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Initial data of effective field theories of relativistic viscous fluids and gravity

Lorenzo Gavassino1, Áron D. Kovács2, and Harvey S. Reall1

  • 1Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, United Kingdom
  • 2Centre for Geometry, Analysis and Gravitation, School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom

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Phys. Rev. D 113, 124022 – Published 9 June, 2026

DOI: https://doi.org/10.1103/ft2b-m8hv

Abstract

There has been recent progress in developing well-posed theories of relativistic viscous hydrodynamics and of gravitational effective field theories. These have in common the feature that they introduce unphysical degrees of freedom. We address the problem of how these should be treated. We propose a “reduction of order” approach which is applied not at the level of equations of motion but only to initial data. This specifies uniquely the data for the unphysical modes in terms of the data for the physical modes. We argue that the apparent breaking of Lorentz invariance associated with this approach is not a problem provided one restricts to Lorentz frames for which the assumptions of effective field theory are manifestly valid.

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Phys. Rev. D 113, 124022– Published 9 June, 2026
  • Received 23 February 2026
  • Accepted 11 May 2026
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