A University of Hull astrophysicist has contributed to a 15-year-long research project in which scientists, using large radio telescopes to observe a collection of cosmic clocks in our Galaxy, have found evidence for gravitational waves that oscillate with periods of years to decades.
The gravitational-wave signal was observed in 15 years of data acquired by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and Physics Frontiers Centre (PFC), a collaboration of more than 190 scientists from the US and Canada who use pulsars to search for gravitational waves.
The research was published today in The Astrophysical Journal Letters,
Dr James McKee, of the EA Milne Centre for Astrophysics at the University of Hull, contributed to the design and operation of the experiment.
A pulsar is the ultra-dense remnant of a massive star's core following its demise in a supernova explosion. Pulsars spin rapidly, sweeping beams of radio waves through space so that they appear to “pulse” when seen from the Earth. The fastest of these objects, called millisecond pulsars, spin hundreds of times each second. Their pulses are very stable, making them useful as precise cosmic timepieces.
“These results are important because they give us an entirely new way of observing the universe - it's like switching on a new telescope for gravity”, says Dr McKee. “We're going to be able to understand the population of supermassive black holes in the universe and search for exotic new physics like cosmic strings and dark matter.”
While earlier results from NANOGrav uncovered an enigmatic timing signal common to all the pulsars they observed, it was too faint to reveal its origin. The 15-year data release demonstrates that the signal is consistent with slowly undulating gravitational waves passing through our Galaxy.
Dr McKee said: “Supermassive black holes lie at the centre of most galaxies, and can weigh billions of times the mass of the sun. This is so massive that, when they orbit each other in pairs, they distort space-time itself. This distortion travels through the universe like a ripple in a pond, called a gravitational wave. The gravitational waves from all of the supermassive black hole binaries in the universe add up to form a gravitational wave 'background' - a constant rippling of space-time.
Einstein’s theory of general relativity predicts precisely how gravitational waves should affect pulsar signals. By stretching and squeezing the fabric of space, gravitational waves affect the timing of each pulse in a small but predictable way, delaying some while advancing others. These shifts are correlated for all pairs of pulsars in a way that depends on how far apart the two stars appear in the sky.
The large number of pulsars used in the NANOGrav analysis has enabled scientists to see what they think are the first signs of the correlation pattern predicted by general relativity.
Observing so many pulsars requires a huge investment in people, infrastructure, and time. In 2004, a small group of astronomers carried out the first set of pulsar observations that would form the foundation for this work. For nearly two decades, the group has been growing in the number of people and diversity of expertise needed to perform this complex gravitational-wave search. Along the way, the NANOGrav collaboration took form, using the members' combined knowledge and skills to expand the data collection and improve the analysis.
Initially, pulsar instrumentation was not precise enough to achieve the sensitivity needed for this experiment. The team worked to develop next-generation instrumentation for both the Arecibo and Green Bank telescopes. They scoured known pulsars to find those precise enough to enable the search for low-frequency gravitational waves and added them to the pulsar timing array. In parallel, there were advances in theory and breakthroughs in data-analysis techniques that are tuned and optimized for modern computing architectures.
In 2020, with just over twelve years of data, NANOGrav scientists began to see hints of a signal, an extra “hum” that was common to the timing behaviour of all pulsars in the array, and that careful consideration of possible alternative explanations could not eliminate. The collaboration felt confident that this signal was real, and becoming easier to detect as more observations were included. But it was still too faint to show the gravitational-wave signature predicted by general relativity. Now, their 15 years of pulsar observations are showing the first evidence for the presence of gravitational waves, with periods of years to decades.
Astrophysicists around the globe have been busy chasing this gravitational-wave signal. Several papers released today by the Parkes Pulsar Timing Array in Australia, the Chinese Pulsar Timing Array, and the European Pulsar Timing Array/Indian Pulsar Timing Array report hints of the same signal in their data. Through the International Pulsar Timing Array consortium, regional collaborations are working together to combine their data in order to better characterize the signal and search for new types of sources.