The early Universe goes so far beyond our understanding of the way the world works that it is difficult to describe in words. Back then, the cosmos was filled not with stars and galaxies, but with a seething broth of quarks and gluons, with a few microscopic black holes thrown in, which occasionally exploded like depth charges. Thus is the early Universe described in a new paper written by researchers from the Free University of Brussels and the Massachusetts Institute of Technology.
Primordial black holes. Source: phys.org
Energy of primordial black holes
Primordial black holes (PBHs) are the subject of much current research. These are hypothetical objects believed to have formed in the first few seconds after the Big Bang and are significantly different from the stellar-mass black holes we can observe today. In the extremely dense environment following the Big Bang, some of the denser regions may have collapsed directly into black holes—ranging in size from microscopic particles to supermassive giants.
In this particular scientific article, the authors focus on low-mass primordial black holes. Although we tend to think of black holes as objects that swallow up light and everything else, they actually emit energy into the surrounding space. However, Hawking radiation—named after the physicist who discovered it, Stephen Hawking—has one distinctive feature. According to the theory, the smaller a black hole is, the hotter it becomes and the faster it evaporates. Black holes with a mass of less than 500 trillion grams (which is relatively small by black hole standards) should have completely evaporated by now. But they don’t fade away quietly on that peaceful night—they vanish with a loud bang.
Modern cosmological theory regarding the demise of supermassive black holes posits that their energy is simply dispersed into the cosmic plasma, creating a stable “hot spot” within the quark-gluon plasma that made up the early Universe. However, according to a new article, the demise of black holes was actually much more violent and dramatic. In particular, the scientists observed the hydrodynamics of the plasma surrounding a dying PBH and realized that the energy released by these microscopic black holes was so immense and concentrated that it created extreme pressure gradients.
Significant pressure fluctuations in a fluid (or plasma) can trigger a shock wave, and in this particular case, scientists believe that is what happened when microscopic PBHs died. In essence, a dying PBH formed a relativistic fireball that rapidly expanded outward into the cosmic soup.
Propagation of the shock wave from a PBH explosion
According to this article, the evaporation process of a primordial black hole can be divided into four distinct phases. In the first phase, when the primordial black hole still has a relatively large mass, it slowly evaporates, forming a stable, expanding plasma bubble. Eventually, it contracts to such a small point that it enters a second phase, where it instantly releases its residual energy, creating an ultrarelativistic explosion that can be simulated using a model known as the Blandford-McKee regime.
As the shock wave propagates outward, it captures more and more of the surrounding plasma and slows down, transitioning into a third phase, which is modeled using a non-relativistic shock wave model known as the Sedov-Taylor regime. Ultimately, even the energy of this shock wave is absorbed by the surrounding plasma and dissipates almost entirely as it transitions into the fourth phase.
Microscopic black holes and the development of the theory of “baryogenesis”
All well and good, but what do microscopic black holes that rapidly die in the early Universe have to do with modern cosmological physics? According to this article, this may be the very answer to the question of baryogenesis.
“Baryogenesis” is a scientific term that explains why physical matter exists at all. According to our best Big Bang theories, matter and antimatter should be created in equal amounts—which means they should also completely annihilate each other. But somehow, what we now know as “matter” won that battle, and this is what we now call baryogenesis, or the creation of baryons (the subatomic particles—protons and neutrons—that make up ordinary matter).
We believe that somewhere in the early Universe, a sudden disruption of thermal equilibrium occurred, resulting in more matter than antimatter being preserved. The authors of this article point to a property of the early Universe known as electroweak (EW) symmetry as a possible explanation. If the plasma temperature of the early Universe had fallen below 162 GeV, EW symmetry would have been broken.
The authors believe that shock waves from the explosions of primordial black holes could have temporarily raised the temperature above this threshold, creating EW symmetry centers within a moving plasma “bubble.” Such a mechanism, operating under conditions of disequilibrium, is precisely what is needed to create an imbalance between matter and antimatter in the Universe—and this is so fascinating that the same research group explores the implications of this phenomenon in a related article.
In short, according to this new theory, the early Universe may have been formed by violent explosions of tiny black holes, and literally everything we see in the Universe—including ourselves—consists of matter created by these explosions. So instead of saying that we are made of stardust, perhaps we could start saying that we are made of black hole shock waves, even though that doesn’t sound quite the same.
According to phys.org