The Future Universe

In the previous section, we associated the present era with the age of stars. However, at some future point in time, possibly some 10-100 trillion years, the last hydrogen-fuelled star will cease the process of fusion. No matter how dramatic this may appear from our current perspective, the life span of the universe may have barely started. At this moment in time, we are currently less than 0.15% into the stelliferous era of stars. As we enter the far future, the distribution of stars will have changed radically:

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An accurate prediction of the nature of a far future universe will, like the early universe, require additional advances in fundamental physics. So before discussing just one scenario, it is possibly worth outlining a few other possibilities:

  • The Big Crunch:
    If the energy density currently attributed to `dark energy` diminishes to a point where the universe is closed, i.e. [k>0], it would be possible for the expansion of the universe to reverse and the universe contract back towards a hot, dense state, analogous to a time-reversal of the Big Bang. This is often discussed as part of an oscillating universe, similar in scope to simple harmonic motion. Note, the collapse of the universe under gravity is assumed by this hypothesis, but not necessarily explained by it, as there appears to be an implicit suggestion that gravity is either pulling matter towards some centre, analogous to a black hole, or can somehow reverse the expansion of space.

  • The Big Rip:
    This scenario is associated with the speculation that the energy density of dark energy increases without limit over time. As a consequence, the expansion rate [H] of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, the solar system and ultimately the Earth would be torn apart as all gravitational cohesion is lost to expansion. Eventually, the expansion would increase so that the electromagnetic forces holding people, molecules and atoms together would be overcome. Finally, even atomic nuclei will be torn apart and the universe as we know it will end.

  • Vacuum Meta-stability Disaster:
    If our universe is in a very long-lived false vacuum, it is possible that the universe will collapse into a lower energy state. In this case, all structures will be destroyed instantaneously, without any forewarning and be worthy of its foreboding name.

  • Heat Death:
    This scenario is generally considered to be the most likely and is the one that will underpin our discussion. Essentially, it assumes that the universe continues to expand based on k=0. As such, the universe must eventually enter a high entropy state, in which galaxies collapse into black holes that subsequently evaporate via Hawking radiation. In this case, the universe will ultimately consist of only uniform radiation.