Question: how many particles are there in the universe

Susan Skelton answered on 12 Nov 2013:

Really cool question!! It’s quite hard to answer, but I’ll give it a go!

The problem is that we don’t know how big the universe really is. It is possible that the universe is infinite, in which case the answer is easy: the number of particles would be infinite too!

But let’s assume that we want to calculate how many particles are there in the VISIBLE universe?

Then the answer depends on what you mean by particles…

Simon mentioned how many atoms scientists think there might be in the universe. But atoms aren’t the smallest particles…

If we look inside atoms, we see that atoms consist of protons, neutrons and electrons, which themselves are made up of quarks. But actually, when we want to calculate the number of particles in the universe, none of these particle really matter, because the number will be dominated by billions of tiny particles with no mass at all called photons (particles of light) and neutrinos.

Most of the universe is just empty space that is filled with photons and neutrinos left over from the Big Bang. There are about 100 neutrinos and 400 photons per cubic centimetre, compared to about about one proton per cubic meter, so you can see that this doesn’t really change our answer at all.

When you work out the number of neutrinos and photons in the universe you get 2.5 x 10^89 elementary particles in the visible universe – that is 250000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.

Just to make that even more complicated, visible matter only makes up 4% of the total matter – the other 96% is what is called ‘dark matter’. We can’t see or measure dark matter, but it’s likely that would contribute a whole load more particles to our calculation!