At long last, all of the universe’s ordinary
matter seems to be present and accounted for.

Astronomers have taken a new census of
matter in the universe by examining how bright flashes of radio waves from
other galaxies, called fast radio bursts, are distorted by particles on their
way to Earth. This analysis shows that about half of the universe’s ordinary
matter, which has eluded detection for decades, is lurking in intergalactic
space, researchers report online May 27 in Nature.

The mystery of the missing matter has
vexed cosmologists for some 20 years. This elusive material isn’t the invisible,
unidentified dark matter that makes up most of the mass in the universe. It’s
ordinary matter, composed of garden-variety particles called baryons, such as protons and neutrons (SN: 10/11/17).

Observations of light emitted when the
universe was young indicate that baryons should make up roughly 5 percent of
all the mass and energy in the cosmos. But in the modern universe, all the
matter that astronomers can easily see, like the stars and gas in galaxies,
adds up to only about half of the expected amount of matter.

Scientists have long suspected the
missing matter is hiding between galaxies, along filaments of gas strung
between galaxy clusters in a vast cosmic web (SN: 1/20/14).
“But we haven’t been able to detect it very well, because it’s really, really
diffuse, and it’s not shining brightly,” says Jason Hessels, an astrophysicist
at the University of Amsterdam not involved in the new work.

Some intergalactic matter is detectable by how it absorbs the light of distant, bright objects called quasars (SN: 10/25/02). But the only way to take inventory all of the baryons hanging out in intergalactic space relies on mysterious blasts of radio waves from other galaxies, possibly generated by energetic activity around neutron stars or black holes (SN: 2/7/20).

Even though no one knows what causes
these blasts, called fast radio bursts or FRBs, they can make useful baryon detectors (SN:
7/25/14
). A burst’s high-frequency, high-energy radio waves zip through
intergalactic matter faster than its low-frequency waves. The more
intergalactic matter that a radio burst’s waves pass through, the farther its
lower-frequency waves fall behind — creating a detectable smear in the radio
signal by the time it reaches Earth.

Astrophysicist J. Xavier Prochaska of the
University of California, Santa Cruz and colleagues examined five fast radio
bursts from five galaxies, all detected by the Australian Square Kilometre Array Pathfinder (SN: 6/27/19). For each FRB, the researchers compared the
arrival times of radio waves of different frequencies to tally up the number of
baryons that the burst encountered on its journey through intergalactic space. Then,
using the distance between the FRB’s host galaxy and the Milky Way, Prochaska’s
team could calculate the baryon density along that path.

Bright blasts of radio waves from other galaxies, called fast radio bursts or FRBs, have helped astronomers find previously undetectable ordinary matter. FRBs make good matter detectors because radio waves are affected by the particles they encounter as they cross the universe. Although radio waves all travel at the same speed through empty space, higher-frequency waves (shown in purple) zip through intergalactic matter faster than lower-frequency waves (shown in red). By measuring when radio waves of different frequencies arrive on Earth, astronomers can figure out how many particles of matter the FRB encountered on its journey through the cosmos. That has allowed them to identify matter in the shadowy regions between galaxies that was previously considered missing.  

The average density of matter between the
Milky Way and each of the five FRB host galaxies came out to about one baryon per
cubic meter. The material in the Milky Way is about 1 million times as dense as
that, Prochaska says, making the intergalactic stuff “a very wispy medium.” But
all that wispy material, taken together, is enough to account for all the
universe’s missing matter — bringing ordinary matter up to about 5 percent of
the modern universe’s overall matter and energy, the researchers say.

Astrophysicist J. Michael Shull of the
University of Colorado Boulder cautions that “five is an awfully small number”
of FRB observations from which to draw conclusions about the number of baryons throughout
the modern universe. But “once they get their error bars beaten down with many,
many more bursts … I think that will really be the nail in the coffin on this
baryon problem,” he says.

Using more fast radio bursts as cosmic weigh
stations will also be useful for pinpointing exactly where all the matter in
the universe is located, says Shami Chatterjee, a radio astronomer at Cornell
University not involved in the work.

Right now, all the researchers can say about
the lost-and-found matter is that it’s between galaxies. But with thousands of
FRB observations, astronomers could start teasing out the slight variations in baryon
density along the sight lines between the Milky Way and other galaxies to map
out the cosmic web, Chatterjee says.