Black holes are among the most compelling mysteries of the universe.
Nothing, not even light, can escape a black hole. And at the center of
nearly every galaxy there is a supermassive black hole that's millions to
billions of times more massive than the sun. Understanding black holes, and
how they become supermassive, could shed light on the evolution of the
universe.
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.
A cosmological phase transition is akin to a more familiar type of phase
transition: bringing water to a boil. When water reaches the exact right
temperature, it erupts into bubbles and vapor. Imagine that process taking
place with a primordial state of matter. Then, shift the process in reverse
so it has a cooling effect and magnify it to the scale of the universe.
"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 happened with the
known particles because they interact often. But what if there are
not-yet-known particles out there performing differently?"
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."
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.
"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."
Such a phase transition would be a dramatic event, even for something as
spectacular as the universe.
"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.
Reference:
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
Tags:
Space & Astrophysics