Sagittarius A* is the SMBH at the centre of the Milky Way
Hawking's Knot
Naturally occurring black holes are objects in space that are so dense that within its Schwarzschild radius or event horizon (see diagram), its gravitational field does not let particles or even electromagnetic radiation such as light escape. Hence the event horizon shields our view of any events or what happens to objects that fall into the hole.
Black holes are formed from the collapse of a giant star with a mass greater than 8M (where M is 1 solar mass) in a supernova explosion, or further collapse of a neutron star. For a typical black hole with a mass of 10M, the Schwarzschild radius is approximately 30 km.
An object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it, a fifth dimension effect known as gravitational time dilation. The object will also most likely be ripped apart by tidal forces in a process sometimes referred to as spaghettification or the "noodle effect" before crossing the event horizon. An entire star was observed undergoing spaghettification.
At the center of a black hole is a gravitational singularity, a region where the spacetime curvature becomes infinite. The singularity is a single point (0d) with zero volume and infinite density. In this strange place it might be possible to exit to any point in spacetime, whether it's another universe or instance. Naturally occurring singularities of infinite curvature might only join two random points for an extremely short time before closing, so to keep them open would require extremely advanced technology like creating a white hole by threading a black hole with exotic matter.
A white hole is like a black hole but with a singularity that cannot be entered from the outside, although its negative energy-matter can escape from it. It is therefore the reverse of a black hole, which can be entered only from the outside and from which its positive energy-matter cannot escape. So white holes "blow" and black holes "suck". White hole and black hole singularities work together to form wormholes.
The ergosphere is a region located outside a rotating black hole's event horizon. It is possible to extract energy and mass from this region using the Penrose process. Negative energy exists in this region.
Black holes can also contain some forms of exotic matter that are found in neutron stars. It is possible that the entire region within the event horizon are actually fuzzballs composed of strings, which are the ultimate building blocks of matter and energy. These are at Planck length scale, and have densities similar to degenerate matter on the scale of 4.0×1017 kg/m3.
The centers of many galaxies contain supermassive black holes (SMBH), which form in the earliest period of a galaxy's existence. These are in the order of millions to billions of M. Almost every large galaxy has a SMBH at the galaxy's center. The Milky Way has one too, which corresponds to the location of Sagittarius A*. Accretion of interstellar gas onto SMBHs is responsible for powering active galactic nuclei and quasars. A tidal disruption event occurs when a star approaches sufficiently close to a SMBH and is pulled apart by the black hole's tidal force, experiencing spaghettification. Currently the largest known SMBH, classed as an ultramassive black hole (UMBH), powers the quasar TON 618 and is 66 billion solar masses. A stupendously large black hole (SLAB) could also exist around the original point of the Big Bang, at least 1 trillion times the mass of the sun, or at the centre of an extremely large galaxy such as IC 1101.
An accretion disk is a disk-like structure (seen in orange in the diagram of Sagittarius A*) formed by material in orbital motion around a massive central body; the most spectacular accretion disks found in nature are those of active galactic nuclei and of quasars. Friction, irradiance, magnetohydrodynamic effects, gravitational and other forces induce instabilities causing orbiting material in the disk to spiral inward towards the body. These forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. While no energy can escape from beyond the event horizon around the black hole, this energy is released from the material as it falls in. Accretion onto a black hole is a very efficient process for emitting energy from matter, releasing up to 40% of the rest-mass energy of the material falling in. Only an antimatter-matter collision is more efficient at 100%. Black hole accretion disks contain material of high temperatures generated by high accretion rates, allowing nuclear fusion and nucleosynthesis to take place
Artificial black holes can be made in various ways, such as using a very powerful explosion to implode a large object or wormhole, or blowing up a star, or striking objects with impactors moving close to the speed of light. According to Allen Guth, producing one Planck energy in a tiny region of space would create a black hole. A halo of dark energy would need to surround that black hole in order to prevent it from collapsing.
Quantum black holes may have been created around 10−43 seconds after the Big Bang, or may even be created by god-level civilizations that are able to manipulate or breach the fabric of reality.
Black holes have many important industrial uses, including power generation in Hawking's Knots, the production of gravity wells for artificial planets, enhanced space-time curvature generation for starships, and deep space garbage disposal.
When the Hawking's Knot is "closed" or "tied," the distortion field works to redirect the flow of radiation back into the hole, thus maintaining its mass and preventing it from evaporating completely. When the Knot is "opened," the space-time metric is modulated to segregate antiparticles from particles and channel these away from the Knot to receiver/storage devices. Since this results in a decrease in the mass of the black hole, a constant stream of "feeder" particles is required to prevent the Knot from evaporating.
To quote Lawrence Krauss from The Physics of Star Trek: "A remarkable property of black holes that the more massive they are, the less dense they need be when they form. For example, the density of the black hole formed by the collapse of an object 100 million times as massive as our Sun need only be equal to the density of water. An object of larger mass will collapse to form a black hole at a point when it is even less dense. If you keep on extrapolating, you will find that the density required to form a black hole with a mass equal to the mass of the observable universe would be roughly the same as the average density of matter in the universe! We may be living inside a black hole."
If a universe indeed has a black hole structure, this implies that black hole universes are within larger structures called multiverses, and they could be within megaverses, and so on, ad infinitum.