On the trail of ultra-low gravitational waves (Astronomy)

Analyzed in detail a promising signal that could be due to the so-called gravitational wave background produced by the gravitational energy released by pairs of supermassive black holes in mutual approach. The study, which was also attended by researchers from INAF, represents a step forward on the road to the detection of gravitational waves of very low frequency, of the order of one billionth of a Hertz.

The collaboration Epta (European Pulsar Timing Array) published today in Monthly Notices of the Royal Astronomical Society an article in which the detailed analysis of a promising signal is reported that could be due to the so-called gravitational wave background ( Gwb ), due to astronomers around the world have been chasing for some time, produced by the gravitational energy released by pairs of supermassive black holes as they approach each other, eventually leading them to merge. The results of the study were made possible thanks to the pulsar data collected, in twenty-four years of observations, with five large-aperture European radio telescopes – including the 64-meter-diameter Sardinia Radio Telescope ( Srt ), located near Cagliari.

The beams of radiation emitted by the magnetic poles of pulsars rotate with the star, and we observe them as radio pulses as they cross our line of sight, like the beams of light from a distant beacon. Pulsar Timing Arrays ( Pta ) are made up of an array of pulsars that have a very stable rotation, and for this property they are used as gravitational wave detectors on a galactic scale. In the presence of a gravitational wave, space-time is in fact deformed and the very regular cadence of the radio pulses of a pulsar is therefore in turn altered. PTAs are sensitive to very low frequency gravitational waves, in the one-billionth hertz regime: a gravitational wave of this type makes a single oscillation in about 30 years.

The Pta are therefore able to widen the window of observability of gravitational waves, currently limited only to the high frequencies (of the order of hundreds of hertz), which are studied by the detectors on the ground Ligo, Virgo and Kagra. These instruments are able to pick up the gravitational signals generated in short-lived collisions involving stellar-mass black holes and neutron stars, while PTAs can detect gravitational waves produced by binary systems of supermassive black holes located in the center of galaxies. during their slow spiraling motion of mutual approach. The cumulative effect of the signals produced by this population of extreme celestial objects is, in fact, the background of gravitational waves.

“The presence of a gravitational wave background”, explains Andrea Possenti , researcher at INAF in Cagliari and co-author of the work, “manifests itself in the form of very low frequency fluctuations in the rhythm of radio pulses coming from all pulsars, a sort of Additional “noise” that disturbs the regular pulse pattern, which we could otherwise compare to the ticking of a very precise clock. Speaking in a very simplified way, an experiment such as the one conducted by Epta therefore consists in the repeated observation of the array of pulsars, every few weeks and for many years, in search of a very low frequency “noise” that afflicts their ticking in a common way. all pulsars, and that it is not attributable to causes other than gravitational waves ».

Artist’s impression of the Epta experiment. A group of European radio telescopes observed a network of pulsars spread across the sky. The variation recorded in the time of arrival on Earth of the radio pulses emitted by these celestial bodies allows astronomers to study the smallest perturbations of space-time. These perturbations, called gravitational waves, propagate relentlessly from the most remote and therefore oldest borders of the universe, as the first galaxies merged with each other and the supermassive black holes housed in their central regions orbited each other. and they produced them © INAF

In fact, the expected amplitude of the “noise” due to the gravitational background is incredibly small, from a few tens to a couple of hundred billionths of a second of advance or delay in the arrival times of the radio pulses: in principle many other effects they could induce a similar “noise”. In order to reduce the role of the other sources of perturbation and validate the results, the analysis of the data collected by Epta’s measurements therefore made use of two completely independent procedures, with three different modeling of the corrections due to the bodies of the Solar System, and adopting different statistical treatments. This allowed the team to pinpoint a clear signal that could potentially be identified as belonging to the gravitational wave background. Particularly,

“Epta had already found indications of the presence of this signal in the data set published in 2015,” recalls Nicolas Caballero , researcher at the Kavli Institute for Astronomy and Astrophysics in Beijing and lead co-author of the publication. “Since the results were then affected by large statistical uncertainties, they were strictly discussed only as upper limits for the amplitude of the signal. Our new data now clearly confirms the presence of this signal common to all pulsars, making it a candidate for the gravitational wave background. ‘

The general relativity Einstein predicts very specific relationship between the deformation of space-time experienced by radio signals from pulsars located in different directions in the sky. Scientists call this the “spatial correlation” of the signal. Its detection will uniquely identify the observed noise as due to a gravitational wave background. “At the moment, the statistical uncertainties in our measurements do not yet allow us to identify the presence of the predicted spatial correlation for the background signal of gravitational waves. To confirm the nature of the signal, ”explains Siyuan Chen , researcher at Lpc2Eof the French CNRS in Orleans, first author of the study, “we therefore need to include more pulsar data in the analysis. However, we can say that the current results are very encouraging ».

The Cagliari team that participated in the study. From left: Andrea Possenti, Marta Burgay and Delphine Perrodin. Credits: G. Alvito, P. Soletta / Inaf Cagliari

Epta is a founding member of the International Pulsar Timing Array ( Ipta ). Since the independent data analyzes performed by the other IPTA partners – i.e. the NanoGrav and Ppta experiments – have also indicated the presence of a similar signal, the IPTA members are working together to better prepare the next steps, thanks to progress. obtained by comparing all their data and methods of analysis.

“As it was for high frequency gravitational waves in 2015, the detection of very low frequency gravitational waves would be an epochal achievement for physics, astrophysics and cosmology”, concludes Delphine Perrodin, researcher at the INAF of Cagliari and co-author of the work. «In particular, the discovery and study of the gravitational wave background will give us direct information on the size and evolution of supermassive black holes, and on their contribution in shaping galaxies and the current universe. A challenge in which INAF has been immersed since 2006, the year of the birth of the Epta collaboration, and which now makes use of the asset represented by Srt and its involvement as part of Leap, the Large European Array for Pulsars, in which the Epta’s telescopes work in a synchronized way to reach the capabilities of a single 200 meter diameter antenna, and thus greatly increase Epta’s sensitivity to gravitational waves ».

Featured image: The five large European radio telescopes used in this study. From top left: The Effelsberg radio telescope in Germany, the Nancay radio telescope in France, the Sardinia Radio Telescope in Italy, the Westerbork Synthesis Radio Telescope in the Netherlands and the Lovell Telescope in the United Kingdom © INAF

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