When it comes to primordial black holes being dark matter suspects, their alibi may be falling apart. Tiny black holes, created seconds after the birth of the universe, may survive longer than expected, reigniting a suspicion that primordial black holes could account for dark matter, the universe's most mysterious stuff.
Dark matter currently represents one of the most pressing problems in physics. That is because, despite making up an estimated 85% of the matter in the cosmos, dark matter remains effectively invisible to our eyes because it doesn't interact with light.
Because the particles that comprise atoms that compose "everyday" stuff we can see, like stars, planets, and our own bodies, clearly do interact with light, this has prompted the search for dark matter particles outside the Standard Model of particle physics. Many scientists believe the answer could still lie within the Standard Model, however, if we consider a diminutive cousin of cosmic objects we usually view as tremendously massive, and even monstrous: Black holes.
Max Planck Institute scientists Valentin Thoss and Ana Alexandre are two researchers who have recently been involved in such studies. They posit tiny black holes born over 13.8 billion years ago, just after the Big Bang, that are no larger than a proton, could cluster to become suspects for dark matter without the need for new physics.
Not only has a recent change in thinking regarding how black holes "evaporate" prompted a reassessment of primordial black holes' viability as dark matter suspects, but as the search for a dark matter particle continues to mostly draw a blank, more researchers could begin to look at the primordial black hole dark matter theory more seriously.
What are primordial black holes?
"As the name suggests, 'primordial black holes' are a type of black hole that is formed at the beginning of the universe," Thoss told Space.com. "Within the first fraction of a second of the universe, in fact."
He explained that all the structures we observe in the universe, from superclusters of galaxies to the galaxies within themselves, are formed from slight overdensities in space present during the early universe. If the early universe experienced much stronger density fluctuations than the those which created these features, and these fluctuations collapsed at an earlier time than galaxies indeed formed, then those overly dense patches could have spurred primordial black holes.
Thoss added that, depending on the time at which this collapse may have happened as well as the scale of the collapse, these primordial black holes would have very different masses. The primordial black holes that Thoss and Alexandre considered possible dark matter candidates would have masses ranging between a few tons and a thousand tons, to be specific, 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 to date, known as stellar-mass black holes, have masses equivalent 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 zeroes) tons — these primordial black holes are incredibly tiny.
Like their larger black hole counterparts formed from either the collapse of massive stars or the merger of relatively smaller black holes, according to Alexandre, primordial black holes would have a light-trapping outer boundary called an event horizon. The diameter of this horizon is determined by the mass of the black hole, which means the event horizon would be incredibly small in those cases. "Smaller than the radius of a proton," Alexandre said.
Small, primordial black holes had previously been ruled out as dark matter candidates because all black holes are thought to "leak" a type of thermal radiation first theorized by Stephen Hawking in 1974 and later named "Hawking radiation."
The smaller a black hole, the more rapidly it should leak Hawking radiation and, thus, the faster it should evaporate. That means if primordial black holes ever existed, the smallest examples shouldn't be around today — yet, dark matter clearly is.
"Primordial black holes with the masses Ana and I are now considering had been essentially previously considered ruled out because they were assumed to have evaporated fully by this time in the universe," Thoss said.
Recent work by Giorgi Dvali, a theoretical physicist at the University of Munich who has collaborated with Thoss and Alexandre, has suggested that the evaporation process breaks down at a certain point, however. This means primordial black holes of the masses the scientists considered could achieve a semi-stable state.
"In order to decrease its mass through the emission of Hawking radiation, the black hole has to 'rewrite' its information, or something else. This rewriting process takes time," Alexandre explained. "It is called 'memory burden' because of this memory that now has to be passed along to something else, and that just kind of slows down the evaporation process overall. So it's a kind of stabilization."
And that "rescue mechanism" means primordial black holes are back as potentially dark matter candidates!
A dead ringer for dark matter?
The fact that primordial black holes could exist in the universe today, however, doesn't immediately mean they should be considered dark matter suspects. As it happens, there are other reasons to link these tiny hypothetical black holes to the universe's mysterious matter content.
Perhaps the most obvious connection is dark matter's lack of interaction with light. Dark matter doesn't emit or reflect light, and the event horizon that bounds all black holes represents the point at which the escape velocity necessary to cross it exceeds the speed of light. That means primordial black holes would "trap" all incident light, resulting in an apparent lack of interactions.
"If they are light enough, somewhere around a planetary mass, primordial black holes behave like particles of dark matter for all purposes that we are interested in," Thoss said. "Dark matter is 'collision-less' in standard models, so dark matter particles do not interact with each other to such a degree that it affects the universe."
He added that if primordial black holes are lighter than planetary masses, then, even on cosmic timescales, they would be so small they'd very rarely collide. These primordial black holes could rather 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.
Yet, if primordial black holes have to cluster to account for the effects of dark matter, what would prevent these black holes from drawing together and merging to create larger black holes? Wouldn't a cluster of tiny black holes eventually just become one tremendous black hole? Thoss said this has been investigated, and the answer is simply: "No."
"Even if you take into account clustering, the time scales for the merger are so long that they would only merge into really 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 don't require an extension to the Standard Model of particle physics, the best explanation we have of the universe on subatomic scales.
Still, primordial black holes are going to be incredibly difficult to confirm as dark matter, if they really do explain the phenomenon. Again, their light-trapping nature means they are effectively invisible. Plus, at such diminutive sizes, they don't have the same immense gravitational effects as their stellar and supermassive brethren.
Even then, should a cluster of primordial black holes be detected, there is no real way to tell the difference between lots of little black holes and one large black hole.
Despite this difficulty, Thoss and Alexandre intend to stay hot on the tail of primordial black holes — theoretically, at least. If particle candidates for dark matter continue to fail to manifest, perhaps the answer is to get more physicists to start looking over the metaphorical fence between particle physics and cosmology.
"I wouldn't say that primordial black holes were ever dismissed as dark matter candidates; they were ignored for a while though," Alexandre said. "Now, lined up with the fact that we don't really have any detection of particle dark matter, I think it just becomes more and more relevant to consider this option."
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