Astronomers discover background of gravitational waves – 06/29/2023 – Science

The universe is permeated by subtle and ultra-long gravitational waves, with crests and troughs light-years apart, coming from all directions, producing a backdrop to the cosmos whose observation could reveal secrets of some of the most colossal objects ever to appear. in it and maybe even bring information about its birth, the Big Bang.

The first conclusive detection of this so-called gravitational wave background was announced this Thursday (29) in a panel organized by the American-Canadian collaboration NanoGrav, with support from the NSF (National Science Foundation of the USA). Independent groups in Australia, China, Europe and India also reported their results, corroborating the finding.

The data, the result of 15 years of observations, were presented in a set of eight scientific articles published simultaneously in the journal Astrophysical Journal Letters and represent a very different finding from that made by gravitational wave detectors such as the American Ligo (acronym in English for Observatório de Laser Interferometry Gravitational Waves), responsible for the first detection of this phenomenon, in 2015.

Gravitational waves are one of the many predictions of the general theory of relativity, formulated by Albert Einstein in 1915. By suggesting that the force of gravity was a distortion in space (and time), the theory indicated that objects with mass moving through the universe would produce ripples in the void itself, generating fluctuations and periodically shortening and lengthening distances—much like stones thrown to the surface of a lake which, when touching the water, generated concentric waves from the point of impact.

LIGO was built to detect these tiny fluctuations by shooting a laser down two perpendicular 4km arms. As gravitational waves passed through the Earth (and through the detector), generating a rapid shrinking and stretching of the arms (less than the size of an atom), this would cause a detectable misalignment between the laser beams, denouncing the passage of the ripple through that region of space -time.

With its size, LIGO (and other similar detectors, such as Virgo) is sensitive to gravitational waves with relatively short lengths (between 43 km and 10,000 km), which are produced mainly when two high-mass objects (for example, a pair of stellar-sized black holes) are spiraling at high speed towards each other, until finally merging into one, emitting large amounts of energy in the form of ripples through space-time.

To detect much longer wavelength gravitational wave signals, the NanoGrav collaboration (an acronym for North American Nanohertz Observatory for Gravitational Waves) needed a detector that was comparable in size to our galaxy, the Milky Way.

Impossible to build something like this, of course, but fortunately nature provides a very valuable resource to put this idea into practice: millisecond pulsars. They are the remains of high-mass stars that exploded as supernovae, leaving behind an ultra-compressed lump of matter that narrowly missed collapsing completely to become a black hole.

These pulsars spin extremely quickly, sending periodic radio pulses toward Earth with each rotation (hence their name, by the way). With that, they are, so to speak, the most accurate watches that we can find in space.

It turns out that, when bathed by gravitational waves, these pulses can end up being slightly delayed or advanced, due to the pull and pull of space-time. And that’s what the NanoGrav researchers sought to observe when collecting data from dozens of pulsars with large arrays of radio telescopes, such as the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the VLA (acronym for Very Large Array), in New Mexico.

Started with a modest effort by a small group in 2004, the project needed to accumulate 15 years of observations before researchers were able to assert that the pulsation pattern of 67 pulsars spread across tens of thousands of light-years indicates the presence of a gravitational waves background.

“The large number of pulsars used in the NanoGrav analysis allowed us to see what we believe are the first signs of the pattern of correlation predicted by general relativity,” says Xavier Siemens, researcher at Oregon State University and co-director of the collaboration.

According to the group’s estimates, the chance of this correlation being a coincidence, a false negative, is between one in a thousand and one in 10,000, which gives the researchers a lot of confidence in the finding.

“The result reveals that the technique works to detect gravitational waves and there is a detectable gravitational wave background in the nanohertz frequency range,” he told Sheet Odylio Aguiar, a researcher at Inpe (National Institute for Space Research) who specializes in gravitational waves and was not involved in the work.

This is a very low frequency, which is equivalent to a very long length, with a distance of several light years between each crest of the wave.

The source of the signal

Where are the waves that make up this bottom coming from? Researchers have a favorite candidate: binary supermassive black holes.

We know that each galaxy has a giant black hole in its interior, with a mass equivalent to millions or billions of suns. And we know that galaxies often collide with each other, giving the opportunity for two of these supermassive black holes to meet and orbit each other.

This slow spiraling (a few million years before they merge and become one) would be able to produce the gravitational waves necessary to generate the pattern of variation observed in pulsars.

And since there must be binaries of supermassive black holes everywhere, what was detected at the moment would be the joint action of all of them, in the form of this background of gravitational waves.

However, the researchers were adamant in saying that they still cannot establish that this is indeed the origin of the signal. There are other hypothetical objects present in exotic theories that would be able to produce the same result. The way to resolve this doubt is to collect more data, with more precision. “With the improvement in sensitivity, the origin of this background will probably be determined”, says Aguiar.