Probing String Theory with Modulated Cosmological Fluctuations

TL;DR

The paper investigates how light moduli from string theory during inflation can imprint observable cosmological fluctuations beyond standard inflaton perturbations. It shows that moduli fluctuations generate spatial variations in couplings via $\delta\alpha/\alpha \sim \sum_a (\partial \log \alpha/\partial \chi_a)\,\delta\chi_a$, with $\delta\chi_a$ of order $H$; these lead to modulated isocurvature and, through reheating dynamics, to modulated curvature perturbations, potentially even when inflaton fluctuations are suppressed. The authors analyze the post-inflation evolution of moduli, distinguishing potentials with minima from runaway potentials, and discuss how modulated couplings affect freeze-out and preheating to produce large-scale curvature/isocurvature signals. They highlight observational implications for CMB/LSS and emphasize the string-theory–cosmology connection, while noting moduli stabilization and consistency with BBN as important considerations.

Abstract

Superstring theory, models with extra dimensions and other SUSY models generically predict that the coupling constants are in fact vacuum expectation values of fields like the dilaton, moduli etc. Assuming some of these fields are light during inflation, we get generation of small classical inhomogeneities in these fields from inflation. Consequently, coupling constants inherit small inhomogeneities at scales much larger than the causal horizon in the early universe. After the moduli get pinned down to their minima, the spatial variations of coupling constants in the late time universe will be erased. However, inhomogeneities in coupling constants in the very early universe would generate modulated large scale fluctuations in all relic species that are produced due to interactions and freezing out. Moreover (p)reheating of the inflaton field results in modulated curvature fluctuations. Even if the standard inflaton fluctuations are suppressed, in this picture we may have pure curvature cosmological fluctuations entirely generated by the modulated spatial variations of the coupling constants during preheating.