It is believed that across all space and time, there’s a vibration – a low-frequency hum due to gravitational waves created by some of the most massive black holes, or from what happened right after the Big Bang. It is called the gravitational wave background, and a tentative detection of this signal might have finally happened.
The background is too faint to be observed with our current gravitational wave observatories, as these gravitational waves are light-year scale. So astronomers have to be inventive. The universe has provided them with incredible “clocks” for this task: millisecond pulsars.
These objects are an extreme version of neutron stars, the end product of some supernovae. Pulsars are neutron stars emitting beams of radiation as they rotate, acting a bit like a lighthouse if you’re staring at them in the right direction. Millisecond pulsars rotate hundreds of times per second and rotation is constant over a long time, making them precise clocks.
If gravitational waves pass between us and the pulsars, the timing of these pulses will be slightly altered, and that’s what the researchers looked for. Combining three data sets from The European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA), the second data release of the International Pulsar Timing Array (IPTA) was published.
As reported in Monthly Notices of the Royal Astronomical Society, the signal looks like what’s expected for the gravitational wave background. However, currently, the analysis cannot prove that this is truly what is being seen and not something else, such as another source or mistakes in the modeling that have not been considered.
“This is a very exciting signal! Although we do not have definitive evidence yet, we may be beginning to detect a background of gravitational waves,” Dr Siyuan Chen, a member of the EPTA and NANOGrav, and the leader of the IPTA DR2 search and publication said in a statement.
A crucial find would be spatial correlations between pulsar pairs. If this background is there, pulsars signals should respond in certain specific ways depending on their position in space. To measure that, longer data collection is needed as well as more pulsars. In general, just bigger data sets should do the trick.
Thanks to data from radio observatory MeerKAT and from the Indian Pulsar Timing Array (InPTA), which just joined IPTA, the data sets will grow and the researchers are confident that they will be able to prove that this is the gravitational wave background.
“The first hint of a gravitational wave background would be a signal like that seen in the IPTA DR2. Then, with more data, the signal will become more significant and will show spatial correlations, at which point we will know it is a gravitational wave background. We are very much looking forward to contributing several years of new data to the IPTA for the first time, to help achieve a gravitational wave background detection,” explained Dr Bhal Chandra Joshi, a member of the InPTA.
Being able to measure this signal could be revolutionary to our understanding of the Universe. It can probe some of the most extreme events in the history of the cosmos.
“The detection of gravitational waves from a population of massive black hole binaries or from another cosmic source will give us unprecedented insights into how galaxy form and grow, or cosmological processes taking place in the infant universe,” Professor Alberto Vecchio, Director of the Institute for Gravitational Wave Astronomy at the University of Birmingham, and member of the EPTA. explained.
“A major international effort of the scale of IPTA is needed to reach this goal, and the next few years could bring us a golden age for these explorations of the universe.”