We demonstrate some shortcomings of “fixing the equations,” an increasingly popular remedy for time evolution problems of effective field theories (EFTs). We compare the EFTs and their “fixed” versions to the UV theories from which they can be derived in two cases: K-essence and nonlinear Proca theory. We find that when an EFT breaks down due to loss of hyperbolicity, fixing does not approximate the UV theory well if the UV solution does not quickly settle down to vacuum. We argue that this can be related to the EFT approximation itself becoming invalid, which cannot be rectified by fixing.

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• Alternative gravity theories
• Numerical relativity