First look
The idea, in plain words
Start with the Sun. It is about 4.6 billion years old and in the middle of its life. When it is 10 or 11 billion years old it will swell into a red giant, then puff its outer layers away and leave a small, dim ember called a white dwarf. This real photograph shows a star like the Sun that has already done it. That is the end of our Solar System’s daylight, more than 5 billion years from now.
The galaxy will change too. This is Andromeda, the nearest big galaxy, 2.5 million light years away. For years it was thought certain to crash into the Milky Way. In 2025 a team ran the sums again with the newest measurements and found the chance is about even: a 50-50 chance of the two meeting within the next 10 billion years. If they do, hardly any stars will touch, but the two spirals will become one great ball of stars.
Then the lights begin to go out. Stars are made from gas, and the galaxies are slowly using it up. The smallest stars burn so gently that they can last up to about 14 trillion years, a thousand times the present age of the universe. But by about 100 trillion years no new stars are being made anywhere, and the last small ones fade. The sky goes dark for good.
What is left is cold. Dead stars, planets and black holes drift in the dark. On the longest view anyone has worked out, even these do not last. The matter in dead stars may slowly fall apart, and black holes very slowly leak away to nothing. The largest would take a number of years written as 1 followed by a hundred noughts. After that there is only a thin, cold haze, spreading for ever.
That is the ending the measurements so far point to, and it is sometimes called the Big Freeze. It is not the only one on the table. If the push that is speeding up the universe grows stronger with time, it could one day tear apart galaxies, then stars, then atoms: the Big Rip. If the push fades or turns round, gravity could win, and everything could fall back together: the Big Crunch.
Which it will be depends on dark energy, the name for that push, and nobody knows what dark energy is. It was found in 1998 by measuring exploding stars like this one in far galaxies. In 2025 a survey of nearly 15 million galaxies found signs that it may be changing with time. The signs are not yet strong enough to count as a discovery. The end of the universe is still an open question.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- White dwarf
- The hot, dense core left behind when a star like the Sun dies: about the size of the Earth, and no longer making energy.
- Red dwarf
- A small, cool, faint star. Red dwarfs are by far the commonest kind of star.
- Planetary nebula
- The glowing shell of gas blown off by a dying star like the Sun. It has nothing to do with planets.
- Dark energy
- The name for whatever is making the expansion of the universe speed up.
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Galaxy merger
- Two galaxies falling together under gravity and settling into one.
- Power of ten
- A step of ten times bigger or ten times smaller, the only practical way to count the sizes of things in space.
- Supernova
- The explosion of a star. For a few weeks it can outshine a whole galaxy.
Where this comes from
- ESA, Gaia reveals the past and future of the Sun (11 August 2022)
- ESA/Hubble, Apocalypse when? Hubble casts doubt on certainty of galactic collision (2 June 2025)
- NASA, Types of stars
- Adams and Laughlin, A dying universe: the long-term fate and evolution of astrophysical objects (1997)
- Caldwell, Kamionkowski and Weinberg, Phantom energy and cosmic doomsday (2003)
- University of California, Santa Cruz, on the DESI results of 19 March 2025
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.