What Happens to a Black Hole When it Stops Being a Black Hole Due to Hawking Radiation?

Article Via FORBES / Author: Astrophysicist Ethan Siegel

Black holes are one of the universe’s phenomena that are had scientists baffled for years, and probably will do for a number of more years. But, one thing that experts are certain of is that they won’t last forever and that one day they will cease to be black holes because of Hawking radiation.

To understand this a bit better, first, let’s think about what a black hole is. Generally, a black hole will form during the collapse of a massive star’s core. As a drop in radiation pressure occurs in the core the outer layers experience some form of fusion reaction, blowing apart the star in the supernova. But, if the mass is too great, the neutrons will compress and condense into such a dense state that a black hole will form. Another way in which a black hole can form is if a neutron star accumulates enough mass from a neighboring star to warrant it being a black hole. Really, all that’s needed for a black hole to form is enough mass squeezed into a small enough space so that nothing (not even light) can escape.

The exterior spacetime to a Schwarzschild black hole, known as Flamm’s Paraboloid, is easily calculable. But inside an event horizon, all geodesics lead to the central singularity. Wikimedia Commons user AllenMcC.
The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation.

Escape velocity is the speed in which you would need to achieve in order to escape the gravitational pull at a given distance from its center of mass. But, if the mass is so great that the speed needed to achieve at a certain distance from the center of mass is the speed of light or faster, then absolutely nothing could escape it as nothing is faster than the speed of light.  The size of the black hole’s event horizon is determined by the distance from the center of mass where the escape velocity equals the speed of light. The problem is the fact that there’s a matter under these circumstances, which means it must then collapse down to a singularity. Wherever you are inside the event horizon, all light-like curves will lead to the center. Hence, when a black hole is formed with an event horizon all the matter ends up getting crunched into a singularity.

You may also think that as nothing can escape the realms of a black hole that it would stay a black hole forever, but that’s not the case, and that’s all down to quantum physics.  Because of quantum physics, there’s something called the quantum vacuum to consider where in no regions is the curvature greater than near the singularity of a black hole. It’s the combination of quantum physics and the General Relativistic space-time around a black hole that created the phenomenon of Hawking radiation. Using the quantum field theory calculation in curved space brings about a surprising result in that the blackbody radiation is emitted in the space around the event horizon. What this means is the smaller the event horizon, the greater the curvature of space, and therefore the greater the Hawking radiation rate. Essentially this means that the smaller black holes will decay much faster than the larger ones.

A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum. Derek B. Leinweber

Right up until a black hole’s final moments it still has an event horizon. Once a singularity has been formed it remains a singularity right up until the point where its mass goes to zero.  The very last second of a black hole’s life will result in a big burst of energy being released that’s greater than any particle the LHC has ever created. What remains after that is just outgoing radiation. Space is now back to its non-singular state and there will be no other stars or light left after this happens for the first time in our Universe either. So, if you did happen to see the last black hole in our Universe evaporate, all you would be left with is an empty void of space, that displayed no light or signs of activity for many, many years, then all of a sudden a massive outrush of radiation of a certain magnitude and spectrum would appear, leaving a single point in space at 300,000 km/s.

More News to Read