Perturbations In A Non-Uniform Dark Energy Fluid: Equations Reveal Effects of Modified Gravity and Dark Matter

TL;DR

This paper introduces a unified fluid model combining dark matter and dark energy effects, revealing new gravity modifications and providing a framework that aligns with large-scale cosmology while fitting galaxy rotation curves.

Contribution

It proposes a general Nu-Lambda fluid theory unifying dark matter and dark energy, including new models and modifications to existing theories like MOND and ΛCDM.

Findings

Nu-Lambda fluid models resemble ΛCDM on large scales

Perturbed Einstein equations in specific models show dark matter-dark energy coupling

Potential to test Nu-Lambda theories through cosmological simulations

Abstract

We propose a unified single-field description of the galactic Dark Matter and various uniform scalar fields for the inflation and cosmological constant. The two types of effects could originate from a fluid of both spatially and temporally varying Vacuum Energy if the vacuum has an uneven pressure caused by a photon-like vector field (of perhaps an unstable massive boson). We propose a most general Lagrangian with a {\bf N}on-{\bf u}niform Cosmological Constant for this vacuum fluid (dubbed as a Nu-Lambda fluid), working within the framework of Einsteinian gravity. This theory includes a continuous spectrum of plausible dark energy theories and gravity theories, e.g., inflation, quintessence, k-essence, f(R), Generalized Einstein-Aether f(K), MOND, TeVeS, BSTV etc. theories. It also suggests new models such as a certain f(K+R) model, which suggests intriguing corrections to MOND depending of redshift and density. Some specific constructions of the Nu-Lambda fluid (e.g., Zhao's V-$\Lambda$ model) closely resemble the $\Lambda$CDM cosmology on large scale, but fit galaxy rotation curves as good as MOND. Perturbed Einstein Equations in a simple $f(K_4)$ model are solvable and show effects of a DM coupled to DE. Incorporating the perturbation equations here into standard simulations for cosmological structure growth offers a chance to falsify examples of the Nu-Lambda theories.