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arXiv · 2603.27386

Thermal channels of scalar and tensor waves in Jordan-frame scalar--tensor gravity

David S. Pereira·Francisco S. N Lobo·José Pedro Mimoso
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Abstract

We study first-order scalar and tensor perturbations of Jordan-frame scalar--tensor gravity about a spatially flat FLRW background using the Einstein-like effective-fluid decomposition of the scalar sector. In the scalar-gradient frame, we derive the perturbed effective density, pressure, heat flux, and anisotropic stress, and show that they admit an exact Eckart-type constitutive identification at linear order. We then show that these same quantities appear explicitly and exhaustively in the linearized field equations: the scalar Hamiltonian, momentum, trace, and traceless Einstein-like equations are governed, respectively, by the effective density, heat-flux, pressure, and anisotropic-stress channels, while the tensor propagation equation is governed by the transverse-traceless anisotropic-stress channel. In particular, the Jordan-frame modification of gravitational-wave damping is identified with the effective transverse-traceless anisotropic stress of the scalar sector. We also derive the perturbed evolution equation for the invariant product $\kappa T$, clarify its gauge behavior, and show that flux matching on FLRW fixes only the background value $\overline{\kappa T}$, not its perturbation. These results leave open the possibility that gravitational waves in scalar--tensor gravity admit a deeper thermodynamic characterization, perhaps even an intrinsic one, although the present analysis establishes this only at the level of an effective constitutive description.

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David S. Pereira, Francisco S. N Lobo, José Pedro Mimoso. 2026-03-28. Thermal channels of scalar and tensor waves in Jordan-frame scalar--tensor gravity. https://doi.org/10.1103/xwys-64rz

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