The International Collaboration of Radio Astronomers has announced a discovery that could become one of the most important achievements of modern cosmology. We are talking about a new type of gravitational waves, which are caused by the merger of supermassive black holes. These waves were discovered by a method proposed nearly half a century ago by Soviet astronomer Mikhail Sazhin, the study says. published in the Astrophysical Journal Letters.
The phenomenon of gravitational waves, i.e. disturbances of the metric of spacetime by massive bodies moving with varying acceleration, was predicted by Albert Einstein in 1916. Experimental confirmation has not was first obtained only in 2015 using the LIGO gravitational wave observatory, which picked up a signal from the merger of two stellar-mass black holes, for which the Nobel Prize in Physics was awarded in 2017.
However, scientists have long hoped to capture gravitational waves from much more massive sources – so-called supermassive black holes, which are located at the center of most galaxies. To capture waves from black holes and neutron stars, the LIGO project uses a laser interferometer capable of measuring the vibrations of test bodies with an amplitude less than the size of an atomic nucleus and frequencies on the order of hundreds of hertz. But this recording method is not suitable in the case of galactic black holes, which are billions of times more massive than stellar-mass holes, and generate much longer waves – the distance between the neighboring crests of these waves are tens of light-years away, and the frequencies are measured in nanohertz. Not a single laboratory experiment on Earth is able to pick up the arrival of such low-frequency waves, so they tried to search for its manifestations in space.
Mikhail Sazhin, an employee of the department of relativistic astrophysics of the Sternberg State Astronomical Institute of Moscow State University, was the first to express such an idea in 1978 (died April 2023). In the article “Possibilities of detecting ultra-long gravitational waves”, (Astronomical Journal) he Free to use for this as a precise clock pulsars – compact rotating objects that emit electromagnetic signals of strict periodicity. The principle is that gravitational waves bend spacetime, so the speed of time in different regions of space where the wave reaches can change. Therefore, if a gravitational wave passes between Earth and the pulsar, then the frequency of the “accurate time” signal received on Earth must change.
To use this approach, the international collaboration of radio astronomers North American Nanohertz Observatory for Gravitational Waves (NANOGrav) was created. Using telescopes such as Arecibo, the Green Bank Telescope and the Very Large Array, astronomers have spent 15 years monitoring minute changes in the arrival of pulses from dozens of pulsars to identify glitches that indicate an anomaly. gravitational waves. “Pulsars are pretty weak radio sources, so we need thousands of hours of observations each year with the largest telescopes in the world to conduct the experiment,” said Maura McLaughlin of West Virginia University. .
After 15 years of observations, scientists from several scientific organizations from different countries participating in the collaboration announced that long-term analysis of the signals revealed the presence of nanohertz gravitational waves with periods ranging from years to decades. . So far, it is more of a “hum”, which does not identify the waves to a specific source. “Now that we have evidence for gravitational waves, the next step is to use our observations to investigate the sources that produce this hum,” explained collaboration member Sarah Vigeland.
The most likely source of the nanohertz hum are pairs of supermassive black holes moving in a “death” spiral towards each other. Such encounters are possible during the collision of galaxies, in the center of which there are usually black holes with a mass of millions of solar masses. As a result, these giant objects begin to revolve around a common center of mass, gradually moving closer to each other due to friction against interstellar gas and stars.
A number of theoretical works suggest that, having approached to a distance of 1 parsec (about 3 light-years), black holes stop losing energy, and should stop approaching for billions of years. This is called the last parsec problem, which assumes that only a small fraction of supermassive black holes eventually merge into one.
Their loss of energy for the emission of gravitational waves is a mechanism capable of driving the holes to final fusion. “When the holes get close enough to affect the arrival of pulsar signals, nothing can stop them from merging within a few million years,” explained Luke Kelly, a member of the collaboration from the University of California at Berkeley. Detecting nanohertz waves can confirm this prediction and solve the last parsec problem, the scientists believe.
“There is no doubt that the discovery of ultra-long gravitational waves will be crowned with a Nobel Prize – perhaps as early as next year, if the data over 15 years is convincing enough, or in the years to come. As a general rule, the Nobel Prize is awarded both to the theorist who predicted the phenomenon and to the observers. From this point of view, Mikhail Sazhin is an obvious candidate for half the Nobel Prize”, writing in 2022 astrophysicist Nikolai Gorkavy.
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