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Study of supermassive black hole binaries places limit on the strength of gravitational waves

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Posted April 7, 2014
This news or article is intended for readers with certain scientific or professional knowledge in the field.

The attempts of direct gravitational waves detection are ongoing in the major global centers of astrophysics. One of such initiatives is the North American Nanohertz Observatory for Gravitational Waves, or the NANOGrav project, which currently monitors and analyzes the data from 43 different pulsars using the Green Bank and Arecibo radio telescopes. The scientists participating in this project have not reported any evidence for the presence of a detectable continuous gravitational waves yet; however, this scientific venture yielded some other interesting results related to the physics of gravitational forces.

A graphic representation of two black holes in a binary system generating gravitational waves. Credit: CSIRO

A graphic representation of two black holes in a binary system generating gravitational waves. Credit: CSIRO

The NANOGrav team currently analyzes the precise timing of an array of so-called millisecond pulsars (MSPs) that are most sensitive to gravitational waves in the 10-9 – 10-7 Hz, or nanohertz frequency range. According to the scientists, the gravity waves of such frequency could be theoretically emitted by supermassive black hole binary systems, among other sources such as cosmic strings or events caused by inflation of the universe. Despite the fact that no successful detection of continuous waves has yet been made, the astrophysicists used the data accumulated over a 5-year period of observations to constrain the upper limit of the strength of gravitational waves.

Linearly polarised gravitational wave: relative change of distance between points corresponds to the amplitude of the wave. Credit: Raoul NK/Wikimedia Commons

Linearly polarised gravitational wave: relative change of distance between points corresponds to the amplitude of the wave. Credit: Raoul NK/Wikimedia Commons

In total a subset of 17 pulsars, which are believed to be related to binary supermassive black holes orbiting in circular orbits, was selected for the theoretical analysis. The authors of the study employed existing standard pulsar timing models together with time and pulse-shape parameters – the same that are used for gravitational waves observation. They performed statistical computations to find out the probability distributions for the data with an existing set of potential gravitational signal parameters according to this information. The statistical models showed that 95% upper limit of the gravitational wave induced strain amplitude could be placed approximately at h≤ 3.8*10-14 at a frequency of 10 nHz.

What does this magnitude mean? The value of the mentioned parameter – the amplitude of a gravitational wave – is a relative quantity, related to how much gravitational waves change the otherwise ‘normal’ distance between points in spacetime. In simple terms, if you have two or more free particles and monitor the spacetime interval between them, any gravitational wave will change that interval as it passes through them (see the animation above). The value of an order 10-14 is really small to most of us (i.e. equivalent 10-12 % change in spacetime), but from scientific point of view this upper limit is even three times larger compared to the previously published studies. Some additional estimates for comparison purposes are given in the table below.

Some gravitational strain amplitude estimates (for supernova, and neutron star and massive black hole binary systems). Here H denotes amplitude, f - wave frequency, M - mass of the object that generates waves, R - radius of the object, r - radius of particular binary system. Source

Some gravitational strain amplitude estimates (for supernova, and neutron star and massive black hole binary systems). Here H denotes amplitude, f – wave frequency, M – mass of the object that generates waves, R – radius of the object, r – radius of particular binary system. Source

Most likely, the results of the current study will not provide any practical outcomes anytime soon. Nonetheless, at least a part of such knowledge is an important consideration in development of gravitational wave detectors, not mentioning the fact that it is simply interesting to know how colossal forces of nature may influence the physics of the universe we live in.

The study has been published on arXiv.org.

By Alius Noreika, source: www.technology.org

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