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

Understand

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 $G_{\rm eff}<G_{\rm N}$ 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.