The Bass of the Universe: The Ensemble of the Milky Way's Most Mysterious Objects

The artist's concept above shows stars, black holes and nebulae laid out on a grid representing the fabric of space-time. The ripples in this fabric are called gravitational waves.

The NANOGrav collaboration has detected evidence of gravitational waves from a black hole billions of times more massive than our sun. Image credit: NANOGrav collaboration; Aurore Simonet

At the center of the Milky Way, there is a supermassive black hole with a mass equivalent to 4 million suns, about 26,000 light-years away from us. It is called Sagittarius A (abbreviated as SgrA), and is one of the most mysterious celestial bodies in the Milky Way.

As a celestial body with extremely strong gravity, black holes can swallow surrounding matter and light, making it impossible for them to escape. But black holes are not alone, they also interact with other black holes.

When two black holes merge, they create something called gravitational waves, just as two rocks dropped on water create ripples. Gravitational waves are distortions of spacetime created when objects spin or collide in space, and they can change the path of light and cause distant stars to flicker or dim.

Gravitational waves can help us explore the mysteries of black holes and the universe, but they are very difficult to detect because they are very weak and they decay with distance. Currently, scientists have captured gravitational wave signals from different frequencies and sources using detectors on the ground and in space.

In July this year, an international collaboration project called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) announced a major discovery: they found the first evidence that the long-wavelength gravitational wave background is ubiquitous in the universe. This gravitational wave background is produced by supermassive black holes with masses up to billions of times that of the sun circling each other for hundreds of millions of years before merging.

This signal is like a cosmic bass, allowing us to hear the ensemble music played by supermassive black holes in space and time.

The existence of gravitational waves was predicted a century ago by Albert Einstein, whose general theory of relativity described how matter and energy bend space-time to produce the phenomenon we know as gravity.

But it wasn't until 2015 that humans detected gravitational waves for the first time. This historic discovery was made by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, whose two detectors captured an extremely short gravitational wave signal from the merger of two black holes, lasting less than 1 second. It arrived on Earth on September 14, 2015 after a long journey of 1.3 billion years. Since then, LIGO and the European Virgo gravitational wave detector (Virgo) have also detected multiple similar signals, as well as gravitational wave signals from the merger of two neutron stars.

NANOGrav, the protagonist of this major discovery, is an international cooperation project consisting of more than 190 scientists from the United States and Canada, which uses a pulsar timing array (PTA).

Pulsars are strange, compact stars that spin hundreds of times per second and produce radio waves like a lighthouse. These pulses are so stable that they allow us to capture tiny changes in time caused by the stretching and squeezing of space-time. With pulsars, scientists can predict time to tens of nanoseconds, with the same accuracy as atomic clocks in some cases. If multiple pulsars are monitored and the same pattern of time changes is found between them, that is, the timing residuals between the model prediction and the actual observation, then we can infer the presence of gravitational waves.

There are currently three major pulsar timing array projects detecting the cosmic gravitational wave background. In addition to NANOGrav, there are the European Pulsar Timing Array (EPTA) and the Parkes Pulsar Timing Array (PPTA) in Australia. Recently, new forces such as the China Pulsar Timing Array, the Indian Pulsar Timing Array (InPTA) and the South African Pulsar Timing Array (SAPTA) have been added. These projects use the world's largest and most sensitive radio telescopes, such as the Very Large Array (VLA) in New Mexico, the Parkes Observatory in New South Wales, Australia, and the Five hundred meter Aperture Spherical Telescope (FAST) in Guizhou Province, China. These telescopes observe more than 100 pulsars every few weeks and record the precise time when they emit radio pulses.

NANOGrav has spent 15 years collecting high-precision data from multiple radio telescopes, including the Green Bank Telescope in West Virginia, the Arecibo Observatory in Puerto Rico, and the Very Large Array radio telescope, observing 68 millisecond pulsars.

This is a milestone achievement because it is the first time that humans have detected a low-frequency gravitational wave background and the first time that gravitational wave signals generated by the merger of supermassive black holes have been detected.

The gravitational waves detected by LIGO have much higher frequencies than those registered by NANOGrav (NANOGrav gets its name from the fact that it detects low-frequency gravitational waves in the nanohertz range, or one cycle every few years). High-frequency waves come from smaller black hole pairs orbiting each other quickly in the last few seconds before they collide, while low-frequency waves are thought to be produced by giant black holes at the centers of galaxies, up to billions of times more massive than our sun, which orbit slowly and have millions of years to go before they merge.

In the new study, scientists believe that NANOGrav has discerned the collective "hum" of gravitational waves from many pairs of merging supermassive black holes throughout the universe. "People compare this signal to more of a background noise than the loud shout that LIGO detects," explained Katerina Chatziioannou, an assistant professor of physics at the California Institute of Technology (Caltech) and a member of the LIGO team and a member of the NANOGrav team.

The low-frequency gravitational wave background is important because it can help us better understand the mysteries of black holes and the universe. The low-frequency gravitational wave background can tell us the frequency and characteristics of supermassive black hole mergers, which is very important for studying the evolution of the Milky Way and the universe.

Scientists believe that supermassive black holes may have grown by swallowing and merging other black holes during the formation and evolution of the Milky Way, and they have an important impact on the structure and dynamics of the Milky Way. By detecting the low-frequency gravitational wave background, we can estimate the rate and distribution of supermassive black hole mergers, and thus infer the history and future of the Milky Way and the universe.

By detecting the low-frequency gravitational wave background, we can test whether the predictions of general relativity and other theories are consistent at different scales and conditions, and thus discover possible new physical phenomena or laws.

The supermassive black hole at the center of our own Milky Way, Sagittarius A*, may also be on a collision course with other black holes, or will collide in the future, which would change its mass and shape and affect the evolution of our galaxy.

A series of papers detailing the latest NANOGrav results have been accepted for publication in The Astrophysical Journal Letters. The paper describing the gravitational wave evidence, titled "NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background," was co-led by two former JPL postdocs: Sarah Vigeland, now at the University of Wisconsin, Milwaukee, and Stephen Taylor, now at Vanderbilt University.