The flattened road to acceleration in string theory

Stages of accelerated expansion during the history of the universe – both a very late and very early variant – have very strong support from cosmological data. A phase of (almost) de Sitter space-time can describe both stages of acceleration, both the extremely tiny dark energy of today and the stage of cosmological inflation early on. As both forms of dark energy are radiatively unstable, there is a strong motivation to understand positive vacuum energy in string theory. Constructing models of de Sitter space or inflation in string theory does not come easy due to the presence of extra dimensions and moduli. We will discuss a general phenomenon in string theory that the build-up of positive vacuum energy tends to ‘flatten out’ with increasing amount of supersymmetry breaking due to the backreaction effects. We will show how this effect occurs in string theory models of inflation, flattening the inflaton potential and suppressing the amount of primordial gravitational waves generated. We will then argue that the same flattening effects occur in the construction of static de Sitter minima as well. These backreaction effects render the construction of a positive cosmological constant in string theory more difficult