  We consider the dynamics of a charged particle (e.g., an electron)
oscillating in a laser field in flat spacetime and describe it in terms of the
variable mass metric. By applying Einstein's equivalence principle, we show
that, after representing the electron motion in a time-dependent manner, the
variable mass metric takes the form of the
Friedmann-Lema\^itre-Robertson-Walker metric. We quantize a massive complex
scalar or pseudo-scalar field in this spacetime and derive the production rate
of electrically neutral, spinless particle-antiparticle pairs. We show that
this approach can provide an alternative experimental method to axion searches.
