Scalar-tensor theory with Lagrange multipliers: Away of understanding the cosmological constant problem, and future singularities

The use of Lagrange multipliers in the context of quintessence/phantom scalar fields allows us to constrain the behavior of the scalar field, which provides a powerful tool, not only for the reconstruction of cosmological solutions but also for the study of some problems in cosmology and gravitational physics. In the present paper, we focus on the reconstruction of cosmological solutions capable of controlling the cosmological constant value by imposing a constraint on the scalar field, providing a relaxation mechanism of the value of the cosmological constant. The formalism is also extended to the study of phantom scalar fields with a future singularity and their conformal transformation to the Jordan frame, where a type of modified gravity, constrained by the Lagrange multiplier, can be constructed.