When a massive star explodes, one of two things happens. Its core compacts into a space no bigger than a city, creating an ultra-dense object called a neutron star, or it or collapses entirely into a black hole from which nothing can escape.
The star's fate seemed like and either-or proposition. But maybe that's wrong, according to a group of Italian researchers who just published a paper. You can thank the weird world of quantum physics.
Neutron stars and black holes play by the rules of Einsteinian physics. For the most part they obey the laws of gravity it sets forth through the interactions of normal particles and matter—a neutron star, for instance, is called that because it's a thick plasma soup of neutron-based matter. But within each particle are subparticles called quarks, which is where things get weird at the subatomic level.
Say you had a star that was just more than three times the mass of our sun. Here, according to the researchers from the International School for Advanced Studies (SISSA), is the point typically seen as the dividing line between a neutron star and a black hole. And here is where a quantum effect could begin to come into play.
Quantum vacuum polarization provides an additional repulsive force that counteracts the gravity drawing the massive object toward a black hole. You can think of it as an actual black hole sun: The SISSA researchers believe it would behave like a black hole but would not have an event horizon, the black hole's point of no return.
This type of object—which has not been discovered, only theorized—could be observed in the next few decades. The SISSA researchers believe that the gravitational waves emitted by the object would be different enough to differentiate them from black holes with an event horizon, at which point we may begin to better understand their nature.