The LIGO-Virgo-KAGRA Collaboration has published the largest catalog of black hole and neutron star fusions to date. From the gravitational waves resulting from such events, members of the international research collaboration, including Hungarian researchers, observed 35 new gravitational waves in the recent observation period. This has already increased the number of detected gravitational wave signals to 90. In the second half of 2022, another, fourth observation period may begin, with four detectors instead of the previous three – it turns out on Monday from its communication.
Quickstart: what is it all about?
One of the most significant scientific projects of recent years has been the hunting of gravitational waves. The first wave was identified in 2015, a scientific sensation at the time of the 2016 announcement that won the Nobel Prize in an unusual way the following year, and researchers have won a whole new tool to learn about the universe.
Gravitational waves are ripples in the fabric of spacetime: wave-like elongations and contractions of spacetime created by fast-moving, massive masses. They can be formed in a number of ways, mostly from the merging of two black holes, but they have also been observed in the collision of neutron stars – the latter, the 2017 discovery, was a scientific milestone as we ushered in the era of multichannel astronomy. About this and the operation and significance of gravitational waves you can read more in detail here.
The area itself, of course, dates back many decades – the 2016 discovery confirmed Albert Einstein’s 1916 theory – but only in recent years, with hundreds of scientists and a lot of money, have we been able to build sensitive instruments to detect wave signals: the two American LIGOs -, the Virgo detector in Italy and most recently the KAGRA detector in Japan. These are called laser interferometers: multi-kilometer vacuum tubes arranged in an L-shape in which a laser beam cycles back and forth between mirrors. These rays are the rulers for detecting waves: since the speed of light of their photons is constant if they reach their target in a longer or shorter time than usual, it is known that the spacetime is temporarily elongated or contracted, that is, a gravitational wave has passed. The more detectors detect a signal, the more accurately it can be determined from where it came from.
Hungarian researchers also participate in the international collaboration, there is a LIGO member group at ELTE and SZTE, and the staff of the Wigner Physics Research Center are members of the Virgo group.
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We learn more and more about the evolution of stars
The catalog expands the list of gravitational waves with events observed between November 2019 and March 2020. Detections were achieved with the two LIGO and Virgo detectors.
Of the 35 detected signals, 32 were most likely due to the fusion of black holes. The fused black holes were of various sizes, the largest weighing 90 times the mass of our Sun. Several of the black holes formed from the mergers exceed 100 solar masses in size, so they can be classified as black holes of so-called intermediate mass. Astrophysicists have long been concerned with this type on a theoretical level, but experimental evidence for its existence has only emerged due to gravitational waves.
Recent observations confirm that this new class of black holes is much more common in the universe than previously thought.
Two of the gravitational waves observed during this period probably had a different source: they could have formed when a neutron star and a black hole merged. These are very rare events that were only observed for the first time in the most recent data collection period of LIGO and Virgo. In one of the two events, a huge black hole with a mass of 33 solar masses merged with a very small neutron star with a mass of only 1.17 solar masses. It is one of the lightest neutron stars ever detected using gravitational or even electromagnetic waves.
The mass of black holes and neutron stars provides a key clue as to how high-mass stars evolve or perish in supernova explosions. With the discoveries, we are just beginning to see the diversity of black holes and neutron stars. Recent results demonstrate that such doubles exist in a wide variety of sizes and pairings. Some long-standing mysteries have been solved, but in the meantime new ones have emerged. The observations have brought researchers closer to solving issues related to stellar evolution, according to an ELTE paper.
Gravitational waves can rewrite previous theories
One of the gravitational waves in the catalog came from the fusion of two objects, one of which was almost certainly a black hole (weighing 24 solar masses) and the other was either an extremely light black hole or a very heavy neutron star with a mass of 2.8 solar masses. Researchers have concluded that this is probably a black hole, but they cannot be entirely sure.
A similar event in question also discovered LIGO and Virgo in August 2019. The mass of the smaller object is mysterious in both cases, as experts say the maximum mass a neutron star can reach before it collapses into a black hole is about 2.5 times the mass of our Sun. However, with conventional electromagnetic observations, no black hole less than 5 solar masses has been discovered so far. This was previously the starting point for theories that stars do not collapse into black holes in this mass range.
New gravitational wave observations indicate that these theories are likely to be revised.
Research on gravitational waves is also of great importance in the field of cosmology: the rate of expansion of the universe since the Big Bang has been With Hubble constant custom, and the catalog now published also helped to define this more precisely, in which ELTE researchers also participated.
New types of signals can also be hidden in the data
Since the first detection of gravitational waves in 2015, the number of observations has exploded as the sensitivity of the detectors continues to improve, thanks in part to increased laser power and a technological innovation in the use of compressed light. For the third observation period, observations have become commonplace: on average, they are weekly, but researchers may observe multiple events in a single day. As the number of its detection increases, data evaluation techniques are also being developed to make the results as accurate and reliable as possible.
One of the major achievements of the third observation period was that the researchers sent a public alert to other observatories and detectors around the world as early as the minutes following the observations. Thus, the network of neutrino detectors and light-detecting telescopes was able to focus on the area from which the gravitational waves came. The electromagnetic and neutrino equivalents of gravitational wave signals are rare, and finding them is extremely challenging, so rapid alarm is a huge advantage. No electromagnetic or neutrino match was found for any of the newly announced gravitational waves, the only such source still being the 2017 neutron star fusion.
Detectors from LIGO and Virgo are currently undergoing development before the upcoming fourth observation period, which is expected to begin in the second half of 2022. The Japanese KAGRA Observatory will join this period. Deep under a mountain, KAGRA successfully completed its first observation period in 2020, but has not yet joined the joint observations of LIGO and Virgo. With more detectors, determining the celestial position of events will also be more accurate.
As more and more confirmed observations are added to the LIGO-Virgo-KAGRA gravity wave catalog, researchers are learning more and more about these astronomical phenomena. Before the next observation period, further analysis of the existing data will provide even more information about neutron stars and black holes, and according to the ELTE researchers, there is even a chance to discover new types of signals hidden in the data series.
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