Fast Radio Bursts Shed Light On The Universe's "Missing Matter"

Most of what exists in the universe is invisible to us. We think the universe is made of normal (baryonic) matter, dark matter, and dark energy. The subatomic particles that make people, planets, and stars are called "baryons", and they make up just a fraction of the matter of the universe, which is dominated by the more mysterious dark matter and dark energy, neither of which we've found. But even when it comes to detecting baryonic matter, that's been difficult too.

We know this "missing" matter exists, because we know how much matter there was in the beginning of the universe, thanks to the measurements of the Big Bang. However, when researches went looking for this matter, they could only find half of it. The baryons that we can see in the form of galaxies are a small minority, most of this matter is spread across intergalactic space in such low density that it took decades to find. Now, researchers have used the equally mysterious Fast Radio Bursts (FRBs) to detect this missing "normal" matter.

FRBs are intense millisecond-long emissions of radio waves, the origins of which are not exactly clear. Some are likely the product of extremely magnetized stars, others might be an emission from powerful supernovae. Despite the mystery, these cosmic events are ideal to measure the density of intergalactic material. The new study, published in the journal Nature, provides the best census yet of how much of this baryonic matter there is out there, using FRBs as "cosmic weigh stations".

The study builds on previous work that used the light of a distant quasar as a way to illuminate the spread-out matter between galaxies. The light emitted by quasars is sensitive only to certain atoms, which leads to uncertainties in the estimates on how much baryonic matter is between galaxies. The team, led by Associate Professor Jean-Pierre Macquart from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), focused on how to improve such measurements and decided to use FRBs to perform a similar analysis.

“The radiation from fast radio bursts gets spread out by the missing matter in the same way that you see the colors of sunlight being separated in a prism,” Professor Macquart said in a statement.

“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.”

Using the data, the team estimates that the baryons make up 5 percent of the energy-matter density of the universe. This is consistent with estimates derived from the cosmic microwave background and from the amount of matter that formed in the Big Bang.

“Intergalactic space is very sparse,” Macquart said. “The missing matter was equivalent to only one or two atoms in a room the size of an average office. So it was very hard to detect this matter using traditional techniques and telescopes.”

The breakthrough was possible thanks to a mixture of excellent observations and new theories. The team used the ASKAP radio observatories that can discover and pinpoint FRBs with great precision. They also studied how FRBs signals would change as a function of distance from us due to the no-longer-missing baryons across galaxies.