A Minimal Axio-dilaton Dark Sector (2410.11099v2)

Abstract: In scalar-tensor theories it is the two-derivative sigma-model interactions that like to compete at low energies with the two-derivative interactions of General Relativity (GR) $\unicode{x2014}$ at least once the dangerous zero-derivative terms of the scalar potential are suppressed (such as by a shift symmetry). But nontrivial two-derivative interactions require at least two scalars to exist and so never arise in the single-scalar models most commonly explored. Axio-dilaton models provide a well-motivated minimal class of models for which these self-interactions can be explored. We review this class of models and investigate whether these minimal two fields can suffice to describe both Dark Matter and Dark Energy. We find that they can $\unicode{x2014}$ the axion is the Dark Matter and the dilaton is the Dark Energy $\unicode{x2014}$ and that they robustly predict several new phenomena for the CMB and structure formation that can be sought in observations. These include specific types of Dark Energy evolution and small space- and time-dependent changes to particle masses post-recombination that alter the Integrated Sachs-Wolfe effect, cause small changes to structure growth and more.

• The paper introduces a dual-scalar framework where the axion acts as Dark Matter and the dilaton drives Dark Energy.
• It employs effective field theory and numerical simulations to investigate gravitational dynamics beyond General Relativity.
• The study identifies parameter ranges that align with observational data, influencing cosmic structure and CMB fluctuations.

Overview of "A Minimal Axio-dilaton Dark Sector"

The paper "A Minimal Axio-dilaton Dark Sector" addresses the challenge of describing both Dark Matter and Dark Energy within a unified scalar field framework in cosmology, specifically through axio-dilaton models. These models posit two interacting scalar fields, the axion and the dilaton, which have the potential to simultaneously address gravitational phenomena beyond General Relativity (GR) and provide a comprehensive account of the dark sector.

Theoretical Framework

The theoretical basis for this paper employs scalar-tensor theories, which have long been considered as modifications to GR to explore alternative measures of gravity. Traditional models often consider single scalar fields; however, the authors argue that axio-dilaton models, featuring two scalar fields, offer a more comprehensive alternative that could also encompass dark sector dynamics. The axion component follows the role of Dark Matter through coherent oscillations, while the dilaton component is considered to describe Dark Energy, drawing parallels to quintessence models.

Axio-dilaton Model Dynamics

The paper details the dual-scalar axio-dilaton model derived from effective field theories (EFT). The axion, viewed as a pseudo-Goldstone boson, possesses properties allowing it to act as Cold Dark Matter, while the dilaton, associated with potentially scale-invariant dynamics, influences the gravitational interaction metrics. The kinetic coupling of these fields is highlighted, particularly through functions like $W(\chi)$ and potentials $V(\chi,)$. These fields compete and interact effectively at low energy scales, surpassing the derivative interactions of GR models.

Cosmological Implications

The research provides robust modelling for analysing cosmic structures, background dynamics, perturbations, CMB fluctuations, and baryon density perturbations. Numerical simulations are utilized to compute specific cosmological consequences if the axion and dilaton simultaneously describe Dark Matter and Dark Energy. The variability of scalar field parameters, including axion mass and decay constant $f$, impact structure growth and cosmic microwave background (CMB) observations. These cosmologies predict nontrivial changes to the Integrated Sachs-Wolfe (ISW) effect, mass evolution dynamics, and power spectrum deviations at small multipoles.

Empirical Analysis

Extensive numerical evaluations elucidate distinctive outcomes depending on whether the dilaton's potential has exponential or more complex forms. With exponential potentials, coupling parameters like $\zeta$ can influence structure formation and baryon mass variance, essential for matching observational data constraints, such as solar-system fifth-force tests and CMB power spectra. The paper identifies ranges for these parameters that produce results consistent with empirical evidence, establishing a key step towards unifying dark sector physics.

Impact and Future Directions

The paper concludes by acknowledging the conceptual promise of axio-dilaton models in solving dark sector questions. It outlines how the dilaton-matter coupling parameter, constrained by limits from local gravitational measurements, still allows for meaningful cosmological analyses.

Future explorations may focus on integrating these findings with broader scalar fields' implications for naturalness problems and vacuum energy phenomenology. Potential developments could include observational strategies to test for axion-related Dark Matter signature or detecting minute time-variance in fundamental constants influenced by the dilaton field. This framework, in particular, holds promise for enriching the comprehensive understanding of fundamental cosmology, bridging theoretical predictions with empirical observations in ongoing and future astrophysical surveys.