Astrophysicists from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Collaboration have found evidence for gravitational waves that oscillate with periods of years to decades, according to a series of papers published in the Astrophysical Journal Letters.
An artist’s interpretation of an array of pulsars being affected by gravitational ripples produced by a supermassive black hole binary in a distant galaxy. Image credit: Aurore Simonnet / NANOGrav Collaboration.
In 1916, Albert Einstein proposed space-time as a four-dimensional fabric, and that events such as exploding stars and merging black holes create ripples — or gravitational waves — in this fabric.
Almost a century later, in 2015, astrophysicists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations made the first direct observation of gravitational waves caused by the collision of two stellar-mass black holes.
The new research from the NANOGrav Collaboration is the first evidence of gravitational waves at very low frequencies.
The authors transformed our region of the Milky Way Galaxy into an immense gravitational-wave antenna using millisecond pulsars.
NANOGrav’s endeavor involved collecting data from 68 pulsars, fashioning a pulsar timing array — a distinctive type of detector.
The researchers examined 15 years of data from three radio observatories: Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.
“It’s incredibly exciting to have helped open a new window to the Universe,” said SETI Institute researcher Michael Lam.
The team’s analysis provides evidence that the variations in the ‘ticking rate’ of such millisecond pulsars are caused by low-frequency gravitational waves.
The spatial distortion from the gravitational waves creates the appearance that the pulsars’ radio-signal ticking rates are changing.
But really, it’s the stretching and squeezing of space between Earth and the pulsars which causes their radio pulses to arrive at Earth billionths of seconds earlier or later than expected.
“These are by far the most powerful gravitational waves known to exist,” said West Virginia University astrophysicist Maura McLaughlin, co-director of the NANOgrav Physics Frontiers Center.
“Detecting such gargantuan gravitational waves requires a similarly massive detector, and patience.”
“The likely source of these waves are distant pairs of close-orbiting, ultra-massive black holes,” said Vanderbilt University astrophysicist Stephen Taylor, chair of the NANOGrav Collaboration.
“Detecting a ‘chorus’ of low-frequency gravitational waves, as NANOGrav has done, is a key to unlocking the mysteries of how structures are formed in the cosmos,” said Oregon State University astrophysicist Jeff Hazboun.
“We’ve opened up this new area of the spectrum for gravitational waves.”
“We’ve seen low-frequency waves, from a completely different part of the spectrum, which tells us that they’re a ubiquitous physical phenomenon and that we can look for them anywhere.”
The new results are providing new insights into how galaxies evolve and how supermassive black holes grow and merge.
The widespread spacetime distortion revealed in their findings implies that extremely massive pairs of black holes may be similarly widespread across the Universe, numbering perhaps in the hundreds of thousands or even millions.
“While our early data told us that we were hearing something, we now know that it’s the music of the gravitational Universe,” said NANOGrav co-director Xavier Siemens, an astrophysicist at Oregon State University.
“As we keep listening, individual instruments will come to the fore in this cosmic orchestra.”
