When stars cease to fuse hydrogen at the end of their lifetime, they collapse in on themselves, condensing into a singularity of infinite density. The surrounding material — whether gas, dust, or even other stars — is gravitationally pulled towards the singularity, it passes a point of no return. This is referred to as the event horizon or Schwarzschild radius. At this distance, something must be traveling faster than the speed of light to escape the black hole’s pull, which is impossible. This is why black holes are black. Not even light can escape once it gets too close, so they often look like dark, silent voids in the black of space.

For this reason, black holes are considered the universe’s ultimate endpoints, where matter, energy, and time itself are thought to vanish into oblivion.

But what if black holes aren’t the end? What if they are, instead, the beginning of something entirely new?

A new study by researchers at the University of Sheffield and Complutense University of Madrid suggests that black holes might not be the eternal traps we once imagined. Instead, they could transform into “white holes” — theoretical objects that eject matter, energy, and even time back into the universe.

A Physics Problem

Singularities are problematic. They are places where the laws of physics, as we know them, break. This has led physicists to search for ways to resolve these cosmic dead-ends.

One promising avenue is quantum gravity, a field that seeks to unify general relativity with quantum mechanics. In this new study, researchers applied quantum gravity to the interior of a black hole. They focused on a specific type of black hole — a planar anti-de Sitter black hole — which has a simpler geometry that makes it easier to study.

The technique hinges on a concept called unitarity. In quantum mechanics, unitarity means that the total probability of all possible outcomes of a system must always add up to one. This principle ensures that information is never lost, even in the chaotic environment of a black hole.

By requiring unitarity, the researchers found that the classical singularity could not exist. Instead, the black hole’s interior would transition into a quantum state whereby the singularity is replaced by a region of intense quantum fluctuations known as a “quantum bounce”.  Here, space and time don’t end. Instead, they transition into a white hole, a theoretical object that expels matter and energy rather than consuming them.

“Hypothetically, you could have an observer — a hypothetical entity — go through the black hole, through what we think of as a singularity, and emerge on the other side of the white hole,” said Dr. Steffen Gielen, a co-author of the study from the University of Sheffield. “It’s a highly abstract notion of an observer, but it could happen, in theory.”

White Holes

This idea isn’t entirely new. Some physicists have speculated that black holes could transition into white holes. But this study provides a concrete mathematical framework for how it might happen. The researchers also showed that this process would be consistent with the principles of quantum mechanics.

If black holes don’t actually end in singularities, it could change our understanding of the universe’s most extreme environments. It might also help resolve long-standing puzzles, such as the black hole information paradox. Anything in the physical world can run both ways — it’s one of the deepest features of the laws of physics, reflecting elemental symmetries of space, time and causality.

If you send all the parts of a system into reverse, what was done will be undone. The information required to wind back the clock is always preserved. But if black holes don’t preserve information — like they should given the way we describe them as bottomless pits — then we have a problem.

Dark Energy and Time

Another interesting consequence of this research is that time is driven by dark energy. Dark energy, the mysterious force driving the accelerated expansion of the universe, could serve as a universal reference point for measuring time, the scientists wrote in their study.

This idea is a radical departure from traditional notions of time, which are often tied to the observer’s perspective. Instead, the study suggests that time is intrinsically linked to the fabric of the universe itself. Dark energy acts as its clock.

“In quantum mechanics, time as we understand it cannot end as systems perpetually change and evolve,” said Gielen.

“While time is, in general, thought to be relative to the observer, in our research time is derived from the mysterious dark energy which permeates the entire universe,” the researcher added.

“We propose that time is measured by the dark energy that is everywhere in the universe and responsible for its current expansion.”

Implications and More Questions

This connection between dark energy and time could have far-reaching implications. It not only offers a new way to understand the behavior of black holes but also provides a potential bridge between quantum mechanics and general relativity. These are two pillars of modern physics that have long resisted unification.

But what does a black hole turning into a white hole mean? Could white holes be connected to wormholes, hypothetical tunnels through spacetime? Could they offer a glimpse into other dimensions or universes? The new research doesn’t answer any of these questions, so it remains rather speculative. The theory of unimodular gravity, which the researchers used in this study, is also a speculative branch of general relativity. So, take all of this with a big grain of salt.

While many questions remain, one thing is clear: black holes are even stranger than we imagined.

The findings appeared in the journal Physical Review Letters.