Who You Gonna Call? Runaway Ghosts, Higher Derivatives and Time-Dependence in EFTs

Abstract

We briefly review the formulation of effective field theories (EFTs) in time-dependent situations, with particular attention paid to their domain of validity. Our main interest is the extent to which solutions of the EFT capture the dynamics of the full theory. For a simple model we show by explicit calculation that the low-energy action obtained from a sensible UV completion need not take the restrictive form required to obtain only second-order field equations, and we clarify why runaway solutions are nevertheless typically not a problem for the EFT. Although our results will not be surprising to many, to our knowledge they are only mentioned tangentially in the EFT literature, which (with a few exceptions) largely addresses time-independent situations.

12 pages

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• Generalized Galileons: All scalar models whose curved background extensions maintain second-order field equations and stress tensors
• The String Landscape and the Swampland
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Cited by in corpus (12)

• Exploring gravitational theories beyond Horndeski
• Shaken, not stirred: kinetic mixing in scalar-tensor theories of gravity
• UV completions, fixing the equations and nonlinearities in $k$-essence
• Unitarity and the Vainshtein Mechanism
• Intro to Effective Field Theories and Inflation
• Effective field theory and classical equations of motion
• Heavy Fields and Gravity
• Non-Gaussianities in the Extended EFT of Inflation
• Dynamical renormalization and universality in classical multifield cosmological models
• Higher-derivative supersymmetric effective field theories
• Stable non-linear evolution in regularised higher derivative effective field theories
• Ghost condensation and Ostrogradskian instability on low derivative backgrounds