The early universe is so far outside our understanding of how the world works that it is difficult to describe in words. At that time, the universe was not filled with stars and galaxies but with a boiling soup of quarks and gluons, with several black holes that were thrown, sometimes exploding like depth charges. That is the first universe compiled by a new paper, available in preprint from arXiv, from researchers at the Vrije Universiteit Brussel and MIT however.
Primordial Black Holes (PBHs) are the subject of much research today. They are hypothetical objects that could have formed in the first seconds after the Big Bang, and are very different from the types of stellar mass black holes we see today. In the denser environment after the Big Bang, more dense objects would have fallen directly into black holes – ranging in size from microscopic to giants.
For this research paper, the authors focus on low-mass PBHs. While we think of black holes as absorbing light and everything possible, they actually add energy to the space around them. However, Hawking radiation, named after Stephen Hawking, the physicist who discovered it, is still catching. According to the theory, the smaller the black hole, the hotter it is, and the faster it evaporates. PBHs weighing less than 500 trillion grams (which is relatively small by black hole standards) would have completely evaporated in our current time. But they don’t go quietly into that good night – they go out with a bang.
Fraser talks about ancient black holes and how important they were in the early universe.
The current cosmological theory about the death of PBHs simply involves dissipating their energy out of the cosmic fluid, creating the same “hot spot” of quark-gluon soup that the early universe was made of. But, according to a new paper, the reality of how black holes die was very violent and impressive. In particular, they observed the hydrodynamics of the plasma around the dying PBH and realized that the energy released by these super-small black holes was so large and concentrated that it created extreme pressures.
Very strong pressures in the liquid (or plasma) can cause a wave, and in this case, the researchers believe that is what happened when the very small PBHs died. In fact, the dying PBH created a fireball that quickly expanded into the cosmic soup.
According to the paper, this PBH vaporization process can be divided into four different phases. In the first phase, when the PBH is still very large, it slowly evaporates, creating a stable, expanding plasma bubble. It eventually shrinks to a point small enough to enter the second phase, where it releases its remaining energy immediately, creating a massive explosion that can be made using a mechanism known as the Blandford-McKee regime.
Fraser also refers to PBHs as remnants of the early universe.
As the wave expands outward, it sweeps away more of the surrounding plasma and eventually descends into the third phase, which is governed by a non-coherent shock system known as the Sedov-Taylor regime. Finally, even the shockwave whose energy is absorbed by the surrounding plasma, actually loses its energy completely when it enters the fourth phase.
OK, but what do violently dying black holes in the early universe have to do with the physics of the universe today? According to the paper, it may hold the answer to baryogenesis.
Baryogenesis is a fancy word for why there is any physical matter at all. According to our best theories of the big bang, matter and anti-matter should have been created equally – which means they should also have perfectly annihilated each other. But somehow, what we know today as “matter” somehow won that battle, which is what we now call baryogenesis, or the formation of baryons (subatomic particles (protons and neutrons) common matter is made of).
Fraser talks about where all the antimatter has gone.
Our best guess is that, somewhere in the early universe, there was a drastic shift away from thermal equilibrium that caused more matter than antimatter to exist. The authors of the current paper point to a property of the early universe called Electroweak (EW) symmetry as a possible explanation. If the plasma temperature of the early universe had dropped below 162GeV (and yes, cosmologists measure temperature in electron-volts – but that’s a story for another time), the EW symmetry would have been broken.
The authors believe that shock waves from the PBH explosion would have temporarily pushed the temperature above that threshold, creating pockets of EW symmetry within the moving plasma bubble. It’s exactly the kind of breakthrough that would be needed to create a matter-antimatter imbalance in the universe — and it’s interesting enough that one research group is exploring what that means in an accompanying paper.
In short, according to this new theory, the first universe may have been created by a violent explosion of small black holes, and that basically everything we can see in the universe, including us, is made of matter created by that explosion. So instead of saying we’re made of star matter, maybe we can start saying we’re made of black shockwaves – though there’s no ring to it.
Learn more:
M. Vanvlasselaer et al. – Shocks from Exploding Primordial Black Holes in the Early Universe
UT – Signal from the Stars
UT – Where are the Primordial Black Holes?
UT – Is There a Connection Between Primordial Black Holes, Neutrinos, and Dark Matter?
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