A subtle symphony of ripples in spacetime Astronomers use dead stars to measure gravitational waves produced by ancient black holes

An international team of astronomers has detected a faint signal of gravitational waves reverberating through the universe. Using dead stars as a giant array of gravitational-wave detectors, the collaboration called NANOGrav was able to measure a low-frequency hum from a chorus of spacetime ripples.

I am an astronomer who studies and has written on cosmology, black holes and exoplanets. I studied the evolution of supermassive black holes using the Hubble Space Telescope.

While the team members behind this new discovery aren’t certain yet, they strongly suspect that the background hum of the gravitational waves they measured was caused by countless ancient merger events of supermassive black holes.

Pulsars are rotating dead stars that emit strong beams of radiation and can be used as accurate cosmic clocks.

Using dead stars for cosmology

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

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

The NANOGrav collaboration is also trying to detect spacetime ripples, but on an interstellar scale. The team used pulsars, rapidly rotating dead stars that emit a beam of radio emissions. Pulsars are functionally similar to a lighthouse as they rotate, their beams can pass through the Earth at regular intervals.

The NANOGrav team used pulsars that rotate incredibly fast up to 1,000 times per second, and these pulses can be timed like the ticking of a highly accurate cosmic clock. When gravitational waves pass through a pulsar at the speed of light, the waves will slightly expand and contract the distance between the pulsar and Earth, slightly changing the time between ticks.

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

A giant white reflective dish with a receiver.
The NANOGrav team has used a number of radio telescopes, including the Green Bank Telescope in West Virginia, to listen for pulsars for 15 years.
NRAO/AUI/NSF, CC BY

Finding a buzz within the cacophony

The first thing the NANOGrav team had to do was check for noise in its cosmic gravitational wave detector. This included noise in the radio receivers used and the subtle astrophysics that affect the behavior of pulsars. Even accounting for these effects, the team’s approach was not sensitive enough to detect gravitational waves from single binaries of supermassive black holes. However, it had sufficient sensitivity to detect the sum of all massive black hole mergers that have occurred anywhere in the universe since the Big Bang up to a million overlapping signals.

In a musical analogy, it’s like being in a crowded downtown and hearing the faint sound of a symphony somewhere in the distance. You can’t choose just one instrument due to the noise of cars and people around you, but you can hear the hum of a hundred instruments. The team had to extract the signature of this gravitational wave background from other competing signals.

The team was able to detect this symphony by measuring a network of 67 different pulsars over 15 years. If any disruption in a pulsar’s ticking was due to gravitational waves from the distant universe, all the pulsars the team was observing would have been similarly affected. On June 28, 2023, the team released four papers describing its design and the evidence it found against the background of gravitational waves.

The buzz found by the NANOGrav collaboration is produced by the merger of black holes that are billions of times more massive than the Sun. These black holes rotate around each other very slowly and produce gravitational waves with frequencies of a billionth of a hertz. This means that spacetime ripples have a wobble every few decades. This slow oscillation of the wave is why the team needed to rely on the pulsars’ incredibly accurate timekeeping.

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

If you think of black holes as a tuning fork, the smaller the event, the faster the tuning fork vibrates and the higher the pitch. LIGO detects gravitational waves that resonate in the audible range. The mergers of black holes that the NANOGrav team found a ring with a frequency billions of times too low to hear.

A sky full of stars with many spiral galaxies.
The James Webb Space Telescope has allowed astronomers to look back in time and study the first galaxies that formed after the Big Bang.
NASA, ESA, CSA, STScI

Giant black holes in the early universe

Astronomers have long been interested in studying how stars and galaxies first emerged in the aftermath of the Big Bang. This new discovery by the NANOGrav team is like adding another color of gravitational waves to the picture of the early universe that is just starting to emerge, thanks in no small part to the James Webb Space Telescope.

One of the James Webb Space Telescope’s main scientific goals is to help researchers study how the first stars and galaxies formed after the Big Bang. To do this, James Webb was designed to detect the faint light from impossibly distant stars and galaxies. The further away an object is, the longer it takes for light to arrive at Earth, so James Webb is effectively a time machine that can look back more than 13.5 billion years to see the light from the universe’s first stars and galaxies.

He has been very successful in his research, having found hundreds of galaxies that flooded the universe with light in the first 700 million years after the big bang. The telescope also detected the oldest black hole in the universe, located at the center of a galaxy that formed just 500 million years after the Big Bang.

These discoveries are challenging existing theories about the evolution of the universe.

It takes a long time to grow a huge galaxy. Astronomers know that supermassive black holes are found at the center of every galaxy and have a mass proportional to their host galaxies. So these ancient galaxies almost certainly have the corresponding massive black hole at their core.

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

These new results from the NANOGrav team have emerged from the first opportunity for astronomers to listen to gravitational waves from the ancient universe. The findings, while tantalizing, aren’t strong enough to claim a definitive discovery. That will likely change, as the team has expanded its pulsar network to include 115 pulsars and is expected to get results from this next survey around 2025. While James Webb and other research challenge existing theories about how galaxies evolved, the ability of studying the era after the Big Bang using gravitational waves could be an invaluable tool.

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Image Source : theconversation.com

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