Scientists “hear” the hum of gravitational waves

On Earth, a visible ripple effect occurs when a stone is thrown into water. A similar phenomenon occurs in space. However, instead of creating waves that can be seen by the human eye or optical telescopes, the shock waves produced by merging gravitational energy are called gravitational waves.

Physicist Albert Einstein first theorized about the existence of gravitational waves in 1916. But it wasn’t until almost a century later, in 2015, when LIGO (Laser Interferometry Gravitational-Wave Observatory) detected gravitational waves for the first time in the universe. Since then, the discovery of gravitational waves has allowed scientists to peer inside neutron stars and discover the wobbly black hole ever detected.

Yesterday, scientists announced compelling evidence that they’ve discovered a huge “buzz” of low-frequency gravitational waves rippling through the universe and think it’s very likely they come from the merger of two supermassive black holes, each of which can vary ranging in size from 100,000 to tens of billions of times more massive than our Sun. The news, which may significantly advance our fundamental understanding of the universe, comes from a series of five papers published June 28 in The Astrophysical Journal Letters.

“We have evidence for what we call ‘the gravitational-wave background,’ which is a gravitational-wave hum from all gravitational-waves in the universe,” Dr. Sarah Vigeland, an astrophysicist at the University of Wisconsin, Milwaukee and co . -head of the Gravitational-Wave Background Paper, told Salon. “Every single source emits a single note and you can’t hear individual notes, but it’s a hum, a web of gravitational waves all together.”

“We have evidence of what we call ‘the gravitational-wave background,’ which is a gravitational-wave buzz coming from all gravitational-waves in the universe.”

The discovery by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center is the result of an international collaboration of scientists who used pulsars to search for gravitational waves. (Pulsars are highly magnetized rotating neutron stars that emit beams of electromagnetic energy.)

The first gravitational waves LIGO detected were high-frequency waves: fleeting, rapid, and frequent. The newly detected gravitational waves occur over longer timescales, from months to decades, and are produced by different sources. The discovery was made by scientists who collected more than 15 years of observations with the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.

Very large array;  Radio telescopeComet Hale-Bopp behind the Very Large Array (VLA) radio telescope near Socorro, New Mexico. (Education Images/Universal Images Group via Getty Images)NANOGrav scientists detected these low-frequency waves by looking at the stars. Together, they observed pulsars, which are the ultra-dense remnants of massive supernova stars. Thanks to their rapid rotations, they can be detected as rhythmic pulses of radio waves. Vigeland described pulsars as “clocks” spread across the galaxy. The clock metaphorically ticks faster or slower, driven by gravitational waves.

“By measuring the changes, we can measure gravitational waves and then we can learn about the sources that are producing them,” Vigeland said. In fact, scientists have compelling evidence that the source of this gravitational wave is two supermassive black holes merging in a type of explosive collision almost too big and too powerful to imagine.

“Before this discovery, we weren’t even sure that supermassive black holes merge, in fact there’s a paper from the 1980s that says supermassive black holes will never merge because there isn’t enough material to interact with for merge within the age of the universe,” NANOGrav scientist Chiara Mingarelli told Salon. “And now, not only are they merging, but we’ve found this signal that comes from hundreds or thousands of these supermassive black holes merging.”


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But supermassive black holes are just one candidate as a source of the massive gravitational hum.

“We’ve found the chorus, but we don’t know who’s singing to us, pop stars are supermassive black holes, they’re the ones who are the most obvious candidates,” Mingarelli explained. “However, there are other potential sources of gravitational waves, such as quantum fluctuations in the early universe that have been blown up to the size of the entire universe by inflation.”

Mingarelli said if quantum fluctuations in the early universe are the source, there would be a “complete paradigm shift” in cosmology.

“We’ve found the chorus, but we don’t know who’s singing it. Pop stars are supermassive black holes”

“It would mean that we probably lived in an ekpyrotic universe, which means that the universe would undergo these infinite sequences of expansion and contraction phases forever,” Mingarelli said. “That would be very upsetting and completely different from the current paradigm.”

Another possible explanation is that the hum also comes from cosmic strings, which are remnants of the early universe when it cooled rapidly and left cracks floating through space.

late-stage galaxy merger and its two newly discovered central black holesThis artist’s conception shows a late-stage merger of galaxies and its two newly discovered central black holes. (ALMA (ESO/NAOJ/NRAO) / M. Weiss, NRAO/AUI/NSF)“It’s these ultra-dense, almost string-like things that are really infinitely small, but very, very dense filaments of energy of matter floating around in the universe,” Mingarelli said. “This is also what’s really exciting is that the least revolutionary way to interpret the data is that we’ve found this evidence for hundreds of thousands of supermassive black hole mergers, and this is the most boring way to interpret the result.”

If quantum fluctuations in the early universe are the source, there would be a “complete paradigm shift” in cosmology.

Still, even if it’s supermassive black holes that are causing the buzz, such a discovery will tell scientists more about how supermassive black holes grow and evolve.

“We know from observations that most massive galaxies have massive or supermassive black holes at their centers,” Vigeland said. “And the properties of those galaxies are related to the properties of their black holes because the two are evolving together. We also know that galaxies merge, but there are still a lot of questions about how these things happen.”

Next, the scientists will try to measure the strength of the gravitational hum which could hopefully lead to confirmation of the source.

“When we have more data, more pulsars, potentially from combining all of our data with Europe and Australia, we might be able to tell very soon, maybe in the next five years, what exactly is the source of this gravitational background,” Mingarelli said. . “It could be a combination of all of these things or maybe none of these things.”

Mingarelli added that scientists will also map gravitational waves.

“I think, as theorists, we’ll be in business for a long time thinking about all the different sources of these gravitational waves,” Mingarelli said. “And then think about how they interact with light and how we can use the interaction between the two to learn more about the physics of the universe.”

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