by When primordial black holes are suspected of being dark matter, their alibi may be falling apart. Tiny black holes, created seconds after the birth of the universe, may live longer than expected, changing doubts that primordial black holes could account for dark matter, the stuff that mysterious in the universe.
Dark matter represents one of the most pressing problems in physics at the moment. That’s because, despite making up an estimated 85% of the matter in the cosmos, dark matter remains invisible to our eyes because it doesn’t interact with light.
Because the particles that make up atoms that make up “everyday” stuff we can see, like stars, planets, and our own bodies, clearly interact with light, this has fueled the search for dark matter particles outside of the Standard Model of particle physics. Many scientists believe that the answer may still lie within the Standard Model, however, if we consider a small cousin of cosmic objects we are used to being amazingly massive, and even terrifying: Black holes.
Related: New view of the supermassive black hole at the heart of the Milky Way hints at a disturbing hidden element (image)
Max Planck Institute scientist Valentin Thoss and Ana Fernandes Alexandre from the University of Lisbon are two researchers who have recently been involved in such studies. They sit tiny black holes that were born more than 13.8 billion years ago, just after the Big Bang, which is no bigger than a proton, clustered to be a dark matter suspect without the need for new physics.
Not only has a recent shift in thinking about how black holes “evaporate” prompted a reassessment of the viability of primordial black holes as dark matter is doubted, but as the search for a dark matter particle continues to draw white mainly, more researchers could start on. look at the primordial black hole dark matter theory more seriously.
What are primordial black holes?
“As the name suggests, a ‘primordial black hole’ is a type of black hole that forms at the beginning of the universe,” Thoss 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 made of a small overdensity in space present during the early universe. If there were much stronger density fluctuations in the early universe than those that created these features, and if these fluctuations collapsed earlier than galaxies were actually formed, those overdense patches could have fueled primordial black holes .
Thoss said that depending on the time this collapse might occur as well as the scale of the collapse, these initial black holes would have very different masses. The primordial black holes that Thoss and Fernandes Alexandre considered possible dark matter candidates would have masses between a few tons and a thousand tons, specifically, which is less than the mass of a planet and more in the small asteroid category.
Considering how the smallest black holes scientists have discovered so far, known as stellar mass black holes – have masses equal to between 3 and 50 times that of the sun – which is 2.2 times 10 power of 27 (22 and later 26). zero) tons – these primordial black holes are extremely tiny.
Like their larger black hole counterparts formed from the collapse of massive stars or the merger of relatively smaller black holes, according to Fernandes Alexandre, primordial black holes would have an outer light-trapping boundary called an exit horizon. The diameter of this horizon is determined by the mass of the black hole, which means that the event horizon would be extremely small in those cases. “Smaller than the radius of a proton,” said Alexandre Fernandes.
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 first theorized by Stephen Hawking in 1974 and known as “Hawking radiation .”
The smaller the black hole, the faster it would emit Hawking radiation and, therefore, the faster it should evaporate. That means if primordial black holes ever existed, there shouldn’t be even the tiniest examples around today – yet, dark matter clearly exists.
“Primordial black holes with the masses Ana and I am now thinking that it was considered basically before it was excluded because it was assumed that they had completely evaporated by this time in the Universe,” said Thoss.
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 stops at a certain point, however. This means primordial black holes of the masses that the scientists considered could reach a semi-stable state.
“In order to reduce its mass by emitting Hawking radiation, the black hole has to ‘rewrite’ its information, or something else. This rewriting process takes time,” explained Fernandes Alexandre. “It’s called ‘memory load’ because of this memory that now needs to be passed on to something else, and that actually slows down the overall evaporation process. So it’s a form of stabilization.”
And that “rescue mechanism” means that primordial black holes are back as potential candidates for dark matter!
A dead ringer for dark matter?
The fact that primordial black holes may exist in the universe today, however, does not immediately mean that they should be considered suspicious dark matter. As it turns out, there are other reasons to connect these hypothetical little black holes with the mysterious matter of the universe.
Perhaps the most obvious connection is the lack of interaction of dark matter with light. Dark matter neither emits nor reflects light, and the event horizon that bounds each black hole represents the point at which the escape velocity necessary to traverse it 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, somewhere around the mass of a planet, primordial black holes behave like dark matter particles for all purposes we are interested in,” Thoss said. “Dark matter is ‘collisionally less’ in standard models, so dark matter particles don’t interact with each other so much that it affects the universe.”
He added that if primordial black holes are lighter than planetary masses, then, even on cosmic time scales, they would be so small that they would rarely collide. These primordial black holes could cluster to create the gravitational effects we currently define for dark matter, such as providing the gravitational influence that prevents rapidly spinning galaxies from flying apart .
But, if primordial black holes had to cluster to account for the effects of dark matter, what would prevent these black holes from pulling together and merging to form larger black holes? Wouldn’t a cluster of small black holes be a super black hole after all? Thoss said this has been investigated, and the answer is simple: “No.”
“Even if you take clustering into account, the timescales for mergers are so long that they would only merge into supermassive black holes over the entire age of the universe,” he continued.
Thoss said the beauty of using primordial black holes as an explanation for dark matter is that, unlike proposing a hypothetical particle as an axiom to explain the mystery, primordial black holes do not require an extension of the Standard Model of particle physics. , our best explanation of the universe at sub-atomic scales.
RELATED STORIES
— Dark matter was detected hanging from the cosmic web for the 1st time
— How the successor to the Large Hadron Collider will pursue the dark universe
– A massive galaxy is a cosmic puzzle with no dark matter
Still, it will be extremely difficult to confirm primordial black holes as dark matter, if they actually explain the phenomenon. Again, their light-trapping nature means they are effectively invisible. Furthermore, at such small sizes, they do not have the same massive gravitational effects as their supermassive and supermassive brethren.
Even then, if a cluster of primordial black holes is discovered, 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 stay hot on the tail of primordial black holes – at least in theory. If particle candidates for dark matter still fail to demonstrate, the answer may be to get more physicists to start looking at the metaphorical fence between particle physics and cosmology.
“I wouldn’t say that primordial black holes were ever dismissed as candidates for dark matter; however, they were ignored for a while,” Fernandes Alexandre said. “Now, in line with the fact that we don’t really have any detection of dark matter particles, I think it will become more and more relevant to consider this option.”
