A black hole that swallows matter might not just evaporate over billions of years: it could pop like a soap bubble and turn into its opposite: a white hole. This scenario emerged from numerical experiments in semiclassical gravity. The simulations used a spherical black hole with two boundaries: an outer one (the event horizon) and an inner one. It turned out that the inner horizon of a black hole is unstable due to quantum effects. It collapses, and the entire trapped region vanishes much faster than with ordinary Hawking evaporation.
Then, where the trap disappeared, an anti-trap appears — a dynamical white hole. Everything gets flung outward. The authors of the new work built an analytical model of two-dimensional collapses (simplified but capturing the essence) and computed how the vacuum energy changes. The key was a negative energy flux running from inside outward along the escape direction. Its amplification is what makes the black hole flip sign.
Interestingly, the resulting white hole itself emits fluxes, but now inward, along the infall direction. These fluxes can spawn a new black hole. This creates a cascade of back-and-forth transitions that could end in a horizonless spacetime — pulsing like a heartbeat. The analytical approach allowed the authors to separate universal features of the mechanism from details of the geometry. Although a full picture requires accounting for the backreaction of quantum effects on gravity, it is already clear: black holes may be just a phase in the life of a compact object.
The Hawking lifetime of a solar-mass black hole is $10^{67}$ years. But the transition to a white hole in the new scenario could take seconds.
In Interstellar, the heroes dive into a black hole and pop out of a white one, a similar trick, just without the quantum cascade.