Breakthrough in search of slowly oscillating gravitational waves

June 29, 2023

(Nanowerk News) For the first time, astrophysicists have found convincing evidence of gravitational waves oscillating with time spans from years to decades. This is demonstrated in five articles published in Astrophysics Journal Letter (“15-Year NANOGrav Dataset: Searching for Signals from New Physics”).

For this, the researchers evaluated data that had been collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) over 15 years. Prof Kai Schmitz from the University of Münster and Dr Andrea Mitridate from DESY in Hamburg are involved in one of the research articles. This publication discusses the hypothesis that NANOGrav saw the gravitational waves generated in the Big Bang. From Germany, in addition to teams from the University of Münster and DESY, the team involved in NANOGrav are from the Max Planck Institute for Gravitational Physics in Hannover, and from the University of Mainz. Artist’s impression of how the pulsar string is affected by gravitational waves emanating from a pair of supermassive black holes from a distant galaxy. (Image: Aurore Simonnet for the NANOGrav Collaboration)

“This is key evidence for gravitational waves at very low frequencies,” said Dr. Stephen Taylor of Vanderbilt University, who co-led the search and is the current Chair of the collaboration. “After years of work, NANOGrav opens a whole new window on the gravitational wave universe.”

The NANOGrav collaboration, which brings together more than 190 researchers, observes pulsars in our Galaxy with large radio telescopes, looking for gravitational waves in the process. A pulsar is the extremely dense remnant of a massive star’s core after its destruction in a supernova explosion. The pulsar rotates rapidly, sweeping a beam of radio waves through space so that it appears to “pulse” when viewed from Earth.

“When the pulsar is oriented properly, this highly regular signal can be measured from Earth. The effect is comparable to a cone of light from a lighthouse flashing at regular intervals – except that the pulsar blinks more rapidly; and in the case of the pulsars observed by NANOGrav, they actually blink at millisecond intervals,” explained Prof. Kai Schmitz, Associate Professor at the Institute of Theoretical Physics at the University of Münster, who is a member of the NANOGrav Collaboration.

Albert Einstein’s General Theory of Relativity predicted exactly how gravitational waves should affect pulsar signals. By stretching and compressing the fabric of space, gravitational waves influence the timing of each beat in small but predictable ways, delaying some while advancing others. Deviations according to specific patterns caused by low-frequency gravitational waves now appear in data from the 68 pulsars observed – data the Collaboration has amassed over 15 years of research work.

Previous results from NANOGrav have actually uncovered mysterious time signals that are common to all observed pulsars. However, it was too weak to draw any conclusions about its origins.

The new NANOGrav data set shows growing evidence for gravitational waves with periods of years to decades. These waves could arise from the most massive pair of orbiting black holes in the entire Universe: billions of times more massive than the Sun, with a size greater than the distance between Earth and the Sun. From the superposition of signals from many individual pairs of black holes, a diffuse background noise of gravitational waves is generated. Future study of this signal will open a new window on the gravitational wave universe, providing insight into titanic black holes merging at the hearts of distant galaxies, among other exotic sources.

Background: Unlike low-frequency gravitational waves that can only be detected via pulsars, transient high-frequency gravitational waves can be observed using ground-based instruments such as LIGO (Laser Interferometer Gravitational-Wave Observatory). Rainer Weiss, Barry Barish, and Kip Thorne were awarded the Nobel Prize in Physics in 2017 for the first direct measurement of high-frequency gravitational waves with the LIGO detector in 2015. The new NANOGrav results now open up a new frequency band within the gravitational wave spectrum – the band that corresponds to the frequency band LIGO is what long radio waves do to visible light in the electromagnetic spectrum. In addition, NANOGrav does not track transient gravitational waves, but rather continuous background noise that reaches Earth continuously and from all directions.

The publication of the NANOGrav results has been coordinated with a team of researchers working on gravitational waves around the world; each of these teams also presented their new results on June 29. In addition to the NANOGrav, this is a further so-called collaborative pulsar timing array from Australia, China, Europe and India which is jointly hosted on the International Pulsar Timing Array (IPTA).

The signal seen by NANOGrav may also be receiving cosmological contributions in the form of gravitational waves from the early universe. This hypothesis was examined in detail in one of five published studies led by Kai Schmitz and Dr. Andrea Mitridate, postdoc at DESY in Hamburg. “In our work,” Andrea Mitridate explains, “we looked at the possibility that NANOGrav saw gravitational waves generated a fraction of a second after the Big Bang – rather than signals of astrophysical origin emitted by giant black holes orbiting each other. in the galactic centers.” Such a primordial background of gravitational waves should be seen as the gravitational counterpart of the cosmic microwave background – the “leftover light” from the Big Bang that was discovered in the 1960s.

“Many theories related to new physics beyond the Standard Model of particle physics predict the generation 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 that, “In this sense, the NANOGrav data allows us to examine new physics models at energies that are unattainable in laboratory experiments on Earth.” However, further studies are needed to determine which argument will ultimately prevail through: an astrophysical interpretation in the form of a supermassive black hole binary system, or a cosmological interpretation in the form of the gravitational waves from the Big Bang.

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