Stealth dark energy in scordatura DHOST theory

Understand

A stealth de Sitter solution in scalar-tensor theories has an exact de Sitter background metric and a nontrivial scalar field profile.

• Recently, in the context of Degenerate Higher-Order Scalar-Tensor (DHOST) theories it was shown that stealth de Sitter solutions suffer from either infinite strong coupling or gradient instability for scalar field perturbations.
• The sound speed squared is either vanishing or negative.
• In the first case, the strong coupling scale is zero and thus lower than the energy scale of any physical phenomena.