Scientists Glimpse Background ‘Hum’ of Spacetime In Major Breakthrough

In a major breakthrough, scientists have discovered the first hints of an entirely new class of gravitational waves from the early universe using a detector that measures the clockwork pulses of dead stars across the galaxy.

This evidence of an elusive “hum” in the fabric of spacetime will likely offer the first direct glimpse of supermassive black holes in the early universe, more than 10 billion light years from Earth, marking a new era in gravitational wave astronomy.

Though the detection still needs to be fully confirmed, scientists with the International Pulsar Timing Array (IPTA) consortium, which snagged the milestone results, believe that the preliminary observations are now solid enough to share with the world as unprecedented evidence that we have tuned into a gravitational wave “background” that has never been explored, according to a study published on Wednesday in the Astrophysical Journal Letters.

“This will be the first direct detection of these supermassive black holes in the early universe,” if confirmed, said Robert Ferdman, who is an astrophysicist at the University of East Anglia and a co-author of the study, in a call with Motherboard. “This is definitely like a snapshot of the state-of-things in the early universe, and that is what's really exciting—it is a brand new window into that point in time.”

The putative evidence could shed light on a host of mysterious phenomena, including the evolution of galaxies, the emergence of the “cosmic web” that links the universe, and perhaps even events that unfolded right at the dawn of time. The technique pioneered by IPTA researchers will also advance our basic understanding of reality by testing cosmic models like Albert Einstein’s theory of general relativity and string theory, which proposes that there are hidden dimensions in the universe. 

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which is part of the IPTA, will announce more details about the results in a livestream scheduled for 1pm Eastern Time on Thursday.

Gravitational waves are ripples in spacetime that are produced by momentous cosmic events, such as exploding stars or the interactions between massive objects like black holes. Einstein initially predicted the existence of these ripples in 1916, but it took a century before scientists were able to detect the first gravitational wave in 2015 using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO), an achievement that earned the 2017 Nobel Prize in Physics. 

LIGO, and other detectors like Virgo in Italy, have since snagged dozens of gravitational waves made by exotic objects, like black holes and neutron stars, that can not be accessed by traditional light-based astronomy. Existing detectors like LIGO are attuned to high-frequency waves created by individual catastrophic events involving stellar-mass objects, which are typically no larger than a few dozen times the mass of the Sun. 

In contrast, the IPTA consortium has spent nearly two decades trying to eavesdrop on the constant background “hum” of low-frequency waves collectively produced by countless pairs of supermassive black holes in the early universe that are billions of times more massive than the Sun. 

Supermassive black holes are at the center of all known large galaxies, including the Milky Way, but nobody really knows the backstory of these important objects. Our best guess is that smaller black holes continually merged to form the huge cosmic behemoths that we now see enthroned within modern galactic cores, but the exact details remain a mystery. 

As supermassive black holes in the early universe circled each other before merging, their interactions are predicted to produce gravitational waves at extremely low “nanohertz” frequencies. These ripples have wavelengths on the scale of light years and periods that last for years or decades, as opposed to the much shorter periods of LIGO’s waves, which are on the order of seconds.

To detect these expansive waves, scientists with IPTA have spent years examining a special class of dead stars known as pulsars, which spin hundreds of times every second and shoot out beams of light that serve as extremely precise cosmic clocks. By monitoring the rhythmic beat of pulsars across the Milky Way, the IPTA team hoped to spot the extraordinarily subtle influence of passing gravitational waves that might result in light from the pulsars arriving at Earth ​​a fraction of a second earlier or later. 

The IPTA consortium has pooled the work of multiple groups, including NANOGrav, the European Pulsar Timing Array, the Indian Pulsar Timing Array Project, and the Parkes Pulsar Timing Array. Now, the researchers think they have arrived at their first major goal with the presumed detection of the gravitational wave background.

“This is evidence for what we expect the signature of these ultra-low frequency gravitational waves to look like,” Ferdman said. “It's more than tantalizing, but we don’t want to put the cart before the horse. I think the next step is to take all of our collaboration’s data, put them all together, and continue observing and getting more data.” 

“Hopefully, in the near future, we’ll have a very convincing and acceptable statistical level with which we can claim a detection,” he added. “So this is evidence for detection, but we won’t go as far as to say it is a detection.”    

If the IPTA really has captured these low-frequency waves, it means that we have tapped into a completely different source of information about the universe. While there are predictions about what to expect from the gravitational wave background, every new observational advance unveils total surprises. 

Scientists are eager to solve long-standing riddles about galaxies and black holes, but they might also potentially sense the vibrations of exotic cosmic strings, the fallout of the Big Bang that birthed the universe, and other events that are currently beyond imagination.  

“There’s this parallel way of looking at the universe that we couldn't do before because all we could rely on was the light that we get from these objects,” Ferdman concluded. “We can learn things that we wouldn't be able to even know were there before, in ways that we could never probe before. It's super exciting.”