Tiny Black Holes May Have Existed in the Primordial Universe. Could They Explain Dark Matter?

All the way back in the '70s, Stephen Hawking and Bernard Carr suggested that tiny black holes may have come into existence coinciding with the birth of the universe. Hawking and Carr reasoned that in the universe’s first fractions of a second, small fluctuations in density could have given certain regions additional mass. These regions may have then collapsed into a black hole.

If they exist, these primordial black holes were probably created in large numbers in the first second of the Big Bang, about 13.77 billion years ago. While the smallest would have disappeared by now, through the process of emitting Hawking radiation, Hawking’s calculations also indicated that black holes with a mass bigger than that of a small asteroid could possibly still be lurking in the universe today.

Dark Matter? What?

Possibly the most important, or at least the most interesting, implication of the current research is that these black holes may account for dark matter, which is thought to comprise about 85% of all matter in the universe, yet we have no idea where it originated or how it exists in general. These tiny black holes themselves could be the elusive dark matter in and of themselves. We are unable to see or study these primordial black holes, but their combined mass may mean they are responsible for one of the biggest mysteries in astrophysics: the origin of dark matter.

"Our study shows that without introducing new particles or new physics, we can solve mysteries of modern cosmology from the nature of dark matter itself to the origin of supermassive black holes," Nico Cappelluti, an assistant professor at the University of Miami and co-author of the new study, notes.

In addition to explaining how these black holes could account for dark matter, this research suggests, "If most of the black holes formed immediately after the Big Bang, they could have started merging in the early Universe, forming more and more massive black holes over time."

"According to this model, the Universe would be filled with black holes all over. Stars would start to form around these clumps of ‘dark matter,' creating solar systems and galaxies over billions of years. If the first stars indeed formed around primordial black holes, they would exist earlier in the Universe than is expected by the ‘standard’ model. Primordial black holes could then be the seeds from which all black holes form, including the one at the center of our own Milky Way galaxy," according to the European Space Agency

What we guestimate about primordial black holes and the existence of early black holes

Before we delve into primordial black holes, we must understand how the most typical black holes are structured and how they work. As mentioned, stellar-mass black holes form when a massive star, much larger and more massive than our Sun, reaches the end of its life. All of its remaining gas is spewed into space, forming a supernova remnant, but something interesting happens to the core. Assuming the star was massive enough to overcome becoming a white dwarf, a neutron star, or a pulsar, it begins to collapse in on itself completely, eventually forming something called a singularity.

A singularity is one of the most significant parts of a black hole. There's also an event horizon, which is the point at which nothing -- not even light -- can escape the black hole's gravitational grasp.

This can be understood by thinking about escape velocity - how fast something needs to move to escape the gravity of another object. There are two things that affect the escape velocity – the mass of the object and the distance to the center of that object. The more dense and smaller the object, the greater velocity is needed to reach escape velocity. At a certain point, the velocity needed to escape would be greater than the speed of light. But since the speed of light is the cosmic speed limit, it would be impossible to escape at that point. Any matter trapped by a black hole's gravity but traveling below the escape velocity becomes the black hole's lunch.

The singularity is a lot more mysterious. It's believed that all matter is "destroyed" or "consumed" once it enters the singularity, which will increase the size/mass of the black hole. Currently, the main idea behind black hole singularities is that we can't even retrieve any information about the matter that entered the black hole, as the singularity is believed to be an infinitely dense point, or close to it. However, another of Stephen Hawking's theories suggests that some of this information bleeds out through something known as Hawking radiation, this loss of information eventually causes small black holes to "evaporate". 

Supermassive holes may have been created when stellar-mass black holes collide with other intermediary black holes. In the process, they would exhibit something called gravitational waves, when their individual gravities finally begin to combine. There's one problem though, according to the study.

“Black holes of different sizes are still a mystery. We don’t understand how supermassive black holes could have grown so huge in the relatively short time available since the Universe existed,” researcher Günther Hasinger notes.

So, how do primordial black holes come about, and are they dangerous?

The formation of primordial black holes is a little more complex than stellar, intermediate, or supermassive black holes. In fact, they challenge our beliefs of how black holes form because they are much too small, believed to be perhaps just 1,000 times larger than an atom, to have formed from dying stars. However, it's possible that they formed just a second after the big bang, and grew larger as they consumed more matter, thus explaining the mystery of how massive black holes came about before the first stars became large enough to collapse. 

In discussing how small a primordial black hole is, whether they are detectable if they could account for dark matter, and how it would affect the human body if one were to pass through us, Scientific American explains, 

"PBHs do not reflect sunlight and cannot be identified this way ahead of impact. They do glow faintly in Hawking radiation, but their luminosity is lower than a mini light bulb of 0.1 watts for masses above a millionth of the mass of the moon. Is this invisibility a reason for concern?"

"In particular, if PBHs in the allowed mass range make up the dark matter, one may wonder whether they pose a threat to our life. An encounter of a PBH with a human body would represent a collision of an invisible relic from the first femtosecond after the big bang with an intelligent body—a pinnacle of complex chemistry made 13.8 billion years later. Although this constitutes a meeting of an extraordinary kind between the early and late universe, we would not wish it upon ourselves"

"The attractive gravitational force induced by a PBH of the abovementioned mass would shrink our entire body by several inches during its quick passage. The pull would be impulsive, lasting 10 microseconds for the typical PBH speed of 100 miles per second in the dark matter halo of the Milky Way galaxy. The resulting pain would feel as if a tiny vacuum cleaner with a tremendous suction power went quickly through our body and shrunk its mussels, bones, blood vessels, and internal organs. The dramatic bodily distortion would create severe damage and cause immediate death."

Yikes. While that doesn't sound like much fun, it's estimated to be highly unlikely that anyone on Earth will experience death by a primordial black hole, so sleep tight with the knowledge that at least one astrophysical object bears little chance of killing you.