Chapter 10: The Ultimate End: Dark Energy And The Fate Of The Universe
The story of where our Universe came from is a remarkable one, and the fact that we have reached the point where we understand as much of it as we do is perhaps equally remarkable. But up until very recently, the fate of our Universe — how it would continue to evolve in the future — was very much unknown. We have already explored (back in Chapters 4 and 8; see Fig. 4.2) the idea that there are three possible fates for the Universe, dependent on the relationship between the amount of matter-and-energy present and the rate of the initial expansion. These fates are:
The Big Crunch: an initially large expansion rate causes the young Universe to grow incredibly quickly, but the density of matter and energy is also tremendously large, slowing the expansion rate down over time. Eventually, there is just enough gravitation to overcome the expansion completely, causing the Universe to reach a maximum size and cease expanding altogether. Once it reaches this turning point, the gravitation from all the matter and energy present causes the Universe to begin contracting. After the same amount of time passes again that it took the Universe to expand from the Big Bang until reaching maximum size, it will recollapse into an arbitrarily hot, dense state again: the Big Crunch. A Universe that ends in a Big Crunch will have a positive (closed) spatial curvature.
The Big Freeze: the initial conditions of the Universe — the initial expansion rate and the matter-and-energy density — are practically 322 Beyond The Galaxy indistinguishable from the Big Crunch conditions early on. The Universe expands incredibly rapidly and slows down to a minuscule fraction of its initial value. But rather than there being just enough gravitation to stop and reverse the initial expansion, it is the other way around: the initial expansion is just slightly too great for the amount of matter and energy in our Universe. As a result, the expansion rate remains positive at all times, and the galaxies that are not gravitationally bound to ours recede farther and farther away as time goes on. Inevitably, the Universe keeps expanding and cooling forever and ever. A Universe that ends in a Big Freeze will have a negative (open) spatial curvature.
A Critical Universe: at most, the departure from a perfect balance between the early matter-and-energy density of the Universe and the initial expansion rate must be less than one part in 1025, so why couldn't the balance be exactly perfect? Perhaps the Universe really is right on the border that separates an eternal expansion from a recollapse, where only one more proton in the Universe would change our fate. A critical Universe will have the expansion rate drop, asymptotically, towards zero, but the expansion rate will never reverse itself. A Universe that possesses this exact critical density of matter-and-energy will have zero (flat) spatial curvature.
