What exactly is The Big Crunch? I want to know exactly what happens. Does the universe come back in on itself entirely? I did research and it says it could result in a pre-Big Bang particle. The universe is expanding quicker so the chances are smaller of it happening but what chance could it still happen.
It's what was thought would happen when it was thought that the universe's expansion was slowing down, and would eventually reverse. Measurements of the acceleration parameter for the universe, then expected to be negative (deceleration) since any matter should exert gravitational attraction which was thought would slow down the expansion, were made and lo and behold the answer came back the opposite. This was discovered in 1998 and is well accepted now. The acceleration is positive, the expansion is accelerating, and it is expected it will expand even faster as time passes.
The mass and energy density was measured (by counting galaxies and estimating masses, and interstellar mass, radiation energy was also estimated and known to be much lower), but still enough mass that eventually it would slow down the expansion. When the acceleration was discovered it was realized that there must be some other source that is accelerating the expansion: this was called dark energy. It is not clear what it is but it is repulsive rather than attractive, and it is known that it now constitutes about 75% of the total mass/energy of the universe. Dark matter was also found, it is attractive, but it only constitute, along with visible matter, about 25% of the mass/energy of the universe. So there are some mysteries remaining, but the measurements of the expansion and acceleration are unambiguous (well, there are some opposing theories, but the standard cosmology model is that) and predict a constantly accelerating universe.
The opposite of the Big Crunch is the so called Big Rip, that spacetime will rip apart as it accelerates infinitely, a long long time from now. Again, there's opposing theories, but the data indicates the continuing acceleration. Another possibility, is that it will continue expanding forever, and the universe will become very very empty of matter and energy (except dark energy), and have a high enough entropy that nothing much will happen with the matter it contains after, it'll be all dispersed. This is called the Heat Death (really an entropy death). If we ever understand more about the microscopic structure of spacetime, in a quantum theory of gravity, we might be able to say more.
See some of this and references for more in https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_the_universe
In models of the evolution of the universe in which there exists enough mass to completely stop its expansion via gravity, the universe then reverses itself and contracts all the way back down to nothingness. Since this piles all the matter in the universe on top of itself, everything gets smashed together into a big fireball- hence the name, the "Big Crunch". Best estimates of the mass density in the universe suggest this will not be its fate, however.
For an body to escape another body's gravitational influence, the 1st body's velocity has to exceed the 2nd body's escape velocity, e.g. to get to a speed where the sum of an object's kinetic energy and its gravitational potential energy is equal to zero. The formula for escape velocity is given as V=the square root of 2GM/r, where G is the gravitational constant, M is the mass of the 2nd body, and r is the distance from the 2nd body's center of mass to the 1st body.
If the speed of the universe's expansion does not exceed the escape velocity of the universe's mass, then the universe will begin to collapse in on itself by its own gravity. The temperature of the cosmic microwave background will begin to rise and large galactic structures will start merging together from superclusters all the way to galaxies. When the objects are close enough together for stars to merge, the temperature of the CMB will have risen enough so that it stars will not be able to expel their internal heat and so will explode into gas clouds which then will dissolve into their respective subatomic particles because of the increasing heat. Eventually, everything will either collapse into a black hole or be swallowed into one with all matter in the universe becoming one coalesced singularity.