Astrophysicists present first evidence of gravitational wave ‘background’

Wed, 06/28/2023
A quintet of new studies offers the first direct evidence of a gravitational wave background, confirming standard models of black hole growth.
By Jim Shelton

Researchers have found the first direct evidence of a “background” of gravitational waves in the universe — a sign that gravitational waves from slowly merging pairs of supermassive black holes, or possibly from the early universe, can be detected from Earth in a background field of low-frequency energy.

The discovery, made by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), helps confirm the accuracy of standard models of galaxy formation and black hole growth.

NANOGrav scientists, including Yale’s Chiara Mingarelli, published the discovery in a quintet of new studies June 29 in The Astrophysical Journal Letters.

These are big questions we’re trying to answer about how the universe evolved,” said Mingarelli, an assistant professor of physics in Yale’s Faculty of Arts and Sciences. “I’ve been interested in helping to find these answers since I was a kid. It’s awe-inspiring.”

Gravitational waves are ripples in the fabric of space-time, which can be caused by the merging of two black holes. Albert Einstein predicted the existence of gravitational waves in 1915 as part of his general theory of relativity. A century later, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first observation of gravitational waves.

LIGO scientists, however, were only able to detect waves from the higher frequency end of the gravitational wave spectrum. In order to look for waves at the lower end of the gravitational wave spectrum — such as more distant and powerful waves from supermassive black hole collisions — a different detection method was necessary.

The NANOGrav project, which began in 2007 and includes more than 170 researchers from more than 70 institutions, spearheaded a detection method centered around pulsars.

Pulsars are rapidly rotating neutron stars — the collapsed cores of massive stars that have exploded. Pulsars send out radio emissions that can be timed to the millisecond, making each one something of an ultra-accurate, cosmic clock.

With help from several ground-based telescopes in the United States and Canada, NANOGrav created a network of precisely timed pulsars, which allows NANOGrav researchers to measure and track previously undetected gravitational waves at low frequencies as they make their way to Earth. Gravitational waves, as they wash over the pulsars, alter the distances between the pulsars in the array and the Earth — meaning the normally-stable pulsar signals reach Earth early and then late.

Gravitational waves leave an indelible but challenging-to-detect signal within pulsar timing signals.

The five new studies elucidate the detection of the gravitational wave background field, the data that NANOGrav collected, a characterization of the data, tests of fundamental physics, and an astrophysical interpretation of the data in terms of supermassive black hole physics, respectively.

The findings “open a tantalizing new window — the gravitational wave window — and offer a first glimpse into the population of supermassive black holes,” said Priyamvada Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy and professor of physics in Yale’s Faculty of Arts and Sciences and chair of the Department of Astronomy, who is a co-author of the detection study.

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