Were we born from exploding primordial black holes? New study suggests so

How was the early universe after its birth? It didn’t have stars and galaxies. Instead, it was a simmering soup of particles such as quarks and gluons, with a few microscopic black holes popping up and then dying through violent explosions. Now, researchers at Vrije Universiteit Brussel and Massachusetts Institute of Technology have described this picture of the early universe in a new paper, available in pre-print on arXiv. Among other topics, primordial black holes (PBHs) constitute much of modern-day astronomical research. Their existence is still hypothetical. They are thought to have formed in the very first seconds after the Big Bang. They are completely different from the types of stellar-mass black holes, which form after the death of massive stars. After the Big Bang, the environment in which the particles floated around was extremely dense. Denser regions in such an environment collapsed directly into black holes, spawning microscopic as well as supermassive giants. 

In the new paper, the researchers focused on low-mass PBHs. Black holes conjure up an invisible monster that swallows light and matter. But black holes also leak energy into space around them. Such energy emission is called Hawking radiation, a phenomenon named after physicist Stephen Hawking. According to the theory, black holes get hotter as they become smaller, causing them to evaporate faster. By that logic, PBHs that weighed under 500 trillion grams (which is comparatively small by black hole standards) would have completely evaporated by our current time. But then again, they wouldn't have done that quietly.

According to current cosmological theory, PBHs die, diffusing their energy out into the universe’s plasma. This creates a consistent “hot spot” in the quark-gluon mixture that made the early universe. But the new paper depicts that the deaths of such black holes are more violent and dramatic. The researchers studied the hydrodynamics of the plasma around a dying PBH. They found that such a dying black hole created an extreme pressure gradient. It is known that massive pressure gradients in a fluid (or a plasma) can trigger shock waves. This was exactly what happened when microscopic PBHs died. While dying, a PBH created a relativistic fireball that expanded and engulfed the cosmic soup. PBH evaporation process has four distinct phases.

While the PBH is still relatively massive in the first phase, it slowly evaporates, creating a steady, expanding bubble of plasma. Then, it shrinks to a small point and enters the second phase. At this stage, it releases the remaining energy instantaneously, creating an ultra-relativistic blast. It can be modeled using a framework known as the Blandford-McKee regime. Next, the shock wave continues to expand, causing it to sweep up more of the surrounding plasma. Then it slows down into the third phase, which is modeled by a non-relativistic shock wave framework known as the Sedov-Taylor regime. After dissipating all its energy to the surrounding plasma, it enters the fourth phase. But why do we need to study the microscopic black holes’ death to understand the early universe? According to the new paper, it might shed light on baryogenesis. 

Baryogenesis is a process that took place in the early universe and created matter-antimatter imbalance immediately after the Big Bang. Initially, matter and antimatter were thought to be created in equal proportion, which means they should have annihilated each other. But matter, somehow, had an edge over antimatter through baryogenesis, or the creation of baryons or subatomic particles such as protons and neutrons that make normal matter as we know it today. So how did matter win? Researchers guess a violent departure from thermal equilibrium caused matter to defeat antimatter. The authors of the new paper point to electroweak symmetry (EW), a property of the early universe. If the temperature of the early universe's plasma sank below 162GeV (giga-electron volts), EW symmetry would have been shattered. 

The researchers note that the shock waves created by PBH explosions may have temporarily sent temperatures over that threshold, thereby resulting in the creation of EW symmetry in a moving bubble of plasma. This lack of equilibrium is likely what caused the matter-anitmatter imbalance in the universe. To put it simply, it indicates that the early universe may have been molded by the explosions of tiny black holes, and all of us, including everything we see around us, are made of matter from those explosions.

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