Scalar-Induced Electromagnetic Radiation: Comparison with Axion-Like Particles and Implications for Modified Gravity

The scalar-tensor theory of gravity, a modified gravity theory, introduces a fundamental scalar field that can serve as dynamical dark energy, driving the late-time accelerated expansion of the Universe. In this work, we analyze electromagnetic (EM) radiations arising from scalar fields and compare these features with those induced by axion-like particles (ALPs). Scalar and ALP fields couple differently to the EM field due to their distinct parity properties, $\phi F_{\mu\nu} F^{\mu\nu}$ for scalar fields and $\phi F_{\mu\nu} \tilde{F}^{\mu\nu}$ for ALPs. Building on analytical methods developed for ALPs, this work presents a theoretical feasibility analysis that demonstrates how the scalar field could produce observable EM signatures from oscillating field configurations. We also show that resonance effects can amplify the EM radiation for the scalar field under specific conditions, and that the enhancement mechanisms depend on the coupling structure and the configuration of the background magnetic field. Resonance phenomena can accentuate the differences in signal strength and spectral features, potentially aiding future observations in distinguishing scalar fields from ALPs. Although our studies apply to general scalar fields, we embed them within the framework of scalar-tensor theory and discuss the mass and coupling parameter in the context of testing modified gravity. This work provides a theoretical framework for studying generic pure and pseudo-scalar fields on an equal footing and suggests new avenues for observational tests of modified gravity scenarios alongside ALP models.