Artist’s impression of the IPTA experiment – An array of pulsars around the Earth embedded in a gravitational wave background from supermassive black hole binaries. The signals from the pulsars measured with a network of global radio telescopes are affected by the gravitational waves propagating through spacetime, allowing astronomers to study the origin of the background. (Credit: Tonia Klein / NANOGrav)
In data gathered and analyzed over 13 years, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has found an intriguing low-frequency signal that may be attributable to gravitational waves.
NANOGrav researchers studying the signals from distant pulsars – small, dense stars that rapidly rotate, emitting beamed radio waves, much like a lighthouse – have used radio telescopes to collect data that may indicate the effects of gravitational waves, as reported recently in The Astrophysical Journal Letters.
NANOGrav has been able to rule out some effects other than gravitational waves, such as interference from the matter in our own solar system or certain errors in the data collection. These newest findings set up direct detection of gravitational waves as the possible next major step for NANOGrav and other members of the International Pulsar Timing Array (IPTA), a collaboration of researchers using the world’s largest radio telescopes.
“It is incredibly exciting to see such a strong signal emerge from the data,” says Joseph Simon, lead researcher on the paper. “However, because the gravitational-wave signal we are searching for spans the entire duration of our observations, we need to carefully understand our noise. This leaves us in a very interesting place, where we can strongly rule out some known noise sources, but we cannot yet say whether the signal is indeed from gravitational waves. For that, we will need more data.”
Gravitational waves are ripples in space-time caused by the movements of incredibly massive objects, such as black holes orbiting each other or neutron stars colliding. Astronomers cannot observe these waves with a telescope like they do stars and galaxies. Instead, they measure the effects passing gravitational waves have, namely tiny changes to the precise position of objects - including the position of the Earth itself.
NANOGrav chose to study the signals from pulsars because they serve as detectable, dependable galactic clocks. These small, dense stars spin rapidly, sending pulses of radio waves at precise intervals toward Earth. Pulsars are in fact commonly referred to as the universe’s timekeepers, and this unique trait has made them useful for astronomical study.
But gravitational waves can interrupt this observed regularity, as the ripples cause space-time to undergo tiny amounts of stretching and shrinking. Those ripples result in extremely small deviations in the expected times for pulsar signals arriving on Earth. Such deviations indicate that the position of the Earth has shifted slightly.
By studying the timing of the regular signals from many pulsars scattered over the sky at the same time, known as a “pulsar timing array,” NANOGrav works to detect minute changes in the Earth’s position due to gravitational waves stretching and shrinking space-time.
“NANOGrav has been building to the first detection of low frequency gravitational waves for over a decade and today’s announcement shows that they are on track to achieving this goal,” said Pedro Marronetti, NSF Program Director for gravitational physics. “The insights that we will gain on cosmology and galaxy formation are truly unparalleled.”
NANOGrav is a collaboration of U.S. and Canadian astrophysicists and a National Science Foundation Physics Frontiers Center (PFC). Maura McLaughlin, Co-Director of the NANOGrav Physics Frontiers Center, added, “We are so grateful for the support of the NANOGrav PFC, that’s allowed us to dramatically increase both the number of pulsars being timed and the number of participants working on data analysis over the past six years.”
NANOGrav created their pulsar timing array by studying 47 of the most stably rotating “millisecond pulsars,” as reported in the January 2021 issue of the Astrophysical Journal Supplements. Not all pulsars can be used to detect the signals that NANOGrav seeks – only the most stably rotating and longest-studied pulsars will do. These pulsars spin hundreds of times a second, with incredible stability, which is necessary to obtain the precision required to detect gravitational waves.
Of the 47 pulsars studied, 45 had sufficiently long datasets of at least three years to use for the analysis. Researchers studying the data uncovered a spectral signature, a low-frequency noise feature, that is the same across multiple pulsars. The timing changes NANOGrav studies are so small that the evidence isn’t apparent when studying any individual pulsar, but in aggregate, they add up to a significant signature.
Potential Next Steps
In order to confirm direct detection of a signature from gravitational waves, NANOGrav’s researchers will have to find a distinctive pattern in the signals between individual pulsars. At this point, the signal is too weak for such a pattern to be distinguishable. Boosting the signal requires NANOGrav to expand its dataset to include more pulsars studied for even longer lengths of time, which will increase the array’s sensitivity. In addition, by pooling NANOGrav’s data together with those from other pulsar timing array experiments, a joint effort by the IPTA may reveal such a pattern.
At the same time, NANOGrav is developing techniques to ensure the detected signal could not be from another source. They are producing computer simulations that help test whether the detected noise could be caused by effects other than gravitational waves, in order to avoid a false detection.
“Trying to detect gravitational waves with a pulsar timing array requires patience. We’re currently analyzing over a dozen years of data, but a definitive detection will likely take a couple more. It’s great that these new results are exactly what we would expect to see as we creep closer to a detection,” says Scott Ransom, from the National Radio Astronomy Observatory, and the current Chair of NANOGrav.
Like light from distant objects, gravitational waves are a cosmic messenger signal – one that holds great potential for understanding “dark” objects, like black holes. In 2015, NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct observation of gravitational waves.
LIGO and its counterparts Virgo in Europe and Kagra in Japan use purpose-built interferometry facilities to detect high-frequency gravitational waves. However, unlike the transient signals detected by LIGO/Virgo/Kagra, low-frequency gravitational waves are persistent, requiring many years of data to detect. Over the past decade, NANOGrav has used existing radio telescopes to search for evidence of these low-frequency gravitational waves, which have the potential to help answer longstanding questions in astrophysics, including how massive black holes form and how galaxies merge.
Throughout its work, NANOGrav has utilized data from two NSF-supported instruments: the Green Bank Telescope in West Virginia and Arecibo Observatory in Puerto Rico. With the recent collapse of the Arecibo Observatory’s 305-meter telescope, NANOGrav will be seeking alternate sources of data and working even more closely with their international colleagues. Although NANOGrav does not expect the situation to result in significant delays in detection due to years of very sensitive Arecibo data already contributing to their datasets, the loss of Arecibo is a terrible blow to science, and will impact NANOGrav’s ability to characterize the background and detect individual sources in the future. NANOGrav members are also saddened by the collapse and its impact on the staff and the island of Puerto Rico.