How much does the universe weigh? It’s a simple question with an answer so complex that scientists have devised several ways to calculate the mass of, well, everything and arrived at many different answers. Luckily, some help is arriving from a galaxy some 8 billion light-years away in the form of ultra-bright radio waves.
In October 2022, astronomers detected a fast radio burst (FRB) — an extremely powerful blast of radio waves releasing huge amounts of energy in just a few milliseconds — in a high-redshift galaxy (meaning it was extremely far away). This wasn’t any ordinary burst, as the FRB released about 3.5 times more energy than should be possible, at least according to current theories. Now, in a new study, Macquarie University astrophysicist Stuart Ryder and his colleagues say that the bright flash of radio waves is a great example of a relatively new idea that FRBs could help calculate how much matter exists in the universe.
“One of my personal interests and reasons why I am excited about FRBs is to use them to study the intergalactic medium and the host galaxy interstellar medium gas (especially at high redshift), and combine that with other ways to study these media at other wavelengths,” Ryder tells Inverse.
Ryder and his colleagues published their work today in the journal Science.
A Blast From the Past
Ryder and his team, including astrophysicists from the International Centre for Radio Astronomy Research and other universities around the world, spotted the bright flash of radio waves with the Australian Square Kilometer Array Pathfinder (ASKAP), an array of 36 radio antennas in western Australia. ASKAP is extremely good at pinpointing the exact spot in the sky where a signal originates, so scientists first traced the fast radio burst to a tiny spot in space and then took a closer look with the Very Large Telescope, an optical telescope in Chile’s Atacama Desert.
The fast radio burst came from a galaxy in the throes of a very messy galactic merger. In fact, the merger is so messy and so distant that Ryder can’t be sure whether it’s actually two colliding galaxies or three. Because this is the most distant fast radio burst ever detected, that means its also the oldest, and it’s likely that the released energy came from a restless dead star when the universe was less than half its current age.
Along with its impressive age and distance, this particular fast radio burst was also surprisingly powerful. Based on what astrophysicists know about fast radio bursts, models can predict how much energy FRBs should release, and this one was at least three times more energetic than the theoretical maximum.
“I suspect that this is an extreme example of the other FRBs that we have seen with ASKAP,” Ryder says. “Previous models had included upper limits on the maximum energy an FRB could have, and those need to be revised. The FRBs are a trillion times more energetic than pulsars observed in our own galaxy, so a truly extreme process needs to produce the bursts.”
Putting the Universe on the Scale
So, how exactly do radio waves help weigh the universe?
As radio waves travel through intergalactic space, they pass through a lot of stuff but mostly very, very sparse patches of electrically charged gas known as plasma. Because of its electrical charge, the plasma distorts the radio waves just slightly, making the sharp, clear signal slightly wider and fuzzier. If astronomers know where a fast radio burst originated, they can measure how fuzzy and dispersed it looks and then calculate how much plasma exists between the FRB’s point of origin and Earth. This means every FRB is essentially a quick snapshot of the universe’s structure — at least along a straight line between Earth and the original galaxy.
While there are several ways to estimate the mass of the universe, each technique produces a slightly different answer — which means scientists still aren’t sure how much stuff is out there (meaning normal matter; don’t even get us started on dark matter).
“If we count up the amount of normal matter in the Universe – the atoms that we are all made of – we find that more than half of what should be there today is missing,” says Swinburne University astrophysicist Ryan Shannon, a coauthor of the recent study, in a statement. “We think the missing matter is hiding in the space between galaxies, but it may just be so hot and diffuse that it’s impossible to see using normal techniques.” FRBs could help resolve that.
Ryder’s team measured how dispersed the radio waves were in their bright, newly discovered FRB. When they subtracted the effect of all the interstellar gas from the final leg of the journey (the Milky Way), they were left with a measurement of how much plasma lay in between the Milky Way and the distant merging galaxies – but they saw more than they expected.
According to Ryder and his colleagues, that could mean there's more plasma than usual in the intergalactic space the FRB passed through, which isn’t too surprising; as Ryder puts it, "the cosmic web is not expected to be perfectly smooth." Some of the gas may also have been part of the FRB's host galaxy.
"Higher resolution observations of the host with space telescopes like Hubble/JWST will help better understand the nature of the host," says Ryder.
A Weigh Forward
A fast radio burst from 8 billion light years away is near the limit of what modern radio telescopes can detect, but Ryder and his colleagues expect the next generation of telescopes to be able to spot even older, more distant FRBs.
ASKAP is exactly what it says on the tin: a pathfinder, or precursor, for a much bigger array of radio antennas called the Square Kilometer Array (SKA), which will span a total of one square kilometer (about a third of a square mile), split between sites in South Africa and Australia. The SKA will be able to detect longer radio waves from more distant galaxies — which includes FRBs from earlier in cosmic history. And because those FRBs will be traveling across even longer stretches of space, they’ll help astronomers measure how much matter is floating around in the distant universe.
The SKA should be online sometime in the next few years. Meanwhile, construction crews on a Chilean mountaintop are building a bigger, newer telescope called the Extremely Large Telescope (the latest in a long line of deeply uncreative but accurate telescope names), which will see the universe in infrared and visible light. Like the Very Large Telescope, the Extremely Large Telescope will help astronomers study the galaxies that FRBs come from.
