A group of physicists is convinced that black holes originated at the beginning of time — in line with theories of the renowned astrophysicist Stephen Hawking — just six years after the phenomena's existence was confirmed in an unbelievable advance that rocked the astronomy field.
Since then, physicists from around the world have been carefully examining incoming records of black holes, and in a new development detailed in the January edition of Physics of the Dark Universe, researchers noticed that these black holes lack an expected feature — spin — suggesting that they are primordial.
The year 2015 marked the beginning of a new era in astrophysics: The LIGO Observatory installation in the U.S., the world's largest gravitational wave observatory, detected waves that were the result of two black holes colliding. That confirmed that black holes were more than just a hypothesis. More recently, LIGO and its Italian sister site the Virgo Observatory have continued to observe the phenomena.
“We realized that there is no spin in LIGO/Virgo experiments and this, I think, is tremendously important,” said Juan García-Bellido, the paper’s lead author and a theoretical physics professor at the Autonomous University of Madrid. “It tells us that a significant fraction of the black holes detected by LIGO/Virgo must be primordial.”
García-Bellido explained to The Academic Times how his team proved that primordial black holes, formed shortly after the Big Bang, do not have a characteristic spin through Bayesian analysis. This statistical method compares the legitimacy of hypotheses “without prejudices,” according to the researcher.
It solidified that primordial black holes truly would lack spin, a result that suggests they’re real, still here and being observed by LIGO/Virgo Interferometers.
In the scientific community, primordial black holes are only a theory, heavily alluded to by Hawking in the 1970s. He believed that only such black holes could contain the inordinate amount of dark matter in the universe. Dark matter's nature is completely unknown, even though it accounts for roughly 80% of the universe’s mass.
“I think once you discover that it’s probable that primordial black holes are there, it's inevitable that you will convince yourself they are also a significant fraction of the dark matter,” García-Bellido said.
He cites his work from 2015, three years before Hawking died. In it, García-Bellido notes that primordial black holes can be any size, and that gigantic ones could clearly solve the perplexing dark matter question. It would also mean that physicists can study dark matter with the physics they already have, such as general relativity and thermodynamics.
But before coming to this conclusion, the researcher spelled out why primordial black holes lack spin. It has to do with what is called a causal horizon, which can be thought of as a physical limit in the universe for matter and energy.
“In the universe, information cannot travel more than a distance called the causal horizon. In the expanding universe, that horizon has a certain size, which is given by the time it took for light to travel that distance, up until that moment” when one is trying to observe it, García-Bellido explained.
The causal horizon today is so large that it’s irrelevant. But at the time of the universe’s creation, the causal horizon was small enough to impact the compression of matter, a process specific to the spin of black holes.
“You cannot compress more than the size of the horizon, because of causality,” García-Bellido said. “The radius of the [primordial] black hole is exactly the size of the horizon, so there’s no significant compression.”
Primordial black holes could fit inside a horizon almost perfectly, like a mold. This made them rather symmetrical and made the need for a spin obsolete, because very little compression of matter was involved.
Later stars that collapsed into black holes tell a different story: They weren’t constrained by the causal horizon’s structure, so matter and energy would bounce around, making compression necessary and consequential.
“In the case of a star, it would typically be slightly asymmetrical. When you compress something like this, there will be a final spin on the object,” García-Bellido said. “Some of the spin may be shed away with the supernova, but the rest of the black hole, which is the core, will collapse and end highly spinning.”
The team concluded that LIGO/Virgo black holes that do not have a spin couldn’t be the product of the implosion of an asymmetrical star, because to lack a spin is to never have been highly compressed; to escape compression is to have symmetry.
And to have symmetry is to be a primordial black hole.
The findings provide concrete footing to Hawking’s theory that primordial black holes are lurking around space today and could be carrying dark matter. However, the unclear origin of primordial black holes is a large factor in why their existence — and Hawking's theory — is debated. After all, the universe is flat, relatively in sync and uniform — three features not conducive to spontaneous, massive collapses of matter.
Pointing out his original work in the same domain from 1996, well before LIGO's discovery, García-Bellido suggests a potential route of generation: quantum diffusion, another concept directly related to Hawking’s propositions.
The idea is that before the Big Bang, the universe was smaller than a single atom. At this size, tons of quantum particles created a quantum fuzz, buzzing around. It’s thought that once the universe finally started to expand, by a process called cosmic inflation, these quantum fluctuations maintained enough strength to cause extremely high-density points of matter. Those points would have immediately collapsed into a black hole, leading to their primordial emergence.
“The possibility of forming a black hole was very natural after those quantum fluctuations. I think it's a revolution in our understanding,” García-Bellido said.
Hawking’s contribution here is “Hawking radiation.” It suggests that there are quantum particles that radiate, or “evaporate,” off black holes after they’re created. Perhaps they’re the remnants of ancient quantum fluctuations.
As a final point to support the theory of primordial black holes, one espoused in his previous work, García-Bellido discussed how if LIGO/Virgo experiments were to detect a black hole below a certain size, it would most definitely be primordial.
The threshold stems from the Chandrasekhar mass, which denotes the lowest size of a standard black hole: 1.4 solar masses. This restriction is due to the Pauli Exclusion Principle, which says that only a certain number of quantum values can exist in a given location at one time.
If a star-induced black hole is below the limit, its respective quanta would invade the space. However, primordial black holes have a compression-related loophole: They’re mostly made up of photons.
“In the early universe, you had photons and you could compress them without end," García-Bellido explained. "In the late universe, you have fermions, and those you cannot."
The researcher believes that LIGO/Virgo is very close — at most a year — to encountering a hole of this size, and scientists will have no choice but to begin thinking about the observed phenomena as primordial. This could rewrite the narrative of black holes, dark matter and general relativity as it is known.
The paper, "Bayesian analysis of the spin distribution of LIGO/Virgo black holes,” was published in the January 2021 edition of Physics of the Dark Universe. It was authored by Juan García-Bellido and José Francisco Nuño Siles, Autonomous University of Madrid; and Ester Ruiz Morales, Polytechnic University of Madrid.