Moduli Dynamics in Effective Nested Warped Geometry in Four Dimensions and Some Cosmological Implications

TL;DR

This paper investigates the dynamics of moduli fields in nested warped extra-dimensional models, exploring their potential for driving inflation and analyzing the implications of brane cosmological constants on the effective four-dimensional theory.

Contribution

It introduces a detailed analysis of moduli potentials and kinetic terms in nested warped geometries, highlighting their role in inflation and the impact of brane curvature.

Findings

Metastable minima exist for the first modulus in dS branes.

Slow roll inflation is most viable along the first modular direction.

Higher moduli require unphysical hierarchies for successful inflation.

Abstract

We analyze the effective four-dimensional dynamics of the extra-dimensional moduli fields in curved braneworlds having nested warping, with particular emphasis on the doubly warped model which is interesting in the light of current collider constraints on the mass of the Kaluza-Klein graviton. The presence of a non-zero brane cosmological constant ($\Omega$) naturally induces an effective moduli potential in the four-dimensional action, which shows distinct features in dS ($\Omega>0$) and AdS ($\Omega<0$) branches. For the observationally interesting case of dS 4-branes, a metastable minimum in the potential arises along the first modulus, with no minima along the higher moduli. The underlying nested geometry also leads to interesting separable forms of the non-canonical kinetic terms in the Einstein frame, where the brane curvature directly impacts the kinetic properties of only the first modulus. The non-canonicity of the scenario has been illustrated via an explicit computation of the field space curvature. We subsequently explore the ability of curved multiply warped geometries to drive inflation with an in-built exit mechanism, by considering predominant slow roll along each modular direction on a case-by-case basis. We find slow roll on top of the metastable plateau along the first modular direction to be the most viable scenario, with the higher-dimensional moduli parametrically tuning the height of the potential without significant impact on the inflationary observables. On the other hand, while slow roll along the higher moduli can successfully inflate the background and eventually lead to an exit, consistency with observations seemingly requires unphysical hierarchies among the extra-dimensional radii, thus disfavouring such scenarios.