How higher-spin gravity surpasses the spin-two barrier

Abstract

International audienceAiming at nonexperts, the key mechanisms of higher-spin extensions of ordinary gravities in four dimensions and higher are explained. An overview of various no-go theorems for low-energy scattering of massless particles in flat spacetime is given. In doing so, a connection between the S-matrix and the Lagrangian approaches is made, exhibiting their relative advantages and weaknesses, after which potential loopholes for nontrivial massless dynamics are highlighted. Positive yes-go results for non-Abelian cubic higher-derivative vertices in constantly curved backgrounds are reviewed. Finally, how higher-spin symmetry can be reconciled with the equivalence principle in the presence of a cosmological constant leading to the Fradkin-Vasiliev vertices and Vasiliev's higher-spin gravity with its double perturbative expansion (in terms of numbers of fields and derivatives) is outlined

One-loop effective actions and higher spins. Part II

“…On the one hand, there are different "no-go" theorems putting serious constraints on such theories, especially in flat spacetime (see e.g. [48] and references within). On the other hand, we have significant higher spin results: free fields can be constructed in the same manner as in lower spin cases (see, e.g.…”

On conformal higher spin wave operators

“…In section 3 we test the consistency of a factorized conformal wave operator in generic backgrounds 2 See e.g. [22] and references therein for a review of various no-go theorems and [33] for a stronger version of the Coleman-Mandula theorem in flat space.…”

The other effective fermion compositeness

“…Most importantly, the experimental resolution, limited by statistical errors at present, is not enough to resolve the SM-BSM interference term in the cross section, which is instead dominated by the quadratic new physics contribution. 22 For this reason, positivity constraints do not improve sizably our constraints on m * and g * .…”

“…[22] for a review). Yet, the roots of these arguments are based on the IR properties of the states, i.e.…”

“…Moreover, it could be possible that a UV completion in the unbroken phase needs to be formulated in AdS space, given the need to inject a positive contribution to the vacuum energy to move to the broken phase. Notice that massless higher spins in AdS pose no problem [22,25,26].…”