Astronomy & Cosmology
The End of the Universe
This theory suggests that the average density of the Universe is sufficient, ultimately, to stop the expansion of the Universe gravitationally, and cause it to start to collapse back on itself. Just as the Big Bang started with a singularity, so the Big Crunch ends with one, although the exact nature of either singularity is not explained, and must certainly consider Quantum effects. The collapse would look very different to a time reversal of the expansion. As the Universe contracts, and matter gets closer together, more and more black holes would form. As these approach each other, they coalesce into ever larger black holes until ultimately everything in the Universe is contained in a singe black hole. The fact that the Universe is expanding at an increasing rate does not bode well for this theory.
There are, however, ways around this, good examples being the Steinhardt-Turok cyclic universe discussed in Alternatives to the Big Bang, or that dark energy is a form of quintessence that could change value over time and permit gravity to take hold once more.
The cosmologist Andrei Linde and his physicist wife Renata Kallosh go much further and in 2002 suggested that dark energy is, or will become, negative, and thus attractive. This would pull the Universe back on itself causing a big crunch. They even suggest a rather short timescale of between ten and twenty billion years; a time comparable to the current age of the Universe.
The Big Crunch