Stellar Bounds on Light Spin-2 Particles in Bimetric Theories

TL;DR

This work investigates light massive spin-2 particles as dark matter within ghost-free bimetric gravity, focusing on their production in stellar interiors and subsequent energy loss. The authors compute emission rates from photoproduction and bremsstrahlung in a non-degenerate solar plasma, including plasma screening, and derive bounds on the coupling ratio $G'/G$ as a function of mass $m$ by comparing to stellar cooling limits. Horizontal-branch stars provide the strongest constraints in the 3–20 eV range, with solar bounds and indirect photon searches offering complementary coverage at other masses, and inverse-square-law tests constraining the very light regime; together these bounds carve out substantial parameter space for light spin-2 dark matter. The results are broadly applicable to any weakly coupled, propagating spin-2 field and highlight the potential of stellar physics to probe new gravitationally coupled dark sectors, while suggesting extensions to degenerate stellar environments in future work.

Abstract

Using the bimetric formalism, we compute the production and emission rates of light spin-2 particles in non-degenerate stellar interiors through photoproduction and bremsstrahlung processes, including the effects of plasma screening. By comparing the resulting energy-loss rates with observational limits on stellar cooling, we derive bounds on the coupling strength and mass of the spin-2 particle. Assuming these particles are the dark matter of the Universe, the obtained constraints are competitive with existing astrophysical and cosmological limits, excluding a wide region of parameter space in the mass range 5-30 eV.

Figures (2)

• Figure 1: Left panel: Differential flux at Earth of the number of spin-2 particles produced in the Sun via bremsstrahlung and photoproduction, for $G'/G=10^{16}$, compared to the corresponding flux of axions produced in the Sun via the Primakoff process, for $g_{a\gamma\gamma}=10^{-10}\,\text{GeV}$. Right panel: Branching ratio of the scalar polarization mode ($\lambda = 0$) relative to the total solar emission of spin-2 particles. The result shows that the emission is dominated by the tensor components ($\lambda = \pm 2$), and consequently, the spectrum of light spin-2 particles produced in the solar plasma closely follows that of ordinary gravitons, differing only by an overall normalization factor $G'/G$.
• Figure 2: Constraints on the coupling ratio $G'/G$ as a function of the mass, $m$, of the spin-2 particle. The stellar energy-loss bounds derived in this work from the Sun and HB stars are shown in orange, together with limits on dark matter decay into photons from the Leo T dwarf galaxy Wadekar:2021qae and from ultraviolet observations of dwarf galaxies with HST Todarello:2024qci. The figure also includes bounds on deviations from Newtonian gravity Chen:2014oda, parametrized by the Yukawa potential in Eq. \ref{['eq:yukawa']}. For convenience, the upper axis shows the corresponding interaction range for each mass, while the right axis indicates the effective Planck mass associated with the spin-2 field.