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

Understand

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.