Anisotropy Strikes Back: Modified Gravity and Dark Matter Halos

TL;DR

This paper investigates how modifications to gravity theories and symmetry properties influence the emergence of effective dark matter and anisotropic stresses in spherically symmetric models, with implications for understanding dark matter halos.

Contribution

It introduces a deformation of the Hamiltonian in GR and HL gravity, revealing how anisotropic stresses and nonconservation laws can produce effective dark matter-like behavior.

Findings

Potential deformations induce anisotropic stress energy contributions.

HL gravity can produce positive scaling dark matter densities.

The analysis identifies conditions for consistent dark matter emergence in HL gravity.

Abstract

We explore dark matter like fluids in a spherically symmetric Lemaitre Tolman Bondi (LTB) minisuperspace, tracking how symmetry properties of the Hamiltonian constraint control the emergence of effective dark sources in General Relativity (GR) and Horava Lifshitz (HL) gravity. We first deform the GR Hamiltonian by adding an extra weight $+1$ density to the potential. We show that potential deformations of this type leave the (reduced) Dirac algebra unchanged and the modification is naturally reinterpreted as an effective anisotropic stress energy contribution. While the fluid reproduces an isothermal-like mass scaling, its pressure anisotropy prevents it from giving flat rotation curves. We then turn to HL gravity, where the deformed Dirac algebra induces a controlled nonconservation law for an emergent dust component. Generalizing earlier results, we identify a restricted class of LTB backgrounds for which the HL source term yields a positive scaling dark matter density, consistent with ghost-freedom, and recovery of GR in the infrared. The analysis is conditional on a prescribed background: obtaining a fully backreacted areal radius solution consistent with the HL field equations is left as a natural direction for future work.