Kaluza-Klein towers in the early universe: Phase transitions, relic abundances, and applications to axion cosmology

Abstract

We study the early-universe cosmology of a Kaluza-Klein (KK) tower of scalar fields in the presence of a mass-generating phase transition, focusing on the time-development of the total tower energy density (or relic abundance) as well as its distribution across the different KK modes. We find that both of these features are extremely sensitive to the details of the phase transition and can behave in a variety of ways significant for late-time cosmology. In particular, we find that the interplay between the temporal properties of the phase transition and the mixing it generates are responsible for both enhancements and suppressions in the late-time abundances, sometimes by many orders of magnitude. We map out the complete model parameter space and determine where traditional analytical approximations are valid and where they fail. In the latter cases we also provide new analytical approximations which successfully model our results. Finally, we apply this machinery to the example of an axion-like field in the bulk, mapping these phenomena over an enlarged axion parameter space that extends beyond those accessible to standard treatments. An important byproduct of our analysis is the development of an alternate "UV-based" effective truncation of KK theories which has a number of interesting theoretical properties that distinguish it from the more traditional "IR-based" truncation typically used in the extra-dimension literature.

Deciphering the archaeological record: Cosmological imprints of nonminimal dark sectors

“…Alternatively, a time dependence for the ðm 2 l Þ eff can also arise directly as a consequence of dynamical processes which do not involve modulation of the VEVs of other dark-sector fields-e.g., from a cosmological phase transition. Even considering only the effect on the VEVs, multiple fields receiving such dynamical mass contributions exhibit an array of possible behaviors [75][76][77]. Furthermore, when the ðm 2 l Þ eff evolve in this way, the pattern of intra-ensemble decays could be drastically altered, thereby modifying the ultimate shape of the resulting dark-matter phase-space distributions.…”

Multiple-axion framework

“…These results motivate a future study of the multifield dynamics and perturbations. The corresponding initial conditions can be sourced by upward transitions or other mechanisms (e.g., [54,55]). Note that while n s and r are independent of the overall scale of the potential, the power spectrum depends on the scale, and so to match observation, trajectories with smaller r must occur in models with correspondingly smaller values for the Λ I .…”

Deciphering the Archaeological Record: Cosmological Imprints of Non-Minimal Dark Sectors

“…Alternatively, a time-dependence for the (m 2 ) eff can also arise directly as a consequence of dynamical processes which do not involve modulation of the VEVs of other dark-sector fieldse.g., from a cosmological phase tran-sition. Even considering only the effect on the VEVs, multiple fields receiving such dynamical mass contributions exhibit an array of possible behaviors [75][76][77]. Furthermore, when the (m 2 ) eff evolve in this way, the pattern of intra-ensemble decays could be drastically altered, thereby modifying the ultimate shape of the resulting dark-matter phase-space distributions.…”