Given that primordial black holes are suspects of dark matter, their excuse may be falling apart. Small black holes formed seconds after the birth of the universe may survive longer than expected, reigniting suspicion that primordial black holes could explain dark matter, the universe’s most mysterious substance.
Dark matter currently represents one of the most pressing problems in physics. This is because, despite making up an estimated 85% of the matter in the universe, dark matter remains virtually invisible to our eyes because it does not interact with light.
This has led to the investigation of dark matter particles outside the Standard Model of particle physics, as the particles that include the atoms that make up the “everyday” things we can see, such as stars, planets, and our own bodies, clearly interact with light. Many scientists believe the answer can still be found in the Standard Model, but if we consider a tiny cousin of the cosmic objects we usually view as extremely large or even monstrous: Black holes.
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Max Planck Institute scientist Valentin Thoss and Ana Fernandes Alexandre from the University of Lisbon are two researchers who have recently participated in such studies. They suggest that small black holes no larger than a proton, born 13.8 billion years ago, just after the Big Bang, could cluster together to become suspicious for dark matter without the need for new physics.
A recent shift in thinking about how black holes “evaporate” has not only led to a reassessment of the viability of primordial black holes as suspected dark matter, but as the search for dark matter particles continues to mostly leave a void, more researchers may begin to do so. Take a more serious look at the primitive black hole dark matter theory.
What are primitive black holes?
“As the name suggests, ‘primordial black holes’ are a type of black hole that formed at the beginning of the universe,” Thoss told Space.com. “It’s actually the first fraction of a second of the universe.”
He explained that all the structures we observe in the universe, from galaxy superclusters to galaxies within themselves, are formed from slight overdensities in space that existed in the early universe. If the early universe experienced density fluctuations much stronger than those that created these features, and if these fluctuations collapsed earlier than galaxies formed, then these extremely dense spots may have promoted primitive black holes.
Thoss added that depending on when this collapse might have occurred and the scale of the collapse, these primordial black holes could have very different masses. The primordial black holes that Thoss and Fernandes Alexandre consider as possible dark matter candidates would have masses ranging from a few tons to a thousand tons, less than the mass of a planet and more than the mass of a small category of asteroids, to be more specific.
Given that the smallest black holes scientists have discovered to date, known as stellar-mass black holes, have masses ranging from 3 to 50 times that of the Sun, itself 2.2 times 10 to the 27th power (22 followed by 26). zero)ton – these primitive black holes are incredibly small.
According to Fernandes Alexandre, like their more massive black hole counterparts formed from the collapse of massive stars or the merger of relatively smaller black holes, primordial black holes will have an outer boundary that traps light, called the event horizon. The diameter of this horizon is determined by the mass of the black hole; This means that in these cases the event horizon will be incredibly small. “It’s smaller than the radius of a proton,” said Fernandes Alexandre.
Small, primordial black holes had previously been overlooked as candidates for dark matter because all black holes were 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 is, the faster it must leak Hawking radiation and therefore the faster it must evaporate. This means that if primordial black holes existed, the smallest examples should not be around today; But dark matter clearly exists.
“Primordial black holes with the masses that Ana and I are currently considering were previously thought to have been ruled out, as they were assumed to have completely evaporated by this time in the universe,” Thoss said.
Recent work by Giorgi Dvali, a theoretical physicist at the University of Munich who collaborated with Thoss and Fernandes Alexandre, suggested that the evaporation process breaks down at a certain point. This means that primordial black holes of the masses scientists consider could reach a metastable state.
“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,” Fernandes Alexandre explained. “It’s called ‘memory load’ because that memory now has to be transferred to something else, and that kind of slows down the evaporation process in general. So it’s a kind of stabilization.”
And this “rescue mechanism” means primordial black holes are potentially returning as dark matter candidates!
A dead call for dark matter?
But the fact that primordial black holes may exist in the universe today does not mean that they should be considered direct dark matter suspects. In fact, 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 that dark matter does not interact with light. Dark matter does not emit or reflect light, and the event horizon surrounding all black holes represents the point at which the escape velocity required to pass it exceeds the speed of light. This means that primordial black holes would “capture” all incoming light, resulting in an apparent lack of interaction.
“If they get light enough, somewhere around a planetary mass, primordial black holes behave like dark matter particles for all purposes we’re interested in,” Thoss said. “In standard models, dark matter is ‘collisionless’, so dark matter particles do not interact with each other to a degree that would affect the universe.”
He added that if primordial black holes were less massive than planets, they would be so small that they would collide so rarely, even on cosmic time scales. These primitive black holes may choose to cluster to create the gravitational effects we currently attribute to dark matter, such as providing the gravitational effect that keeps rapidly spinning galaxies from pulling apart.
But if primordial black holes need to cluster to account for the effects of dark matter, what’s to stop these black holes from clumping together and merging to form larger black holes? Wouldn’t a cluster of small black holes eventually turn into a giant black hole? Thoss said this has been investigated and the answer is simply “No.”
“Even if you take clustering into account, the time scale of the merger is so long that they will only become 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 primordial black holes do not require an extension to the Standard Model of particle physics, as opposed to proposing a hypothetical particle like the axion to explain the mystery. It’s the best description we have of the universe at subatomic scales.
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However, it will be incredibly difficult to confirm that primordial black holes are dark matter if they truly explain this phenomenon. Again, their light-trapping nature means they are effectively invisible. Moreover, at such small sizes, they do not have the same enormous gravitational effects as their stars and supermassive siblings.
Even then, even if a group of primitive black holes were 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 intend to remain in the hunt for primitive black holes, at least in theory. If candidate particles for dark matter continue to fail to emerge, perhaps the solution is for more physicists to start looking over the metaphorical fence between particle physics and cosmology.
“I wouldn’t say primordial black holes have been ruled out as dark matter candidates, but they were ignored for a while,” Fernandes Alexandre said. said. “Now, given the fact that we have no detection of dark matter particles, I think it is becoming increasingly meaningful to consider this option.”