Multi-fluid cosmology: An illustration of fundamental principles

TL;DR

This paper develops a relativistic multi-fluid formalism to model the Universe's evolution, allowing for anisotropic phases and potential observable signatures from fluid interactions and transitions.

Contribution

It introduces a multi-fluid variational approach to cosmology, extending beyond perfect fluids to include heat flow and dissipative effects, with detailed non-linear solutions.

Findings

Identifies a transition from anisotropic to isotropic universe due to fluid dominance change.

Shows the potential for observable imprints at the transition scale.

Demonstrates the formalism's applicability to coupled matter and radiation fluids.

Abstract

Our current understanding of the Universe depends on the interplay of several distinct "matter" components, which interact mainly through gravity, and electromagnetic radiation. The nature of the different components, and possible interactions, tends to be based on the notion of coupled perfect fluids (or scalar fields). This approach is somewhat naive, especially if one wants to be able to consider issues involving heat flow, dissipative mechanisms, or Bose-Einstein condensation of dark matter. We argue that a more natural starting point would be the multi-purpose variational relativistic multi-fluid system that has so far mainly been applied to neutron star astrophysics. As an illustration of the fundamental principles involved, we develop the formalism for determining the non-linear cosmological solutions to the Einstein equations for a general relativistic two-fluid model for a coupled system of matter (non-zero rest mass) and "radiation" (zero rest mass). The two fluids are allowed to interpenetrate and exhibit a relative flow with respect to each other, implying, in general, an anisotropic Universe. We use initial conditions such that the massless fluid flux dominates early on so that the situation is effectively that of a single fluid and one has the usual Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime. We find that there is a Bianchi I transition epoch out of which the matter flux dominates. The situation is then effectively that of a single fluid and the spacetime evolves towards the FLRW form. Such a transition opens up the possibility of imprinting observable consequences at the specific scale corresponding to the transition time.