Hamilton, NY — August 19, 2026 — A mysterious background of extremely low-frequency gravitational waves detected by networks of pulsars may carry information about events that began more than 13 billion years ago — including the formation of some of the first supermassive black holes in the Universe.
In a new study published as a Letter in Physical Review D, Sohan Ghodla and Cosmin Ilie of Colgate University investigate whether supermassive black holes formed in the early Universe could ultimately produce a substantial fraction of the gravitational-wave background now observed by Pulsar Timing Arrays, or PTAs.
Their results establish a direct connection between two seemingly very different observational frontiers: observations of unexpectedly massive black holes in the young Universe and gravitational waves produced by supermassive black-hole binaries billions of years later.
Remarkably, the researchers find that one possible population of early black-hole seeds — black holes left behind by supermassive Dark Stars — could potentially account for a dominant contribution to the observed PTA signal. "Pulsar timing arrays are usually thought of as probes of supermassive black-hole binaries in the relatively recent Universe," said Ilie. "What our work shows is that the signal may also contain information about how the ancestors of those black holes formed at cosmic dawn. In that sense, gravitational waves observed today could provide a new window onto the birth of the first supermassive black holes."
Using cosmic clocks to detect enormous black holes
Pulsar Timing Arrays use rapidly rotating neutron stars called pulsars as extraordinarily precise cosmic clocks. Passing gravitational waves subtly alter the arrival times of radio pulses reaching Earth. By monitoring many pulsars over years, collaborations around the world have detected evidence for a stochastic gravitational-wave background at nanohertz frequencies.
The leading astrophysical explanation is a cosmic population of inspiraling supermassive black-hole binaries. Black holes with combined masses greater than about a billion times the mass of the Sun are particularly important contributors at PTA frequencies. But building such enormous black holes raises another question: Where did their original seeds come from? Observations with facilities including the James Webb Space Telescope and Chandra have revealed massive black holes surprisingly early in cosmic history, intensifying interest in mechanisms capable of producing massive black-hole seeds rapidly.
Ghodla and Ilie asked whether descendants of such early seeds could survive, grow with their host galaxies, eventually form binaries, and generate the gravitational-wave background measured billions of years later.
A possible gravitational-wave signature of Dark Stars
The researchers considered two early black-hole formation channels: direct-collapse black holes and the collapse of supermassive Dark Stars.
Dark Stars are a proposed type of primordial star whose principal energy source is heating associated with dark matter rather than ordinary nuclear fusion. In the WIMP dark-matter scenario considered in the study, Dark Stars can remain comparatively cool and extended while continuing to accrete matter, potentially reaching masses of a million Suns or more before collapsing into massive black holes.
In this work Ghodla and Ilie followed the cosmological evolution of black holes produced by such seeds, modeled the halos in which they reside, calculated their merger rates and predicted the resulting gravitational-wave background. They find that if supermassive-Dark-Star remnants have a number density of order (10^{-3} {rm Mpc}^{-3}), their descendants can make a major — and potentially dominant — contribution to the PTA gravitational-wave signal.
The competing direct-collapse-black-hole population considered in the study is expected to be far rarer, with characteristic densities around (10^{-6} {rm Mpc}^{-3}), and consequently produces a much smaller contribution.
Turning gravitational waves into a census of the early Universe
The key insight of this work is the fact that existing PTA measurements can be used to place an upper limit on how abundant the early seeds of Supermassive Black holes could have been. "Produce too many of these massive seeds and you end up over-producing the PTA-detected signal. Produce too few, and you need other sources to efficiently assemble these supermassive black holes later in the life of the universe to match PTA observations," said Ghodla.
For the models explored in the study, the researchers find that seed densities approximately in the (10^{-2})–(10^{-1} {rm Mpc}^{-3}) range would begin to overproduce the measured gravitational-wave background, with the precise constraint depending strongly on the masses of the dark-matter halos in which the seeds formed. This means PTA observations can potentially do something unexpected: constrain populations of objects that existed at redshifts greater than 10, even though the gravitational-wave-producing mergers of their descendants occur much later. The calculation also confirms a previous result that binaries with total black-hole masses roughly above (10^9) solar masses dominate the predicted PTA signal. Lower-mass binaries contribute considerably less.
The result provides a new observational connection among dark matter physics, the formation of the first luminous objects, the origin of supermassive black holes and gravitational-wave astronomy.
"Dark Stars were originally proposed as objects that might be seen directly at cosmic dawn," Ilie said. "This work points to a completely different way of testing their possible role in cosmic history. Their descendants could leave a gravitational-wave imprint that persists all the way to the present-day Universe."
Future improvements in PTA measurements, combined with better constraints on high-redshift black-hole populations and their host galaxies, could therefore help distinguish among different scenarios for the origin of the Universe's first supermassive black holes.
