A 15-year study has revealed the first evidence of a cosmic “gravitational wave background,” and it’s louder than expected. Immediately, five independent groups of radio astronomers—North American, European, Indian, Chinese, and Australian teams—published a series of papers and presented the evidence here. The universe is filled with gravitational waves created by the collision of supermassive black holes.. Gravitational waves are ripples in the fabric of space-time that move at the speed of light in the universe. Although Albert Einstein predicted their existence as early as 1916, it took almost a century for spatio-temporal oscillations to be detected on Earth by the Laser Interferometric Gravitational-Wave Observatory (LIGO) in 2015. It turns out that they are literally everywhere in the universe and permeate it.

opening details

The North American Nanohertz Gravitational Waves Observatory (NANOGrav) explains that scientists have observed pulsars in their research.

The newly discovered gravitational waves – ripples in the fabric of space-time – are the most powerful waves ever measured: they carry about a million times more energy than single bursts previously detected from black hole-neutron star mergers, NANOGrav scientists report. “It’s like a chorus of all these supermassive black hole pairs singing at different frequencies.”– says the scientist from Chiara Mingarelli. Currently, NANOGrav can only measure the overall background of gravitational waves, not the emission of individual “duets”.. But even that brought surprises.

The gravitational wave background is about twice as loud as we expected. This is the upper bound of what our computer models predict for supermassive black holes.
– says Mingarelli.

This noise could be the result of errors in the experiment, or, more interestingly, the fact that there are actually more supermassive black holes than previously thought. But there is also the possibility that strong gravitational waves were produced by something else, for example, mechanisms predicted by string theory or alternative explanations for the birth of the universe. Which leads us to the fact that in the long run the discovery will help prove string theory.

These gravitational waves are unlike any previously measured. Unlike the high-frequency wave bursts detected by the ground-based LIGO and Virgo instruments, the main background of gravitational waves is ultra-low frequency. It may take years or even decades for one of the waves to rise and fall. Since gravitational waves travel at the speed of light, in this way the length of a wave can be tens of light years.

Earth-based instruments couldn’t detect such massive waves, so the NANOGrav team looked at the stars instead. Scientists are watching closely for pulsars, the super-dense remnants of massive stars. As mentioned earlier, they emit radio waves at regular intervals. When the gravitational wave passes between the Earth and the pulsar, it breaks the synchronization of the radio waves. This is because gravitational waves stretch and compress space, as Albert Einstein predicted. So over the course of 15 years, scientists first determined the exact timing of pulses from our galaxy’s 67-millisecond pulsars.

Pulsars are actually very weak sources of radio emissions, so we need thousands of hours a year in the world’s largest telescopes to perform this experiment.
says Maura McLaughlin of West Virginia University, co-director of the NANOGrav Center for Physical Limits.

Gravitational waves cause space to expand and contract. By carefully measuring how objects in space change their position relative to each other, scientists can come to a conclusion about the passage of a gravitational wave. LIGO watched how the length of the 4-kilometer-long tunnels changed by less than one-thousandth the size of a proton. Thanks to this engineering feat, in 2015 researchers discovered gravitational waves produced by black holes with a mass ten times greater than the Sun. But to detect the low-frequency rumble of gravitational waves produced by supermassive black holes billions of times larger than the Sun, A detector much larger than Earth size is required.

As a result, it was possible to single out the interference, an additional “hum” common to all pulsars in the data set. Detection of this background hum is of great importance as it could revolutionize understanding of the early days of the universe.

For example, electromagnetic radiation does not give a picture of the universe before its final scattering (about 400,000 years after the Big Bang). However, gravitational waves can only give us about ∼10 of information up to the beginning of inflation.^-32 seconds after the big bang,
– Said Susan Scott of the Australian National University and the ARC Center of Excellence for the Discovery of Gravitational Waves.

The most likely sources of the gravitational wave background are pairs of supermassive black holes rapidly orbiting each other. These black holes are truly gigantic, containing the masses of billions of suns. Almost all galaxies, including the Milky Way, have at least one of these giants at their centers. When two galaxies merge, their supermassive black holes can meet and begin to orbit each other. Over time, their orbits narrow until the holes finally collide. Fortunately, black holes collide a few million years after they start emitting gravitational waves strong enough to be detected by pulsars.

Because supermassive black hole pairs are formed by the merging of galaxies, the abundance of gravitational waves will help cosmologists estimate how often galaxies have collided throughout the history of the universe. The conclusions so far are that there are hundreds of thousands, and possibly a million, pairs of supermassive black holes in the universe.

Therefore, although the researchers do not claim to have detected gravitational waves themselves, they have obtained a very promising fingerprint of their existence.