An article published in the journal “Nature Astronomy” reports the use of observations of the neutron stars merger recorded on August 17, 2017 to try to calculate the value of the Hubble constant, which measures the speed of the universe expansion. That event is the most famous of those recorded so far for gravitational waves due to the importance it had for the so-called multimessenger astronomy but it has already proved useful also to offer an additional way to measure the expansion of the universe that is alternative to the two that are providing a discrepancy in their results.
The speed of the universe expansion is also called the Hubble constant after Edwin Hubble, the astronomer who provided the first evidence of the expansion of the universe. What was defined as a tension in the field of astrophysics is given by the discrepancy between the measurements made using what are called standard candles, variable stars called Cepheid variables that have a very close correlation between their period of variability and their absolute brightness, and the ones carried out by studying the early universe with the Planck Surveyor space probe. An alternative to cepheids are supernovae but their use hasn’t changed the terms of the problem.
Already in an article published in the journal “Physical Review Letters” in February 2019 a team of researchers proposed to use gravitational waves generated by neutron star mergers to calculate the speed of the universe expansion. The problem was that they estimated that it would take 50 of those events to get enough data on their gravitational waves to reach that result. Now, however, another team led by Kenta Hotokezaka of Princeton University offered a first estimate of the Hubble constant based on the event recorded on August 17, 2017 and cataloged as GW170817 estimating that 15 more events of that type with observations of a comparable completeness to the first one could solve the problem.
The quality of the data collected is the key to solving the problem caused by the discrepancy between the results of the two methods used so far to measure the speed of the universe expansion. The event GW170817 was extraordinary because it was the first neutron star merger discovered, it confirmed the then theoretical concept of kilonova and was observed at many electromagnetic wavelengths as well.
The radio wave detections allowed to study the relativistic jet following the kilonova with further information reported in an article published in the journal “Science” in March 2019. The information obtained thanks to these studies allowed Kenta Hotokezaka’s team to take a significant step forward in estimating the speed of the universe expansion based on that event.
Using only the gravitational wave data, the team estimated the Hubble constant between 66 and 90 km/s per megaparsec with a probability peak at 74, a rather crude estimate with considerable margins of error. Using the data of the radio wave detect as well with the information on the orientation of the neutron star pair involved in the event GW170817 the speed was estimated between 65.3 to 75.6 km/s per megaparsec with a probability peak at 70.3.
The image (Courtesy K. Hotokezaka et al., Nature Astronomy (2019). All rights reserved) shows the distribution of the Hubble constant (H0) obtainable from the observation of neutron star mergers with gravitational wave data only (in orange) or with the combination of gravitational and electromagnetic wave data (in blue). The vertical bars indicate the value ranges estimated using the Planck Surveyor space probe (green) and supernovae (pink) data.
Even the new estimate isn’t very precise with margins of error still wide but the combination of gravitational and electromagnetic wave information has already considerably improved the result. This explains the optimism of Kenta Hotokezaka’s team regarding the number of events needed to solve the problem.
The LIGO and VIRGO experiments began a new season of scientific activity on March 4, 2019 and now the gravitational wave event candidates are made public on the Internet very quickly. By now, on average, every week a new event is discovered but generally those are black hole mergers: only the April 25 one seems a new neutron star merger and the April 26 one could be a merger between a neutron star and a hole black. In any case, no electromagnetic waves from those events have been observed, so they’re not useful to improve the new Hubble constant estimate.
In essence, it could take years to measure the speed of the universe expansion using this new method with enough precision to help figure out the correct value and the reason for the discrepancy. The event GW170817 was relatively close, about 130 million light years away, and this made it easier to detect electromagnetic emissions. There will certainly be progress because gravitational wave astronomy is just beginning and the results will help to better understand some extreme phenomena and perhaps even the force of gravity and some fundamental characteristics of the universe.