The theory of relativity predicted the existence of black holes and neutron stars. Einstein receives recognition for the theory based on his work published in 1915, although the work of other scientists contributed to it. Regardless of the minds behind them, theory predicted black holes, neutron stars, and the gravitational waves from their fusions.
It took about a hundred years, but scientists finally observed these fusions and their gravitational waves in 2015. Since then, the collaboration between LIGO and Virgo has discovered many of them. The collaboration has released a new catalog of discoveries along with a new infographic. The new infographic shows the black holes, neutron stars, fusions, and the other unsafe compact objects behind some of them.
LIGO stands for Laser Interferometer Gravitational Wave Observatory. LIGO is actually two facilities in the United States built and operated by Caltech and MIT. LIGO's cooperation partner is Virgo, an interferometer in Italy. When they observed between 2002 and 2010, they did not notice any gravitational waves or fusions. Eventually the facilities were modernized and in 2015 their first merger was noted.
This event was called GW150914 and resulted from a fusion between a black hole with 36 stellar masses and a black hole with 29 stellar masses. That was a big deal. Three scientists behind the first observation received a Nobel Prize, and the observation promised to open a whole new window into astronomy and cosmology. Now the LIGO / Virgo collaboration detects a fusion and gravitational waves about every five days.
The infographic accompanies the new catalog of gravitational waves and fusions published by LIGO / Virgo. The catalog is referred to as GWTC-2 or Gravitational Wave Transient Catalog-2. While the previous catalog only contained 11 signals, this new one contains 50.
"We get a better picture of the population of the gravitational wave sources."
Frank Ohme, head of the independent Max Planck research group at AEI Hannover
The signals come from all combinations of fusions between black holes and neutron stars.
The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and O3a). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (gray) that have been detected by their gravitational wave emission. Each fusion of a binary system corresponds to three compact objects shown: the two merged objects and the result of the fusion. For comparison, a selection of black holes (purple) and neutron stars (yellow) that have been discovered through electromagnetic observations is shown. Photo credit: LIGO Virgo Collaboration / Frank Elavsky, Aaron Geller / Northwest
The new catalog contains some surprises. A couple of the discoveries came from fusions of low mass objects.
“One of our new discoveries, GW190426_152155, could be the fusion of a black hole of around six solar masses with a neutron star. Unfortunately, the signal is rather weak, so we cannot be absolutely sure, ”explains Serguei Ossokine, senior scientist at AEI Potsdam. "GW190924_021846 is certainly from the fusion of the two lightest black holes we have seen so far. One had the mass of 6 suns, the other that of 9 suns. There are signals of fusions with less massive objects like GW190814, but we don't know exactly whether they are black holes. "
The new identifications in the catalog result from improvements in the LIGO / Virgo collaboration. The raw data processing and the handling of malfunctions or malfunctions have been improved. A press release states that these improvements will enable LIGO / Virgo to "listen deeper into the cosmos than ever before".
"One of the keys to finding a new gravitational wave signal roughly every five days for six months was the upgrades and improvements to both LIGO detectors and the Virgo detector," says Karsten Danzmann, Director at the Max Planck Institute for Gravitational Physics (Albert) Einstein- Institute (AEI) and director of the Institute for Gravitational Physics at Leibniz University Hannover. “The high-performance lasers developed at AEI Hannover, new mirrors and the reduction of background noise sources play important roles. This increased the volume with which our detectors could pick up the signal from merging neutron stars, for example, by a factor of four! "
The laser interferometer's gravitational wave observatory consists of two detectors, one in Livingston, La., And one near Hanford, Wash. The detectors use huge arms shaped like an "L" to measure tiny waves in the tissue of the universe. Photo credit: Caltech / MIT / LIGO Lab
At first glance, each of these events may look similar. They are all the result of fusions of black holes and / or neutron stars. According to Frank Ohme, head of an independent Max Planck research group at AEI Hannover, the observations are revealing more and more details.
“If you look at the catalog, all events have one thing in common: They come from the amalgamation of compact objects such as black holes or neutron stars. But if you take a closer look, they are all very different, ”said Ohme. "We get a more complete picture of the population from gravitational wave sources. The masses of these objects span a very wide range of masses from about our Sun to more than 90 times, some of them are closer to Earth, some of them are very far away. "
LIGO / Virgo researchers have also published four articles on their findings. All three are available from arxiv.org, a pre-print server, and none have been peer-reviewed.
One of the highlights in the catalog is GW190521, the most massive fusion of binary black holes with a total mass of 150 suns and the first observation of the birth of a medium-mass black hole. Another is GW190425, which is most likely the second observation of a binary neutron star fusion.
Visualization of the coalescence of two black holes that inspire and merge and emit gravitational waves. One black hole is 9.2 times more massive than the other and both objects do not rotate. Photo credits: N. Fischer, S. Ossokine, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), simulation of the collaboration between eXtreme Spacetimes (SXS)
Over time, technology has developed ways to test the predictive accuracy of Einstein's theories. One of the new articles examines how well the LIGO / Virgo detections agree with general relativity. It is entitled "General Relativity Tests with the Compact Binary Signals from the LIGO Virgo Catalog GWTC-2". Einstein's theory has – again – held up well, and according to a press release, the study found "no evidence of new physics beyond this theory".
Another paper uses fusions and gravitational waves to develop a new measurement for Hubble's constant. The title is "A gravitational wave measurement of the Hubble constant after the second observation run by Advanced LIGO and Virgo". Instead of using standard candles, "standard sirens" are used on paper. The standard siren refers to how the distance to a fusion event is encoded in the gravitational waves that result from the event.
This new catalog covers the first half of the third observation run (O3), which lasted from November 1, 2019 to March 27, 2020. LIGO / Virgo researchers issued warnings for another 23 events in O3b, the second half of the third observation run. However, they are only possible gravitational waves and have not yet been confirmed. After a more detailed analysis, some of them can be published.