The big-crunch hypothesis is asymmetrical about the ultimate fate of the Universe. Unfortunately, the discovery of dark energy has dealt a fatal blow to Big Crunch, as it suggests that the Universe is eternally expanding. We have no idea what dark energy is, but every year, that dark energy causes the expansion rate to accelerate.
If dark energy increases in power, this may cause an inflation-like state followed again by a new, really hot Big Bang. Instead, 68% of the dark energy might be converted to matter and radiation, akin to when cosmic inflation ends and the warm start. The 68 % dark energy is related to zero-point energy in a quantum vacuum and would decay, breaking up the Universe as we know it.
The first thing to know about this scenario is that the vacuum, in this context, does not refer to empty space; it refers to a lower-energy potential state. The fourth and final prediction of the apocalypse is what is called vacuum decay. Black holes will vaporize, matter itself will ultimately decay to radiation, and the Universe will remain a cold, dark, lifeless place for the rest of eternity.
As the Universe itself continues to age, the final sources of light will come from black holes evaporating. Only those final bright spots from the evaporation of black holes will light up the Universe; all the rest is now cold, dead, and decaying. As existing stars exhaust their fuel and cease their glow, the Universe will gradually and inexorably become darker.
The endgame will start in about five billion years when the Sun begins running low on fuel. Like previous lower-mass stars, many of these will burn through their fuel in just a few trillion years and then glow as a white dwarf for another hundred trillion to quadrillion years, then dim into black. It is akin to the choke point for all astrophysical activity because the fuel to grow and reproduce becomes so dispersed that it is no longer usable.
If more than enough matter-energy is available, then the Universe expands up to a certain maximal size, and expansion turns into contraction, with a collapsed universe. If the Universe contains enough matter, including dark matter, then the combined gravitational pulls from all things will slowly stop that expansion, precipitating a final collapse. Gravity can force the Universe back into a collapsing state, creating a reverse Big Bang called a Big Crunch (we will explain that one in a bit).
The Big Crunch will rip through galaxies, followed by black holes, stars, and even our own planet. Factors to be considered when determining the Universe's origins and ultimate destiny include average galaxies movements, the shape and structure of the Universe, and the amount of dark matter and dark energy that the Universe contains. With the advent of dark energy, several of the earlier contenders for our ultimate destiny are now far less likely.