Discovery of Gravitational Waves through “Cosmic Clocks” and Spatial Distortions

by Liam O'Connor
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Caption for Image:
In this interpretation by Aurore Simonnet for the NANOGrav Collaboration, gravitational waves emitted by a pair of supermassive black holes (top left) create ripples in the fabric of space-time. These waves affect the paths of radio waves emitted by pulsars (white). Recent measurements of the radio waves have led scientists to detect the background of gravitational waves in the universe.

Radio telescope observations of pulsars in the Milky Way unveil spatial distortions believed to be caused by colossal gravitational waves permeating the cosmos.

Although imperceptible to sight and touch, the universe, including our own bodies, is continuously undergoing a process of expansion and contraction. A recent study conducted by the U.S. National Science Foundation’s NANOGrav Physics Frontiers Center reveals that this phenomenon is a result of gravitational waves passing through our galaxy.

The research findings, published in The Astrophysical Journal Letters, originate from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), an extensive collaboration among researchers from over 50 institutions in the United States and abroad. The team investigated millisecond pulsars, celestial remnants that rapidly rotate hundreds of times per second and emit radio pulses resembling the precise ticks of cosmic clocks. By analyzing data from radio telescopes spanning 15 years and observing over 60 pulsars, they identified variations in the pulsars’ ticking rates. These variations are attributed to low-frequency gravitational waves that distort the fabric of space-time, known as spacetime.

An illustration by Keyi “Onyx” Li/U.S. National Science Foundation showcases gravitational waves emanating from a pair of closely orbiting black holes (visible on the distant left). These waves pass near multiple pulsars and Earth (on the right).

The NANOGrav team’s study suggests that the spatial distortions caused by gravitational waves alter the appearance of the pulsars’ radio signals. In reality, it is the stretching and squeezing of space between Earth and the pulsars that leads to minute deviations in the arrival times of their radio pulses, occurring billions of seconds earlier or later than expected. These results provide the initial evidence of a gravitational wave background—a pervasive soup of spacetime distortions theorized by scientists to exist throughout the entire universe.

“The NSF NANOGrav team effectively created a galaxy-wide detector that unveils the gravitational waves permeating our universe,” remarked NSF Director Sethuraman Panchanathan. “This collaboration involving research institutions across the U.S. demonstrates how world-class scientific innovation can and does extend to every corner of our nation.”

Gravitational waves were initially theorized by Albert Einstein in 1916, but their existence was not confirmed until 2015 when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected spacetime ripples passing through Earth. However, the spatial distortions detected by LIGO resulting from the collision of two distant black holes were smaller than an atomic nucleus.

In contrast, the pulsar time shifts observed by the NANOGrav team are several hundred billionths of a second, signifying a spacetime flexing between Earth and the pulsars equivalent to the length of a football field. These spacetime distortions are induced by gravitational waves of such magnitude that the distance between two wave crests spans 2-10 light-years or approximately 9-90 trillion kilometers.

“These gravitational waves are, by far, the most potent ones known to exist,” emphasized Maura McLaughlin, an astrophysicist from West Virginia University and co-director of the NANOGrav Physics Frontiers Center. “Detecting such colossal gravitational waves necessitates an equally immense detector and unwavering patience.”

The NANOGrav team utilized 15 years’ worth of astronomical data collected by radio telescopes at NSF-supported observatories, including the Green Bank Observatory in West Virginia, the Very Large Array in Socorro, New Mexico, and the Arecibo Observatory in Puerto Rico. By creating a “detector” consisting of 67 pulsars scattered across the sky, they compared the ticking rates of various pulsar pairs. Through sophisticated data analysis, they deduced the presence of a gravitational wave background responsible for the distortion of space and subsequently explained the apparent timing variations of the pulsars.

Stephen Taylor, an astrophysicist from Vanderbilt University and chair of the NANOGrav collaboration, commented, “These are the first signs of gravitational waves at these low frequencies. The probable source of these waves is distant pairs of ultra-massive black holes orbiting closely.”

Sean L. Jones, the NSF Assistant Director for Mathematical and Physical Sciences, remarked, “There is much about the physical nature of the universe that we have yet to comprehend, and that is why the National Science Foundation supports bold team endeavors like NANOGrav—to expand our knowledge for the betterment of society.”

The team’s findings offer fresh insights into the evolution of galaxies and the growth and merging of supermassive black holes. The widespread spatial distortions unveiled in their study suggest that extremely massive pairs of black holes may be similarly widespread throughout the universe, potentially numbering in the hundreds of thousands or even millions. In the future, the NANOGrav team anticipates the ability to identify specific pairs of supermassive black holes by tracing the gravitational waves they emit, and they may even uncover traces of gravitational waves from the early universe.

“While our initial data hinted at something significant, we now understand that we are listening to the symphony of the gravitational universe,” affirmed Xavier Siemens, co-director of NANOGrav and an astrophysicist from Oregon State University. “As we continue to listen, individual instruments will emerge in this cosmic orchestra.”

For more information on this research, refer to the following publication:
“Gravitational Waves From Merging Supermassive Black Holes ‘Heard’ for First Time”
Reference: “The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background” by Gabriella Agazie et al., The Astrophysical Journal Letters, 29 June 2023.
DOI: 10.3847/2041-8213/acdac6

Frequently Asked Questions (FAQs) about gravitational waves

What are gravitational waves?

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of massive objects. They were first predicted by Albert Einstein in 1916 as a consequence of his theory of general relativity.

How are gravitational waves detected?

Gravitational waves are detected using specialized observatories equipped with extremely sensitive instruments. These instruments, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), can measure minuscule changes in the distance between objects caused by passing gravitational waves.

What are cosmic clocks?

Cosmic clocks refer to millisecond pulsars, which are highly precise celestial objects that emit regular radio pulses. These pulsars act as natural clocks in space, allowing scientists to measure any variations in their ticking rates caused by the effects of gravitational waves.

What is the NANOGrav collaboration?

The NANOGrav collaboration is a team of researchers from over 50 institutions in the United States and abroad. They work together to study gravitational waves using pulsar timing arrays and analyze data collected from radio telescopes to detect and understand the properties of these waves.

What is the significance of detecting the gravitational wave background?

Detecting the gravitational wave background provides evidence for the existence of pervasive spacetime distortions throughout the universe. It offers insights into the evolution of galaxies, the behavior of supermassive black holes, and potentially traces of gravitational waves from the early universe, contributing to our understanding of the fundamental nature of the cosmos.

More about gravitational waves

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