This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: So far, observations by instruments including the James Webb Space Telescope have revealed the presence of surprisingly massive black holes in the early universe. Even less than a billion years after the Big Bang, some of these bodies already weighed billions of times the mass of the sun, and to date, astronomers are unclear as to how they could have grown so large so quickly.
Through new research published in Physical Review Letters, a team led by Yiqin Chen at Beijing Normal University has used a pulsar timing array to place the first direct limits on pairs of supermassive black holes in this early era, known as the 'cosmic dawn.' When two supermassive black holes orbit each other, they slowly stir the surrounding spacetime, sending gravitational waves rippling across the universe. For pairs this massive, a single orbit can take around a year, producing waves far too slow for Earth-based detectors. Instead, astronomers search for them using pulsar timing arrays: networks of rapidly spinning neutron stars spread across our galaxy, which flash with the regularity of cosmic clocks.
A passing gravitational wave subtly shifts the timing of these flashes, in a pattern shared across the whole array. By picking out signals from individual pairs at great distances, astronomers could reveal how much the earliest supermassive black holes grew through mergers. However, standard searches ignore the expansion of the universe, which stretches gravitational waves as they travel.
If the resulting redshift is ignored, it could lead to misjudgments in the masses of distant pairs. As a result, these searches have remained limited to the nearby universe. To tackle the challenge, Chen's team developed a new approach that could account for the strong redshift in signals originating from the distant early universe.
A redshifted wave arriving at Earth was originally emitted at a higher frequency, when the black holes were orbiting more tightly and producing stronger waves. Surprisingly, this brightening can outweigh the dimming caused by distance. To apply the idea, the researchers analyzed 18 years of observations of 30 pulsars from the Parkes Pulsar Timing Array in Australia.
They then combined their search with the known positions and redshifts of two targets. The first was an ultrabright quasar, observed less than 900 million years after the Big Bang. The second was one of the most distant galaxies ever spotted by JWST, observed under 300 million years after the Big Bang.
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