A fast radio burst (FRB) is a transient radio pulse with a length ranging from a fraction of a millisecond to a few milliseconds created by an unknown high-energy astrophysical activity in radio astronomy. Astronomers believe that the average FRB emits as much radiation in a millisecond as the Sun does in three days. While incredibly intense at their origin, the signal strength reaching Earth has been reported as 1,000 times less than that of a mobile phone on the Moon. The first FRB was discovered in 2007 by Duncan Lorimer and his student David Narkevic while going through archival pulsar survey data, and it is now known as the Lorimer Burst.
For nearly a century, astronomers have debated the rate of the universe’s expansion. A new independent way of measuring that rate could be the deciding factor. Astronomers estimated the Hubble constant — the rate at which the universe expands — for the first time using measurements of cosmic flashes known as rapid radio bursts, or FRBs. While the results are preliminary and the uncertainties are significant, researchers indicate on arXiv.org that the technique could evolve into a potent tool for determining the elusive Hubble constant.
Ultimately, if the uncertainties in the new method can be reduced, it could help settle the long-standing debate that holds our understanding of the universe’s physics in the balance. “I see great promises in this measurement in the future, especially with the growing number of detected repeated FRBs,” says Stanford University astronomer Simon Birrer, who was not involved with the new work.
Our data agrees a little bit better with the CMB side of things than with the supernova side. But the error bar is pretty huge, so you can’t really say anything. I think quick radio bursts have the potential to be as precise as the other approaches.
Steffen Hagstotz
The Hubble constant is commonly measured in two methods by astronomers. One method is to use the cosmic microwave background, which is the light that was released shortly after the Big Bang in the distant cosmos. The other makes advantage of neighboring supernovae and other stars. These methodologies now differ by a few percentage points. For each megaparsec, the new value from FRBs expands at a pace of approximately 62.3 kilometers per second (about 3.3 million light-years). While lower than the other approaches, it appears to be closer to the figure from the cosmic microwave background, or CMB.
“Our data agrees a little bit better with the CMB side of things than with the supernova side,” says Steffen Hagstotz, an astronomer at Stockholm University. “But the error bar is pretty huge, so you can’t really say anything. I think quick radio bursts have the potential to be as precise as the other approaches,” he says.
No one knows for certain what creates FRBs, though explosions from extremely magnetic neutron stars are one possibility. During the few milliseconds that FRBs emit radio waves, their exceptional brightness allows them to be seen across vast cosmic distances, allowing astronomers to study the void between galaxies.
As a FRB signal travels through the dust and gas that separates galaxies, it scatters in a predictable fashion, causing some frequencies to arrive slightly later than others. The signal becomes more diffused as one moves away from the FRB. Hagstotz and colleagues computed the distances to nine FRBs using measurements of this dispersion. The Hubble constant was established by comparing those distances to the rates at which the host galaxies of the FRBs are retreating from Earth.
The new method’s biggest flaw is that it doesn’t know exactly how the FRB signal disperses as it left its home galaxy and enters intergalactic space, where the gas and dust content is better understood. With a few hundred FRBs, the team estimates that it could reduce the uncertainties and match the accuracy of other methods such as supernovas.
“Given that it’s a first measurement, it’s not unexpected that the current results aren’t as constraining as other more matured probes,” Birrer says.
New FRB data could be available soon. Many new radio observatories are coming online, and larger surveys, such as those proposed for the Square Kilometre Array, have the potential to find tens of thousands of FRBs every night. Hagstotz believes that in the next year or two, there will be enough FRBs with distance estimations to reliably compute the Hubble constant. FRB data like this could potentially assist scientists figure out what’s producing the spectacular outbursts.
“I am quite pleased about the new opportunities that will be available to us soon,” Hagstotz says. “It’s only just getting started.”
