Gravitational radiation: rhythms of the Universe

Gravitational waves, predicted by Albert Einstein back in the early 20th century, remained for a long time an unattainable phenomenon for scientists. However, modern technology has finally made it possible to detect and study these space-time flows, which influence the very structure of the Universe.

Recently published studies by several international research groups confirm that the Universe rhythmically stretches and compresses space-time. One such study was conducted by a team from the North American NANOGrav nanohertz gravitational wave observatory and published in The Astrophysical Journal Letters. The scientists concluded that the Universe is “humming” from gravitational radiation, which rhythmically affects space-time and the matter in it.

Researchers from around the world, who also published their work in June this year, came to the same conclusion. They studied millisecond pulsars in the Milky Way and found that the precise rhythms of these objects are affected by the stretching and compression of spacetime caused by gravitational waves at very low frequencies.

It is interesting to note that gravitational waves were only discovered in 2015 by the Laser-Interferometer Gravitational-Wave Observatory (LIGO), although their existence was already predicted by Einstein. However, until recently, scientists did not have access to sufficiently sensitive instruments to detect such waves.

It was previously found that short-wavelength “space-time” changes are caused by the merger of low-mass black holes and neutron stars. However, the question arose as to whether long gravitational waves are also created by black holes. The NANOGrav paper provides evidence that the hum of the universe is caused by hundreds of thousands of pairs of supermassive black holes that have gotten close enough to each other throughout the history of the universe that a merger is inevitable.

Detecting gravitational waves requires a galaxy-sized array of antennas. A set of millisecond pulsars has become such a grid for scientists. The length of gravitational waves is measured in light-years, so such large-scale instruments are needed to detect them.

As reported in a number of papers, in addition to black holes, background gravitational waves could be caused by dark matter axions, primordial black holes left over from the early universe, and cosmic strings. These theories have yet to be tested and investigated.

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