Pulsar timing signal from ultralight axion in $f(R)$ theory

TL;DR

This paper explores how ultralight axion dark matter could produce detectable pulsar timing signals within $f(R)$ gravity models, showing that the gravitational potential's amplitude can vary, affecting observability.

Contribution

It analyzes the detectability of axion-induced signals in $f(R)$ theories, especially the Hu-Sawicki model, revealing conditions where such models are constrained or excluded.

Findings

Gravitational potential amplitude varies with $f(R)$ parameters.

Detection prospects depend on the axion mass and $f(R)$ model.

Certain $f(R)$ models are excluded based on pulsar timing constraints.

Abstract

An ultralight axion around $10^{-23}$ eV is known as a viable dark matter candidate. A distinguished feature of such a dark matter is the oscillating pressure which produces the oscillation of the gravitational potential with frequency in the nano-Hz range. Recently, Khmelnitsky and Rubakov pointed out that this time dependent potential induces the pulse arrival residual and could be observed by the Square Kilometre Array (SKA) experiment. In this paper, we study the detectability of the oscillating pressure of the axion in the framework of $f(R)$ theory, and show that the amplitude of the gravitational potential can be enhanced or suppressed compared to that in Einstein's theory depending on the parameters of the $f(R)$ model and mass of the axion. In particular, we investigate the Hu-Sawicki model and find the condition that the Hu-Sawicki model is excluded.