Importance of being nonminimally coupled: Scalar Hawking radiation from regular black holes

Abstract

In curved space-time, a scalar field $\phi$ is generically expected to couple to curvature, via a coupling of the form $\xi\phi^2R$. Yet in the study of Hawking emission from regular black holes (RBHs), where scalar fields are often introduced as simple probes of the geometry, and the Ricci scalar is generically non-zero, this non-minimal coupling is almost always ignored. We revisit this assumption by studying scalar Hawking emission from four representative RBHs (the Bardeen, Hayward, Simpson-Visser, and D'Ambrosio-Rovelli space-times), within two benchmark cases: the conformal case $\xi=1/6$, and a large negative value $\xi=-10^4$ motivated by Higgs inflation. We compute the graybody factors and emission spectra, showing that the latter can be either enhanced or suppressed, even by several orders of magnitude. A crucial role is played by the sign of the term $\xi fR$, with $f(r)=-g_{tt}$ in Schwarzschild-like coordinates, as it determines whether the non-minimal coupling suppresses or enhances the geometric potential barrier. For the D'Ambrosio-Rovelli case with large negative $\xi$, the low-energy emission spectrum is enhanced by up to five orders of magnitude, since $\xi fR<0$ throughout the space-time, leading to a deep potential well which broadens the transmissive window. The deviations we find can be particularly relevant in the case where primordial RBHs are dark matter candidates, given the impact of the non-minimal coupling on their evaporation history.