Why we study black holes and gravitational waves

In June 2023, scientists announced they had found evidence to suggest that the universe is replete with a "cosmic symphony" of massive gravitational waves caused by pairs of black holes spinning around each other in a very slow dance.   

Hundreds of scientists have tuned into this field of study in recent years because black holes and gravitational waves could be a key to unlock the universe's biggest secrets, including invisible dark matter.

"We are trying to clarify how supermassive black holes formed in the universe," says Keitaro Takahashi at Kumamoto University in Japan. 

Takahashi, who was part of the study, says that may help scientists "detect the primordial gravitational waves [that were] generated at the beginning of the universe and approach the mystery of the origin of the universe."

What is a gravitational wave? 

Imagine gently placing a ping-pong ball on the surface of a pond. It floats on the surface by causing a dent on it.

The physicist Albert Einstein said this was how matter causes gravity. Objects in space — or to be precise, "masses" — cause a curvature of space-time, the fabric of our universe.

In Einstein's General Theory of Relativity, he proposed that space and time behave like the surface of water. 

Now, imagine throwing the ping-pong ball onto the pond with all your force. Ripples will expand from the point of impact into all directions. 

When heavy masses in the cosmos accelerate rapidly, they cause an invisible yet incredibly fast ripple in space, and scientists call them gravitational waves.  

The study published in June suggests the universe is replete with such waves. So, why don't we feel them? The answer is that their effect on us is practically insignificant.

"When a gravitational wave, as seen by our experiment, passes through us (and everything around us) it stretches and squeezes us by an amount that compares to the size of one neutron in the whole body," explains Michael Kramer, director at the Max Planck Institute for Radio Astronomy in Germany. Kramer is part of the international collaboration that observed the gravitational waves.

Travelling at the speed of light, gravitational waves can squeeze and stretch anything in their path and in all directions, making it hard for us to detect without sophisticated equipment.

Some of the origins of these waves include the asymmetrical explosion of a star, a supernova, a massive binary system of stars or black holes. And scientists can use these gravitational waves to spot massive objects moving in the cosmos.  

But why study gravitational waves? 

To explain why we study gravitational waves, let's return to the idea of a symphony for a moment. Think of an orchestra. Then pick two musical instruments — say, a violin and a cello. They are both string instruments, with a similar shape, and we can quickly see that they vary in size.

But to truly appreciate the differences between them, we must listen to them. And that's how we come to understand they are two different instruments.  

Now, let's get back to gravitational waves, and say the universe is our orchestra. In order to understand the universe fully, we must look at it from different perspectives. We must see and hear it.

The international team working on the study published in June used pulsar technology to do just that.

"We listen to the noise from the universe, and we can see the visible light from stars using telescopes," says Bhal Chandra Joshi, a senior astrophysicist at the Tata Institute of Fundamental Research in India, is behind the Indian Pulsar Timing Array (InPTA). 

"We understand the cosmos by studying electromagnetic radiations like ultraviolet, infrared, radio, X-ray and gamma rays. We do particle astronomy by studying the neutrinos darting across the universe. Studying gravitational waves is the third way we understand the universe," Joshi says.

Gravitational waves help our understanding of "how matter in the universe is organized the way it is," Joshi says.

In other words, they could give us information about an invisible mass of really distant objects in the universe, known as dark matter.

Most of the universe is said to be comprised of dark matter.

Dark matter does not emit light or any electromagnetic radiation and that makes it hard to study it. We can't see it. But gravitational waves could change that. 

Didn't we already detect gravitational waves? 

Yes, we did. Scientists at the Laser Interferometer Gravitational Observatory (LIGO) in the US were the first to detect gravitational waves.  Their 2015 study of a gravitational wave that emerged from a small black hole merger 1.3 billion years ago won them a Nobel Prize in Physics.  

But the LIGO detectors can only measure high-frequency gravitational waves, which usually occur when two small black holes spin around each other at high speeds. When two super-massive blackholes spin around each other, they do so slowly and lethargically. The subsequent low frequency gravitational waves send out very light ripples into space, avoiding human observation.  

"With LIGO one can see black holes that are 100 times as heavy as the sun. With our experiment, the pulsar timing array, you can observe black holes that are 10 billion times heavier and merging in the center of distant galaxies," says Kramer.  

In a bid to discover these slower gravitational waves, scientists used six large radio telescopes around the world to capture deviancies in pulsars — distant, rapidly-rotating neutron stars that emit pulses of radiation, observed from the Earth as bright flashes of light, like a lighthouse.

The pulses of light are extremely evenly paced, enabling scientists to use pulsars as "cosmic clocks." It is like doctors using the rhythm of your heartbeat to get an picture of your health.

When scientists watched 25 pulsars over a 15 year period, they noticed discrepancies ranging in millionths of seconds. And scientists from all the observatories noticed the same deviation: some of the signals from these neutron stars arrived a little early, while a few others were late. 

"This is key evidence for gravitational waves at very low frequencies," says Vanderbilt University's Stephen Taylor, who co-led a collaborative study by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). 

It is these irregularities that show the presence of gravitational waves.

"When we can detect gravitational waves of even lower frequency, we can peer back in time into the grand cosmological events that took place shortly after the Big Bang," says Joshi.

Which is what it is all about.  

Edited by: Zulfikar Abbany