Implications of alternative universes
Taming limitations of general relativity, such as the big bang singularity, by formulating theories that admit bouncing or cyclic universes.
On Earth, solar, and galactic length scales, Einstein’s theory of General Relativity is unbeatable. However, the theory breaks down at the smallest length scales, due to the presence of singularities and the lack of a complete quantum description.
In order to rectify these shortcomings, modifications to General Relativity are necessary. Alternative theories predict signatures that may be observable, or at least falsifiable. For instance, some theories require that the Universe undergoes cyclic phases of contraction and expansion, as opposed to the usual Big Bang. It is possible that physical quantities preserve memory through the bounces, and could hence be measured. Furthermore, the scale at which modifications become important in classes of modified theories of gravity can be identified, signifying where and when deviations should show up.
A full theory of quantum gravity, and a unification of gravity with the other fundamental forces of nature is the holy grail of modern astrophysics and cosmology. The fact that such a successful theory as General Relativity needs modification demonstrates how precise our understanding of the Universe has become, and understanding these fine details may illuminate other mysteries, such as the nature of dark matter and dark energy.
The number of particles in a higher derivative theory of gravity relates to its effective mass scale, which signals the theory’s viability.
In an infinitely bouncing Universe, the scalar field driving the cosmological expansion and contraction carries information between phases.