Observations, including with the James Webb telescope, have shown that unexpectedly massive black holes existed in the early Universe. Less than a billion years after the Big Bang, some already weighed billions of solar masses. How they managed to grow so fast remains unclear.
A new study led by Yiqin Chen of Beijing Normal University, published in the journal Physical Review Letters, has for the first time directly constrained the mass of pairs of supermassive black holes in the era of "cosmic dawn" - the early period of the Universe's history.
When two supermassive black holes orbit each other, they disturb space-time and emit gravitational waves - ripples in space-time. For such heavy pairs, one orbit can take about a year, and the waves come out too slow for ground-based detectors.
That is why astronomers search for them using pulsar arrays - networks of rapidly rotating neutron stars (pulsars) in our Galaxy. Pulsars flash with the regularity of cosmic clocks. A passing gravitational wave slightly shifts the timing of these flashes, and the shift appears consistently across the entire array.
Conventional searches do not account for the expansion of the Universe, which stretches gravitational waves along their path. This stretching is called redshift. If it is ignored, the masses of distant pairs can be misestimated, so such searches remained limited to the nearby Universe.
Chen's team developed a method that accounts for the strong redshift of signals from the distant early Universe. A redshifted wave that reaches Earth was originally emitted at a higher frequency - when the black holes orbited closer together and radiated more strongly. This amplification can outweigh the weakening due to distance.
The researchers analyzed 18 years of observations of 30 pulsars from the Australian Parkes Pulsar Timing Array. They then matched the search against the known positions and redshifts of two objects. The first is an ultrabright quasar observed less than 900 million years after the Big Bang. The second is one of the most distant galaxies ever spotted by James Webb, visible less than 300 million years after the Big Bang. This made it possible to calculate limits on the true masses of possible black hole pairs in these systems.
For both objects, the team ruled out pairs with a mass of about 10 billion solar masses and more. These are the first direct gravitational-wave limits on pairs of black holes in the cosmic dawn era.
When Parkes combines its data with other arrays, and future instruments such as the Square Kilometer Array improve the precision of pulsar timing, the limits should become stricter. Together with the search for short gravitational-wave bursts, this will bring observations closer to less extreme pairs as well.
The results show that large distances do not necessarily prevent the detection of black hole pairs. This could provide a new way to search for them and help explain how such massive black holes appeared so early in the history of the Universe.