When giant stars die, they don't just disappear. They collapse, leaving behind a compressed stellar remnant, usually the size of a metropolis, which is a superdense ball of neutrons called a neutron star.
Most theorists believe that in exceptional cases, a giant dying star forms a black hole - a point "singularity" with virtually infinite density and a gravitational field so powerful that there is not even light, the fastest thing in the universe.
The new study offers an alternative idea that hypothetical objects such as black stars or gravastars could exist halfway between neutron stars and black holes. If so, then exotic stellar corpses should be almost identical to black holes, except for one key circumstance - they do not irrevocably absorb light.
There are good reasons to look for such alternatives because black holes raise many theoretical problems. For example, their singularities are supposedly hidden behind invisible boundaries known as event horizons. Throw something into a black hole, and as soon as it passes the event horizon, it will disappear forever. But there are laws of physics that suggest that information cannot be destroyed, including information encoded inside anything that falls into black holes.
Models of black stars and gravastars developed over the past two decades suggest that these objects will not have singularities and event horizons. But it is still unclear whether such objects can actually form and remain stable.
New research by theoretical physicist Raul Carballo-Rubio of the International School of Advanced Research in Italy proposes a mechanism that allows black stars and gravastars to exist.
The scientist investigated an unusual phenomenon known as quantum vacuum polarization. Quantum physics, which gives the best description of how all known subatomic particles behave, assumes that reality is uncertain, limiting how accurately you can know the properties of the most basic units of matter - for example, you can never absolutely know the position and momentum of a particle in one and the same same time. One strange consequence of this uncertainty is that the vacuum is never completely empty, but has so-called "virtual particles" that continually oscillate as they go in and out of existence.
Promotional video:
With a gigantic amount of energy released by the explosion of a huge star, these virtual particles can polarize or order depending on their properties, just as magnets have north and south poles. Carballo-Rubio calculated that the polarization of these particles can create an amazing effect inside the powerful gravitational fields of dying giant stars - a field that repels, does not attract.
According to Einstein's general theory of relativity, the material and energetic curvature of space-time leads to the formation of gravitational fields. Planets and stars have a positive amount of energy, and the resulting gravitational fields are attractive in nature.
“However, when virtual particles are polarized, the vacuum they occupy can, on average, have negative energy, and they warp spacetime in such a way that the associated gravitational field becomes repulsive,” says Carballo-Rubio.
This, of course, could prevent the formation of a black hole. This phenomenon causes relatively light stellar remnants to form neutron stars instead of black holes. Their gravitational fields are not strong enough to destroy neutrons at the singularity.
Two previous models suggested that repulsive gravity could cause stellar remnants to collapse to form black holes. One of the simulated stellar remnants instead formed gravastars - objects filled with a quantum vacuum covered with a thin shell of matter. Another model suggested that these collapses resulted in black stars, where matter and quantum vacuum alternate throughout the structure in careful balance. Both objects still have powerful gravitational fields that deeply warp light, making them appear as dark as black holes.
According to Carballo-Rubio, there was previously great uncertainty about the properties of black stars and gravastars. In a new work, he created a mathematical framework that incorporates the effects of repulsive gravity into equations describing the expansion and contraction of stars. Previously, it was believed that this could only be interpreted with the help of computers. His new model assumes the existence of a hybrid of a black star and a gravastar - an object where matter and quantum vacuum are distributed throughout the structure, but with matter in higher concentrations in the envelope than in the core.
The research is published in the journal Physical Review Letters.