Strong primordial inhomogeneities induced by axion-like scalar field

TL;DR

The paper investigates how axion-like scalar fields associated with domain walls can induce strong primordial inhomogeneities in an anisotropic spacetime. It combines thin-shell domain-wall formalism with a Kantowski-Sachs–type metric in the distant-observer limit and a cosine-type ALP potential to derive a closed set of equations. A power-law ansatz $Y = X^n$ and $\phi \propto a^{l}$ reduces the system to a nonlinear scalar equation for $\theta = \phi/f$, whose numerical solutions reveal $l$-dependent scalar-field behavior and an approximately constant equation of state for the cosmic fluid alongside anisotropic stresses. The results illustrate how ALP dynamics on domain walls can affect spacetime structure and hint at observable consequences for primordial gravitational-wave backgrounds and anisotropic cosmologies.

Abstract

Our goal is to consider Axion-like particle (ALP) model to investigate the behaviour of space-time in the vicinity of the domain wall, induced by axion-like field. Here we present first-step approximation in our analysis and discuss the applicability of thin-shell approximation.

Figures (2)

• Figure 1: The plot of the solution of \ref{['thetaEq']} for different values of of $l$, where the interesting observation is the impact of $l$ changing slowly for $l<-2$.
• Figure 2: The plot of the equation of state parameter for the scalar field. We see that scalar field could behave differently depending on the value of $l$ and in particular, if $l=-1.5$ then scalar field behaves like non-relativistic matter.