A pencil dropped on the floor hits with its entire surface at once. But if the floor were quantum, made of countless vibrating grains of sand, the pencil would never touch a single point. That's roughly how the core of a black hole works in a new study: instead of an infinitely dense point (a singularity), there's a region where space itself becomes elastic, like foam rubber.
The authors relied on loop quantum gravity — an approach where spacetime is not continuous but assembled from tiny 'atoms'. In the spherically symmetric case, without gravitational waves, they managed to decompose the dynamics into independent shell-like layers, like an onion. Time was measured with an auxiliary 'dust' field: something like an hourglass that just ticks without interfering with gravity. It turned out that the equations admit a solution with an anti-de Sitter core — a region where the geometry resembles the inside of a bouncy ball that pushes matter apart.
Then the researchers checked whether a bounce of matter appears in simplified versions of the model. It turned out that finite approximations can lose it, like watching a trampoline through frosted glass and not seeing the acrobat jump. The full theory, however, shows that a black hole can end up with no horizon at all, turning into a traversable tunnel with a fluffy core.
The work concludes by constructing a four-dimensional action — a kind of instruction manual for assembling such a geometry from simpler pieces. This opens the way to searching for such objects in the real Universe: if they exist, they might leave characteristic imprints in gravitational waves or in the motion of stars around them.
If our Sun suddenly became a black hole with an anti-de Sitter core, its size would be about 6 kilometers — a distance light travels in two hundredths of a second.
In Stanislaw Lem's novel 'Solaris', the planet's ocean behaves like a giant quantum object, generating stable structures from the vacuum — somewhat akin to a self-sustaining core without a singularity.