Breakthrough in the search for slowly oscillating gravitational waves

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Astrophysicists from the NANOGrav consortium have for the first time found convi
Astrophysicists from the NANOGrav consortium have for the first time found convincing evidence of gravitational waves at very low frequencies. This artist’s rendering shows how a series of pulsars are affected by gravitational waves originating from a pair of supermassive black holes in a distant galaxy. © Aurore Simonnet for the NANOGrav Collaboration

Data from 15 years provide first convincing evidence for the existence of low-frequency background noise from gravitational waves in the universe / Physicist Kai Schmitz from the University of Münster involved in consortium

Astrophysicists have for the first time found convincing evidence for the existence of gravitational waves that oscillate with periods ranging from years to decades. This is according to five papers published June 29 in The Astrophysical Journal Letters. To do so, the researchers evaluated 15 years of data collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Kai Schmitz of the University of Münster and Andrea Mitridate of DESY in Hamburg are involved in one of the research articles. This publication deals with the hypothesis that NANOGrav sees gravitational waves generated in the Big Bang. In addition to the team from the University of Münster and DESY, the NANOGrav consortium also includes researchers from the Max Planck Institute for Gravitational Physics in Hannover and the University of Mainz.

"This is important evidence for gravitational waves at very low frequencies," emphasizes Stephen Taylor of Vanderbilt University, who co-led the search and currently chairs the collaboration. "After years of work, NANOGrav opens a new window on the gravitational wave universe."

The NANOGrav consortium, an association of more than 190 scientists, observes pulsars in our galaxy with large radio telescopes, searching for gravitational waves. A pulsar is the extremely dense remnant of the core of a massive star after its life has ended in a supernova explosion. Pulsars spin rapidly and send beams of radio waves through space, so they appear to pulsate as seen from Earth. "If the pulsar is oriented correctly, this very regular signal can be measured from Earth. You can compare the effect to the light cone of a lighthouse that flashes at a certain rate - except that pulsars flash much faster; in the case of the pulsars observed by NANOGrav, even at millisecond intervals," illustrates Kai Schmitz, assistant professor at the Institute for Theoretical Physics at the University of Münster, who is a member of the NANOGrav consortium.

Albert Einstein’s General Theory of Relativity predicts exactly how gravitational waves should affect pulsar signals. By stretching and compressing the structure of space, gravitational waves affect the arrival time of each pulse in a small but predictable way, delaying some pulses and allowing others to reach Earth sooner. Deviations following a specific pattern that can be attributed to slowly rippling (low-frequency) gravitational waves are now emerging in data from 68 observed pulsars compiled by the consortium over 15 years of research. Earlier results from NANOGrav had already revealed a puzzling signal in the measured time series that was common to all observed pulsars. But it was too weak to draw any conclusions about its origin.

The latest data set from NANOGrav now shows increasingly clear evidence of gravitational waves with periods ranging from years to decades. These waves could emanate from orbiting pairs of the most massive black holes in the entire universe: They are billions of times more massive than the Sun and larger than the distance between Earth and the Sun. Superposition of the signals from many individual black hole pairs results in a diffuse gravitational wave background noise. Future studies of this signal will open a new window on the gravitational-wave universe and, among other things, provide insights into the merger of giant black holes in distant galaxies.

Background: In contrast to low-frequency gravitational waves, which can only be detected with pulsars, volatile high-frequency gravitational waves can be observed by ground-based instruments such as LIGO (Laser Interferometer Gravitational-wave Observatory). The first direct measurement of high-frequency gravitational waves with the LIGO detector in 2015 earned Rainer Weiss, Barry Barish, and Kip Thorne the 2017 Nobel Prize in Physics. The new NANOGrav results now open up a new frequency band in the gravitational wave spectrum that is related to the LIGO frequency band in the same way that long-wavelength radio waves in the electromagnetic spectrum are related to visible light. Moreover, NANOGrav is not tracking fleeting gravitational waves, but a continuous background noise that reaches the Earth permanently and from all directions.

The publication of NANOGrav’s results is coordinated with gravitational wave research teams from around the world, each of which will also present new results on June 29. In addition to NANOGrav, these are other so-called pulsar timing array consortia from Australia, China, Europe, and India, which are organized together in the International Pulsar Timing Array (IPTA).

Contribution of Kai Schmitz and his group (University of Münster) and Andrea Mitridate (DESY) to the NANOGrav publications of June 29.

The signal seen by NANOGrav could also receive a cosmological contribution in the form of gravitational waves from the early universe. This hypothesis is explored in detail in one of the five studies now published, which Kai Schmitz led with Andrea Mitridate, a postdoctoral researcher at DESY in Hamburg. "In our work," Andrea Mitridate elaborates, "we take a close look at the possibility that NANOGrav sees gravitational waves generated in the Big Bang - instead of a signal of astrophysical origin emitted by giant black holes orbiting each other in the center of galaxies." Such a primordial gravitational wave background should be seen as the gravitational counterpart to the cosmic microwave background - the "afterglow" of the Big Bang discovered in the 1960s.

"Many theories of new physics beyond the Standard Model of particle physics predict the origin of gravitational waves in the Big Bang, including phenomena such as cosmic inflation, cosmological phase transitions, or so-called cosmic strings," explains Kai Schmitz. Andrea Mitridate adds, "In this sense, the NANOGrav data allow us to study models of new physics at energies that are unattainable in laboratory experiments on Earth." However, further research is needed to determine whether the astrophysical interpretation in the form of binary systems of extremely massive black holes or the cosmological interpretation in the form of gravitational waves from the Big Bang will ultimately prevail.

Research Funding

NANOGrav’s work has received funding from the U.S. National Science Foundation (NSF) Physics Frontiers Center (award numbers 1430284 and 2020265), the Gordon and Betty Moore Foundation, NSF (AccelNet award number 2114721), a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Institute for Advanced Research (CIFAR). The Arecibo Observatory is an NSF facility. It is operated under a cooperative agreement (#AST-1744119) by the University of Central Florida (UCF) in collaboration with Universidad Ana G. Méndez (UAGM) and Yang Enterprises (YEI), Inc. The Green Bank Observatory and the National Radio Astronomy Observatory are NSF facilities operated under cooperative agreements through Associated Universities, Inc.

Original publication with participation of the working groups of the University of Münster and DESY.

Adeela Afzal et al./ The NANOGrav Collaboration (2023): The NANOGrav 15-year Data Set: Search for Signals from New Physics. The Astrophysical Journal Letters, Volume 951, Number 1; DOI: 10.3847/2041-8213/acdc91.