Researchers at the Research Center for the Early Universe (RESCEU) and the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) at the University of Tokyo have recently leveraged quantum field theory—traditionally applied to minute, subatomic phenomena—to explore the early universe. Their findings suggest that miniature black holes, specifically primordial black holes (PBHs), may be significantly rarer than previously thought, challenging existing models and assumptions.
Quantum field theory is a well-established framework used to describe the interactions of fundamental particles. By applying this theory to cosmological phenomena, researchers have uncovered insights into the conditions of the universe's infancy. Their work, published in Physical Review Letters and Physical Review D, posits that the early universe may not have produced as many PBHs as current models predict. Confirming this hypothesis through observational data is a next step that researchers anticipate.
The universe's age is estimated to be around 13.8 billion years, originating from a hot, dense state often referred to as the Big Bang. Following this event, the universe experienced a rapid expansion known as inflation, evolving from a uniform state to one with rich structure and detail. Despite the vast emptiness of space, there is a discrepancy between the observed mass of the universe and the mass inferred from gravitational effects. This discrepancy is attributed to dark matter, a mysterious form of matter that does not emit light and is undetectable by conventional means.
Primordial black holes have been hypothesized as potential candidates for dark matter. These are black holes thought to have formed in the early universe, before the creation of stars and galaxies. Jason Kristiano, a graduate student involved in the research, highlights their significance: “We call them primordial black holes (PBHs), and many researchers feel they are a strong candidate for dark matter, but there would need to be plenty of them to satisfy that theory.”
The recent advancements in gravitational wave astronomy have detected binary black hole mergers, which could be explained by the existence of numerous PBHs. However, the anticipated abundance of PBHs has not been directly observed, raising questions about existing models. Kristiano, along with Professor Jun'ichi Yokoyama, director of Kavli IPMU and RESCEU, scrutinized various PBH formation models. They found that leading models did not align with observations of the cosmic microwave background (CMB)—a relic radiation from the Big Bang.
Using quantum field theory, the researchers developed a revised model of PBH formation that better fits current CMB observations. This novel approach could be validated with data from upcoming gravitational wave observatories.
Professor Yokoyama explains the significance of their findings: “At the beginning, the universe was incredibly small, much smaller than the size of a single atom. Cosmic inflation rapidly expanded that by 25 orders of magnitude. During this period, waves traveling through this tiny space could have had relatively large amplitudes but very short wavelengths. What we have found is that these tiny but strong waves can translate to otherwise inexplicable amplification of much longer waves we see in the present CMB.”
Their research suggests that the coherence of early short-wavelength fluctuations could influence the larger-scale fluctuations observed in the CMB. This insight provides a rare example of a theory developed for one scale effectively explaining phenomena at a vastly different scale.
If early small-scale fluctuations do impact larger-scale structures in the universe, it could lead to a revised understanding of cosmic evolution. Moreover, by constraining the extent of these short-wavelength fluctuations, the researchers indirectly constrain other phenomena, including PBH formation.
Kristiano notes, “It is widely believed that the collapse of short but strong wavelengths in the early universe is what creates primordial black holes. Our study suggests there should be far fewer PBHs than would be needed if they are indeed a strong candidate for dark matter or gravitational wave events.”
Currently, global gravitational wave observatories such as LIGO (U.S.), Virgo (Italy), and KAGRA (Japan) are actively searching for evidence of small black holes, including PBHs. The outcomes of these observations will provide critical data to either support or refine the researchers' theoretical model. As the scientific community continues to explore the cosmos, these findings may bring us closer to understanding the true nature of dark matter and the early universe.
Source: University of Tokyo