White Holes — The Cosmic Object Nothing Can Ever Enter

There is an equation written down in 1916, on a battlefield in Russia, by a physicist dying of an autoimmune disease. The equation describes a black hole. But the same equation has a second property. If you run it forward in time, you get a region of space from which nothing can ever escape. If you run it backward in time, you get a region of space into which nothing can ever enter.

A perfect mirror. A spacetime that only spits things out, light, matter, energy, and into which no signal, no photon, no thought, can ever fall.

This object has a name. It is called a white hole. For a century, most physicists treated it as a mathematical phantom, a solution the equations allow but that nature surely forbids. Today that consensus is cracking. Loop quantum gravity, the Planck star hypothesis, and the long story of how a black hole dies all now suggest white holes may be real, may be hiding in the universe, and may carry the answer to one of the deepest paradoxes in modern physics. Where does information go when a black hole evaporates?

This is the equation that birthed both objects. The quantum gravity scenario that may make the white hole physical. And the question the picture cannot finally settle.

Schwarzschild on the Russian front

Begin with the equation. General relativity, written down by Albert Einstein in 1915 and completed in field-equation form in 1916, says that the curvature of spacetime is determined by the distribution of matter and energy:

Gμν + Λgμν = (8πG/c⁴) Tμν

Spacetime tells matter how to move, and matter tells spacetime how to curve. That single sentence is general relativity.

Einstein finished the equation in late 1915. He did not believe a closed-form solution would be found in his lifetime. Within months, a German physicist named Karl Schwarzschild, serving as an artillery officer on the Russian front during the First World War, computed one anyway. He sent it to Einstein in a letter. Schwarzschild was already terminally ill with pemphigus, an autoimmune disease that left him with weeks to live. He died a few months later. The solution he left behind became the foundation of every black hole physics paper written since.

The Schwarzschild solution describes the geometry of empty spacetime outside a non-rotating spherical mass. At a certain critical radius, the solution does something strange. The radial coordinate becomes the time coordinate, and the time coordinate becomes spatial. This critical radius is the Schwarzschild radius:

rₛ = 2GM/c²

For one solar mass, this radius is about 2.95 kilometers. For the Earth, it is just under 9 millimeters. For the supermassive black hole at the center of our galaxy, Sagittarius A*, with a mass of roughly 4.3 million solar masses, the Schwarzschild radius is about 13 billion meters, smaller than the orbit of Mercury. For TON 618, one of the largest known black holes at 66 billion solar masses, the radius is around 200 billion meters, larger than the orbit of Pluto.

Inside this radius is the event horizon. No information can cross it from inside to outside. No light. No signal. No object.

This is the black hole. But Schwarzschild’s solution, taken seriously, contains a second region.

The second half of the equation

If you map the full spacetime, what physicists call the maximally extended Schwarzschild solution, you find that the geometry has a second horizon. A second region. A second singularity. And around that second region, the time direction is reversed. The arrow of time runs the wrong way. Light streams outward. Matter streams outward. Nothing can fall in.

This second region is the white hole. It is not a separate solution. It is half of the same solution. The equation that gives you a black hole, run all the way through, also gives you a white hole.

For decades after Schwarzschild’s death, this second region was treated as a mathematical curiosity. Roger Penrose proved his singularity theorem in 1965, showing that under broad assumptions about energy and causality, gravitational collapse must produce a singularity. Penrose’s work established that the black hole side of the Schwarzschild geometry was physically realizable. A massive enough star collapses. The collapse cannot be stopped. A singularity must form.

The white hole side seemed to require the time-reverse of this, an exploding singularity that has always existed. As the relativist James Bardeen observed, a white hole that has existed forever is not a solution of physically reasonable initial conditions. So the black hole was the physics. The white hole was the math. The two were inseparable on the page, but only one of them, it seemed, could happen in the universe.

In 1963, the New Zealand mathematician Roy Kerr extended the Schwarzschild solution to rotating black holes. The Kerr metric describes the spacetime around a rotating mass and predicts an even richer geometry, with multiple horizons, an ergoregion where space itself is dragged along with the rotation, and inside, a ring singularity rather than a point. The Kerr solution, like Schwarzschild’s, contains a maximally extended structure with white hole regions in its mathematics.

The Event Horizon Telescope, in 2019, released the first direct image of a black hole’s shadow, in the galaxy M87. The shadow’s diameter matched the Kerr prediction to within a few percent. In 2022, the same collaboration imaged Sagittarius A*. The images of both black holes are consistent with general relativity. They show ring shapes, accretion disks, gravitational lensing, exactly what Kerr and Schwarzschild geometry predict. But none of the images show any anomaly consistent with the white hole half of the solution.

This does not prove white holes do not exist. It only proves that, at the resolution of the Event Horizon Telescope, the supermassive black holes in M87 and the Milky Way are not actively emitting matter through a time-reversed horizon. The mathematics is consistent. The observations are consistent. The white hole, if it exists, exists somewhere else.

Hawking changes the picture

The mathematical phantom might have stayed a phantom forever. It did not, because in 1974, Stephen Hawking applied quantum field theory to the curved spacetime around a black hole and discovered something that broke the simple picture entirely.

Hawking showed that black holes are not perfectly black. Quantum field theory in the curved spacetime near the horizon predicts that a black hole emits radiation, now called Hawking radiation, with a temperature inversely proportional to its mass. A solar-mass black hole has a Hawking temperature of about 60 nanokelvin, far colder than the cosmic microwave background, meaning it absorbs more than it emits. But the emission is real. Given enough time, and a sufficiently cold cosmic environment, a black hole will slowly lose mass and eventually evaporate entirely.

Hawking radiation appears to be thermal. Pure thermal radiation, in standard quantum mechanics, carries no information about the system that produced it. But the matter that fell into the black hole carried information, the precise quantum state of every particle. If the black hole evaporates entirely into thermal radiation, that information appears to be lost. A pure quantum state goes in. A mixed thermal state comes out.

This violates one of the deepest principles of quantum mechanics, unitarity, which says that the evolution of any closed quantum system preserves information. If Hawking is right, unitarity is broken.

This is the black hole information paradox. It is not a paradox of metaphor. It is a paradox of equations. Two of our most successful theories, general relativity and quantum mechanics, predict different things about what happens at the end of a black hole’s life. Both cannot be right.

Entropy and holography

There is a precise number that captures the strangeness. The Bekenstein-Hawking entropy of a black hole is given by:

S_BH = (k c³ A) / (4 G ℏ)

where A is the area of the horizon. The entropy is proportional to the horizon area, not the volume. For ordinary thermal systems, a hot gas, a piece of metal, entropy scales with volume. For a black hole, entropy scales with surface area. For a one-solar-mass black hole, the entropy is roughly 10⁷⁷ in units of Boltzmann’s constant. For a supermassive black hole the size of Sagittarius A*, the entropy is around 10⁹⁰. For comparison, the total entropy of the visible universe, excluding black holes, is around 10⁸⁸. A single supermassive black hole carries more entropy than all the rest of the visible universe combined.

This surface-area scaling led the Dutch physicist Gerard ‘t Hooft and the American physicist Leonard Susskind in the 1990s to propose the holographic principle: the information content of any region of spacetime is encoded on its two-dimensional boundary, not in its three-dimensional volume. The Bekenstein-Hawking formula was the first hint. Holography became the framework. It now sits at the center of every serious attempt to resolve the information paradox.

But notice what holography does not yet specify. How does the information stored on the horizon actually escape? Hawking radiation, in his original derivation, is thermal. Thermal radiation contains no information. Either Hawking’s derivation is incomplete, and the radiation actually carries the information through subtle quantum corrections, or the picture changes near the end of evaporation, when the black hole becomes Planck-scale and quantum gravity takes over.

That last possibility is where the white hole returns.

Loop quantum gravity and the bounce

We do not have a complete theory of quantum gravity. We have candidates. Loop quantum gravity was developed in the 1980s and 1990s, primarily by Abhay Ashtekar, Carlo Rovelli, Lee Smolin, and their collaborators. It begins from a reformulation of general relativity in terms of new variables, called Ashtekar variables, that make the geometry of space look more like a gauge field. When you quantize this reformulated gravity, the result is a theory in which space itself is granular. Areas and volumes come in discrete units, with the smallest non-zero area being roughly the Planck area, around 10⁻⁷⁰ square meters. The full machinery is captured in objects called spin networks.

In 2006, Ashtekar, Tomasz Pawlowski, and Parampreet Singh applied loop quantum gravity to cosmology. They studied a simple, homogeneous, isotropic universe, the kind described by the Friedmann equations of standard Big Bang cosmology. They found that when the energy density of matter approaches a critical density of about 0.41 times the Planck density, the equations no longer drive the universe toward a singularity. Instead, they produce a bounce. The Big Bang singularity is replaced by a smooth transition from a previous contracting phase.

The next question was natural. If loop quantum cosmology can remove the Big Bang singularity, can loop quantum gravity remove the black hole singularity?

A series of papers throughout the 2000s answered yes. Leonardo Modesto, in 2004, showed that loop quantum effects in a simplified Schwarzschild model produce a bounce at the singularity. Dah-Wei Chiou, in 2008, refined the picture. Chiou showed that loop quantum gravity, applied to the interior of a Schwarzschild black hole, produces a bounce that connects the black hole interior to a white hole interior. The classical singularity is replaced by a finite-density quantum region across which the geometry continues. On one side of the bounce, the geometry looks like a collapsing black hole. On the other side, it looks like an expanding white hole. They are joined at the Planck core.

This is the loop quantum gravity black hole bounce. It depends on assumptions about how to quantize the relevant equations and on a particular choice of variables. Different choices give different details. But the qualitative picture is robust. A collapsing black hole, in this framework, does not end in a singularity. It ends in a bounce. And what emerges from the bounce, on the far side, is a white hole.

The Planck star

In 2014, Carlo Rovelli and Francesca Vidotto published a paper that gave this picture a physical name. They called the bouncing core a Planck star. A Planck star is what sits at the center of a black hole when collapse is halted at Planck density by quantum gravity. It is roughly Planck-sized in terms of its core but, in the curved geometry of the interior, the Planck star can have an enormous internal volume, an internal volume that grows with time as the black hole ages.

The interior of a black hole, in the Planck star picture, is much larger than its exterior. This sounds paradoxical but is a real geometric feature of the Schwarzschild interior. The space inside grows with time. By the time a black hole has lived for billions of years, its interior contains a vast region of curved spacetime in which the information of everything that fell in is stored. The Planck star waits in that interior. When the bounce occurs, the Planck star expands outward, and a white hole is born.

Rovelli and Vidotto computed estimates for how long the bounce should take from the perspective of an outside observer. The answer depends on the mass of the black hole. For a small black hole, the bounce is quick. For a large black hole, the bounce is delayed by the enormous time dilation near the horizon. From the perspective of someone outside, the black hole appears to be slowly evaporating through Hawking radiation. Meanwhile, deep inside near the Planck core, almost no time passes. The bounce can be triggered, in principle, at any point during the evaporation.

Black hole fireworks and primordial signatures

In 2015, Hal Haggard and Carlo Rovelli published a paper titled “Black hole fireworks.” It made the picture more concrete. After collapse, they argued, the black hole undergoes a quantum tunneling event. Across the tunneling, the geometry flips into the geometry of a white hole. The white hole then explodes, releasing the trapped matter, the information, and the energy stored during the long Hawking evaporation. Hence the name.

Haggard and Rovelli estimated that the tunneling timescale for a typical astrophysical black hole might be much shorter than the full Hawking evaporation time, possibly short enough that primordial black holes formed in the early universe could be exploding now.

If a primordial black hole formed shortly after the Big Bang is reaching the end of its life today through this tunneling process, the explosion would produce a brief, intense burst of radiation. The French physicist Aurelien Barrau and his collaborators have suggested that fast radio bursts might be candidates for such events. Fast radio bursts are millisecond-duration pulses of radio emission, of cosmological origin, whose mechanism is still not fully understood. The leading explanation involves magnetars, highly magnetized neutron stars, and that explanation remains dominant. But a small fraction of the community continues to investigate whether some fast radio bursts could be black hole fireworks, the death cry of a primordial Planck star.

To date, no fast radio burst has been confidently identified as a black hole-to-white-hole transition. The signature is hard to extract, and the relevant calculations involve many unknowns. But the search continues.

White holes as remnants and dark matter candidates

In 2018, Eugenio Bianchi, Marios Christodoulou, Fabio D’Ambrosio, Hal Haggard, and Carlo Rovelli published a paper titled “White holes as remnants.” The paper proposes a comprehensive scenario.

Step one: a star collapses to form a black hole. Step two: the black hole emits Hawking radiation over an enormously long timescale, losing most of its mass. Step three: when the black hole is reduced to roughly Planck mass, quantum tunneling converts it into a white hole. Step four: the white hole remnant, although it has a Planck-scale horizon area, has an internal volume that is vast, inherited from the original black hole’s interior. The white hole remnant slowly releases its stored information back into the universe through a quiet, long-lived emission. Unitarity is preserved. The information is not lost.

The Bianchi scenario makes the white hole the physical exit of the information paradox. The radiation Hawking calculated is thermal. The information is not in the radiation. The information is stored inside the black hole, carried through the bounce, and slowly released by the white hole remnant after the original black hole has effectively evaporated. The full evaporation timescale, including remnant decay, may scale as M⁴ in Planck units rather than M³. This pushes the actual end of a black hole’s life vastly into the future.

The scenario has a striking secondary implication. If many primordial black holes formed in the early universe, and a significant fraction have already gone through the bounce, the universe might be populated with a large number of Planck-mass white hole remnants. Their gravitational influence would be significant. Their direct detectability would be essentially zero. They emit only the faintest possible signals.

This is precisely the profile of a dark matter candidate.

Several physicists, including Rovelli, Vidotto, and Barrau, have suggested that white hole remnants could be the dark matter. The idea is speculative. Observational constraints on primordial black holes, set by gravitational lensing, by cosmic microwave background distortions, and by the absence of detected Hawking explosions, restrict the allowed mass range. But Planck-mass remnants, sitting below the lower edge of the constraint window, remain in principle viable. Work continues into 2026, with new analyses by Loeb and others extending the parameter space.

A different route: Einstein-Cartan and baby universes

There is another approach to the same problem, taken outside the loop quantum gravity framework. Nikodem Popławski, a Polish-American physicist working in the Einstein-Cartan formulation of general relativity, has developed a different scenario.

Einstein-Cartan theory differs from standard general relativity by allowing spacetime to have torsion in addition to curvature. In standard general relativity, torsion is set to zero by hand. In Einstein-Cartan theory, torsion is sourced by the spin of matter. Under normal conditions, torsion is utterly negligible. At extremely high densities, however, torsion produces a strong repulsive force that resists further compression. Popławski has argued that this repulsive force prevents singularities. A collapsing star, in his scenario, does not crush into a point. It bounces. And the bounce produces a new region of spacetime, what looks, to an inside observer, like a brand new universe.

Each black hole, in Popławski’s picture, is the seed of a baby universe. The white hole side of the black hole geometry, in the Einstein-Cartan extension, becomes the Big Bang of a new cosmos. Our universe, in this scenario, would be the interior of a black hole in some parent universe. The Big Bang we infer would be the white hole bounce on the far side of that parent black hole.

A closely related idea was proposed earlier by Lee Smolin in the 1990s. Smolin’s hypothesis, cosmological natural selection, suggested that black holes give rise to baby universes whose fundamental constants are slightly different from their parents’. Universes that produce more black holes have more descendants. Over many generations, the population of universes evolves toward parameters that maximize black hole production.

The arrow of time problem

Every white hole scenario shares one unsolved problem. The fundamental microscopic laws, Newtonian mechanics, electrodynamics, general relativity, quantum mechanics, are all time-symmetric. They run as well backward as forward. Yet the macroscopic universe shows a sharp asymmetry. Eggs break but do not unbreak. The cosmic clock runs in one direction. This is the arrow of time.

Roger Penrose has spent decades trying to explain this. His proposal, the Weyl curvature hypothesis, says that the Big Bang was a state of extraordinarily low entropy. The probability of such a low-entropy initial condition, in standard statistical mechanics, is astronomically small. Penrose has estimated it as one part in 10^(10^123). That is a number with no analogy in human experience. It is the most extreme fine-tuning known in physics. Penrose himself has acknowledged that no naturalistic mechanism within standard cosmology comes close to explaining it.

In bounce cosmology, the question becomes: what sets the arrow of time across a bounce? If our universe is the white hole of an earlier collapse, the entropy in the parent universe was already increasing at the moment of the bounce. The bounce must somehow reset the entropy to a low value, ready to start increasing again. In some loop quantum cosmology models, this is argued to happen naturally, through the symmetry of the bounce solution. In others, it requires special conditions that look as fine-tuned as the original problem. The community is divided.

The deepest unexplained feature of our universe, the arrow of time, appears in every quantum gravity scenario for white holes not as a solved problem but as an additional layer of mystery.

The question the picture cannot answer

The white hole scenario assumes a great deal. It assumes that unitarity must hold. It assumes that quantum mechanics applies to the universe as a whole, all the way through a bounce, all the way through a Planck-scale region where space and time themselves break down. It assumes that information is conserved. It assumes the dynamics can be described by deterministic, unitary equations. These are large assumptions, imposed rather than derived.

The picture is beautiful. It is also speculative. We do not know that loop quantum gravity is the correct theory. We do not know that the bounce occurs. We do not know that white hole remnants exist. We do not know that information is preserved through black hole evaporation. What we know is that, given a particular set of assumptions about quantum gravity and unitarity, a self-consistent picture emerges in which white holes return to physical relevance. The mathematical phantom of 1916 finds a home.

What white holes do not do is remove the deepest question. Bounce cosmology, even if correct, does not abolish the need for a beginning. It relocates the question. If our universe arose from a previous one through a bounce, what initiated the chain? An infinite regress of bounces still demands an account of the cycle itself, the laws that govern the bouncing, the constants that take exactly the values needed for stars to form and Planck stars to develop their vast interiors. The bounce changes where the question lives. It does not remove the question.

And the arrow of time problem, the most extreme fine-tuning in physics, gets harder in a bounce framework, not easier.

This is the question the companion documentary on the Sleepy Joe Space YouTube channel takes up at length.

What stays

What stays from the white hole story is a precise statement of how far the mathematics of general relativity can be pushed, and where it stops.

The Schwarzschild solution, written down in 1916 by a dying physicist on the Russian front, contains a black hole region and a white hole region as two halves of the same geometry. For most of the twentieth century, the white hole half was treated as a mathematical curiosity, ruled out by realistic initial conditions. The black hole information paradox, raised by Hawking in 1974, gave the white hole a reason to come back. Loop quantum gravity proposals starting in 2004 showed how the singularity at the center of a black hole could be replaced by a finite-density quantum core. The Planck star hypothesis, articulated by Rovelli and Vidotto in 2014, gave that core a name and a physical interpretation. The Bianchi remnant scenario of 2018 connected it to the resolution of the information paradox and, possibly, to dark matter. The Popławski and Smolin scenarios extended the same machinery to cosmology, proposing that every black hole may seed a new universe.

What also stays is the discipline. Every step of the picture rests on assumptions that have not been confirmed by observation. No white hole has been detected. No Planck star has been identified. No fast radio burst has been confidently attributed to a black hole-to-white-hole transition. The mathematics is consistent. The empirical verdict is open.

Whether the white hole turns out to be a permanent feature of the universe or another mathematical curiosity revealed by the next layer of theory, the questions it raises about information, about the arrow of time, and about the fine-tuning of the conditions that make any of this possible, those questions stand where they always stood. The cycle, if it exists, does not explain itself.

The equation Schwarzschild left behind contains both an object that swallows everything and an object that ejects everything. A century later, we still do not know whether the universe builds both, builds only one, or builds the second exclusively as the dying form of the first.