String theory has several potential applications to cosmology, the study of the universe, covering the entire history of the cosmos.
One line of enquiry focuses on the¬†initial singularity¬†‚Äď what happened when the universe was squeezed to a size close to the Planck scale. This happened shortly after the Big Bang. In principle, this is a period ideally suited to any quantum theory of gravity.
Technically, however, it’s hard to model as the relevant equations depend on time. For this reason, it isn’t possible to use a quantum string theory directly, so the full power of the theory is absent. Currently, study of the initial singularity requires unpalatable approximations. Removing the need for these is an open research problem.
Another research direction involves constructing string models of¬†inflation.¬†This was a period of extremely fast expansion that may have occurred in the early universe. It can explain both why the universe is so flat and homogeneous, and also the origin of the small disparities in density that gave rise to galaxies.
Inflation is often said to be a paradigm in search of a theory. String cosmologists hope to pin down inflation more exactly, making novel predictions that move beyond our limited comprehension from¬†field theory.¬†String descriptions of inflation are typically based on the movements of branes or on geometric changes in¬†extra¬†dimensions.
There are also mysteries in the¬†universe after inflation, covering a wide variety of topics.¬†What is the origin of dark matter, the mysterious stuff that makes up almost a quarter of the universe’s mass? Why is there so little¬†antimatter, particles with opposite properties to ordinary particles? Is there extra hidden energy¬†beyond that predicted by standard cosmology?
This area has close connections to string models of particle physics. Researchers hope that such models will have distinctive features or extra particles that will help elucidate cosmological evolution. There is an overlap with work in pure QFT,¬†but string theorists are additionally constrained by extra dimensions.
Finally some people employ strings to decipher the¬†cosmological constant, a deeply mysterious so-called¬†dark energy that fills spacetime. The cosmological constant drives the current accelerating expansion of the universe and what is most perplexing about it is its size.
The observed cosmological constant is a factor of 1060 smaller than theoretical calculations would suggest: the largest mismatch between experiment and theory anywhere in physics! Some would say that the value of the cosmological constant is the biggest unsolved problem in physics. It’s certainly an open and active area of research in string theory, though sadly no good solutions have emerged yet.