A faint gravitational-wave signal spreading across the universe may contain clues about mysterious stars that existed more than 13 billion years ago, according to new research.
Scientists from Colgate University have investigated whether some of the earliest supermassive black holes could be responsible for much of the extremely low-frequency gravitational-wave background detected today.
Their findings also raise an intriguing possibility: some of those ancient black holes may have begun as hypothetical objects known as Dark Stars.
Gravitational waves are ripples in space-time produced by moving massive objects. Scientists can detect extremely slow gravitational waves using pulsars, rapidly spinning neutron stars that send regular radio pulses toward Earth.
Because their signals arrive with extraordinary precision, pulsars act like cosmic clocks. When a gravitational wave passes between a pulsar and Earth, it can slightly change when those pulses arrive. By monitoring many pulsars for years, scientists have found evidence of a background “hum” of gravitational waves across the universe.
The leading explanation is that this signal comes largely from pairs of supermassive black holes slowly orbiting each other before eventually merging. Black-hole pairs with combined masses exceeding roughly a billion suns are expected to be especially important.
But this creates another mystery: How did black holes become so enormous?
Astronomers using powerful observatories, including the James Webb Space Telescope, have discovered surprisingly massive black holes when the universe was still young. Scientists are therefore trying to understand how their original “seed” black holes could have formed and grown so rapidly.
Researchers Sohan Ghodla and Cosmin Ilie examined two possible origins for these early seeds: direct-collapse black holes and black holes created by collapsing supermassive Dark Stars.
Dark Stars are theoretical stars that may have existed shortly after the Big Bang. Unlike ordinary stars such as our sun, which are powered by nuclear fusion, Dark Stars could have been powered largely by heat associated with dark matter.
Under some theories of dark matter, these unusual stars could remain relatively cool and enormous while continuing to collect material. Some might eventually grow to more than a million times the sun’s mass before collapsing into massive black holes.
The researchers modeled how these black holes could grow along with their galaxies, form pairs and eventually produce gravitational waves.
Their calculations suggest that if supermassive Dark Stars were sufficiently common in the early universe, the black holes they left behind could eventually produce a large—and possibly dominant—share of the gravitational-wave background detected today.
The alternative direct-collapse black holes examined in the study were predicted to be much rarer and therefore produced a substantially weaker signal.
The results also provide scientists with a new way to test theories about the early universe. If too many massive black-hole seeds formed, their descendants should produce more gravitational waves than astronomers actually observe. Current measurements can therefore place limits on how common these ancient objects could have been.
The study does not prove that Dark Stars existed. Instead, it shows that modern gravitational-wave observations could potentially test their role in cosmic history.
As pulsar timing measurements become increasingly precise, the faint gravitational-wave hum surrounding us may reveal something remarkable: traces of objects born during the universe’s earliest chapters, preserved across more than 13 billion years of cosmic evolution.
