Against Horndeski

          @misc{ scivideos_PIRSA:24070055,
            doi = {10.48660/24070055},
            url = {https://pirsa.org/24070055},
            author = {Burgess, Cliff},
            keywords = {Cosmology, Strong Gravity, Mathematical physics},
            language = {en},
            title = {Against Horndeski},
            publisher = {Perimeter Institute for Theoretical Physics},
            year = {2024},
            month = {jul},
            note = {PIRSA:24070055 see, \url{https://scivideos.org/index.php/pirsa/24070055}}
          }
          

Abstract

The Horndeski program is motivated by arguing that scalar-tensor modifications to gravity should have two properties: effective interactions that are at most second-order in time derivatives and only a single scalar. I will argue against both of these criteria. First I argue why the low-energy limit of known well-behaved theories can have more than two-derivative field equations. Second I argue why the scalar-tensor interactions most likely to be found competing with gravity at very low energies typically are those with two derivatives, at least when semiclassical methods are justified, and this suggests exploring multiple-scalar models.