Four new gravitational wave detections made by the National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector have revealed with new accuracy the locations of black hole collisions. But are these observations enough to dissuade critics of LIGO’s initial 2015 discovery of gravitational waves?
The observation of the black holes spiralling together and colliding in an event of extraordinary violence was made between the end of November 2016 and the end of August 2017. It brings the total number of such collisions detected by LIGO and VIRGO thus far to ten for stellar-mass black holes with a single detection of a neutron star merger.
The findings describing coalescing cosmic bodies such as black holes and neutron stars were presented to physicists at a workshop held on the first day of December.
Albert Lazzarini, Deputy Director of the LIGO Laboratory, said: “The release of four additional binary black hole mergers further informs us of the nature of the population of these binary systems in the universe and better constrains the event rate for these types of events.”
From Sept. 12, 2015, to Jan. 19, 2016, during the first LIGO observing run since undergoing upgrades in a program called Advanced LIGO, gravitational waves from three binary black hole mergers were detected.
The second observing run, which lasted from Nov. 30, 2016, to Aug. 25, 2017, yielded one binary neutron star merger and seven additional binary black hole mergers, including the four new gravitational-wave events being reported now. The four new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected.
GW170729, detected on 29th July 2017 is the most distant and massive source of gravitational waves detected thus far. In this coalescence, which happened roughly 5 billion years ago, an equivalent energy of almost five solar masses was converted into gravitational radiation.
The event GW170817, found on the 17th August 2017, was the first time that gravitational waves were ever observed from the merger of a binary neutron star system. What’s more, this collision was seen in gravitational waves and light, marking the first example of multimessenger astronomy, in which cosmic objects are observed simultaneously in traditional forms of electromagnetic radiation and an alternative source.
The new detections also enabled researchers to break new ground in the study of gravitational waves. GW170814, the first binary black hole merger measured by the three-detector network, allowed for the first tests of gravitational-wave polarization (the orientation of the waveform).
LIGO and Virgo researchers were also able to make very precise predictions regarding the black hole sources of event GW170818. Located 2.5 billion light-years from Earth, the black hole binary system was identified in the sky with a precision of 39 square degrees, making it the next best localized gravitational wave source after the aforementioned neutron star merger.
EGO Director Stavros Katsanevas remarked: “We find that almost all black holes formed from stars are lighter than 45 times the mass of the Sun.
“Thanks to more advanced data processing and better calibration of the instruments, the accuracy of the astrophysical parameters of the previously announced events increased considerably.”
Papers detailing these discoveries and a catalogue of gravitational wave detections by the LIGO/Virgo network are currently being prepared for distribution on the arXiv repository of electronic preprint studies.
Criticism of LIGO’s gravitational wave discovery
The revelation of these new discoveries comes at a time that must be fairly pleasing to the teams working at LIGO. The first detection of gravitational waves in February 2015 has been called in to question by a team of physicists at the Niels Bohr Institute in Copenhagen.
The Danish physicists spent two and a half years performing independent data-analysis of LIGOs 2017 Nobel prize-winning work. They concluded that the LIGO team had mistaken random patterns in the data for a signal.
The claim reflects that made by physicist Andrew Jackson in a 2016 press conference later codified in a paper published in the Journal of Cosmology and Astroparticle Physics in August of that year. The debate has raged amongst physicists ever since.
LIGO spokesperson David Shoemaker of MIT thinks the controversy all boils down to a misunderstanding about LIGO’s methods for analyzing its data. “Jackson came to it sceptically, and I think scepticism in science is a really important thing. You have to question results.
“But these are complicated data; they are not simple to understand. Certainly nothing they’ve done gives us any reason to doubt our results.”
LIGO has also been criticised for its secrecy and the organisation’s seemingly exasperated and at times almost condescending response to criticism. Thus, the decision to hold this workshop and be more open with the data should be considered a positive move.