Astronomers may have found a new way to investigate one of the biggest mysteries in cosmic history: how the Universe’s first supermassive black holes were born. A faint gravitational-wave “hum” detected through networks of pulsars could contain clues about objects that existed more than 13 billion years ago. A new study by researchers at Colgate University suggests that hypothetical Dark Stars, strange early stars powered partly by dark matter, could have left behind black-hole descendants that contribute significantly to this signal. The study, published as a Letter in Physical Review D, explores whether these ancient black-hole seeds could eventually grow, merge and produce part of the gravitational-wave background being measured today.
A cosmic hum detected through pulsars
The signal comes from extremely low-frequency gravitational waves, at frequencies of around nanohertz. Unlike the gravitational waves detected by instruments such as LIGO, these waves are tracked indirectly using Pulsar Timing Arrays (PTAs). Pulsars are rapidly rotating neutron stars that emit extraordinarily regular pulses of radio waves. They behave much like cosmic clocks. When a gravitational wave passes between a pulsar and Earth, it can produce tiny changes in the arrival time of those pulses. By monitoring many pulsars over long periods, astronomers have found evidence for a widespread, stochastic gravitational-wave background. The leading explanation is that the signal is produced largely by pairs of supermassive black holes slowly spiralling towards one another after galaxies merge. But the new research asks a deeper question: where did those enormous black holes come from in the first place?
The mystery of the first giant black holes
Some extremely massive black holes appear to have existed surprisingly early in the Universe’s history. Their presence has challenged astronomers to explain how black holes could become so massive so quickly. Ghodla and Ilie investigated two possible types of early black-hole seeds: direct-collapse black holes and black holes formed from the collapse of hypothetical supermassive Dark Stars. Dark Stars are theoretical objects proposed in some models of the early Universe. Rather than being powered primarily by ordinary nuclear fusion, they could have received substantial energy from interactions involving dark matter. In the model examined by the researchers, Dark Stars could become enormous, potentially reaching masses of a million Suns or more, before eventually collapsing into black holes. Those black holes could then grow alongside their galaxies, form binary systems and merge. Billions of years later, their descendants could contribute to the gravitational-wave background detected by PTAs.
Could dark stars be behind the signal?
The researchers’ modelling suggests that remnants of supermassive Dark Stars could potentially make a dominant contribution to the PTA gravitational-wave background if they existed at sufficiently high numbers in the early Universe. The study considered a Dark Star remnant density of roughly 10⁻³ per cubic megaparsec. By comparison, the direct-collapse black-hole population examined in the study was much less abundant, at characteristic densities around 10⁻⁶ per cubic megaparsec, and therefore contributed considerably less to the predicted signal. The researchers also found that producing too many massive early seeds could create a gravitational-wave background stronger than the one observed. That means current PTA measurements can potentially place constraints on how common these ancient objects could have been.
A new window into the cosmic dawn
The intriguing part is that the gravitational waves being measured today would not necessarily have been produced when the original Dark Stars existed. Instead, the proposed chain of events stretches across cosmic history: Dark Star → massive black-hole seed → growing supermassive black hole → black-hole binary → gravitational waves detected today. That gives astronomers a possible indirect way to investigate an era that is otherwise extremely difficult to observe. The findings also connect several major questions in cosmology, the nature of dark matter, the first stars, the origins of supermassive black holes and the gravitational-wave background. However, Dark Stars remain hypothetical. The study does not establish that they existed or that they are responsible for the gravitational-wave signal. Rather, it shows that, under particular theoretical assumptions, their descendants could leave a measurable imprint. As pulsar timing observations become more precise, future data may help researchers determine whether this ancient “hum” carries evidence of the Universe’s first giant black-hole seeds or whether another population is responsible.
