An international scientific team led by researchers from Beijing Normal University has announced the first direct gravitational-wave constraints on the masses of binary supermassive black holes that existed during the so-called cosmic dawn. The study utilized 18 years of accumulated observations from the Australian Parkes Pulsar Timing Array project, which allowed researchers to exclude the presence of binary black hole systems with a combined mass on the order of 10 billion solar masses in two distant systems. According to the authors, these are the first such limits for objects in such an early cosmological epoch, when the age of the Universe was only hundreds of millions of years.
How pulsars serve as cosmic clocks
The Parkes Pulsar Timing Array is a network of dozens of millisecond pulsars—rapidly rotating neutron stars that emit strictly periodic radio pulses. Due to the exceptional stability of their rotation, pulsars function as natural chronometers: a passing nanohertz gravitational wave from an orbiting binary supermassive black hole pair introduces characteristic deviations in the arrival time of pulses. A key condition for detection is that such deviations must appear simultaneously in the observations of several pulsars, which allows the signal to be separated from local noise. This method has been the primary tool for searching for gravitational waves with orbital periods of several years for the past several years.
Redshift correction changes the picture
The central methodological innovation of the study was the inclusion of cosmological redshift in the calculation of the pulsar method's sensitivity. Previous approaches hardly accounted for the stretching of gravitational waves due to the expansion of the Universe. The authors demonstrated that a distant binary system might appear brighter to an Earth-based observer than predicted by a simple model of signal attenuation with distance: due to redshift, the wave is received at a lower frequency than that at which it was emitted, meaning that at the moment of emission, the black holes were closer to each other, moved faster, and generated a more intense signal. At a sufficiently high redshift, this amplification partially compensates for the attenuation caused by the enormous distance. As a result, the researchers extended the sensitivity estimate of the pulsar method to objects with a redshift up to 20, which corresponds to the era when the Universe was less than a billion years old.
Two systems at the edge of the observable Universe
To verify the new approach, the authors selected two systems. The first is the bright quasar J0100+2802 at a redshift of 6.327, whose radiation was emitted when the Universe was no more than 900 million years old. The second is the galaxy JADES-GS-z14-0, discovered by the James Webb Space Telescope at a redshift of 14.32, that is, less than 300 million years after the Big Bang. For both systems, it was possible to set an upper limit on the value characterizing the combined mass of the two black holes and determining the parameters of the expected gravitational signal. The obtained data exclude the existence of systems with a mass on the order of 10 billion solar masses and above, although current sensitivity is not yet sufficient to definitively determine whether these objects themselves are binaries.
Significance for cosmology and prospects
The result opens a fundamentally new channel for studying how supermassive black holes managed to increase their mass to billions of solar masses in the earliest eras of the Universe's existence—a question that remains one of the central mysteries of modern cosmology. The authors emphasize that as data from the Parkes Pulsar Timing Array are combined with results from other pulsar arrays, and in the future with observations from the Square Kilometre Array radiotelescope, the constraints on the masses of binary systems will become significantly more accurate. This will bring astrophysicists closer to directly testing competing scenarios for the growth of early black holes, including scenarios in which they formed and increased in mass primarily through sequential mergers.
