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  • Open Access

Can weak-gravity, causality-violation arguments constrain modified gravity?

Stephon Alexander* and Heliudson Bernardo†

Nicolás Yunes‡

  • Brown Theoretical Physics Center and Department of Physics, Brown University, Barus Building, 340 Brook Street, Providence, Rhode Island 02912, USA

  • *Contact author: stephon_alexander@brown.edu
  • †Contact author: heliudson_bernardo@brown.edu
  • ‡Contact author: nyunes@illinois.edu
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Phys. Rev. D 112, 104042 – Published 17 November, 2025

DOI: https://doi.org/10.1103/9bmy-qbwt

Abstract

We investigate limitations of causality arguments from flat-spacetime amplitudes, based on the eikonal limit of gravitational scattering, to place constraints on modified gravity. We show that causality constraints are only valid in the weak-gravity regime even for trans-Planckian scattering, and that such constraints are much less stringent than astrophysical ones, obtained for example from gravitational waves emitted in black hole coalescence. Special attention is given to the weakness of causality constraints on dynamical Chern-Simons gravity, but our results apply to other modified gravity theories as well. In the context of that theory, we also discuss how to obtain a time-delay formula from black holes, neutron stars, and shock wave solutions. For scattering with compact objects, we explicitly show that time delays are greatly suppressed by the ratio of the object’s mass to the impact parameter, so time advances only occur greatly outside the cut off of the theory. For the shock wave solution, we find that the time delay is always positive within the regime of validity of the solution. We also comment on the impact of graviton nonlinearities for time-delay calculations in the nonlinear, strong gravity regime. We conclude that amplitude-based causality constraints on modified gravity are typically not stringent relative to other experimental and observational bounds.

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Phys. Rev. D 112, 104042– Published 17 November, 2025
  • Received 13 August 2025
  • Accepted 17 October 2025
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