Astronomers have managed to “hear” the celestial hum of powerful gravitational waves, created by collisions between black holes, echoing through the universe for the first time.
Their observations reveal that waves – including some that undulate slowly as they pass through our Milky Way Galaxy – occur at different frequencies and oscillate over decades.
The discovery could help scientists better understand cosmic phenomena like supermassive black holes and the frequency with which galaxies merge.
Gravitational waves, first predicted by Albert Einstein in 1916, are ripples in spacetime that were first detected in 2015.
Astronomers found the waves by tracking pulsars, or the dense remnants of cores belonging to massive stars after they exploded in a supernova, across the Milky Way.
Pulsars are like stellar lighthouses, spinning rapidly and releasing beams of radio waves that appear to “pulsate” when viewed through Earth-based telescopes. Pulsars can spin hundreds of times a second, and the stable accuracy of the pulses makes them as reliable as cosmic clocks.
When gravitational waves pass between Earth and a pulsar, the timing of the pulsar’s radio waves is disrupted. Einstein theorized that gravitational waves stretch and compress space as they move through the universe, affecting how radio waves travel. This means that some of the pulses reach Earth a fraction of a second earlier or later than expected.
More than 190 scientists set out to discover the frequencies of gravitational waves as part of the collaboration of the North American Nanohertz Observatory for Gravitational Waves, also known as NANOGrav.
They tracked the radio waves of more than 60 pulsars for 15 years using three large radio telescopes: the Arecibo Observatory in Puerto Rico (which is no longer operational), the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.
Their findings appear in a study published Wednesday in The Astrophysical Journal Letters.
Searching for a heavenly choir
The newly detected gravitational waves are the most powerful ever measured. They were likely caused by collisions of supermassive black holes and carry about a million times more energy than the single events detected in recent years that resulted from mergers of black holes or neutron stars.
“It’s like a choir, with all these pairs of supermassive black holes playing at different frequencies,” study co-author and NANOGrav scientist Chiara Mingarelli, assistant professor of physics at Yale University, said in a statement. “This is the first evidence of the gravitational wave background. We open a new window of observation of the universe.”
The gravitational wave background, a type of cosmic noise that has long been theorized but never detected, is made up of ultra-low frequency gravitational waves. As black holes collide across the universe, all these waves vibrate and resonate together in the background.
Gravitational waves travel at the speed of light, but astronomers realized that a single rise and fall of one of the waves could take years or decades to pass due to the ripple effect of space-time.
“We’re using a galaxy-sized gravitational wave detector that’s made of exotic stars (pulsars), which just blows my mind,” said study co-author Dr. Scott Ransom, a staff astronomer at the National Radio Astronomy Observatory, in a statement.
“Our previous data told us we were hearing something, but we didn’t know what. Now we know it’s music coming from the gravitational universe. As we continue to listen, we will likely be able to pick up the notes of the instruments playing in this cosmic orchestra,” said Ransom.
“Combining these gravitational wave results with studies of the structure and evolution of galaxies will revolutionize our understanding of the history of our Universe.”
cataclysmic collisions
Scientists believe that supermassive black holes are primarily responsible for creating the gravitational wave background. Supermassive black holes exist at the centers of most large galaxies. But as galaxies merge, their black holes begin to orbit each other.
These massive objects, containing billions of times the mass of our sun, dance around until they collide. When they do, the ripples spread out from the host galaxy and eventually reach our own.
It is estimated that there are hundreds of thousands, or perhaps millions, of pairs of supermassive black holes throughout the universe.
“At one point, scientists were worried that supermassive black holes in binaries would orbit each other forever, never getting close enough to generate a signal like this,” said study co-author Dr. Luke Kelley, assistant assistant professor of astronomy at the University of California, Berkeley, and chairman of the astrophysics group NANOGrav, in a statement.
“But now we finally have strong evidence that many of these extremely massive and close binaries exist,” Kelley said. “Once the two black holes get close enough to be seen by pulsar time arrays, nothing can stop them from merging in just a few million years.”
But the researchers acknowledge that it’s not impossible that there are multiple origins to the gravitational wave background, just as there are alternative explanations for how the universe began. The team will continue to study the gravitational wave background and try to isolate individual sources to determine their origins.
“The gravitational wave background is about twice as high as I expected,” said Mingarelli. “It really is the upper limit of what our models can create from supermassive black holes alone. What comes next is everything. This is just the beginning.”
In addition, scientists using telescopes in Europe, India, China and Australia reported similar findings also released on Wednesday. Combining data from NANOGrav with international collaborators can provide a broader picture of the gravitational wave background, the researchers said.
“Our combined data will be much more powerful,” said study co-author Stephen Taylor, an assistant professor of physics and astronomy at Vanderbilt University who currently chairs the NANOGrav collaboration, in a statement. “We look forward to discovering what secrets they will reveal about our universe.”
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Source: CNN Brasil
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