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Jan 28, 2022
Several teams hope to use pulsars in the Milky Way to detect ripples in space-time made by distant supermassive black holes.
Image above: Supermassive black holes orbiting each other very closely are expected to produce gravitational waves. Image Credits: NASA’s Goddard Space Flight Center/Science Photo Library.
Astronomers could be on the verge of detecting gravitational waves from distant supermassive black holes — millions or even billions of times larger than the black holes spotted so far — an international collaboration suggests. The latest results from several research teams suggest they are closing in on a discovery after two decades of efforts to sense the ripples in space-time through their effects on pulsars, rapidly spinning spent stars that are sprinkled across the Milky Way.
Gravitational-wave hunters are looking for fluctuations in the signals from pulsars that would reveal how Earth bobs in a sea of gravitational waves. Like chaotic ripples in water, these waves could be due to the combined effects of perhaps hundreds of pairs of black holes, each lying at the centre of a distant galaxy.
So far, the International Pulsar Timing Array (IPTA) collaboration has found no conclusive evidence of these gravitational waves. But its latest analysis — using pooled data from collaborations based in North America, Europe and Australia — reveals a form of ‘red noise’ that has the features researchers expected to see. The findings were published on 19 January in Monthly Notices of the Royal Astronomical Society (1).
“This is a major milestone,” says Michael Kramer, an astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany, who is a leading member of the European team. Although it does not yet constitute a gravitational-wave detection, it is a necessary step towards one, he adds. If the red noise had not been seen at this stage, cosmologists might have had to reconsider their predictions for the prevalence of supermassive black holes and their role in the evolution of the Universe.
Xavier Siemens, a radio astronomer at Oregon State University in Corvallis and a leader of the North American group, agrees that the red noise is not yet a detection. “But it’s reassuring,” he says.
Beyond LIGO
The first direct detection of gravitational waves was achieved in 2015 by the Laser Interferometry Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state. LIGO’s twin antennas measured waves produced in the final moments of the merger of two black holes, each with a mass tens of times that of the Sun. Since then, LIGO and its Italy-based counterpart Virgo have spotted dozens of similar bursts. Those waves peak in frequency at tens to thousands of cycles per second — similar to the lower frequencies of audible sound — and can be sensed for several seconds or, in some cases, minutes.
The IPTA collaboration’s pulsar technique aims to detect longer-lasting gravitational waves that oscillate at much lower frequencies, measured in cycles per year or even per decade (see ‘The gravitational-wave spectrum’). These signals would typically originate from pairs of black holes that orbit each other long-term, rather than from mergers. “This is different from LIGO burst events, where the event occurs very quickly and that particular event will not reoccur,” says radio astronomer George Hobbs at the Australia Telescope National Facility in Epping.
Astrophysicists think that most large galaxies have a supermassive black hole at their centre. When two galaxies merge, their central black holes eventually sink to the centre of the newly formed galaxy and begin to orbit each other. If they get close enough, they will emit intense gravitational waves.
The pulsar technique looks for these gravitational waves as they sweep through our Galaxy, stretching and compressing the space that separates the Solar System from spinning neutron stars called pulsars (see ‘Pulsars as detectors’). Observatories such as LIGO, by contrast, detect gravitational waves as they sweep Earth.
The approach has unique challenges. Whereas LIGO directly measures minute changes in the distance between two mirrors several kilometres apart, changes in the distance between Earth and a pulsar cannot be measured directly, in part because thousands of gravitational-wave crests and troughs are propagating between them. Earth and the pulsar “are not riding the same crest or trough”, explains Maura McLaughlin, an astronomer at West Virginia University in Morgantown who is a leading member of the North American pulsar collaboration. “To estimate the delay, we have to care about the gravitational waves’ effect on the pulsar and on Earth. The stuff in between cancels out,” says McLaughlin.
Such changes should be revealed because, when local space is stretched, the periodic signals from a pulsar will take tens of nanoseconds more or less to reach Earth than they would have otherwise.
Noisy signals
Measuring these delays requires decades of painstaking data gathering, followed by weeks of number crunching on a supercomputer. And it relies on the bizarre physics of the neutron stars known as pulsars.
Many neutron stars — collapsed cores of stars that pack a mass greater than that of the Sun into a sphere just 20 kilometres or so across — spew radiation from their magnetic poles. As a neutron star spins, the beam of radiation circles around like the rotating light of a lighthouse. Some of these beams happen to cross Earth’s path through space, and are detected as radiation pulsating at regular intervals. In the late 1970s, some astronomers pointed out that because they appear at highly regular intervals, some of these beacons could serve as detectors for gravitational waves.
Image Creditd: Nik Spencer/Nature; Milky way: NASA/JPL-Caltech/R. Hurt (SSC/Caltech). (Click on the image for enlarge).
But pulsar signals are noisy, and can be slowed or scattered by clouds of interstellar electrons. To overcome this issue, astronomers must compare the signals from as many of these beacons as possible, forming a ‘pulsar timing array’.
And the baseline position of the Solar System’s centre of mass — which is affected by the motions of the planets — must be calculated to a precision of less than 100 metres.
In the past decade, those estimates have improved greatly thanks to measurements of Jupiter and Saturn’s positions made by NASA’s Juno and Cassini missions. The revisions have reassured some astronomers: earlier, less-precise measurements, together with some overly conservative assumptions, had made some worry that the expected gravitational-wave background wasn’t there.
But with each passing year, researchers have accumulated more data and refined their techniques. In 2020 and 2021, each of the three collaborations began to see a telltale sign of the gravitational-wave background (2),(3),(4). Whereas ordinary, ‘white’ noise includes random fluctuations at all frequencies, red noise is louder at lower frequencies. Such a feature is expected when signals of long wavelength — with periods comparable to the 20-odd years of data that have now been accumulated — are beginning to emerge. The IPTA’s latest joint analysis — made by pooling the regional collaborations’ data on 65 pulsars to improve their sensitivity to gravitational waves — has detected the red noise, too, even though it did not use the most recent data sets that the three groups analysed separately in 2020 and 2021.
The finding doesn’t necessarily indicate the presence of gravitational waves. “Red noise can also be produced by other things,” warns Kramer, such as a previously unsuspected pattern in the way the spinning pulsars gradually slow down.
To claim a discovery, “a crucial component is missing”, says radio astronomer Andrea Possenti, a leading member of the European group who is at the Cagliari Astronomical Observatory in Italy. “These long-term signals must be correlated from one pulsar to another.”
Hobbs agrees. “I personally would like a lot more checks to be done before I’m going to break open the champagne bottle,” he says.
If and when the gravitational-wave background is discovered, “the scientific reward will be immense”, says Monica Colpi, an astrophysicist at the University of Milan–Bicocca in Italy. From the signals, researchers could ultimately get information about how the black holes interacted with dark matter, stars and gas clouds in their galaxies, she says.
Image above: The radio telescope at the Arecibo Observatory in Puerto Rico, which collapsed in December 2020, was part of the International Pulsar Timing Array. Image Credits: Ricardo Arduengo/AFP/Getty.
The worldwide effort to hunt for the waves took a hit in December 2020, when the venerable 300-metre Arecibo Observatory — which played an important part in measuring pulsars — collapsed. Since then, the North American team has rerouted some of the work to its other major facility, the 100-metre Green Bank Telescope in West Virginia. “We have dropped a handful of our weaker pulsars and have gaps in our data set of a few months, but all in all we are weathering the loss of [Arecibo] as well as we can,” Siemens says.
Future efforts will benefit from pulsar-timing data being collected at major radio-astronomy observatories in India and South Africa. Eventually, China’s Five-hundred-meter Aperture Spherical Radio Telescope is expected to join, too.
And the researchers say that the next IPTA paper, expected this year or next, could use the data that already exist to confirm a discovery of the gravitational-wave background produced by supermassive black holes. “Now the time is ripe to bring it all together and make a detection,” says Kramer.
doi: https://doi.org/10.1038/d41586-022-00170-y
Related article:
Arecibo telescope collapses, ending 57-year run
https://orbiterchspacenews.blogspot.com/2020/12/arecibo-telescope-collapses-ending-57.html
References:
1. Antoniadis, J. et al. Mon. Not. R. Astron. Soc. 510, 4873–4887 (2022).
https://doi.org/10.1093%2Fmnras%2Fstab3418
2. Arzoumanian, Z. et al. Astrophys. J. Lett. 905, L34 (2020).
https://doi.org/10.3847%2F2041-8213%2Fabd401
3. Goncharov, B. et al. Astrophys. J. Lett 917, L19 (2021).
https://doi.org/10.3847%2F2041-8213%2Fac17f4
4. Chen, S. et al. Mon. Not. R. Astron. Soc. 508, 4970–4993 (2021).
https://doi.org/10.1093%2Fmnras%2Fstab2833
Related links:
LIGO Lab: https://www.ligo.caltech.edu/
Arecibo Observatory: https://www.naic.edu/ao/
Images (mentioned), Text, Credits: Nature/Davide Castelvecchi/LIGO/JPL-Caltech.
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