When it comes to primordial black holes being dark matter suspects, their alibi may fall apart. Small black holes, created seconds after the universe’s birth, can survive longer than expected, reviving suspicion that primordial black holes could be responsible for dark matter, the universe’s most mysterious stuff.
Dark matter currently represents one of the most pressing problems in physics. That’s because dark matter, despite making up an estimated 85% of the matter in the cosmos, remains effectively invisible to our eyes because it does not interact with light.
Because the particles that make up atoms that form ‘everyday’ things we can see, such as stars, planets and our own bodies, clearly interact with light, this has led to the search for dark matter particles beyond the standard model of particle physics . Many scientists believe the answer may still lie within the Standard Model, but if we look at a little cousin of cosmic objects we usually think of as enormously massive and even monstrous: black holes.
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Max Planck Institute scientist Valentin Thoss and Ana Fernandes Alexandre of the University of Lisbon are two researchers who have recently been involved in such studies. They hypothesize that small black holes born more than 13.8 billion years ago, just after the Big Bang, and no larger than a proton, could cluster and become suspects for dark matter without the need for new physics.
Not only has a recent change in thinking about how black holes “evaporate” led to a reassessment of the viability of primordial black holes, as dark matter suspects, but as the search for a dark matter particle largely comes to nothing, more researchers may begin to look more seriously at the dark matter theory of the original black hole.
What are primordial black holes?
“As the name suggests, ‘primordial black holes’ are a type of black hole that forms at the beginning of the universe,” Thoss told Space.com. “In fact, within the first fraction of a second of the universe.”
He explained that all the structures we observe in the universe, from superclusters of galaxies to the galaxies within themselves, formed from light overdensities in space that were present during the early universe. If the early universe experienced much stronger density fluctuations than the fluctuations that created these features, and these fluctuations collapsed at an earlier time than the galaxies actually formed, then those overly dense patches could have given rise to primordial black holes.
Thoss added that depending on when this collapse occurred and the magnitude of the collapse, these primordial black holes would have very different masses. The original black holes that Thoss and Fernandes Alexandre considered as possible candidates for dark matter would have masses ranging between a few tons and a thousand tons, to be precise, which is less than the mass of a planet and more in the category of a small asteroid.
Considering how the smallest black holes scientists have discovered so far, known as stellar-mass black holes, have a mass equal to between 3 and 50 times that of the Sun – which itself is 2.2 times 10 to the power of 27 (22 followed by 26). zeros) tons – these primordial black holes are incredibly small.
Like their larger black hole counterparts, formed by the collapse of massive stars or the merger of relatively smaller black holes, primordial black holes would have a light-catching outer boundary called an event horizon, according to Fernandes Alexandre. The diameter of this horizon is determined by the mass of the black hole, meaning that the event horizon would be incredibly small in those cases. “Smaller than the radius of a proton,” said Fernandes Alexandre.
Small, pristine black holes were previously ruled out as candidates for dark matter because all black holes are thought to “leak” a type of thermal radiation first theorized by Stephen Hawking in 1974 and later called “Hawking radiation.”
The smaller a black hole, the faster it should leak Hawking radiation and therefore the faster it should evaporate. That means that if primordial black holes ever existed, the smallest examples shouldn’t be around today — but dark matter clearly is.
“Primordial black holes of the masses that Ana and I are now considering were previously considered excluded because they were thought to have completely evaporated into the universe by then,” Thoss said.
However, recent work by Giorgi Dvali, a theoretical physicist at the University of Munich who has worked with Thoss and Fernandes Alexandre, has suggested that the evaporation process fails at some point. This means that the primordial mass black holes could reach a semi-stable state, according to the scientists.
“To reduce its mass through the emission of Hawking radiation, the black hole must ‘rewrite’ its information, or something else. This rewriting process takes time,” explains Fernandes Alexandre. “It’s called ‘memory burden’ because of this memory that now has to be passed on to something else, and that just slows down the evaporation process in general. So it’s a kind of stabilization.”
And that ‘rescue mechanism’ means that primordial black holes are back as potential candidates for dark matter!
A dead letter for dark matter?
However, the fact that primordial black holes could exist in the universe today does not immediately mean that they should be considered dark matter suspects. Coincidentally, there are other reasons to connect these small hypothetical black holes to the mysterious matter content of the universe.
Perhaps the most obvious connection is the lack of interaction of dark matter with light. Dark matter does not emit or reflect light, and the event horizon that bounds all black holes represents the point at which the escape velocity required to pass it exceeds the speed of light. That means that primordial black holes would ‘capture’ all incoming light, resulting in an apparent lack of interactions.
“If they are light enough, primordial black holes somewhere around a planetary mass behave like particles of dark matter for all purposes we are interested in,” Thoss said. “Dark matter is ‘collisionless’ in standard models, so dark matter particles do not interact with each other to the extent that it affects the universe.”
He added that if primordial black holes were lighter than the mass of planets, they would be so small even on cosmic time scales that they would collide very rarely. These primordial black holes could previously cluster to create the gravitational effects we currently attribute to dark matter, such as providing the gravitational influence that prevents rapidly spinning galaxies from flying apart.
But if primordial black holes must cluster to account for the effects of dark matter, what would prevent these black holes from contracting and merging to create larger black holes? Wouldn’t a cluster of small black holes eventually become one huge black hole? Thoss said this has been investigated, and the answer is simply, “No.”
“Even if you take clustering into account, the merger timescales are so long that they would only merge into truly massive black holes over the entire age of the universe,” he continued.
Thoss added that the beauty of using primordial black holes as an explanation for dark matter is that, unlike suggesting a hypothetical particle such as an axion to explain the mystery, primordial black holes do not require an extension of the standard model of particle physics. the best explanation we have for the universe on a subatomic scale.
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Still, it will be incredibly difficult to confirm that primordial black holes are dark matter, if they can truly explain the phenomenon. Again, their light-catching nature means they are effectively invisible. Furthermore, at such small sizes, they don’t have the same immense gravitational effects as their stellar and supermassive brethren.
Even then, if a cluster of primordial black holes is detected, there is no real way to tell the difference between many small black holes and one large black hole.
Despite this difficulty, Thoss and Fernandes Alexandre plan to continue following the tail of the primordial black holes – at least in theory. If dark matter particle candidates still don’t manifest, the answer may be for more physicists to look over the metaphorical fence between particle physics and cosmology.
“I wouldn’t say that primordial black holes have ever been dismissed as dark matter candidates; however, they were ignored for a while,” Fernandes Alexandre said. “Now that we take into account the fact that we have no real detection of dark matter in particles, I think it becomes increasingly relevant to consider this option.”