Scientists have picked up the vibrations of a resounding “cosmic bass note” believed to be the result of slow-motion collisions between supermassive black holes throughout the cosmos, marking the first detection of low-frequency ripples in the fabric of spacetime. This discovery holds the promise of unveiling the secrets hidden within the colossal black holes that reside at the centers of galaxies.

Supermassive black holes, weighing millions to billions of times the mass of the sun, have played a pivotal role in shaping galaxies but have remained enigmatic due to their ability to trap all light within their grasp.

“The significance of this finding cannot be overstated,” exclaimed Dr. Stephen Taylor, astrophysicist at Vanderbilt University and chair of the North American Nanohertz Observatory for Gravitational Waves (Nanograv) consortium, which spearheaded this groundbreaking discovery.

Dr. Michael Keith, a member of the European team that provided independent evidence for the gravitational wave signal, expressed great excitement, stating, “Today’s results mark the start of a remarkable journey into the mysteries of our universe. After decades of collaborative efforts by hundreds of astronomers and physicists worldwide, we are finally witnessing the unmistakable signature of gravitational waves from distant corners of the cosmos.”

The existence of gravitational waves was first predicted by Albert Einstein a century ago, and in 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States provided compelling evidence that space itself can be stretched and squeezed. However, until now, scientists had only managed to capture brief “chirps” of gravitational waves resulting from the mergers of black holes or slightly larger neutron stars.

The latest observations tap into a much lower frequency range, with a complete wave taking approximately 30 years to pass by Earth at the speed of light. This cosmic rumble is believed to be generated by the collective population of supermassive black hole binaries spanning the past 8 billion years.

“We theorize that each pair contributes a small wave, which combines with others to create what we perceive now—a collective murmur,” explained Prof. Alberto Vecchio from the University of Birmingham and a member of the European Pulsar Timing Array (EPTA).

By carefully monitoring over 100 pulsars—exotic stars that spin hundreds of times per second and emit beams of radio waves—scientists were able to detect subtle timing changes caused by the stretching and squeezing of spacetime.

After 12 years of data, the Nanograv team began to observe hints of this gravitational hum in 2020. They collaborated with independent teams in Europe, India, China, and Australia, who cross-verified the findings using their own datasets.

While the likelihood of these results being coincidental is estimated to be as low as one in 10,000, indicating compelling evidence, it falls short of the one-in-a-million gold standard in physics for claiming the discovery of a new phenomenon. Uncertainty remains regarding the precise source of the gravitational waves, with supermassive black hole mergers being the most likely explanation, although alternative possibilities, such as gravitational imprints from the early universe, are still on the table.

Describing the achievement as “important and a considerable technical achievement,” Prof. Dame Jocelyn Bell Burnell, the discoverer of pulsars, emphasized the precision and meticulousness involved in this work. She stated, “The effect is very small—about one part in a thousand million million—so it requires precision. They have been careful and cautious, refraining from premature announcements.”

Prof. Andrew Pontzen, a cosmologist at University College London, expressed his excitement, saying, “It is rare for us to gain a glimpse of the universe through an entirely new lens. After 15 years of patient efforts, Nanograv seems to have accomplished just that. The initial evidence for these gravitational waves is tremendously exciting and will eventually provide us with a wealth of knowledge about supermassive black holes that weigh hundreds of millions of times the mass of the sun.”

The details of these findings are outlined in a series of papers published in the Astrophysical Journal Letters.