Primordial black holes are the earliest black holes thought to exist, and they vanished almost as fast as they came into being.
As Stephen Hawking predicted, black holes don’t only draw particles in, but release particles into space. This Hawking radiation could have scattered into the universe from the earliest black holes.
Though primordial black holes and their escaped particles eluded us, they may be hiding in dark matter and the cosmic microwave background, the first light to emerge in the universe.
When you think of a black hole, what probably comes to mind is the supermassive beast that goes absolutely feral when the smallest particle of matter creeps close enough. But, if our current theories are correct, the first black holes that existed went against that stereotype.
Primordial black holes (which are still theoretical) are thought to have formed just a fraction of a second after the Big Bang during a period called inflation, in which the universe expanded unfathomably fast, and denser areas collapsed in on themselves to become a swarm of tiny black holes (with masses less than a hundred tons each). Then, those micro-black holes vanished.
As legendary physicist Stephen Hawking predicted, because primordial black holes weren’t massive enough to accrete and hold onto material like their monstrous descendants, they ended up evaporating and radiating particles into space in what’s known as Hawking radiation. Now, a team of researchers led by Harvard astrophysicist Christopher Shallue suggests that whatever was left after these objects evaporated influences how the universe formed.
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“Since Hawking radiation produces all kinematically available particle species, evaporating primordial black holes may have populated the universe with particles,” Shallue and his team said in a study recently published in the Journal of Cosmology and Astroparticle Physics. Most excitingly, that means some of the particles were standard particles, while others were dark matter particles (the latter of which came in both massive and massless varieties). The team behind this paper calls the dark matter portion “Hawking relics.”
In the model explored by this paper, right after inflation, energy from the (very light and very fast) particles escaping the primordial black holes reheated the universe into a scorching plasma. Once the plasma started to cool, neutrons and protons fused to form the first atomic nuclei, and eventually, there were enough elements to create what would be the embryos of the first stars and galaxies. However, the Hawking relics—which, according to the researchers, make up at most a mere 2 percent of dark matter—may have been holding them back.
According to the study, the massive Hawking relics may have affected the formation of galaxies and other large structures in space. While most dark matter is assumed to be cold dark matter—the sluggish particles of which don’t get in the way of much—Shallue and his team think the Hawking relics are a type of matter called warm dark matter, which can actually suppress the growth of galaxies because its particles are too light and move too fast to accumulate into anything.
Massive Hawking relics are thought to be WIMPs (weakly interacting massive particles), which are dark matter particles that have weak interactions with other particles. These have been theorized to exist in both warm and cold dark matter, so they make sense in Shallue’s warm dark matter hypothesis. WIMPs are unable to absorb or emit light on their own, but create gamma rays when they collide with each other and self-destruct.
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So, what could have happened to massless Hawking relics? These particles most likely went into cosmic radiation. It may even be possible to someday detect them in the cosmic microwave background (CMB), which is what remains of the earliest radiation in the universe. If they exist, they are probably not the only relic particles in the CMB. Subatomic particles from the birth of the universe, such as neutrinos, might still be hanging around, too.
There are not enough Hawking relics in dark matter to explain where they all went, and they have eluded detection everywhere else, but there are still possibilities for eventually proving that they are, in fact, still out there. Maybe a future telescope will be able to look back 14 billion years—right to when primordial black holes came into being.
“The discovery of a Hawking relic […] would not only be important for early-universe cosmology,” the researchers said in the same study, “but it would also open a new frontier of particle physics.”