Strange radio signals reveal the matter hiding between galaxies

One mystery has helped solve another, with the discovery that strange radio signals from space can help pin down the universe’s missing matter.

While dark matter often grabs the headlines for its elusiveness, regular matter has its own invisibility problem. Based on our understanding of how the universe evolved after the big bang, there should be twice as much normal matter as we see. Efforts to track down this missing matter saw some success in 2017, and now a new study provides more direct evidence of its existence.

Astronomers monitoring fast radio bursts (FRBs) – bright flashes of radio waves of unknown origin coming from space – noted that each burst was slightly spread out and delayed when it arrived, with higher frequencies arriving before lower ones. They found that this must be due to missing matter between galaxies slowing the FRBs.

“There’s a delay in the arrival of the light from the FRB,” says Jason Prochaska from the University of California, Santa Cruz. “That disperses the signal, and the more material you travel through, the greater the dispersion.”

Prochaska and his colleagues detected six FRBs using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope near Perth, Australia. By looking at the distance between Earth and each FRB, the team calculated how long the signals should last if they weren’t being spread out. The researchers found the signals should have a duration of just a few billionths of a second – far less than the half a second they actually measured.

The team say this discrepancy is due to the FRBs interacting with dust and gas spread between galaxies that is about a million times less dense than matter in our galaxy. Lower frequency electromagnetic signals are slowed down by a greater extent than higher ones when they pass through matter, so this would explain the pattern the researchers observed.

Using this same method, the team says we could begin to probe the cosmic web of matter, a lattice of diffuse material thought to be strung between galaxies. “With a sample of 100 FRBs, we should be able to test directly whether the gas is located in this cosmic web structure,” says Prochaska.

Journal reference: Nature, DOI: 10.1038/s41586-020-2300-2

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