The Observable Universe
The observable universe is the part of the universe we are physically able to see. It is a giant ball about 93 billion light years across, centred on Earth, containing roughly 2 trillion galaxies. The reason there is an edge at all is that light only travels at a finite speed, and the universe is only 13.8 billion years old. We can only see things whose light has had time to reach us since the Big Bang. Beyond the cosmic horizon, the universe almost certainly carries on, but we cannot see it.
- Diameterapprox. 93 billion light yearsAcross
- Radiusapprox. 46.5 billion light yearsIn every direction from Earth
- Galaxiesapprox. 2 trillionBest estimate from Hubble and JWST
- Starsapprox. 10²³A hundred billion trillion
- Age of universe13.8 billion yearsSo the horizon is 13.8 billion years away
- Why the size mismatch?Space has been stretchingFaster than the light could travel
Why we can't see the whole universe
Light always travels at the same speed, about 300,000 kilometres per second. That sounds fast, but the universe is enormous. Light from a galaxy 10 billion light years away has had to spend 10 billion years travelling here. Light from a galaxy 14 billion light years away has not had time to reach us yet, since the universe is only 13.8 billion years old. Even if such a galaxy exists, we will not be able to see it.
This gives us a cosmic horizon: the edge of what we can possibly see. Anything beyond it is invisible to us now (although in many cases its light will eventually reach Earth in the far future). The horizon is not an edge of the universe itself; it is an edge of our view of the universe.
Why is it 93 billion light years across?
Naively, you might expect the observable universe to be 27.6 billion light years across (13.8 billion in each direction from Earth, since the universe is 13.8 billion years old). But the real answer is 93 billion light years. How can that be?
The answer is that the universe has been expanding during the 13.8 billion years the light has been travelling to us. Imagine sending an ant to walk across a sheet of rubber that is slowly being stretched. By the time the ant reaches the other side, the rubber is bigger than it was when the ant set off. The same happens with the light from distant galaxies: while their light has been on its way to Earth, the universe has been stretching out, and those galaxies are now much further away than they were when their light first set out. The current distance to the most distant things we can see is about 46.5 billion light years, even though the light itself has only travelled for 13.8 billion years.
What is beyond the horizon?
Almost certainly, the rest of the universe just carries on. Everywhere we look, the universe looks roughly the same: roughly the same number of galaxies per cubic megaparsec, roughly the same kinds of stars, roughly the same chemistry. There is no reason to think the bit we can see is a special spot, so the bit we cannot see probably looks similar.
How much more universe there is beyond the horizon is genuinely unknown. Some measurements suggest the rest of the universe is at least 250 times bigger than the observable part. Other measurements are consistent with the universe being infinite in every direction. We may never know for sure, because we will never be able to see beyond our horizon.
How many galaxies, stars and atoms?
The numbers in the observable universe are difficult to grasp:
- approx. 2 trillion galaxies, based on counts from the Hubble and James Webb space telescopes.
- approx. 1023 stars (a hundred billion trillion). More stars than there are grains of sand on every beach on Earth.
- At least 1024 planets. Some estimates are several times higher.
- approx. 1080 atoms. To write that out fully would take 80 zeros after a 1.
And those numbers are only the observable universe. The rest of the universe, beyond our cosmic horizon, almost certainly adds enormously more.
The edge: the cosmic microwave background
The very edge of what we can see is not actually a galaxy or even a star: it is a faint glow of microwave radiation called the cosmic microwave background. This is the light released about 380,000 years after the Big Bang, when the universe finally cooled enough for the first atoms to form and let light travel freely for the first time. The cosmic microwave background fills the whole sky uniformly in every direction, and it sets the practical edge of the observable universe. We cannot see anything older than it because no light could travel before that point.
Deeper dive: the heat death of the universe
Once you accept that the universe is expanding, and that the expansion is accelerating, you can start to predict the very far future of everything. The current best guess is called the heat death.
In about 5 billion years, our Sun will swell up, swallow the inner planets, and die. In approx. 100 billion years, the accelerating expansion will have pushed every other galaxy beyond the cosmic horizon: only our merged Milky Way + Andromeda galaxy will be visible. In approx. 1014 years, the last of the smallest red dwarf stars will run out of fuel, and the universe will go dark. In approx. 1040 years, all the matter that has fallen into white dwarfs, neutron stars and brown dwarfs will slowly decay, possibly via proton decay. In approx. 10100 years, even the largest black holes will slowly evaporate via Hawking radiation, leaving behind only a thin, cold mist of photons and elementary particles drifting forever in an empty universe.
That is the heat death: a universe so spread out, so empty, so cold and so featureless that nothing of interest can ever happen again. There may be a different ending (a Big Rip, a Big Crunch, or perhaps an entirely new beginning) but the heat death is currently the most likely fate of our universe according to known physics. The good news is that it is a very, very long time away.
For how the universe started, see the Big Bang. For why it is growing, see the expanding universe. For the invisible 95%, see dark matter and dark energy.