Perfect fluid Lagrangian and its cosmological implications in theories of gravity with nonminimally coupled matter fields
Abstract
In this paper we show that the onshell Lagrangian of a perfect fluid depends on microscopic properties of the fluid, giving specific examples of perfect fluids with different onshell Lagrangians but with the same energymomentum tensor. We demonstrate that if the fluid is constituted by localized concentrations of energy with fixed rest mass and structure (solitons) then the average onshell Lagrangian of a perfect fluid is given by L_{m}=T , where T is the trace of the energymomentum tensor. We show that our results have profound implications for theories of gravity where the matter Lagrangian appears explicitly in the equations of motion of the gravitational and matter fields, potentially leading to observable deviations from a nearly perfect cosmic microwave background black body spectrum: n type spectral distortions, affecting the normalization of the spectral energy density. Finally, we put stringent constraints on f (R ,L_{m}) theories of gravity using the COBEFIRAS measurement of the spectral radiance of the cosmic microwave background.
 Publication:

Physical Review D
 Pub Date:
 March 2018
 DOI:
 10.1103/PhysRevD.97.064018
 arXiv:
 arXiv:1802.04760
 Bibcode:
 2018PhRvD..97f4018A
 Keywords:

 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 6 pages, no figures, matches published version