An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations

We consider scalar-vector-tensor (SVT) theories with second-order equations of motion and tensor propagation speed equivalent to the speed of light. Under the sub-horizon and the quasi-static approximations we find analytical formulae for an effective dark energy fluid, i.e., sound speed, anisotropic stress as well as energy density and pressure. We took advantage of our general, analytical fluid description and showed that it is possible to design SVT cosmological models which are degenerate with Lambda CDM at the background level while having gravity strength G(eff) < G(N) at late-times as well as non-vanishing dark energy perturbations. We implemented SVT designer models in the widely used Boltzmann solver CLASS thus making it possible to test SVT models against astrophysical observations. Our effective fluid approach to SVT models reveals non trivial behaviour in the sound speed and the anisotropic stress well worth an investigation in light of current discrepancies in cosmological parameters such as H-0 and sigma(8).

Automated summary

We investigated scalar-vector-tensor (SVT) theories with second-order equations of motion and tensor propagation speed equivalent to the speed of light. By utilizing sub-horizon and quasi-static approximations, we derived analytical formulae for various properties of the effective dark energy fluid, including sound speed, anisotropic stress, energy density, and pressure. We demonstrated that it is possible to design SVT cosmological models that are degenerate with Lambda CDM at the background level while exhibiting different gravity strength and non-zero dark energy perturbations. Furthermore, we implemented SVT designer models in a popular Boltzmann solver called CLASS, enabling the testing of SVT models against astrophysical observations. Our effective fluid approach to SVT models highlights interesting behaviors in the sound speed and the anisotropic stress, which warrant further investigation given the current discrepancies in cosmological parameters such as H-0 and sigma(8).