Unexpectedly Intense: First Detection of Gravitational Waves from Merging Supermassive Black Holes

After 15 years of dedicated pulsar observations, the NANOGrav collaboration has accomplished a remarkable feat by detecting gravitational waves of unprecedented strength, likely originating from pairs of supermassive black holes. This groundbreaking discovery not only provides the initial evidence for the existence of a gravitational wave background but also reveals that it is remarkably louder than anticipated. The amplified signals indicate the potential presence of an abundance of supermassive black holes or alternative sources of gravitational waves.

Over the course of 15 years, scientists from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) meticulously monitored pulsars, acting as celestial metronomes, scattered throughout our galaxy. Finally, they have “heard” the continuous symphony of gravitational waves rippling through the fabric of our universe, representing the most powerful measurements ever recorded. These waves carry energy approximately a million times greater than the sporadic bursts observed from black hole and neutron star mergers by experiments such as LIGO and Virgo.

The recently detected gravitational waves, illustrated in an artist’s interpretation, emanate from a duo of supermassive black holes emitting ripples in the space-time continuum. These gravitational waves cause the paths of radio waves emitted by pulsars to compress and stretch. By carefully analyzing these radio waves, the scientific team has achieved the remarkable feat of identifying the universe’s gravitational wave background. This momentous discovery marks the inauguration of a new era of observations into the cosmos.

The gravitational wave background, a long-theorized concept, holds significant promise for unraveling enigmas that have persisted for ages, including the fate of supermassive black hole pairs and the frequency of galaxy mergers. However, the unexpectedly intense nature of the gravitational wave background, measuring twice the projected strength, has left researchers puzzled. This heightened intensity might be attributed to experimental constraints or the presence of more massive and abundant supermassive black holes. Alternatively, it opens the door to the possibility of other formidable gravitational wave sources, such as those proposed by string theory or alternative theories concerning the birth of the universe. As Dr. Chiara Mingarelli, a NANOGrav scientist, asserts, “This is just the beginning. What’s next is everything.”

Achieving this feat was no small task for the NANOGrav team. The gravitational waves they pursued differ substantially from those previously observed. Unlike the high-frequency waves captured by terrestrial instruments like LIGO and Virgo, the gravitational wave background consists of ultra-low-frequency waves. The passage of a single rise and fall of these waves can take years or even decades. Considering that gravitational waves propagate at the speed of light, a single wavelength can span tens of light-years.

Detecting such colossal waves on Earth is unfeasible, prompting the NANOGrav team to turn their attention to the stars. They meticulously studied pulsars, rapidly spinning neutron stars emitting focused beams of radio waves. These beams sweep across the sky like cosmic lighthouses, resulting in rhythmic pulses of radio waves from our perspective on Earth.

The precise timing of these pulses, akin to a perfectly synchronized metronome, captivated astronomers when the first pulsar radio waves were recorded by Jocelyn Bell in 1967. The presence of a gravitational wave between us and a pulsar distorts the timing of the radio waves. According to Albert Einstein’s predictions, gravitational waves alter space by stretching and compressing it as they traverse the cosmos, subsequently affecting the distance traveled by radio waves.

For 15 years, NANOGrav scientists meticulously timed radio wave pulses from numerous millisecond pulsars in our galaxy, utilizing observatories such as the Arecibo Observatory in Puerto Rico, the Green

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More about Gravitational waves

• NANOGrav Collaboration
• The Astrophysical Journal Letters – “The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background”
• LIGO – Laser Interferometer Gravitational-Wave Observatory
• Virgo Collaboration