What's inside a black hole?

This is the sixty four thousand dollar question. Astronomers, cosmologists and quantum physicists have been pursuing this question for a long time, ever since Einstein's relativity theories predicted the existence of black holes.
You don't have to go back that far to a point where black holes were considered science fiction. Now they are science fact, but what's inside them is open to a lot of speculation.

A black hole is the ultimate astronomical object. It is essentially a massive spherical object that's so dense even light can't get out of it. Black holes result from the core collapse of a massive star, but this process is not well understood. Think of it this way. What remains after a core of a massive star collapses under gravitational force? The obvious answer is that its neutrons are compressed together, but even that's not dense enough to explain a black hole. There has to be some quantum transformation that occurs that allows even denser matter, including what is known as a singularity, to form.

A singularity is another name for something we don't understand. It violates Einstein's relativity equations because it results in dividing by zero and that means infinity, which is a value that does not compute, at least as far as cosmologists are concerned. Mathematician, on the other hand, don't get their knickers in a twist over dividing by zero.

The event horizon is the only thing we can be sure of about a black hole's structure. This is the outer fringe of the dark hole where matter gets trapped in a maelstrom and causes the emission of intense X-ray and gamma ray radiation because of friction. This radiation can be detected, but it doesn't tell us what happens to the stuff that falls past the event horizon. We can't see inside the black hole because no electromagnetic radiation can escape it. In other words, scientists have to guess what's inside there.

Another thing we know about a black hole is that it emits small amounts of thermal radiation that's called Hawking radiation. This is so named because of the noted physicist Stephan Hawking's work. He determined that a black hole should emit particles in a sort of evaporation effect. The smaller a black hole is the more radiation is emits. This means that smaller black holes will disappear due to this evaporation effect. Larger black holes take time to evaporate, perhaps millions, if not billions, of years.

The rest of what we know is open to speculation, and the theories are crazy at best. Since a black hole is thought to have a singularity at its center, spacetime curvature there would become infinite, whatever that means. This could mean that if one could survive entering a black hole one could end up in another part of the universe or in a completely different universe. In other words, it's what would happen if you traveled into an infinitely curved spacetime. I have trouble imagining that.

The problem with a rotating black hole (and they do rotate) is that you must travel faster than light to hover at the event horizon because of a drag on the spacetime near it. This would be a neat trick. All of this plays havoc with not only time but with space because time and space are linked, according to Einstein. The dilemma is that if you approach a black hole, you become spaghettified, essentially stretched into a spaghetti strand. Time essentially stops for you if you're still alive. This is the result of the time dilation effect predicted by Einstein's equations. As you approach at light speed, time slows, and at light speed, time stops. Faster than light speed, time goes backwards. That's why some scientists believe that one could go back in time in a black hole, assuming that one could survive it.

Some astronomers believe that the inside of a black hole is very hot and bright, but the light or the energy can't get out. So, if you did go beyond the event horizon you would be fried if that were the case.

One interesting thing about black holes is that they can only eat stuff (gas and dust) at a given rate. If they eat too fast, they spit out particles and energy in an effect that's known as a quasar, at least that's what they call the effect for a massive black hole at the center of a galaxy.

Some suggest the presence of a firewall (intense energy) just inside the event horizon. This was proposed to understand quantum entanglement effects that exist at or near the inside the event horizon. If a particle pairs (one matter, the other antimatter) are formed, one would fall in and the other would fall out. You can see how complicated this is. It's equivalent to trying to understand Brane theory.

One could think of the inside of a black hole as a strange object where spacetime is curling around, literally spinning at faster than light speed toward the center where it enters an infinitely tiny place, a place where ordinary particles don't exist and where matter is transformed into something else, something that doesn't make sense, something that could be what existed before the Big Bang. This gives me a headache!

Or, you could read @Srgthe1 'Black Hole' book. He uses math to prove that Einstein was trying to tell us that the center of a black hole is where an object would not be subject to gravity.

Thanks for reading.