Two teams of astronomers set out in the early 1990s to measure the universe’s expansion history in order to forecast its future. If the universal expansion slowed significantly, it would eventually come to a halt and reverse itself, resulting in a fiery Big Crunch. If the slowdown was small, the cosmos would continue to expand indefinitely (albeit at a decelerating rate).
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 pace 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 large, the technique could mature into a powerful tool for nailing down the elusive Hubble constant, researchers report.
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 more with the CMB side of things than the supernova side of things. 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. The cosmic microwave background, which was released immediately after the Big Bang in the distant universe, is used. The other makes advantage of neighboring supernovae and other stars. These methodologies now differ by a few percentage points. The new value from FRBs has an expansion rate of approximately 62.3 kilometers per second per megaparsec (about 3.3 million light-years). While it is lower than the other approaches, it is maybe closer to the value from the cosmic microwave background, or CMB.
“Our data agrees a little bit more with the CMB side of things than the supernova side of things,” says Steffen Hagstotz, an astronomer at Stockholm University. “I think quick radio bursts have the potential to be as precise as the other approaches,” he says.
The wavelengths of radio waves are the longest in the electromagnetic spectrum. They range in size from the length of a football to the size of our entire planet. In the late 1880s, Heinrich Hertz demonstrated the existence of radio waves. He employed a spark gap connected to an induction coil as well as a secondary spark gap on a receiving antenna. When waves created by the sparks of the coil transmitter were picked up by the receiving antenna, sparks would jump its gap as well. Hertz showed in his experiments that these signals possessed all the properties of electromagnetic waves.
You can tune a radio to a specific wavelength—or frequency—and listen to your favorite music. The radio “receives” these electromagnetic radio waves and converts them to mechanical vibrations in the speaker to create the sound waves you can hear.
Nobody knows what creates FRBs, however, highly magnetic neutron star explosions 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 manner, causing some frequencies to arrive slightly later than others. The signal becomes increasingly diffused as the FRB moves away. 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 how the FRB signal disperses as it left its home galaxy and enters intergalactic space, where the gas and dust content is better understood. The team predicts that a few hundred FRBs, it might reduce errors and match the precision of existing methods such as supernovas.
“Given that it’s the first measurement, it’s not surprising 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.”
