The LIGO-Virgo-KAGRA Collaboration has published the largest catalog of black hole and neutron star mergers to date. Of the gravitational waves generated by such events, members of the international research collaboration, including the Hungarian researchers, have observed 35 new gravitational waves in the last observation period. This has already increased the number of detected gravitational wave signals to 90. In the second half of 2022, another fourth observing period may begin, with four detectors instead of the previous three – it turned out on Monday from her contacts.
Quickstart: What is it all about?
The search for gravitational waves has been one of the most important scientific projects in recent years. The first wave was recognized in 2015, a scientific sensation at the time of the 2016 announcement that winning the Nobel Prize in unusual fashion the following year, researchers won a whole new tool for learning about the universe.
Gravitational waves are ripples in the fabric of space-time: wave-like elongations and contractions of space-time caused by massive, fast-moving masses. They can form in a number of ways, mostly from the merger of two black holes, but they’ve also been observed to form in the collision of neutron stars – the latter, the 2017 discovery, was a scientific milestone as we began the era of multi-channel astronomy. About this and about the process and significance of gravitational waves You can read more in detail here.
The area itself, of course, goes back many decades — a 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 instruments sensitive enough to detect waveforms: two American LIGO units – , the Virgo detector in Italy and recently the KAGRA detector in Japan. These are called laser interferometers: they are multi-kilometre vacuum tubes arranged in an L-shape where the laser beam rotates back and forth between the mirrors. These rays are the ruler for detecting waves: since the light speed of their photons is constant if they reach their target in a longer or shorter time than usual, it is known that space-time is temporarily elongated or contracted, i.e. gravity. The wave has passed. The more signal detectors there are, the more accurately it will determine where it came from.
Hungarian researchers are also involved in international cooperation, there is a LIGO member group at ELTE and SZTE, and the staff of the Wigner Center for Physics Research are members of the Virgo group.
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. The discoveries were made using the LIGO and Virgo detectors.
Of the 35 signals detected, 32 were most likely due to merging black holes. The merging black holes were of various sizes, the largest of which weighed 90 times the mass of our sun. Many black holes formed from mergers exceed 100 solar masses in size, so they can be classified as medium mass black holes. Astrophysics have long been interested in this species at a theoretical level, but experimental evidence for its existence has emerged only due to gravitational waves.
Recent observations confirm that this new class of black holes is more common in the universe than previously thought.
Two of the gravitational waves observed during this period may have had a different source: they could have been when a neutron star and a black hole merged. These are very rare events that were first observed only in the most recent period of data collection from LIGO and Virgo. In one of these two events, a massive black hole of 33 solar masses merged with a very small neutron star of only 1.17 solar masses. It is one of the lightest neutron stars that have been detected using gravitational waves or even electromagnetic waves.
The mass of black holes and neutron stars provides a key clue to how high-mass stars develop or perish in supernova explosions. With the discoveries, we are just beginning to see the diversity of black holes and neutron stars. Recent results show that such pairs exist in a variety of sizes and pairs. Some old mysteries have been solved, but new ones have appeared in the meantime. The observations have brought the researchers closer to solving problems related to stellar evolution, according to the ELTE paper.
Gravitational waves can rewrite previous theories
One of the gravitational waves in the catalog came from the merging of two objects, one of which was definitely 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. The researchers concluded that this might be a black hole, but they can’t be completely sure.
Similar event in question He also discovered LIGO and Virgo in August 2019. The mass of the smaller object is a mystery either way, with experts saying the maximum mass a neutron star can reach before collapsing into a black hole is about 2.5 times the mass of our sun. However, with conventional electromagnetic observations, a black hole of less than 5 solar masses has not yet been detected. This was previously the starting point for theories that stars do not collapse into black holes at this mass scale.
New gravitational wave observations suggest that these theories may be revised.
The search for 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 the Hubble constant Custom, and the now-published catalog also helped determine this more precisely, which ELTE researchers also took part in.
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 doubled as the sensitivity of detectors continues to improve, thanks in part to increased laser power and technological innovation in the use of compressed light. For the third observation period, observations became common: on average, they are weekly, but researchers may observe multiple events in a single day. As the number of detections increases, data evaluation techniques are also being developed to make the results as accurate and reliable as possible.
A major achievement of the third observational period was that the researchers sent a general alert to observatories and other detectors around the world as early as the minutes after the observations. Thus, the network of neutrino detectors and light-detecting telescopes were able to focus on the region from which the gravitational waves came. The electromagnetic and neutrino equations of gravitational wave signals are rare, and finding them is quite a challenge, so a quick warning is a huge advantage. No electromagnetic or neutrino match has been found for any of the newly reported gravitational waves, the only source still being a 2017 neutron star merger.
LIGO and Virgo detectors are currently in development ahead of the next fourth observational period, which is expected to begin in the second half of 2022. Japan’s KAGRA observatory will join this period. Deep in the mountain, KAGRA successfully completed its first observational period in 2020, but it has not yet joined the joint observations of LIGO and Virgo. With more detectors, determining the celestial location of events will also be more accurate.
As more and more confirmed observations are added to the LIGO-Virgo-KAGRA catalog of gravitational waves, researchers are learning more and more about these astronomical phenomena. Before the next observation period, further analysis of the existing data will provide more information about neutron stars and black holes, and, according to the ELTE researchers, there is an opportunity to discover new types of signals hidden in the data series.