Ancient 'Dark Stars' May Explain a Mysterious Hum in Spacetime

Astronomers have long puzzled over a faint, steady background of gravitational waves — ripples in space and time — that shows up in the precise timing of radio pulses from pulsars, a class of rapidly spinning neutron stars. A study published as a Letter in the journal Physical Review D by Sohan Ghodla and Cosmin Ilie of Colgate University proposes that part of this signal could trace back to the collapse of hypothetical objects called Dark Stars, more than 13 billion years ago.

Pulsar Timing Arrays, or PTAs, track many pulsars over long stretches of time to detect gravitational waves at extremely low, nanohertz frequencies. The leading explanation for the background these arrays have detected is a population of supermassive black hole pairs gradually spiraling toward each other; pairs with a combined mass above roughly a billion Suns contribute the most to the signal. But telescopes including the James Webb Space Telescope and Chandra have spotted massive black holes far earlier in cosmic history than expected, leaving open the question of how such large 'seed' black holes could form so quickly.

Ghodla and Ilie modeled one possible answer: Dark Stars, hypothetical primordial stars that, under a widely studied dark matter model, would draw much of their energy from dark matter rather than nuclear fusion. Such stars could stay large and relatively cool while continuing to gather material, potentially growing to a million times the mass of the Sun or more before collapsing into black holes. The researchers found that if the remnants of these Dark Stars existed at a number density of around 10⁻³ per cubic megaparsec, their descendants could account for a large, possibly dominant, share of the gravitational wave background measured today by PTAs. A second possible seed type, black holes formed by direct collapse, appeared far less common in the models, at a density around 10⁻⁶ per cubic megaparsec, and would contribute much less to the signal.

"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. The team found that seed densities between roughly 10⁻² and 10⁻¹ per cubic megaparsec would generate more gravitational wave background than has actually been observed — meaning current data can already rule out very high numbers of these ancient seeds, objects that would have existed at redshifts greater than 10, long before their descendants merged to produce the waves detected now.