Something Strange Happens When You Follow Einstein's Math

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Black Holes

Black holes prevent anything, even light, from escaping beyond the event horizon.
The Schwarzschild metric describes the curvature of spacetime around a single spherically symmetric point mass and has two problem spots: a singularity at the center of the mass and another at the Schwarzschild radius.
Black holes were initially doubted due to the massive amount of mass required to collapse into a tiny space.
Oppenheimer and Snyder showed that for the heaviest stars, there is nothing to prevent them from collapsing indefinitely, suggesting the possibility of black holes.
Einstein initially resisted the idea of black holes due to the apparent freezing of time at the event horizon.
The singularity at the event horizon is an artifact of the chosen coordinate system and disappears under a different coordinate substitution.
Space can be visualized as flowing into a black hole like a waterfall, with photons struggling to escape against the flow.
Inside the event horizon, space falls faster than the speed of light, causing everything to fall into the singularity.

A white hole is the time reverse of a black hole, where things are expelled outwards instead of falling in.
The Schwarzschild black hole solution contains a second universe that has its own independent set of coordinates from our universe.
An Einstein-Rosen Bridge is a hypothetical wormhole that connects two universes and could allow for travel between them.
These wormholes are not stable in time and would collapse before anything could pass through them.

Spinning black holes have a more complex structure compared to non-rotating black holes, as described by the Kerr solution.
The Kerr black hole consists of several layers, including the ergosphere, the outer horizon, and the inner event horizon.
Inside the inner event horizon, there's a region where objects can move around freely and avoid the singularity.
Beyond the inner horizon, there could potentially be a white hole that leads to another universe.
By traveling through a rotating black hole and a white hole, it's theorized that one could access an infinite number of universes.

According to our current understanding, white holes, traversable wormholes, and parallel universes are unlikely to exist, but there's always the possibility of future surprises.