When it comes to primordial black holes being suspected as dark matter, their excuse may be about to fall apart. Tiny black holes, which formed seconds after the universe was born, may persist for longer than expected, raising new suspicions that primordial black holes could be responsible for dark matter, the most mysterious substance in the universe.
Dark matter currently represents one of the most pressing problems in physics. This is because although it makes up an estimated 85% of the matter in the universe, dark matter remains invisible to our eyes because it does not interact with light.
Since the particles that make up the atoms that make up the “ordinary” objects we can see, such as stars, planets and our own bodies, clearly interact with light, this has prompted a search for dark matter particles outside the standard model of particle physics. Many scientists believe the answer still lies in the Standard Model, however, if we look at a smaller cousin of cosmic objects, we usually see them as extremely massive, even monstrous: black holes.
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Max Planck Institute scientist Valentin Thos And Anna Fernandez Alexander From the University of Lisbon two researchers recently participated in such studies. They hypothesize that small black holes that arose more than 13.8 billion years ago, immediately after the Big Bang, and are no larger than a proton, could have coalesced into dark matter suspects without the need for new physics.
Not only has the recent change in thinking regarding how black holes “evaporated” led to a reassessment of the viability of primordial black holes as dark matter is suspected, but as the search for dark matter particles continues to draw a mostly blank, it could begin to… More researchers are looking 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 formed at the beginning of the universe,” Thos told Space.com. “Within the first fraction of a second of the universe, actually.”
He explained that all the structures we observe in the universe, from superclusters of galaxies to galaxies within themselves, are formed by slight excess densities in space that were present during the beginning of the universe. If the early universe experienced density fluctuations much stronger than those that created these features, and these fluctuations collapsed earlier than galaxies actually formed, those extremely dense spots could have stimulated primordial black holes.
Depending on when this collapse occurred as well as the size of the collapse, these primordial black holes will have very different masses, Thos added. The primordial black holes considered by Thos and Fernandez-Alexander as possible candidates for dark matter would have masses 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.
Given that the smallest black holes scientists have discovered so far, known as stellar-mass black holes, have masses equivalent to between 3 and 50 times the mass of the Sun – which itself is 2.2 times 10 to the 27th power (22 followed by 26). (Zeros) tons – These primordial black holes are incredibly small.
Like their larger black hole counterparts that formed from either the collapse of massive stars or the merger of relatively smaller black holes, according to Fernandez-Alexander, 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,” Fernandez-Alexander said.
Small primordial black holes were previously ruled out as candidates for dark matter because all black holes are thought to “leak” a type of thermal radiation that was first hypothesized by Stephen Hawking in 1974 and later called “Hawking radiation.”
The smaller the black hole, the faster the Hawking radiation can escape and, therefore, the faster it must evaporate. This means that if primordial black holes existed at all, the smallest examples should not exist today, but dark matter clearly does.
“Primordial black holes with the masses that Anna and I are studying now were previously considered unlikely because they were assumed to have completely evaporated by this time in the universe,” Thos said.
However, recent work by György Dvali, a theoretical physicist at the University of Munich who collaborated with Thos and Fernandez-Alexander, has suggested that the evaporation process breaks down at some point. This means that primordial black holes with the masses considered by scientists could reach a quasi-steady state.
“In order to reduce its mass by emitting Hawking radiation, the black hole has to rewrite its information, or something else. And this rewriting process takes time,” Fernandez-Alexander explained. “This is called 'memory burden' because that memory now has to move on to something else, and this slows down the overall evaporation process. So it's a kind of stabilization.”
This “rescue mechanism” means that primordial black holes are back as potential candidates for dark matter!
A dead ringer for dark matter?
However, the fact that primordial black holes exist in the universe today does not immediately mean that they should be considered dark matter suspects. In fact, there are other reasons to link these small hypothetical black holes to the mysterious matter content of the universe.
Perhaps the most obvious connection is that dark matter does not interact with light. Dark matter does not emit or reflect light, and the event horizon that borders all black holes marks the point at which the escape velocity needed to cross exceeds the speed of light. This means that primordial black holes would “trap” all incident light, resulting in an apparent lack of interactions.
“If they are light enough, then somewhere around a planetary mass, primordial black holes behave like dark matter particles for all purposes we care,” Thos said. “Dark matter is ‘collision-less’ in the standard models, so dark matter particles do not interact with each other to the point that they affect the universe.”
He added that if primordial black holes were lighter than planetary masses, they would, even on cosmic time scales, be so small that they would rarely collide. These primordial black holes could combine to create the gravitational effects we currently attribute to dark matter, such as providing a gravitational influence that prevents rapidly rotating galaxies from being blown away.
However, if primordial black holes had to aggregate to account for the effects of dark matter, what would prevent these black holes from gathering together and merging to form larger black holes? Wouldn't a bunch of tiny black holes eventually become just a supermassive black hole? This has been investigated, and the answer is simply: “No,” Thos said.
“Even if you take clumping into account, the merger time scales are so long that they would only merge into very massive black holes over the entire lifetime of the universe,” he continued.
Thos added that the beauty of using primordial black holes as an explanation for dark matter is that, unlike proposing a hypothetical particle like the axis to explain the puzzle, primordial black holes do not require an extension of the standard model of particle physics. Our best explanation of the universe is on subatomic scales.
However, it would be very difficult to confirm primordial black holes as dark matter, if they indeed explain this phenomenon. Again, their light-trapping nature means they are effectively invisible. In addition, at such small sizes, they do not have the same massive gravitational effects as their stellar and supermassive brothers.
Even then, if a group of primordial black holes were discovered, there is no real way to tell the difference between a lot of small black holes and one big black hole.
Despite this difficulty, Thos and Fernandez-Alexander intend to stay on the tail end of primordial black holes — at least in theory. If dark matter candidate particles continue to fail to emerge, perhaps the answer is to prompt more physicists to start looking at the metaphorical fence between particle physics and cosmology.
“I can't say that primordial black holes have ever been ruled out as dark matter candidates, but they have been ignored for a while,” Fernandez-Alexander said. “Now, with the fact that we don't really have any detection of particle dark matter, I think it's becoming more and more appropriate to consider this option.”