Two-field Screening and its Cosmological Dynamics

We consider the screening of the axio-dilaton fields when both the dilaton and the axion couple to matter with Yukawa couplings. We analyse the screening of the dilaton in the vicinity of a compact object and find that this can only take place when special boundary conditions at infinity are imposed. We study the cosmological dynamics of the axio-dilaton system when coupled to matter linearly and find that the special boundary conditions at infinity, which guarantee the screening of compact objects, do not generically emerge from cosmology. We analyse the background cosmology and the cosmological perturbations at late time in these models and show that the Baryon Acoustic Oscillations constrain the coupling of the dilaton to matter to be smaller than in its natural supergravity realisation. Moreover we find that the Hubble rate in the present Universe could deviate from the normalised Planck value, although by an amount too small to account for the $H_0$ tension, and that the growth of structure is generically reduced compared to $\Lambda$CDM.