An international group of astronomers has detected a weak signal of gravitational waves projecting into the universe. Using dead stars as a giant network of gravitational wave detectors, a collaboration called NANOGrav was able to measure the low-frequency buzz from a chorus of space-time fluctuations. I am an astronomer who studies and writes about cosmology, black holes and exoplanets. I’ve studied the evolution of supermassive black holes using the Hubble Space Telescope.

While members of the team behind this new discovery aren’t yet sure, they strongly suspect that the background hum of the gravitational waves they’re measuring is caused by numerous ancient supermassive black hole mergers.

Using dead stars for cosmology

Gravitational waves are ripples in space-time caused by large objects accelerating. Albert Einstein predicted their existence in his theory of general relativity, which he proposed that when a gravitational wave passes through space, it causes space to contract and then expand periodically.

Researchers first discovered direct evidence of gravitational waves in 2015, when the Laser Interferometer Gravitational-Wave Observatory, known as LIGO, recorded a signal from a pair of black holes that traveled 1.3 billion light-years to reach Earth.

The NANOGrav collaboration is also trying to detect space-time fluctuations on an interstellar scale. The team used pulsars, which are rapidly rotating dead stars that emit a beam of radio radiation. Pulsars are functionally similar to lighthouses; As it rotates, its rays can pass through the Earth at regular intervals.

The NANOGrav team used pulsars that spin incredibly fast at up to 1,000 times per second, and these pulses can be recorded like the ticking of a highly sensitive cosmic clock. As gravitational waves pass a pulsar at the speed of light, the waves will expand slightly and the distance between the pulsar and Earth will shrink, slightly changing the time between pulses.

Pulsars are clocks so precise that their ticking can be measured within 100 nanoseconds. This allows astronomers to calculate the distance between the pulsar and Earth within 100 feet (30 meters). Gravitational waves change the distance between these pulsars and Earth by tens of miles, making the pulsars sensitive enough to detect this effect.

Find the hum in the cacophony

The first thing the NANOGrav team had to do was control the noise in the cosmic gravitational wave detector. This included the noise in the radios it used and the subtle astrophysics that affected the behavior of the pulsars. Even taking these effects into account, the team’s approach wasn’t sensitive enough to detect gravitational waves from individual pairs of supermassive black holes. However, it was sensitive enough to detect the sum of all major black hole mergers—about a million overlapping signals—that have occurred anywhere in the universe since the Big Bang.

In a musical analogy, it’s like you’re standing in a crowded city center and somewhere in the distance you’re hearing the faint sound of a symphony. You cannot choose a single instrument due to the noise of cars and people around you, but you can hear hundreds of instruments hum. The team had to distinguish the properties of this “background” gravitational wave from other competing signals.

The team was able to detect this symphony by measuring 67 different pulsar networks over 15 years. If any disturbance in a pulsar’s ticking is due to gravitational waves from the distant universe, all pulsars observed by the team will be similarly affected. On June 28, 2023, the team published four papers describing their project and the evidence they found for the gravitational wave background.

The NANOGrav collaboration detected the hum caused by the merger of black holes billions of times larger than the Sun. These black holes revolve around each other very slowly and produce gravitational waves with a frequency of one billionth of a hertz. This means that space-time fluctuations have oscillations every few decades. This slow oscillation of the wave was why the team had to rely on the incredibly precise timing of the pulsars.

These gravitational waves are different from what LIGO can detect. LIGO signals are produced when two black holes 10 to 100 times the mass of the Sun combine into a single rapidly rotating object, creating gravitational waves that oscillate hundreds of times per second.

If you think of black holes as tuning forks, the smaller the event, the faster the tuning fork vibrates and the higher the pitch. LIGO detects gravitational waves that “ring” in the audible range. The NANOGrav team has found a “ring” of black hole mergers at a frequency billions of times too low to be heard.

Giant black holes in the early universe

Astronomers have long been interested in studying how stars and galaxies first appeared after the Big Bang. This new discovery by the NANOGrav team seems to add another color – gravitational waves – to the picture of the early universe that is beginning to emerge, thanks in large part to the James Webb Space Telescope.

The primary scientific purpose of the James Webb Space Telescope is to help researchers study how the first stars and galaxies formed after the Big Bang. To do this, James Webb designed it to detect weak light from incredibly distant stars and galaxies. The further away an object is, the longer it takes for light to reach Earth, so James Webb is an effective time machine that can look back 13.5 billion years to see light from the first stars and galaxies in the universe.

He was very successful in his search, finding hundreds of galaxies that filled the universe with light in the first 700 million years after the Big Bang. The telescope also discovered the oldest black hole in the universe, located at the center of the galaxy and formed just 500 million years after the Big Bang.

These findings challenge existing theories about the evolution of the universe.

It takes a long time to grow a large galaxy. Astronomers know that supermassive black holes are at the center of every galaxy and have a mass proportional to their galaxy. Therefore, it is almost certain that these ancient galaxies had a corresponding large black hole at their centers.

The problem is that the objects found by James Webb are much larger than current theory predicts.

These new results from the NANOGrav team come after the first opportunity for astronomers to listen to the gravitational waves of the ancient universe. The findings, though promising, are not strong enough to be considered a definitive discovery. This will likely change as the team expands the pulsar network to 115 pulsars and will receive results from this follow-up survey around 2025. As James Webb and other researchers challenge current theories of galaxy evolution, being able to study the post-Big Bang era using gravitational waves can be an invaluable tool. Source