Scientists have revealed the influence of gravitational waves on the study of the universe
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- Scientists have revealed the influence of gravitational waves on the study of the universe
In ten years, gravitational-wave astronomy has gone from detecting a single short signal to studying entire populations of black holes and neutron stars. New observations have made it possible to discover previously unknown compact objects, test the general theory of relativity and propose an independent way to measure the expansion rate of the universe. By May 2026, the number of registered gravitational wave signals reached 390. This is stated in the Astronomy magazine review published on August 10.
A split-second signal
About 1.3 billion years ago, two black holes collided in a distant galaxy. According to scientists, their masses were about 36 and 29 times the mass of the Sun. As a result of the merger, a new black hole with a mass of about 62 solar masses was formed, and energy equivalent to three more solar masses spread through the universe in the form of gravitational waves.
One of these waves reached Earth on September 14, 2015. At 05:51 US Eastern time, it was detected by the LIGO Observatory detector in Livingston, Louisiana. Seven milliseconds later, the same signal was detected by a second installation in Hanford, Washington.
Designated GW150914, the signal lasted only a fraction of a second. Its frequency increased rapidly as the black holes approached, and after the collision it gradually decreased when the formed object became stable.
In the audio range, such a signal resembles a short pitch increase, which is why it is often referred to as "twitter." In fact, the detectors do not record sound in space, but measure changes in space-time, after which the data can be converted into a human-audible signal.
The detection was the first direct confirmation of the existence of gravitational waves. Albert Einstein formulated the general theory of relativity in 1915, and predicted the existence of propagating distortions of space-time a year later. Prior to the discovery of LIGO, the theory had been experimentally tested several times, but it had not yet been tested in such a strong and rapidly changing gravity.
The signal was also the first direct observation of the merger of two black holes. Prior to this, scientists had only indirect evidence of the existence of such systems.
What was particularly unexpected was how quickly LIGO achieved the result. The first observation cycle of the upgraded detectors began just two days before the registration of GW150914. The project participants initially assumed that they had encountered an artificial test signal that had been secretly added to the data to test equipment and algorithms.
After several weeks of analysis, the team ruled out malfunctions and interference from researchers. Emanuele Berti, a theoretical physicist at Johns Hopkins University, later called the incident the moment after which "the way we observe the universe changed forever."
How scientists "listen" to space and time
Gravitational waves occur when massive objects move at an accelerated rate. They can be created by rotating pairs of black holes and neutron stars, supernova explosions and other processes accompanied by rapid redistribution of huge masses.
The closer two compact objects are to each other, the faster they rotate. The system loses energy in the form of gravitational waves, so the distance between objects continues to shrink. In the finale, they collide and form a single object.
LIGO uses modified laser interferometers to measure these processes. Each of the two detectors consists of 4 km long perpendicular vacuum tunnels. The laser beam splits into two streams that pass through tunnels, reflect off mirrors, and then reconnect.
Under normal conditions, the distances traveled by both beams are selected so that a certain interference pattern occurs when they connect. A gravitational wave briefly stretches space in one direction and compresses it in the other. As a result, the length of one arm of the installation increases, and the other decreases.
The changes turn out to be much smaller than the size of an atom, but they are enough to disrupt the interference pattern. Additional mirrors cause the light to repeatedly pass through the tunnels, actually increasing the length of the path and the sensitivity of the installation.
Two detectors are needed to distinguish a real cosmic signal from local interference, which can be caused by seismic vibrations, traffic, wind, and even the operation of nearby equipment. The gravitational wave must consistently manifest itself at several installations with a delay corresponding to the speed of light.
The waveform contains a lot of information. By its frequency and duration, scientists determine the masses of colliding objects. Heavier bodies merge faster and create short waves of relatively low frequency. Mergers of less massive objects last longer and reach higher frequencies.
The amplitude allows you to estimate the distance to the source: the further away the event occurred, the weaker the wave is by the time it arrives on Earth. Small fluctuations within the signal provide information about the rotation of objects and the direction of their axes. If there are at least three detectors, the source can be localized by triangulation. Scientists compare the arrival time of the wave at different installations and gradually narrow down the area of the sky in which the collision occurred.
From single signals to a census of black holes
During the first observation cycle, LIGO recorded three confirmed signals. All of them appeared during the merger of binary black holes. In the following years, the detectors were upgraded, and the European Virgo Observatory and the Japanese KAGRA facility joined the observations.
The number of detections began to increase rapidly. In May 2026, the international collaboration presented the GWTC-5.0 catalog. 161 signals registered from April 2024 to January 2025 were added to it. The total number of known gravitational wave events has reached 390.
The accumulated statistics have become enough to move from studying individual collisions to studying populations of compact objects. This transition is comparable to the situation when astronomers receive not a photograph of a single star, but a catalog that allows them to study the patterns of formation and evolution of an entire class of objects.
One of the first surprises was the masses of black holes. Before the advent of gravitational-wave observations, scientists found them mainly by X-ray radiation of matter falling on a black hole from a companion star. Most of these objects were about five to 15 times more massive than the Sun. However, detectors began to regularly register mergers of black holes with masses of 20, 30 and 50 solar masses. This showed that previous observations covered only a fraction of the actual population of black holes.
The latest LIGO–Virgo–KAGRA analysis revealed several features of the mass distribution. In particular, scientists have found an increased number of black holes of about ten solar masses and a change in the distribution pattern of about 35 solar masses. Objects in the region of 35 solar masses are also particularly likely to form pairs with black holes of comparable size.
These patterns may indicate the existence of several mechanisms for the formation of binary systems. Some pairs could initially arise as binary stars, go through all the stages of evolution together and turn into black holes. Other systems could form later due to the accidental convergence of objects in dense star clusters.
The rotation of black holes also provides information about the history of the system. If the directions of their rotation coincide with the direction of their orbit, the system could form from a binary star. Chaotically oriented axes may indicate that the black holes met after their formation. The GWTC 5.0 data reinforced the argument that there are several similar scenarios operating in the universe at the same time.
Black holes in "forbidden" ranges
Gravitational waves have also called into question the existence of supposed discontinuities in the distribution of masses of compact objects. According to electromagnetic observations, the mass of neutron stars usually did not exceed about two solar masses, and black holes began to occur after about three to five solar masses. The gap between them was considered a possible consequence of the physical processes occurring during the collapse of the stellar core.
However, gravitational wave detectors have detected several objects in this range. It is not always possible to determine whether they are particularly heavy neutron stars or light black holes. Nevertheless, observations show that the lower mass gap is either not completely empty, or has more complex boundaries than expected.
The other gap should be between about 50 and 120 solar masses. According to models of stellar evolution, extremely massive stars can become unstable due to the birth of pairs of particles and antiparticles in their cores. This leads to powerful ejections of matter or the complete destruction of the star, after which the black hole does not remain.
Nevertheless, gravitational-wave observations have revealed objects in the proposed upper gap. One possible explanation is sequential or hierarchical mergers. First, two relatively small black holes form a larger one, after which the resulting object finds a new companion and merges again.
Such processes can occur in dense star clusters or near the centers of galaxies. Another possible mechanism is considered to be the growth of black holes due to the absorption of surrounding gas. Thus, the presence of heavy objects does not necessarily refute the death models of single stars, but indicates the existence of additional ways of forming black holes.
To separate these scenarios, scientists need to observe mergers at different stages in the history of the universe. In the early cosmic epochs, black holes had less time for repeated collisions. If massive objects existed even then, it will be more difficult to explain their origin only by hierarchical mergers.
What are neutron stars hiding?
Gravitational waves also provide an opportunity to look inside neutron stars. As the two objects approach, they experience a strong mutual attraction and deform slightly. The degree of this deformation depends on the properties of the substance in their depths.
If the matter is relatively rigid, the star changes shape more faintly. If it compresses more easily, the tidal deformation becomes stronger. These changes leave small traces in the gravitational-wave signal just before the collision.
By analyzing them, scientists can limit the possible equations of state — mathematical models that relate pressure, density, and temperature of matter. In fact, a cosmic collision turns into an experiment in nuclear physics that cannot be carried out on Earth. Additional information can be provided by the moment of merging itself. The resulting object can oscillate for a while before turning into a black hole. The frequencies of these vibrations depend on its internal structure.
Another scenario occurs when a neutron star collides with a black hole. Depending on the mass and rotation of the black hole, as well as the rigidity of the neutron star, the latter can be torn apart even before crossing the event horizon. The amount of ejected matter and the shape of the signal make it possible to refine the properties of superdense matter.
So far, the sensitivity of existing installations is not enough to reconstruct in detail all the stages of most such events. Nevertheless, observations have already limited a number of models, and next-generation detectors should make measurements much more accurate.
A new way to measure the expansion of the universe
Gravitational waves have also become a tool of cosmology. Based on the shape and amplitude of the signal, scientists can calculate the distance to the source without using the traditional "distance ladder" based on sequential comparison of different astronomical objects.
The sources of such signals are called "standard sirens" — by analogy with "standard candles", which include supernovae of a certain type. If scientists manage to identify the galaxy in which the merger took place, its redshift can be compared with the distance determined by the gravitational wave and the Hubble constant calculated.
The main difficulty lies in the fact that most black hole mergers do not have noticeable optical radiation. Their position in the sky is not precisely determined to unambiguously name the parent galaxy. In such cases, scientists analyze catalogs of all galaxies in a likely region and apply statistical methods.
GW170817 remains the only event for which it was possible to confidently determine the exact source galaxy using electromagnetic radiation. However, hundreds of other signals can also be used as "dark sirens". The obtained value is consistent with both measurements of nearby objects and estimates based on the early universe. However, the error is still too large to resolve the so—called Hubble stress, a discrepancy between the main methods for determining the rate of expansion of space.
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