The imperfect fluid behind kinetic gravity braiding

Oriol Pujolàs, Ignacy Sawicki, Alexander Vikman

Research output: Contribution to journalArticleResearchpeer-review

122 Citations (Scopus)


We present a standard hydrodynamical description for non-canonical scalar field theories with kinetic gravity braiding. In particular, this picture applies to the simplest galileons and k-essence. The fluid variables not only have a clear physical meaning but also drastically simplify the analysis of the system. The fluid carries charges corresponding to shifts in field space. This shift-charge current contains a spatial part responsible for diffusion of the charges. Moreover, in the incompressible limit, the equation of motion becomes the standard diffusion equation. The fluid is indeed imperfect because the energy flows neither along the field gradient nor along the shift current. The fluid has zero vorticity and is not dissipative: there is no entropy production, the energy-momentum is exactly conserved, the temperature vanishes and there is no shear viscosity. Still, in an expansion around a perfect fluid one can identify terms which correct the pressure in the manner of bulk viscosity. We close by formulating the non-trivial conditions for the thermodynamic equilibrium of this imperfect fluid. © SISSA 2011.
Original languageEnglish
Article number156
JournalJournal of High Energy Physics
Issue number11
Publication statusPublished - 28 Dec 2011


  • Classical Theories of Gravity
  • Cosmology of Theories beyond the SM
  • Global Symmetries
  • Nonperturbative Effects


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