Black holes are one of the most tremendous destructive forces in the universe. And while opposites in magnetism attract, the concepts of creation and destruction aren't conventionally adjacent when it comes to black holes. But what if black holes created something really, really big?
A team of scientists has proposed a new theory where they do: Black holes might not only bend space and time into a singularity of extremely high density. They may also induce "a continuous transition between the inside of a black hole and the beginning of a new universe," according to a study recently shared on a preprint server. In other words, the study suggests black holes might actually burrow into a kind of multidimensional object called a brane, and give birth to an entirely new universe in another colossally big bang.
However, this idea relies on string theory, a body of ideas with aims to unify all forces in nature. So it's a big "maybe." But the idea alone highlights the intriguing mystery which the unknown internal happenings of black holes presents us. And, barring a magic spacecraft that can take us through a black hole's event horizon (alive), we can still try to approach it with mathematics.
'Patching' a black hole singularity into a big bang
Einstein's vacuum field equations for gravity lead us to a singularity, where the fabric of space-time curves away from the plane. And in black holes, this curvature extends far beyond what we expect in ordinary gravitational fields surrounding stars or planets. It's a place where the laws of classical physics (from Sir Isaac Newton's time) begin to break down. But according to the study, this "breakdown" in classical physics also happens within big bang conditions, when a universe is being born.
Jumping forward a few hundred years, quantum physicists hoped to find a way to integrate Einstein's theory of gravity into a quantum schema. But any attempt to unify all physical theories under quantum physics would also have to give an account for singularities like black holes and big bangs. This is what the recent study purports to do. "[W]e propose a way to simultaneously resolve black hole and cosmological singularities by the addition of a single object to the effective field and theory description of space, time, and matter," said the researchers in their study.
"This object is a[n] S-brane, a relativistic object which occupies a co-dimension one space-like hypersurface of space-time and carries positive tension but vanishing energy density," added the researchers. "This object violates the usual energy conditions and hence enables a resolution of space-time singularities." Using two mathematical representations of the big bang and black holes — called the Penrose diagrams of expanding cosmology, and the Penrose diagram of the Schwarzchild black hole, respectively — the authors attempt to bring the "wavy" singularity of both diagrams together, like a patch. If this is possible (and it's a big "if"), they would create a theoretical description of a black hole, whose singularity leads to a new universe.
String theory could hint at what happens beyond a black hole's event horizon
Obviously, this isn't the first time a scientist has proposed that a black hole might birth a new universe beyond its event horizon, but many of these rely on Einstein's General Relativity to bring the two Penrose models of the big bang and black holes together, which runs into problems. In an attempt to circumvent this, the researchers suppose superstring theory — which hints at the possibility of a unified theory of all forces in nature — may do the trick that General Relativity and quantum physics alone have yet to do.
In string theory, instead of particles in space, we view the universe as extended objects: strings existing in ten space-time dimensions (we live in three, and experience the fourth: time). One object that shows up in the math of string theory, called a brane, is multidimensional. When scientists find new ways of describing branes, they often lead to new advances in string theory as such. Here, the study proposes a specific type of multidimensional object called an S-Brane coming into existence within the impassible horizon of a black hole, at the singularity, which could serve as a means of transit between a black hole and the birth of a new universe. "This provides simultaneous resolution of both the black hole and Big Bang singularities," reads the study.
Admittedly, a lot of high-order mathematics is required to fully grasp the complexities of the researchers' study, which again has yet to receive peer review. And, while this absolutely does not mean that black holes are actually gateways to young universes being born out of the destruction and collapse of stars in our universe — we have to consider all theories to advance our scientific understanding of the cosmos. In other words, we still don't know what happens inside of black holes, but string theory gives us a unique perspective into what might happen beyond the event horizon.