Abstract
We consider the implications of the swampland conjectures on scalar-tensor theories defined in the Einstein frame in which the scalar interaction is screened. We show that chameleon models are not in the swampland provided the coupling to matter is larger than unity and the mass of the scalar field is much larger than the Hubble rate. We apply these conditions to the inverse power law chameleon and the symmetron. We then focus on the dilaton of string theory in the strong coupling limit, as defined in the string frame. We show that solar system tests of gravity imply that viable dilaton models are not in the swampland. In the future of the Universe, if the low energy description with a single scalar is still valid and the coupling to matter remains finite, we find that the scalar field energy density must vanish for models with the chameleon and symmetron mechanisms. Hence in these models dark energy is only a transient phenomenon. This is not the case for the strongly coupled dilaton, which keeps evolving slowly, leading to a quasi de Sitter space-time.
Highlights
The standard model of cosmology, the ΛCDM model, is an excellent description of current cosmological and astrophysical data
We show that chameleon models are not in the swampland provided the coupling to matter is larger than unity and the mass of the scalar field is much larger than the Hubble rate
It requires two ingredients, which call for physics beyond the standard model of particle physics: dark matter and dark energy
Summary
The standard model of cosmology, the ΛCDM model, is an excellent description of current cosmological and astrophysical data. Scalar fields that appear in string theory could be coupled to different matter species with different strengths As such the couplings to dark matter are less constrained than the ones to standard model particles, because local tests of gravitation are not sensitive to dark matter per se. In the case of the interactions to the standard model particles, the coupling of the scalar fields is strongly constrained by the Cassini experiment [30] when the force is long-ranged. Such small couplings are not natural unless they result from the dynamics of the models, i.e., if they follow from a screening mechanism [31].
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