Assisted Chaotic Inflation in Higher Dimensional Theories

TL;DR

The paper tackles the challenge of requiring excessively large initial inflaton values in chaotic inflation and the risk of non-renormalizable terms dominating the dynamics. It introduces assisted inflation in a 4D multi-field framework, where an ensemble of sub-Planckian fields combines to produce inflation via an effective inflaton $ ilde{\phi}=\sqrt{N}\,\phi_1$ with a reduced quartic coupling $\tilde{\lambda}=\lambda/N$. It then provides a concrete higher-dimensional realization by compactifying a $(4+d)$-dimensional theory with a single massive scalar, yielding a KK tower that, if many modes share similar masses, forms the effective inflaton and drives chaotic inflation with $V(\tilde{\phi})\sim m^2\tilde{\phi}^2$; this requires the extra dimensions to have size $L$ such that $m_{\vec n}^2=m^2+\frac{\vec n^2\pi^2}{L^2}$. The results show that initial sub-Planckian constituent fields can sustain inflation (with $M\ge 10^{-5}\,M_P$) and that the natural suppression of the quartic coupling arises from multiplicity, offering a UV-wide mechanism for assisted chaotic inflation across multiple extra dimensions, while highlighting potential constraints from heavy-mode fluctuations.

Abstract

We address the problem of the large initial field values in chaotic inflation and propose a remedy in the framework of the so-called assisted inflation. We demonstrate that a 4-dimensional theory of multiple, scalar fields with initial field values considerably below the Planck scale, can give rise to inflation even though none of the individual scalar fields are capable of driving inflation. The problems arising from the presence of possible non-renormalizable interactions is therefore removed. As a concrete example of a theory with multiple scalar fields, we consider a (4+d)-dimensional field theory of a single, non-interacting massive scalar field whose KK modes play the role of the assisted sector. For the KK modes to assist inflation, the extra dimensions must have a size larger than the inverse (4D) Planck scale.