Sweeping gravitational wave map of the universe could reveal hidden black holes

The most detailed map of the universe ever created using gravitational waves could reveal hidden black holes, merging supermassive black holes and even the large-scale structure of the cosmos.

The study, conducted by a team led by astronomers from Swinburne University of Technology, also presented the largest-ever galactic-scale detector of gravitational waves, which are basically ripples in spacetime.

This research has provided further evidence of a background "hum" of gravitational waves that permeates the universe. As such, it could grant new insights into the universe's earliest black holes, how they grew, and the impact they had on the evolution of the comic structure.

"Studying the [gravitational wave] background lets us tune into the echoes of cosmic events across billions of years," team member and Swinburne University researcher Matt Miles said in a statement. "It reveals how galaxies, and the universe itself, have evolved over time."

The gravitational wave background that Miles refers to was created by merging supermassive black holes in the early and distant universe. It was first revealed by a gravitational wave detector that taps into a multitude of rapidly spinning neutron stars, or "pulsars," and a precise time-keeping instrument called a pulsar timing array as part of the NANOGrav project.

This new study also relied on a pulsar timing array, though with the aid of the MeerKAT radio telescope in South Africa. By improving detection methods, the nanosecond precision of the MeerKAT Pulsar Timing Array granted the Swinburne-led team a stronger signal than before.

"What we're seeing hints at a much more dynamic and active universe than we anticipated," Miles said. "We know supermassive black holes are out there merging, but now we’re starting to ask: where are they, and how many are out there?"

Gravitational wave map delivers suprises

Gravitational waves were originally predicted by Albert Einstein's in his theory of gravity, called "general relativity." The great physicist's magnum opus theory states that objects with mass cause the very fabric of space and time (united as a 4-dimensional entity called "spacetime") to "warp." It is from such warping that gravity arises.

However, Einstein also predicted that when massive objects accelerate, they cause ripples that spread through spacetime at the speed of light. These are known as gravitational waves.

An example of gravitational wave-emitting systems are binary black holes, which set spacetime ringing as two black holes within each of these systems swirl around one another. As gravitational waves surge away from the binary black holes, they carry with them angular momentum. This energy loss causes the black holes to draw together and emit gravitational waves faster and faster — that is, until the gravity of these cosmic titans takes over, and they collide and merge.

This powerful event sets spacetime ringing with a high-pitched "scream" of gravitational waves.

Black hole mergers are important to understanding the evolution of the cosmos. This is how the supermassive black holes with masses equivalent to millions, or even billions, of suns that sit at the hearts of large galaxies are born.

Pulsars can be used to detect gravitational waves because they can spin up to 700 times per second; they also blast out beams of radiation that sweep the cosmos like the illumination of a cosmic lighthouse.

The key, though, is that a pulsar's spin timing is highly regular, meaning that when a large group of pulsars is considered en masse, they can be used as a precise cosmic clock — a clock sensitive enough to detect tiny fluctuations in spacetime caused by the passage of gravitational waves.

This is how the MeerKAT Pulsar Timing Array allowed the team to create a highly detailed gravitational wave map — and that spacetime-ripple-forged cosmic atlas actually revealed a surprise feature.

The researchers discovered an unexpected anomaly in their map: a "hotspot" that appears to exhibit a "directional bias" for gravitational waves.

Previously, researchers had assumed that the universe's gravitational wave background would have no preferred direction and thus be evenly spread across the sky.

"The presence of a hotspot could suggest a distinct gravitational wave source, such as a pair of black holes billions of times the mass of our sun," said team member and Monash University researcher Rowina Nathan. "Looking at the layout and patterns of gravitational waves shows us how our universe exists today and contains signals from as far back as the Big Bang.

"There's more work to do to determine the significance of the hotspot we found, but this an exciting step forward for our field."

The team's findings have opened the door to new, unpredicted discoveries concerning the structure of the universe.

These discoveries could well be delivered by the galactic-sized gravitational wave detector formed by the MeerKAT Pulsar Timing Array, which will now continue to refine its gravitational wave map.

"By looking for variations in the gravitational wave signal across the sky, we’re hunting for the fingerprints of the astrophysical processes shaping our universe," Kathrin Grunthal , team member and a scientist at the Max Planck Institute for Radio Astronomy, said in the statement.