This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 in 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 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," Ilie said.
"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 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 rapidly producing massive black-hole seeds. 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.
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 1 million times the mass of the sun or more before collapsing into massive black holes.
Extract — continue reading at the source.
