In a major discovery, scientists say space-time churns like a choppy sea
The mind-bending finding suggests that everything around us is constantly being roiled by low-frequency gravitational waves
In Einstein’s reimagined universe, space is not serenely empty, and time does not march smoothly forward. Instead, the powerful gravitational interactions of massive objects — including supermassive black holes — regularly ripple the fabric of space and time. The picture that emerges is a universe that looks like a choppy sea, churned by violent events that happened over the course of the past 13 billion-plus years.
The gravitational wave background, as described by the astrophysicists, does not put any torque on everyday human existence. There is not a weight-loss discovery in here somewhere. A burble of gravitational waves cannot explain why some days you feel out of sorts. But it does offer potential insight into the physical reality we all inhabit.
“What we measure is the Earth kind of moving in this sea. It’s bobbing around — and it’s not just bobbing up and down, its bobbing in all directions,” said Michael Lam, an astrophysicist at the SETI Institute and a member of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a team largely based in North America. The NANOGrav team released the findings in five papers that were published Wednesday in the Astrophysical Journal Letters.
Teams in Europe, India, Australia and China also observed the phenomenon and planned to post their studies at the same time. The simultaneous release of papers from far-flung and competitive teams using similar methodology came only after some scientific diplomacy that ensured no group tried to scoop the rest of the astrophysical community.
“We’ve been on a mission for the last 15 years to find a low-pitch hum of gravitational waves resounding throughout the universe and washing through our galaxy to warp space-time in a measurable way,” NANOGrav chair Stephen Taylor of Vanderbilt University said at a news briefing Tuesday.
“We’re very happy to announce that our hard work has paid off.”
Discovery from dead stars
The feat builds on previous discoveries of things in the universe that are invisible to the naked eye — pulsars. A pulsar is a type of neutron star, the ultradense remnant of a dead star. It is called a pulsar because it spins rapidly, hundreds of revolutions per second, and emits radio waves in a steady pulse. Pulsars were discovered only in the 1960s, not long after the invention of large radio telescopes.
NANOGrav gathered data from 68 pulsars using the Green Bank Telescope in rural West Virginia, the 27 telescopes of the Karl G. Jansky Very Large Array in New Mexico, and the now-defunct Arecibo Observatory in Puerto Rico.
The pulses from these bizarre objects reach telescopes on Earth at such predictable frequencies that they serve as cosmic timepieces, nearly as accurate as today’s most advanced atomic clocks, said Chiara Mingarelli, an astrophysicist at Yale and a member of the NANOGrav team.
Theorists believed that low-frequency gravitational waves could throw off the arrival of pulsar signals. Such low-frequency ripples can have crests separated by years, so the search for subtle swells in the sea of space-time required patience. The deviation in the pulsar data is so slight that it took 15 years of observations to come up with solid evidence of these gravitational waves, Mingarelli said.
The NANOGrav team had previously published reports with preliminary suggestions that the background exists, but had said more time was necessary to boost confidence that the signal was real and not just noise.
“Even devising the experiment was a huge mental leap,” Mingarelli said.
The existence of gravitational waves is not in dispute. In 2016, scientists announced that their ambitious four-decade experiment called LIGO, for Laser Interferometer Gravitational-Wave Observatory, had detected waves from the merger of two black holes. But the newly announced waves are not one-shot wonders, and theorists are noodling the many potential explanations for why the cosmic sea ripples in such a fashion.
Supermassive black holes are the favored explanation.
Most galaxies are home to supermassive black holes in or near their central region. These black holes certainly deserve the “supermassive” label: They typically have the equivalent mass of millions or even billions of suns. By contrast, “stellar mass” black holes are pipsqueaks, with masses akin to 10 or 20 or 30 suns.
Galaxies rarely collide, but the universe is vast, there are many billions of galaxies, and they have had plenty of time to drift into one another. During a galactic meetup, theorists say, the supermassive black holes at the cores of the two galaxies first will do a gravitational dance. They can orbit each other for millions of years, Lam said. This pairing is known as a supermassive black hole binary.
The swirling dance disturbs the fabric of space-time sufficiently to generate very low-frequency gravitational waves that travel across the universe at the speed of light, scientists believe. Over time, energy leaks from the dance party, as it were, and the supermassive black holes ease closer together, their orbital period shortening to just a few decades. At that point, the wavelengths begin to reach the frequencies detectable by NANOGrav, Lam said.
“So at this point in our measurements, we cannot definitively state what sources are producing the gravitational wave background signal,” NANOGrav team member Luke Kelley, an astrophysicist at the University of California at Berkeley, said at the Tuesday news briefing. However, he said, the data is a compelling match for theoretical predictions.
Theorists are “having fun” coming up with other possible sources for the low-frequency signal, he added. But “if it’s not coming from supermassive black hole binaries, we would need to come up with some explanation of where those supermassive black holes are hiding, and why we’re not seeing their gravitational waves.”
A new astronomical era
No matter the signal’s source, the announcement of a gravitational wave background represents a milestone in the embryonic field of gravitational wave astronomy.
Just as some astronomers use different wavelengths of light to probe the cosmos, they can now look for different types of gravitational waves. The low-frequency waves announced Wednesday wouldn’t be detectable by LIGO, and the opposite is also true: NANOGrav and similar efforts using pulsars could not detect the kind of high-frequency waves from the unimaginably violent stellar-mass black hole mergers seen by LIGO.
Lam said the next goal is to pair specific gravitational waves with potential supermassive black hole binaries detected through more traditional forms of astronomy. In other words, rather than just saying we’re picking up signs of lots of waves, the astronomers could say this particular wave right here came from that place over there.
The announcement carries an echo of another milestone in the history of cosmology. In 1965, two physicists at Bell Labs reported that they had detected the signal of something previously theorized: the cosmic microwave background radiation. That residual glow offered landmark evidence that the universe was created by the big bang.
Maura McLaughlin, co-director of the NANOGrav Physics Frontiers Center, said at the Tuesday briefing that the next step will be for the international teams to combine their independent data into one “uber data set” that should show an even clearer signal of the gravitational wave background — and maybe even the first detection of a supermassive black hole binary.
“We’re opening up a completely new window … on the gravitational wave universe,” she said.
The work, she said, should offer deeper insight into the ways galaxies form and evolve. It might even reveal exotic new physics that would alter our fundamental understanding of the cosmos: “It should be really, really exciting.”
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