A portal to another universe
Black holes have been theorized to contain wormholes, which could serve as a pathway to alternate universes.
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Black holes result as a solution to Einstein's mathematical equations of relativity. Another, opposite solution, known as a white hole, is also mathematically possible. A white hole is the exact opposite of a black hole: instead of sucking in any matter that crosses its event horizon, a white hole would shoot out matter from its event horizon. According to theory, each black hole could be connected to a white hole. The black hole sucks matter in, and the white hole sends that matter back out. These holes would be linked through a wormhole and could be located in completely different areas of our Universe or even entirely separate universes. In modern physics, spacetime is characterized as one bendable surface. Objects with high gravity rest on the surface and bend it inward toward them. A wormhole would work as a result of this bending. If two spacetime sheets were stacked on top of one another, like two books, a wormhole would travel directly through them as a whole, instead of going all the way around. Due to a black hole's immense gravity, the fabric of spacetime would bend so much that it could break, breaking through to the other layer of spacetime. In this way, a wormhole would skip enormous amounts of space and serve as a portal to a completely different place. Wormholes are a very fringe theory of astrophysics and have many critics, as it is a very difficult theory to prove. However, there is research being done on the possibility of the situation. Wormholes are extremely unstable and would require lots of "negative energy" to go against the pull of gravity threatening to collapse them. Researchers have demonstrated that very small wormholes could become stable and be balanced out by the expansion energy of the Universe. If this were the case, the wormholes would be so small that only the force of gravity could get through. Scientists could test this by very accurately measuring the orbits of stars around a black hole, and detecting any influence that could be the gravity of something from another universe acting upon the stars through the wormhole.
Wormholes are purely theoretical. No direct observation of black hole centers has been made, so we have no clue what is inside of them. A wormhole and a different universe are totally nonsensical ideas with no proof or evidence to back them up. For a wormhole to remain stable, it would require immense amounts of negative energy, but negative energy does not exist. Just because wormholes can exist based on the theory of relativity, does not necessarily mean that they do.
[P1] Objects with high gravity bend spacetime. [P2] A black hole has enough gravity to break through spacetime. [P3] The breaking through of spacetime would create a passage to a different area of spacetime. [P4] This passage, with a white hole at the end, is a wormhole.