploding stars. A certain kind of supernova, the explosion of a white dwarf star, reaches a predictable peak brightness and can be seen billions of light years away. In 1998, two teams using such supernovae found that the expansion of the universe is speeding up, a discovery recognised with the 2011 Nobel Prize in Physics.
Our Galaxy And Its Neighbours
The Sun is one of hundreds of billions of stars in the Milky Way, a disc about 100,000 light years across. Andromeda, the nearest large galaxy, is about 2.5 million light years away. The light from Andromeda visible on a dark night left it before modern humans existed.
Across the observable universe there may be as many as 2 trillion galaxies, though some estimates put the number in the hundreds of billions.
Why The Edge Is 46 Billion Light Years Away
Astronomers can see matter 46 billion light years away in a universe only 13.8 billion years old because space itself has been expanding while the light travelled. The light from the most distant matter set out when that matter was much closer. In the time the light took to reach Earth, the distance between them grew, and keeps growing.
The observable universe is therefore a sphere centred on the observer, defined by how far light has had time to travel. An astronomer in a galaxy billions of light years away would sit at the centre of a different sphere.
Finite Or Infinite
What lies beyond the observable edge cannot be seen, and its size is unknown. Many cosmologists think the whole universe is far larger than the part we observe, and it may be infinite. Some models describe a universe that is finite but has no edge, in the way the surface of a sphere has none.
India’s Part
India’s window on the distant universe is AstroSat, launched on 28 September 2015 on a Polar Satellite Launch Vehicle from Sriharikota. It is India’s first dedicated astronomy satellite able to observe across several wavelengths at once, from ultraviolet light to high-energy X-rays, and it completed 10 years in orbit in September 2025.
The universe that can be seen will not grow for ever. Because expansion is accelerating, galaxies beyond a certain distance will never become visible, and those near the edge will grow redder and fainter until they appear to fade from view. Some of the universe is out of reach for good.
Key facts
- A light year is about 9.46 trillion km, the distance light travels in a year.
- The nearest star after the Sun, Proxima Centauri, is about 4.24 light years away.
- The observable universe is about 93 billion light years across.
- Whether the whole universe is finite or infinite is unknown.