The universe is expanding at an accelerated rate. However, the two methods used to establish the expansion rate don’t produce the same numbers, which has caused many headaches for cosmologists over the years. It is quite a problem.
To resolve the discrepancy, astronomers have been looking for alternative methods to establish the expansion rate. Now, a new method to do just that has used a jet of material produced by the first-observed neutron star collision called GW170817. As the two neutron stars collided, they released a large amount of energy in the form of gravitational waves, which were detected by three observatories – the two LIGO and the Virgo – in August 2017. This was the first time such an event was observed and a gravitational wave signal was spotted with all three detectors. This allowed astronomers to triangulate the source to a small area in the sky and follow it up with regular telescopes.
By combining the gravitational wave information and the data obtained from radio telescopes, they were able to establish a measurement for the expansion rate. As reported in Nature Astronomy, the team found a value of 70.3 kilometers (43.7 miles) per second per megaparsec. However, the uncertainty is so large that it encompasses both values at the center of the current tension.
So what’s the advantage? This method has potential because it is independent of the current models, which means it is not affected by assumptions we may have in our interpretation of the universe. It is also good for having only been based on a single event. Researchers estimate that 15 more events like GW170817 with radio data included could possibly resolve the tension in the expansion rate.
The team claims that between 50 and 100 GW without any follow-up observations could also do the trick. This is promising since LIGO and Virgo have found many more events since they restarted in April. They have even detected a second neutron star collision, but unfortunately only two detectors were online at the time, so they couldn’t pinpoint the event. Despite that, it looks likely that more of these collisions will be found in the future.