A faint background of extremely low frequency gravitational waves detected through networks of pulsars could contain clues to events that unfolded more than 13 billion years ago, including the emergence of some of the Universe's earliest supermassive black holes. In a study published as a Letter in Physical Review D, Colgate University researchers Sohan Ghodla and Cosmin Ilie examined whether supermassive black holes that originated in the early Universe could eventually produce a significant share of the gravitational wave background now being measured by Pulsar Timing Arrays, or PTAs. The findings connect two areas of astronomy that may at first seem far apart.
One involves observations of surprisingly massive black holes that already existed when the Universe was young. The other involves gravitational waves generated billions of years later by pairs of supermassive black holes spiraling toward one another. The researchers found that one possible class of early black hole seeds, remnants left behind by supermassive Dark Stars, could potentially provide a dominant contribution to the 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." Pulsar Timing Arrays rely on rapidly spinning neutron stars known as pulsars, which act as extraordinarily precise clocks in space.
When gravitational waves pass through the cosmos, they cause tiny changes in the timing of the radio pulses that eventually arrive at Earth. By tracking many pulsars over long periods, international research teams have found evidence for a stochastic gravitational wave background at nanohertz frequencies. The most widely accepted astrophysical explanation for this background is a population of supermassive black hole binaries gradually spiraling inward.
Systems whose black holes have a combined mass greater than roughly a billion Suns make especially important contributions at the frequencies detected by PTAs. But explaining those enormous black holes raises a deeper question. Astronomers still want to know how their original seeds formed.
Observatories including the James Webb Space Telescope and Chandra have discovered massive black holes at unexpectedly early stages of cosmic history, increasing interest in formation mechanisms capable of producing large black hole seeds very quickly. Ghodla and Ilie investigated whether the descendants of such ancient seeds could persist through cosmic time, grow alongside their host galaxies, eventually pair up, and generate the gravitational wave background detected billions of years later. Could Dark Stars Seed Giant Black Holes?
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