The Undiscovered Country: The Future of the Cosmos
Determining the solutions to string theory that apply to our universe might allow us to determine which future is most likely. In cosmology, the past and the future are linked together, and the explanation for one is tied to the explanation of the other. With the big bang model in place, there are essentially three possible futures for our universe.
A universe of ice: The big freeze
In this model of the universe’s future, the universe continues to expand forever. Energy slowly dissipates across a wider and wider volume of space and, eventually, the result is a vast cold expanse of space as the stars die. This big freeze has always had some degree of popularity, dating back to the rise of thermodynamics in the 1800s.
The laws of thermodynamics tell you that the entropy, or disorder, in a system will always increase. This means that the heat will spread out. In the context of cosmology, this means that the stars will die and their energy will radiate outward. In this heat death, the universe becomes a static soup of near–absolute zero energy. The universe as a whole reaches a state of thermal equilibrium, meaning that nothing interesting can really happen.
A slightly different version of the big freeze model is based on the more recent discovery of dark energy. In this case, the repulsive gravity of dark energy will cause clusters of a galaxy to move apart from each other, while, on the smaller scale, those clusters will gather closer together, eventually forming one large galaxy.
Over time, the universe will be populated by large galaxies that are extremely far apart from each other. The galaxies will become inhospitable to life, and the other galaxies will be too far away to even see. This variant, sometimes called a cold death, is another way the universe could end as a frozen wasteland. (This timescale is incredibly vast, and humans will likely not even still exist. So no need to panic.)
From point to point: The big crunch
One model for the future of the universe is that the mass density of the universe is high enough that the attractive gravity will eventually overpower the repulsive gravity of dark energy. In this big crunch model, the universe contracts back into a microscopic point of mass.
This idea of a big crunch was a popular notion midcentury in science fiction, but with the discovery of the repulsive dark energy, it seems to have gone out of favor. Because physicists are observing the expansion rate increase, it’s unlikely that there’s enough matter to overcome that and pull it all back together.
A new beginning: The big bounce
The ekpyrotic model brings the big crunch back, but with a twist. When the crunch occurs, the universe once again goes through a big bang period. This is not the only model that allows for such a big bounce cyclic model.
In the ekpyrotic model, the universe goes through a series of big bangs, followed by expansion and then a contracting big crunch. The cycle repeats over and over, presumably without any beginning or end. Cyclic models of the universe are not original, going back not only to 1930s physics, but also to religions, such as some interpretations of Hinduism.
It turns out that string theory’s major competitor — loop quantum gravity — may also present a big bounce picture. The method of loop quantum gravity is to quantize (break up into discrete units) space-time itself, and this avoids a singularity at the formation of the universe, which means that it’s possible that time extends back before the big bang moment. In such a picture, a big bounce scenario is likely.