Half the universe’s lost matter found

BRITAIN: The amount of observable matter in the universe has just doubled. And, say astronomers, that is quite a relief.

By looking at the radiation from the early universe it is possible to work out how much matter there is and what kind it is. But this has led to an apparent paradox: half of the ordinary matter that was there at the big bang nearly 14 billion years ago seems to be missing.

Two separate teams, including scientists from Edinburgh University, have at last tracked it down. They found that the matter exists in fine tendrils of gas which form links between the universe’s galaxies.

This gas is different from socalled dark matter and dark energy, which we know makes up the vast majority of mass in our universe but is invisible. Instead, these particles are of the same kind that make up all the matter we can see.

Astronomers had predicted the gas would be there but until now no instrument had been sensitive enough to pick it up.

The latest work used a phenomenon known as the SunyaevZel’dovich effect, in which the light left over from the big bang fluctuates ever so slightly after passing through the hot gas.

It is still impossible to see this effect by looking at the gap between any two galaxies. However, if you combine the readings from hundreds of thousands of galaxies, overlaying them on top of each other, then cumulatively it becomes apparent.

This is what the researchers did, finding the filaments of gas appearing as a slight dimming caused by the Sunyaev-Zel’dovich effect.

Andrew Pontzen, a reader in cosmology at UCL, who was not involved in these studies, said the study was probably going to be the first of many using this technique to probe the mysteries of the cosmos.

‘‘The difficulty is it is made out of regular particles, but because they are very thinly spread they are hard to see,’’ he said.

‘‘This work is a cross-check to see they really are there in the modern universe, and that something hasn’t gone terribly wrong. If you didn’t find this you’d be really worried.’’

In particular, it was comforting to find out that not only did the matter exist, but that it was broadly where astronomers expected it to be.

Pontzen said cosmologists now wanted to find things that they did not expect in order to solve a far greater mystery.

‘‘What this technique is showing you is a normal gas that is hard to pick up but is fundamentally made out of the same stuff you and I are made out of,’’ he said.

‘‘It’s very helpful to confirm that this stuff is there, but it isn’t dark matter. We’ve got a much bigger problem on our hands, which is that 95 per cent of the universe seems to be missing.’’ –