Could the formation of supermassive black holes, as well as the nature of dark matter, be the result of violent cosmological phase transition in the dark sector of the Universe. Understanding black holes, and how they become supermassive, could shed light on the evolution of the universe.
A Cosmological Phase Transition –One Second After the Big Bang
Three physicists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have recently developed a model to explain the formation of supermassive black holes, as well as the nature of another phenomenon: dark matter. In a paper published in Physical Review Letters, theoretical physicists Hooman Davoudiasl, Peter Denton, and Julia Gehrlein describe a cosmological phase transition that facilitated the formation of supermassive black holes in a dark sector of the Universe.
As the early Universe cooled, subatomic particles interacted through multiple phase transitions. First, quarks and gluons combine to form protons (two up quarks and one down quark) and neutrons (one up; two down) via the strong nuclear force. Then some protons and neutrons coalesced due to the weak nuclear force, producing ionized deuterium and helium nuclei. Eventually, negatively charged electrons and positively charged nuclei paired through the electromagnetic force, yielding neutral hydrogen and helium atoms. The first phase transition between the quark-gluon plasma and sea of protons, neutrons, and electrons occurred about 10^-6 s, or one microsecond, after the Big Bang.
New Insights Into the Physics of Supermassive Black Holes
We Don’t Know How the Universe Cooled Down
“Before galaxies existed, the universe was hot and dense, and that is well established. How the universe cooled down to what we observe today is a matter of interest because we don’t have experimental data describing how that happened,” said Peter Denton. “We can predict what happens with the known particles because they interact often. But what if there are not-yet-known particles out there performing differently?”
The elusive dark matter particles may have interacted more readily during the quark-gluon phase transition in the early, hot, dense Universe than they do today.
In an earlier 2019 paper, Davoudiasi suggested that the black holes observed by the LIGO/Virgo collaboration originate from a first order quark confinement phase transition. In his paper, published in Physical Review Letters, Davoudiasl implemented this idea using a light scalar that could turn out to be a good dark matter candidate. He suggests that primordial black holes (PBH) were created by an abrupt cosmological phase transition, an example of this transition in the early universe could be the cooling of hot plasma made up of quarks and gluons, which might have occurred as the universe expanded, and they began binding into protons and neutrons.
“The general subject of non-standard cosmologies is worth thinking about further,” Davoudiasl said. “Modifying some of our usual assumptions regarding the early universe could potentially lead to new insights about open questions in physics and cosmology.”
Is Dark Matter Only the Tip of an Invisible Universe of Unknown Forces?
Yet-to-be-Discovered Particles of the Dark Sector
To explore this question, the Brookhaven team developed a model for a dark sector of the universe, where yet-to-be-discovered particles abound and rarely interact. Among these particles could be ultralight dark matter, predicted to be 28 orders of magnitude lighter than a proton. Dark matter has never been directly observed, but physicists believe it makes up most of the universe’s matter based on its gravitational effects.
“The frequency of interactions between known particles suggests matter, as we know it, would not have collapsed into black holes very efficiently,” Denton said. “But, if there was a dark sector with ultralight dark matter, the early universe might have had just the right conditions for a very efficient form of collapse.”
Looking for Tell-tale Signatures of Ultralight Dark Matter
“People are actively looking for tell-tale signatures of ultralight dark matter in various different ways,” Denton wrote in an email to The Daily Galaxy. “One way is via black hole superradiance where black holes spin down due to the production of ultralight particles, whether they are the dark matter or not. Another is the macroscopic, and very large, characteristic size they have which modifies the dynamics of galaxies. Some galaxies with particularly clean signatures of this may be showing some hints indicating that dark matter has this macroscopic size, although it is currently too soon to tell for sure.”
Little Known About Growth of Supermassive Black Holes
Recent observations have suggested supermassive black holes formed in the early universe, much earlier than physicists previously thought. This finding leaves little time to account for the growth of supermassive black holes. Physicists know that black holes acquire mass primarily by two means. One way, called accretion, is when matter, mostly dust, falls into black holes. But there’s a limit to the speed by which matter can accumulate in black holes through accretion. The second way is through galactic collisions, during which two black holes can merge; however, in the early universe, galaxies were just starting to form. So, physicists have been left wondering how these ancient cosmological wonders grew so massive so quickly. Ultralight dark matter particles could be the missing piece.
Ultralight Dark Matter Particles the Missing Piece?
“We theorized how particles in the dark sector could undergo a phase transition that enables matter to very efficiently collapse into black holes,” Denton said. “When the temperature of the universe is just right, the pressure can suddenly drop to a very low level, allowing gravity to take over and matter to collapse. Our understanding of known particles indicates that this process wouldn’t normally happen.”
Predicting the Existence of Ultralight Dark-Matter
“These collapses are a big deal. They emit gravitational waves,” Denton said. “Those waves have a characteristic shape, so we make a prediction for that signal and its expected frequency range.”
Current gravitational wave experiments aren’t sensitive enough to validate the theory, but next-generation experiments may be able to detect signals of those waves. And based on the waves’ characteristic shape, physicists could then narrow in on the details of supermassive black hole formation. Until then, Brookhaven theorists will continue to evaluate new data and refine their model.
“The signals of our model are manifold such that different experiments are required to probe this model: We predict the existence of an ultralight dark-matter candidate, a scalar boson with mass around 10^(-19) eV, that can be probed with stellar kinematic data from dwarf spheroidal galaxies”, wrote Julia Gehrlein in an email to The Daily Galaxy. “We also predict the existence of gravitational waves which, in a region of parameter space, can be tested by pulsar timing arrays. The simultaneous observation of our model predictions will allow us to thoroughly probe the proposed mechanism for the formation of supermassive primordial black holes.”
Source: Hooman Davoudiasl et al, Supermassive Black Holes, Ultralight Dark Matter, and Gravitational Waves from a First Order Phase Transition, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.081101
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Peter Denton, Julia Gehrlein, Brookhaven National Laboratory
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.