Gravitational Waves Background Hum: NANOGrav’s Historic Discovery

For the first time in history, scientists have heard the collective rumble of the universe. After 15 years of meticulous observation, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has detected a low-frequency “hum” permeating the cosmos. This background noise is likely generated by the slow, spiraling dance of supermassive black holes merging billions of light-years away.

The 15-Year Search for the Cosmic Hum

In June 2023, the NANOGrav collaboration released a series of papers detailing evidence for the Gravitational Wave Background (GWB). Unlike previous detections of gravitational waves, which were short bursts from smaller events, this discovery represents a constant, steady noise.

Think of it like a choir. Previous detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) heard the “shout” of a single singer during a brief solo. NANOGrav, however, has detected the background murmur of the entire choir warming up before the show.

The team analyzed data collected over a decade and a half. They found that the fabric of space-time is constantly being stretched and squeezed by massive waves passing through our galaxy. This confirms a major prediction of Einstein’s theory of general relativity, but on a scale never measured before.

How Pulsar Timing Arrays Work

To detect these waves, scientists could not build a detector on Earth. The waves are simply too big. Instead, NANOGrav transformed the Milky Way galaxy itself into a galaxy-sized antenna known as a Pulsar Timing Array.

The methodology relies on pulsars. These are the ultra-dense remnants of dead stars that spin rapidly, acting like cosmic lighthouses. They emit beams of radio waves that sweep past Earth at incredibly precise intervals. These “ticks” are so stable that they rival the accuracy of atomic clocks.

Measuring the Distortion

If a gravitational wave passes between Earth and a pulsar, it warps the space between them. This warping causes the radio pulse to arrive slightly earlier or later than expected. The delay is minuscule—often merely nanoseconds over span of years.

NANOGrav monitored 68 specific pulsars scattered across the sky. By looking for a specific pattern of correlated timing deviations across these pulsars, known as the Hellings-Downs curve, the team confirmed the presence of gravitational waves. This specific correlation pattern is the “smoking gun” that proves the timing irregularities are caused by gravitational waves and not some other noise source.

The Equipment Behind the Discovery

The data required for this breakthrough came from some of the most powerful radio telescopes on Earth. The collaboration relied heavily on:

The Green Bank Telescope: Located in West Virginia, this is the world’s largest fully steerable radio telescope.
The Arecibo Observatory: Located in Puerto Rico, this legendary telescope provided crucial data before its tragic collapse in 2020.
The Very Large Array (VLA): A collection of 27 radio antennas in New Mexico used to verify and supplement the findings.

Using these massive instruments, astronomers tracked the arrival times of pulsar signals with extreme precision, looking for the tell-tale signature of space-time stretching.

The Source: Monster Black Holes

The primary suspects generating this background hum are supermassive black hole binaries. These are pairs of black holes, each with a mass millions or billions of times that of our sun, orbiting each other at the centers of merging galaxies.

As these distinct galaxies collide, their central black holes sink toward the center and begin a slow, spiraling dance. As they orbit, they churn up the fabric of space-time, sending out low-frequency gravitational waves in all directions.

Because these waves overlap with waves from millions of other merging binaries across the universe, they create a stochastic background. This is the “hum” NANOGrav detected. It is not a single event but the combined gravitational noise of the universe’s history of galaxy mergers.

Nanohertz Frequencies vs. LIGO

It is important to understand how this differs from the Nobel Prize-winning discovery made by LIGO in 2015.

LIGO: Detects high-frequency waves (around 10 to 1,000 Hertz). These come from stellar-mass black holes (about 30 times the mass of the sun) colliding violently in a fraction of a second.
NANOGrav: Detects nanohertz frequencies. These waves are incredibly long; a single wave crest can take years or even decades to pass Earth. Only supermassive black holes moving slowly over long periods can generate waves of this magnitude.

If LIGO hears the chirp of a bird, NANOGrav hears the deep rumble of thunder rolling in the distance.

A Global Effort

While NANOGrav led the North American effort, they were not working in isolation. The announcement was coordinated with other teams around the world who found consistent results using their own data. These organizations include:

The European Pulsar Timing Array (EPTA)
The Parkes Pulsar Timing Array (PPTA) in Australia
The Indian Pulsar Timing Array (InPTA)

The combined data from these groups strengthens the evidence significantly. The International Pulsar Timing Array (IPTA) is currently working to combine all these datasets into a single, massive analysis to sharpen the view of this new gravitational window.

Frequently Asked Questions

What does the background hum sound like? It is not sound in the traditional sense, as sound cannot travel through a vacuum. However, if we translated the data into audio, it would resemble a static or a low, random rumble, similar to the background noise of a busy restaurant.

Does this hum affect life on Earth? No. While these waves stretch and squeeze everything they pass through, including the Earth and our bodies, the effect is infinitesimally small. The change in length is roughly the width of a proton over the distance to the nearest stars.

Why is the Hellings-Downs curve important? This mathematical curve predicts exactly how the signals from different pairs of pulsars should be correlated depending on their angle in the sky relative to each other. Matching this curve is the only way to prove the signal is coming from gravitational waves and not from telescope errors or changes in the pulsars themselves.

What happens next for NANOGrav? The team continues to collect data. As the observation time span increases, the sensitivity of the array improves. Future goals include identifying individual pairs of supermassive black holes standing out above the background noise and mapping the distribution of these monsters across the universe.