2023-07-01 09:01:01
Astronomers and astrophysicists from five different Pulsar Synchronization Network (PTA, for pulsar timing array) presented data this week that strongly suggests the presence of background noise fromgravitational waves : a constant murmur of low-frequency ripples in spacetime that emanates from some of the most unusual objects in the universe.
Header image: Dead stars called pulsars (shown here) emit beams of radio waves that sweep across the Earth like clockwork. Gravitational waves emitted by pairs of supermassive black holes (top left) are thought to vibrate the fabric of spacetime and alter the timing of pulsars. (Aurore Simonet/ Collaboration NANOGrav)
This discovery confirms previous hypotheses drawn from pulsar synchronization data, that a low-frequency signal from the most powerful gravitational sources in the universe, most likely supermassive black holes doomed to collide, permeates the cosmos.
Animation of pulsars rotating near the Earth. Earth receives light from pulsars that reveal gravitational waves from binary supermassive black holes. (OzGrav)
These new results were obtained by the Chinese Pulsar Timing Array (CPTA), the European Pulsar Timing Array (EPTA), the Indian Pulsar Timing Array (InPTA), the Parkes Pulsar Timing Array (PPTA) and the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). These last four collaborations collectively constitute the International Pulsar Timing Array (IPTA), and although the “consortium of consortia” has already published data twice, it was not involved in this week’s publications (link more down).
Supermassive black hole binaries, i.e. pairs of these incredibly massive objects that orbit each other for hundreds of millions of years and eventually merge in one of the most extremes of the universe, are the main candidates for the origin of a background gravitational wave background (GWB, for gravitational wave background). Although predicted, observations have never confirmed the existence of supermassive black hole binaries.
Artistic representation of two colliding black holes. (Mark Myers/ ARC Center of Excellence for Gravitational Wave Discovery (OzGrav))
The expected signal is the “background random ocean” of these gravitational waves, which is the sum of the waves coming from all binary supermassive black holes in the universe. The observation of this background of gravitational waves has important consequences which make it possible to better understand the history of the formation of our universe, because supermassive black holes are in some way the engines at the heart of galaxies.
THE gravitational waves were first predicted by Einstein in his theory of general relativity. According to Einstein’s description, waves are changes in the gravitational field that travel at the speed of light. Indeed, gravitational waves emerge from the seismic interactions of the most massive and compact objects in the universe. When black holes orbit or collide with each other, or with other very dense objects like neutron starsgravitational waves are produced by the interaction.
Black holes are extremely dense regions of spacetime, with gravitational fields so strong that not even light can escape. THE neutron stars are extremely old stellar remnants, so dense that the electrons revolving around the atoms that compose them have collapsed on the protons of the latter, making the whole star a large neutron. The detection of a merger between a black hole and a neutron star was first confirmed in 2021.
Gravitational waves were first detected in 2015, a century following they were predicted by Einstein, by the Laser Interferometer’s Gravitational-Wave Observatory (or LIGOwhich is now part of the collaboration LIGO–Virgo–KAGRA). LIGO detected ripples in spacetime by very precisely measuring the distance between mirrors located in underground tunnels in Washington and Louisiana.
As gravitational waves pass through Earth, they very slightly alter the distances between LIGO’s mirrors, measured using a laser, indicating that spacetime itself has been squashed or stretched.
Diagram of the LIGO experimental device (LIGO)
But the gravitational wave background is a much more subtle signal than the gravitational waves detected by LIGO. The gravitational waves detected by LIGO come from the mergers (a polite term for violent closeness) of stellar-mass black holes, which are exactly what they sound like: star-sized black holes.
Although a much quieter signal than that emitted by stellar-mass black hole mergers, the gravitational wave background would come from nature’s most massive objects: supermassive black holeswhose mass is equivalent to billions of times that of our sun, which revolve around each other in an ultimately fateful attraction.
According to Luke Kelley, an astrophysicist at the University of California at Berkeley and a member of the collaboration NANOGrav :
No examples of these binaries have been confirmed by electromagnetic studies, although there are many compelling candidates. The properties of the gravitational wave background that we are measuring are quite consistent with theoretical expectations for supermassive black hole binaries. At the same time, however, they are also compatible with the new physics.
We can think of LIGO’s gravitational waves as great waves in the cosmic ocean; to continue the analogy, the constant subtle and dynamic ripples of this ocean are equivalent to the background noise of gravitational waves. The best way to observe this ocean is to use the pulsars, rapidly spinning neutron stars that beam light pulses toward Earth with remarkable temporal reliability. Pulsars act as cosmic beacons to spot the background noise of gravitational waves.
Diagram showing light from pulsars traveling towards Earth amidst a sea of gravitational waves (waves). (NANOGrav/ T. Klein)
Just like a group of antennas of radiotélescopes can form a network, a large telescope, radio wave data from a group of pulsars can be put together to form a galaxy-sized network. According to a statement from the National Science Foundation (NSF), gravitational waves detected by pulsar synchronization networks can be as long as 10 light years (90 trillion kilometers) from peak to peak.
Representation of a pulsar synchronization network (PTA, for pulsar timing array). Earth receives light from pulsars that reveal gravitational waves from binary supermassive black holes. (OzGrav)
The results were published simultaneously in several studies (links below with the same indications). The NANOGrav set covers evidence for the existence of background noise, pulsar observations, characterization of the NANOGrav detector, and work exploring what new physics might be in the team’s data. The results of the CPTA team and the results of the PPTA team have been and are expected to be published in two journals.
The 12.5 year dataset of NANOGrav, published in 2021, was a compelling clue to the existence of gravitational wave background noise, but the new data (their 15-year dataset) includes evidence for the spatial correlations that accompany a gravitational wave signal. This increases the likelihood that the signal they are observing is real.
So far, the researchers are seeing a signal that is basically the same across the sky. As the sensitivity of the devices increases, they will begin to see how the signal is distributed across the sky. This distribution would reveal background noise hotspots, or regions where gravitational-wave background noise sources are particularly “noisy.” This might be due to their mass or their proximity to human detectors.
The researchers add that as the supermassive black hole binaries get closer, their sinusoidal signal of gravitational waves becomes more of a “chirp” to which pulsar synchronization networks are not sensitive. However, the individual binary systems make it possible to determine whether the source of the gravitational wave observed by the PTAs (reminder: pulsar synchronization networks) comes from the binaries rather than from another cosmological source, such as thecosmic inflation.
To detect “chirps” of supermassive black holes regarding to merge, Simon said astronomers will need the Laser Interferometer Space Antenna, or LISA. It is a space mission planned by the European Space Agency (ESA) which will consist of three probes orbiting each other, forming a triangle in space 2.41 million kilometers on a side.
Illustration de la mission LISA (NASA)
According to the researchers:
Our experiments are one of the only ways to find direct evidence for the existence of pairs of supermassive black holes that will eventually merge. By studying these gravitational waves, we can understand in more detail how galaxies have merged throughout cosmic history.
Whether supermassive black hole binaries are solely responsible for the apparent gravitational-wave background noise, or whether they are not, due to new physics, new data is sure to shake the cosmos. The rumbles of the universe’s most imposing objects are finally coming to light, as scientists have managed to create observatories from the stars themselves.
The results were published simultaneously in several studies. The NANOGrav set deals with evidence for the existence of background noise: The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Backgroundobservations of pulsars: The NANOGrav 15 yr Data Set: Observations and Timing of 68 Millisecond Pulsarscharacterization of the NANOGrav detector: The NANOGrav 15 yr Data Set: Detector Characterization and Noise Budget and work exploring what the new physics in the team’s data might look like: The NANOGrav 15 yr Data Set: Search for Signals from New Physics.
The results of the CPTA team were published in the journal Research in Astronomy and Astrophysics: Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar Timing Arrayand the results of the PPTA team have been published in The Astrophysical Journal Letters and are expected to be published in the Publications of the Astronomical Society of Australia available for preprint in arXiv: The Parkes Pulsar Timing Array Third Data Release.
Featured on the NANOGrav Collaboration website: Scientists use Exotic Stars to Tune into Hum from Cosmic Symphony and some National Science Foundation via Eurekalert : Gravitational waves from colossal black holes found using ‘cosmic clocks’.
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