Astronomy & Cosmology
The End of the Universe
The Big Freeze
This theory requires the Universe to be either flat or negatively curved. It maintains that the Universe will expand forever, getting colder and colder. The theory does, however, require that the acceleration of the expansion either remains constant or slows down. It is possible that, depending on the nature of Dark Energy, the Universe's expansion rate could slow in the distant future; for example, if the Dark Energy was Quintessence, it could vary in its affect over time. The timescales for this scenario are immense. Star formation is expected to continue for another 1014 years before all the available material for star formation becomes exhausted. Before the end of that period, probably within the next trillion (1012) years, all the galaxies in the Local Group are expected to have coalesced into a single galaxy. In about two trillion (2 x 1012)years time, the expansion of the Universe would have been such that all the other galaxies would be undetectable due to the red shift of their light. If Grand Unification is correct, and nucleons are found to decay with a half-life of around 1037 years, then by 1040 years time all the neutrons and protons will have decayed to photons and leptons. In addition, black holes will evaporate over the period from 1040 years, for stellar mass black holes, to 10100 years for supermassive black holes. After this, the Universe, now grown to an unimaginably huge volume, is almost empty with just some photons and leptons moving randomly, and essentially never interacting.
There is a subtle difference between a flat Universe and a negatively curved Universe. In the case of Omega equal to one, a flat Universe, the expansion rate slows and slows so that it eventually stops and becomes stationary, although that takes an infinite amount of time. With negative curvature, W less than one, the expansion continues for ever with the Universe just getting bigger and bigger. I guess that, to anyone still around, the differences would be incidental!