The Sound of the Universe: How Scientists Are Listening for the Hidden Collisions of Black Holes

Polina Milekhina
2 days ago
6 min read

Featured image: Artist’s impression of a supermassive black hole binary producing gravitational waves - Image Credit: by NASA’s Goddard Space Flight Center/Scott Noble

Research paper explored: The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background (2023)

Main question: Could the gravitational-wave background detected by NANOGrav be explained by populations of supermassive black hole binaries?

Field: Astrophysics | Gravitational-wave astronomy

For most of human history, the universe has been something we could only observe from a distance.

We looked at the stars, mapped galaxies, and studied the light that travelled across space to reach us.

Every discovery, from planets orbiting distant stars to galaxies formed billions of years ago, came from learning how to interpret the light of the cosmos.

But what if some of the universe’s greatest events were happening beyond what light could reveal?

What if the universe was not only something we could see, but something we could hear?

For billions of years, some of the universe’s most extreme events have been producing invisible ripples that travel through space and time. These ripples, known as gravitational waves, are created by some of the most extreme events in existence, including the movements of black holes.

In 2023, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) released results from 15 years of observations that may have opened a completely new way of exploring the cosmos.

Instead of only looking deeper into the universe, scientists may now be learning how to listen to it.

A prediction hidden inside Einstein’s theory

In 1916, Albert Einstein predicted the existence of gravitational waves as part of his theory of general relativity.

His idea was revolutionary: space and time are not fixed backgrounds where events happen. Instead, they form a flexible structure called spacetime that can be stretched and distorted.

When massive objects accelerate, such as two black holes orbiting each other, they can create ripples in this structure.

These ripples move outward through the universe at the speed of light, carrying information about the events that created them.

For decades, gravitational waves remained one of Einstein’s most fascinating predictions. The challenge was that their effects are incredibly small. By the time they reach Earth, they can change distances by less than the size of an atom.

Detecting them required scientists to develop entirely new ways of observing the universe.

In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves from the merger of two stellar-mass black holes.

But this was only one part of the gravitational-wave universe.

Just as different telescopes reveal different types of light, different detectors allow us to study different types of gravitational waves.

LIGO observes short, powerful signals from individual cosmic events. NANOGrav searches for something much slower and much larger: The background hum of the universe.

The giants hiding at the centres of galaxies

At the centre of many large galaxies are supermassive black holes, objects containing millions or even billions of times the mass of our Sun.

Although we know they exist, many questions about them remain unanswered. How did they become so massive? How did they grow alongside the galaxies around them?

One possible way to learn more is by studying what happens when galaxies collide. When two galaxies merge, their central black holes may eventually become gravitationally connected. Over millions of years, they orbit each other, slowly losing energy and moving closer together. During this process, they release gravitational waves.

However, these mergers happen on timescales far beyond human lifetimes. A single supermassive black hole merger may take millions of years to complete. This means scientists are not expecting to detect one individual event. Instead, they are searching for the combined signal of countless black hole pairs across the universe. A cosmic background: the combined signal produced by some of the largest gravitational interactions in existence.

Turning pulsars into cosmic clocks

Detecting a signal this faint requires a completely different approach.

NANOGrav does not use lasers. Instead, it uses some of the most precise natural clocks in the universe: pulsars.

Pulsars are the remains of massive stars that exploded as supernovae. They are incredibly dense neutron stars that rotate rapidly, sending out beams of radio waves at extremely regular intervals. From Earth, these pulses arrive almost like clock signals. Scientists monitor these pulsars over many years, searching for tiny changes in their timing.

If a gravitational wave passes between Earth and a pulsar, it slightly changes when the pulsar’s signal reaches us. The change is incredibly small, but when scientists observe many pulsars together, they can look for a specific pattern. The key is not one pulsar changing. The key is the relationship between many pulsars changing in the way gravitational waves predict. It is similar to hearing a crowd from outside a stadium. You cannot separate every individual voice, but the combined sound tells you that something is happening inside.

The gravitational-wave background is the universe’s version of that distant noise.

What did NANOGrav discover?

The NANOGrav collaboration analysed 15 years of pulsar timing data and found evidence for a low-frequency gravitational-wave background.

Their results showed that the signal is consistent with what scientists expected from large populations of supermassive black hole binaries. These systems are one of the leading explanations for the observed background.

However, the researchers did not claim to have identified the source with absolute certainty.

This is an important part of science.

Discoveries are rarely the moment when every question disappears. Instead, they create new questions and allow scientists to test better explanations.

The NANOGrav results represent a major step forward, but future observations will help determine exactly what is producing this gravitational-wave background.

Why does this matter?

The significance of this discovery is not only about black holes. It is about changing how we experience the universe.

For centuries, astronomy has relied on light. We have studied the cosmos through visible light, radio waves, X-rays, and other forms of electromagnetic radiation. Gravitational waves give us another sense. They allow us to explore events that may be invisible, but are still shaping the universe around us. Just as Galileo’s telescope revealed worlds that had previously been beyond human observation

Learning to hear the cosmos

The universe has always been filled with hidden stories. The collisions of galaxies, the movements of black holes, and the birth of cosmic structures have been happening for billions of years. The gravitational waves from these events travelled silently across space long before humans existed.

Now, we are developing the tools to detect them.

The most exciting discoveries are not always about finding something new. Sometimes, they are about discovering a new way to understand what has been there all along. The universe was always speaking. We are only now learning how to listen.

Key takeaways

NANOGrav found evidence for a low-frequency gravitational-wave background using 15 years of pulsar timing data.
One possible explanation is the combined signal from populations of supermassive black hole binaries.
The discovery opens a new era of gravitational-wave astronomy, allowing scientists to study parts of the universe invisible to traditional telescopes.

Sources & Further Reading

Research paper

Agazie, G. et al. (2023). The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational-Wave Background. Published in The Astrophysical Journal Letters.

Original paper: arXiv:2306.16220

This article is based on the NANOGrav collaboration’s analysis of 15 years of pulsar timing data and their investigation into the possible origins of the low-frequency gravitational-wave background.

Key concepts explored

Gravitational waves: Ripples in spacetime produced by accelerating massive objects.
Pulsar timing arrays: Networks of pulsars used as highly precise cosmic clocks to detect low-frequency gravitational waves.
Supermassive black hole binaries: Pairs of extremely massive black holes orbiting each other, often formed after galaxies merge.
Gravitational-wave background: A combined signal created by many gravitational-wave sources throughout the universe.