Supermassive black holes, with masses billions of times greater than our Sun, are among the most awe-inspiring and mysterious objects in the universe. We’ve known for a long time that these cosmic giants have been around for eons. Astronomers have detected quasars, extremely luminous objects powered by rapidly growing supermassive black holes, when the universe was less than a billion years old. Now, a new study published in Astrophysical Journal Letters has unveiled a fascinating discovery – there were far more black holes in the early universe than previously estimated. This revelation could shed light on how these colossal objects formed and why they appear to be more massive than expected.
Black holes grow by consuming surrounding matter through a process called accretion, releasing immense amounts of radiation in the process. This radiation imposes a limit on how quickly black holes can grow. The existence of these early, massive quasars posed a significant challenge: given the limited time for accretion, they must have either grown at an impossible rate or been born with extraordinary mass. This has prompted scientists to explore various theories for their formation.
One possibility is the existence of primordial black holes, which may have formed shortly after the Big Bang. While this explanation is plausible for smaller black holes, massive black holes cannot have formed in significant numbers according to the standard model of cosmology. Another possibility is that black holes form during the final stages of the life cycle of massive stars, as verified by gravitational wave astronomy. These stellar mass seeds could grow rapidly if formed in dense star clusters where mergers occur.
However, the rapid growth of these seeds to explain the early quasars remains a puzzle. A compelling alternative proposes the formation of “heavy seeds” with masses thousands of times greater than known massive stars. One mechanism for their creation is “direct collapse”, where early structures of dark matter confined gas clouds, preventing them from forming stars. These clouds then collapsed into black holes. The issue here is that only a small fraction of dark matter halos grow large enough to create such seeds, suggesting that early black holes should be relatively rare. However, the new Hubble study has overturned this expectation.
For years, astronomers had a good understanding of the distribution of galaxies in the early universe. However, detecting black holes within these environments was extremely difficult, with only luminous quasars providing conclusive evidence. Black holes do not consume matter at a constant rate, instead feeding in bursts, resulting in variations in their brightness. The study carefully monitored the brightness of early galaxies over 15 years, generating a census of black holes within these environments. The results revealed a surprising abundance: several times more black holes reside in early galaxies than previously thought. This discovery is consistent with recent observations from the James Webb Space Telescope (JWST) and suggests that the universe was far more teeming with black holes than previously imagined. The current number of observed black holes exceeds the number that could have formed through direct collapse.
There exists another, more exotic, pathway to black hole formation, capable of creating both massive and abundant seeds. Stars form through the gravitational contraction of gas clouds. If a significant number of dark matter particles are captured during this process, the internal structure could be fundamentally altered, potentially preventing nuclear ignition. This extended growth phase could result in the formation of far more massive stars. Despite their size, these “dark stars” would ultimately succumb to the relentless force of gravity and collapse into massive black holes. The researchers believe that similar processes likely occurred to form the numerous black holes observed in the infant universe.
The study of early black hole formation has undergone a significant transformation in the past two years, and the field is just beginning to explore its depths. New space-based observatories, such as the Euclid mission and the Nancy Grace Roman Space Telescope, will provide a more comprehensive census of fainter quasars in the early universe. The NewAthena mission and the Square Kilometer Array will enhance our understanding of the complex processes surrounding black holes in the early cosmos. However, it is the JWST that holds the most promise for the immediate future. With its exceptional sensitivity for imaging, monitoring, and spectroscopy, it is poised to provide detailed insights into the population of black holes as the first galaxies were forming, potentially even capturing black hole formation in action. Astronomers hope to witness the explosions associated with the collapse of the first pristine stars, an event predicted by models and one that would require coordinated and dedicated efforts to observe.
The early universe was a much more active and dynamic environment than previously thought. The discovery of a significant number of black holes challenges our understanding of their formation, and promises to unravel the mysteries of the early cosmos.
