A compelling new study proposes that the universe's earliest and most luminous phenomena, quasars and radio galaxies, may owe their existence to primordial black holes (PBHs). These hypothetical black holes, thought to have formed in the universe's infancy within the first few thousand years after the Big Bang, could have acted as the gravitational seeds for these energetic cosmic structures.
The research, originating from Swinburne University and detailed in a 2025 arXiv publication by Jeremy Mould and Adam Batten, suggests that PBHs emerged from minute fluctuations in the early universe's radiation environment. Their immense gravitational pull would have then drawn in surrounding gas and dust, facilitating their growth into the supermassive black holes that power quasars. This theory aligns with observations of the quasar luminosity function (QLF), which describes the distribution of quasar brightness over time. The QLF's characteristics are consistent with the idea that PBHs played a pivotal role in quasar formation. As these nascent black holes consumed matter, their luminosity would have naturally followed the predicted QLF curve, indicating a decline in brightness at higher redshifts.
Furthermore, the study posits a direct evolutionary link between quasars and radio galaxies. Once a quasar has exhausted its immediate cosmic fuel, it may transition into a radio galaxy, a celestial body characterized by powerful radio emissions. The observed similarities in the luminosity functions of both quasars and radio galaxies, with radio galaxies exhibiting a lower overall amplitude but a longer lifespan, support this evolutionary pathway. This PBH-centric model also offers a solution to the fundamental question of what powers quasars. By engulfing smaller galaxies, PBHs could have provided the necessary energy. The research also opens the door to using quasars as reliable standard candles for cosmological distance measurements, a role currently fulfilled by Type Ia supernovae, by establishing a baseline understanding of their intrinsic brightness.
The capabilities of the James Webb Space Telescope (JWST) are expected to be crucial in verifying this theory. JWST's advanced instruments can peer further back in time than ever before, potentially providing definitive data to either confirm or refute the PBH seeding hypothesis. Recent observations by JWST have indeed revealed surprisingly isolated quasars in the early universe, challenging existing models of how supermassive black holes could have formed so rapidly. These "lonely" quasars, observed between 600 to 700 million years after the Big Bang, suggest that their growth might not have solely depended on dense galactic environments, lending credence to alternative seeding mechanisms like PBHs. For instance, a study published in October 2024 highlighted that some of these ancient quasars appear to exist in voids with few neighboring galaxies, prompting questions about their rapid development. This aligns with the PBH theory, which suggests these black holes could have grown independently by accreting matter from their immediate surroundings, rather than relying on a dense galactic neighborhood. The potential for PBHs to act as seeds for these early supermassive black holes is a significant development in understanding the universe's formative stages.