After 15 Years of Pulsar Observation, Scientists of NANOGrav Hear the Resounding Symphony of Gravitational Waves in the Universe

After meticulously observing pulsars across our galaxy for a span of 15 years, the NANOGrav collaboration has achieved an extraordinary feat—capturing the ceaseless symphony of gravitational waves reverberating throughout our vast universe, marking a momentous milestone in scientific history.

This groundbreaking revelation was made possible by the diligent efforts of scientists affiliated with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), who dedicatedly studied pulsars, acting as celestial timekeepers. The newly discovered gravitational waves, undulating through the fabric of space-time, have surpassed all expectations with their resounding strength. Their energy levels surpass those of isolated bursts resulting from the mergers of black holes and neutron stars—phenomena previously detected by experiments like LIGO and Virgo—by an astounding magnitude of approximately one million.

In this artistic representation, depicted on the top left, a duo of supermassive black holes emits gravitational waves that ripple across the fabric of space-time. These gravitational waves then exert compression and elongation effects on the paths of radio waves emitted by pulsars, illustrated in white. By meticulously measuring these radio waves, a team of scientists has recently achieved the groundbreaking feat of detecting the background of gravitational waves permeating the universe. The credit for this image goes to Aurore Simonnet, in collaboration with NANOGrav.

According to a series of new papers published on June 29 in The Astrophysical Journal Letters, the NANOGrav scientists assert that the majority of these immense gravitational waves are likely generated by pairs of supermassive black holes gradually spiraling toward cataclysmic collisions throughout the cosmos.

Describing the phenomenon, NANOGrav scientist Chiara Mingarelli, who contributed to the latest findings during her tenure as an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City, compares it to a harmonious choir, where diverse supermassive black hole pairs resonate at distinct frequencies. This discovery serves as the first-ever evidence supporting the existence of the gravitational wave background, opening up an entirely new realm of observational possibilities in our understanding of the universe.

The emergence and composition of the gravitational wave background, a phenomenon that has long been speculated but never before detected, has now unveiled a wealth of invaluable insights into enduring inquiries. These range from unraveling the destiny of pairs of supermassive black holes to determining the frequency of galaxy mergers. The image accompanying this excerpt showcases pulsars, which are swiftly rotating neutron stars emitting focused beams of radio waves. Credit for the image goes to NASA’s Goddard Space Flight Center.

Currently, NANOGrav’s measurements pertain to the collective gravitational wave background, rather than isolating radiation from individual sources. Nevertheless, even this comprehensive observation has brought forth unexpected revelations.

“The gravitational wave background is approximately twice as prominent as my initial expectations,” remarks Mingarelli, presently serving as an assistant professor at Yale University. “It exceeds the upper limits of what our models predicted solely from supermassive black holes.” This resounding intensity might be attributed to limitations in the experiment or the existence of more massive and abundant supermassive black holes. However, Mingarelli highlights the possibility that potent gravitational waves may originate from other sources, such as mechanisms anticipated by string theory or alternative explanations concerning the birth of the universe. “The future holds limitless possibilities,” she asserts. “This is merely the inception of a remarkable journey.”

A Galaxy-Wide Experiment

The journey undertaken by the NANOGrav team to reach this milestone has been a formidable challenge spanning several years. The gravitational waves they pursued stand apart from any previously encountered. Unlike the high-frequency waves identified by ground-based instruments like LIGO and Virgo, the gravitational wave background comprises ultra-low-frequency waves. Each individual wave’s cycle of rising and falling may span years, or even decades. Given that gravitational waves propagate at the speed of light, a single wavelength could extend across tens of light-years.

Given the immense scale of these waves, no Earth-based experiment could ever detect them. Hence, the NANOGrav team turned their gaze towards the stars. They meticulously observed pulsars, remnants of massive stars that have undergone supernova explosions, known for their remarkable density. Pulsars serve as celestial beacons, emitting beams of radio waves from their magnetic poles. As these pulsars rotate rapidly, sometimes hundreds of times per second, their beams sweep across the celestial sphere. From our vantage point on Earth, this motion manifests as rhythmic pulses of radio waves.

The radio wave pulses emitted by pulsars reach Earth with impeccable precision, akin to a meticulously timed metronome. The accuracy of their timing was so extraordinary that when Jocelyn Bell first detected these pulsar radio waves in 1967, astronomers entertained the possibility that they might originate from an extraterrestrial civilization.

When a gravitational wave traverses the space between us and a pulsar, it disrupts the timing of the radio waves. This occurs because, as foreseen by Albert Einstein, gravitational waves ripple through the cosmos, causing the stretching and compression of space. Consequently, the distance that the radio waves must travel is altered.

Over a span of 15 years, scientists from the NANOGrav collaboration, hailing from the United States and Canada, dedicatedly monitored and timed the radio wave pulses emitted by numerous millisecond pulsars within our galaxy. To accomplish this, they utilized observatories such as the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico. The recent findings stem from a meticulous analysis of an array comprising 67 pulsars.

“Pulsars are inherently faint radio sources, necessitating the allocation of thousands of hours per year on the world’s largest telescopes to conduct this experiment,” explains Maura McLaughlin from West Virginia University, who serves as the co-director of the NANOGrav Physics Frontiers Center. “These remarkable results are made possible due to the unwavering support of the National Science Foundation (NSF) in maintaining these extraordinarily sensitive radio observatories.”

Detecting the Background

In 2020, after analyzing slightly over 12 years of data, scientists from the NANOGrav collaboration began to observe subtle indications of a signal—an additional “hum” present in the timing patterns of all pulsars within the array. Now, three years have passed since then, and with further observations, they have gathered substantial evidence confirming the existence of the gravitational wave background.

“With the confirmation of gravitational waves, our next objective is to utilize these observations to investigate the sources generating this distinct signature,” states Sarah Vigeland, the chair of the NANOGrav detection working group from the University of Wisconsin-Milwaukee.

The most probable sources of the gravitational wave background are pairs of supermassive black holes locked in a spiral of mutual destruction. These black holes are truly colossal, containing a mass equivalent to billions of suns. Virtually all galaxies, including our own Milky Way, harbor at least one of these behemoths at their core. When two galaxies merge, their supermassive black holes can encounter each other, initiating an orbital dance. Over time, their orbits contract as gas and stars intermingle, siphoning off energy from the black holes. Eventually, the supermassive black holes approach a proximity where energy theft ceases. Theoretical studies have long posited that these black holes may then enter a state of perpetual stagnation when they are approximately 1 parsec apart (roughly equivalent to three light-years). This intriguing situation, known as the final parsec problem, suggests that only rare assemblages of three or more supermassive black holes result in mergers.

However, supermassive black hole pairs may possess a remarkable solution. As they orbit one another, they could emit immensely powerful gravitational waves until they finally collide in a cataclysmic finale. “Once the two black holes draw close enough to be detected by pulsar timing arrays, nothing can impede their merger, a process that takes only a few million years,” explains Luke Kelley, chair of NANOGrav’s astrophysics group from the University of California, Berkeley.

The discovery of the gravitational wave background by NANOGrav appears to lend support to this hypothesis, potentially resolving the long-standing final parsec problem. As supermassive black hole pairs arise from the mergers of galaxies, the abundance of their gravitational waves will enable cosmologists to estimate the frequency of galaxy collisions throughout the history of the universe. Mingarelli, along with postdoctoral researcher Deborah C. Good from the CCA and the University of Connecticut, and their colleagues, examined the intensity of the gravitational wave background. Their estimations suggest that hundreds of thousands, or possibly even millions, of supermassive black hole binaries reside in the universe.

Alternative Sources

While the gravitational waves detected by NANOGrav primarily align with the presence of supermassive black hole pairs, it is important to note that not all of the observed waves necessarily originate from such sources. Other theoretical concepts also propose the existence of ultra-low-frequency waves. For instance, according to string theory, cosmic strings—one-dimensional defects that may have formed in the early universe—could emit gravitational waves as they dissipate energy. Additionally, an alternative hypothesis suggests that the universe did not initiate with the Big Bang but with a Big Bounce, where a previous universe collapsed inward before expanding outward. In such a scenario, gravitational waves from this event would persistently ripple through space-time.

Moreover, there remains the possibility that pulsars, despite being considered reliable detectors of gravitational waves, may exhibit unknown variations that could influence NANOGrav’s results. Mingarelli highlights the inherent limitations in experimenting with pulsars, stating that it is impossible to intervene directly with these celestial objects to investigate any potential discrepancies.

As the NANOGrav team continues its pulsar monitoring, they aim to explore all plausible contributors to the newly discovered gravitational wave background. Their plan involves analyzing the background based on the frequency and spatial origin of the waves, thereby unraveling valuable insights into their nature and sources.

An International Effort

Fortunately, the NANOGrav team is not embarking on their quest alone. Today, various collaborations utilizing telescopes in Europe, India, China, and Australia have published several papers reporting indications of the same gravitational wave background signal in their respective data. By uniting under the International Pulsar Timing Array consortium, these groups are consolidating their data to gain a deeper understanding of the signal and to pinpoint its origins.

“The combination of our collective data will yield significantly enhanced capabilities,” asserts Stephen Taylor from Vanderbilt University, who co-led the new research and currently serves as the chair of the NANOGrav collaboration. “We are eagerly anticipating the revelations that will emerge, unveiling hidden secrets about the vastness of our universe.”

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