Scalar quasinormal modes in emergent modified gravity

Emergent modified gravity is a post-Einsteinian gravitational theory where spacetime geometry is not fundamental but rather emerges from the gravitational degrees of freedom in a nontrivial way. The specific relationship between geometry and these degrees of freedom is unique for each theory, but it is not predetermined. Instead, it is derived from constraints and equations of motion, relying on key aspects of the canonical formulation of gravity, such as structure functions in Poisson brackets of constraints and covariance conditions. As shown in previous work, these new theories allow for two types of scalar matter coupling: (1) minimal coupling, where the matter equations of motion mirror the Klein-Gordon equation on a curved emergent spacetime, and (2) nonminimal coupling, where the equations deviate from the Klein-Gordon form but still respect covariance. Observable features, such as the quasinormal mode spectrum, can help distinguish between different couplings based on how well their predictions match the data. In this work, the spectra of scalar quasinormal modes for both minimal and nonminimal couplings are derived using the third-order WKB approximation. Significant differences are found between the two cases. Notably, the nonminimal coupling allows for vanishing real and imaginary frequency components, and even opposite-sign values for the imaginary part at sufficiently small mass scales, pointing to potential new physical implications. Finally, the high-frequency QNM spectra in emergent modified gravity are identical to the classical result, up to an overall constant, suggesting that the horizon area spectrum remains equispaced.