Cosmic bursts find all that missing matter

Mysterious explosions help solve a longstanding mystery.

Astronomers have used fast radio bursts to finally detect all of the missing “normal” matter in the vast space between stars and galaxies.

Writing in the journal Nature, the Australian-led team describes its success – and relief – in solving a decades-old mystery.

“We know from measurements of the Big Bang how much matter there was in the beginning of the Universe,” says lead author Jean-Pierre Macquart, from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR).

“But when we looked out into the present Universe, we couldn’t find half of what should be there. It was a bit of an embarrassment.”

“Normal” or baryonic matter, as opposed to the elusive dark matter, is what makes up all the stars and planets in galaxies, but also lies between the galaxies. Finding the latter was a task significantly harder than looking for a needle in a haystack.

Intergalactic space is so sparse that the missing matter was “equivalent to only one or two atoms in a room the size of an average office”, Macquart says, making it very tricky to detect using traditional techniques and telescopes.

The solution was itself mysterious. Fast radio bursts are brief flashes of incredible energy that appear to come from random directions in the sky and last for just milliseconds, but scientists don’t yet know what causes them – or even when and where to look.

Macquart says the team used them as “cosmic weigh stations”. “The radiation from fast radio bursts gets spread out by the missing matter in the same way that you see the colours of sunlight being separated in a prism.

200528 missing matter
When travelling through completely empty space, all wavelengths of the FRB travel at the same speed, but when travelling through the missing matter, some wavelengths are slowed down. Credit: ICRAR.

“We’ve now been able to measure the distances to enough fast radio bursts to determine the density of the Universe. We only needed six to find this missing matter.”

The fast radio bursts in question were discovered using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope – a precursor for the future Square Kilometre Array (SKA) telescope – at the Murchison Radio-astronomy Observatory in Western Australia.

The pulse capture system was designed by Keith Bannister from Australia’s national science agency CSIRO, which operates the telescope.

“ASKAP both has a wide field of view, about 60 times the size of the full Moon, and can image in high resolution,” says co-author Ryan Shannon, from Australia’s Swinburne University.

“This means that we can catch the bursts with relative ease and then pinpoint locations to their host galaxies with incredible precision.”

Shannon’s Swinburne colleague Alan Duffy, who was not involved with the research, says Macquart’s team, which brought together researchers from Australia, the US and Chile, has answered a question that fascinated some, but was lost on others.

“Astronomers so often talk about the need to identify the dark matter and dark energy, which collectively make up 96% of our universe, that people may be forgiven for thinking that we have at least found all the normal matter that make up the galaxies, stars and people,” he says.

“Yet nearly half of these atoms were simply unaccounted for, we suspected that they lay between the galaxies in an incredible tenuous state, at such low densities that no telescope could see them directly. And now we know.”

The Royal Institution of Australia has an education resource based on this article. You can access it here.

Nick Carne

Nick Carne

Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.

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