Dark energy from cosmological fluids obeying a Shan-Chen nonideal equation of state

Abstract

We consider a Friedmann-Robertson-Walker universe with a fluid source obeying a non-ideal equation of state with "asymptotic freedom," namely ideal gas behavior (pressure changes directly proportional to density changes) both at low and high density regimes, following a fluid dynamical model due to Shan and Chen. It is shown that, starting from an ordinary energy density component, such fluids naturally evolve towards a universe with a substantial "dark energy" component at the present time, with no need of invoking any cosmological constant. Moreover, we introduce a quantitative indicator of darkness abundance, which provides a consistent picture of the actual matter-energy content of the universe.

Cosmological evolution with quadratic gravity and nonideal fluids

“…3.3. Also, we do not expect any moment of maximum expansion at which ȧ = 0 = H since we are considering a flat everexpanding universe filled with the effective fluid (8). However restricting to a domain of the full solution space consisting of only ever expanding (or ever contracting) solutions, this choice of variables is very advantageous when looking for a physical interpretation of the solutions and connecting with the cosmological observables.…”

“…For example, Born-Infeld-like theories can lead to an effective description of the cosmic matter interpolating between dark matter and dark energy dominated epochs as a consequence of the Friedman equations, in terms of the Chaplygin Gas [3,4] or of the Anton-Schmidt fluid [5,6]. Other thermodynamically-motivated fluid models like the Dieterici [7] or the Shan-Chen [8] can as well exhibit a phase transition from a decelerating to an accelerating phase of the universe; the former from a matterdominated epoch to a dark energy epoch, and the latter from an early radiation-dominated epoch to a dark energy epoch. The Shan-Chen model can also be used for describing the exponential expansion occurring during the inflationary epoch with the advantage of exhibiting a graceful exit mechanism, but for a different choice of the free parameters entering its equations of state than in the former analysis [9].…”

Late-time evolution of cosmological models with fluids obeying a Shan-Chen-like equation of state

“…We refer to Ref. [2] for more details on SC thermodynamics and the microscopic foundations of the SC equation of state.…”

“…The above first order solution does not depend on the SC parameters g and α sc . Therefore, it is worth going to the next order looking for solutions of the form ξ = ǫξ (1) + ǫ 2 ξ (2) and κφ ± = ±λα + ǫκφ (1) + ǫ 2 κφ (2) . Equations (2.16) then imply dξ (2)…”

“…SC-based cosmological models have been successfully applied to account for the current distribution of dark energy as well as to describe the inflationary epoch [2,3]. Constraints on the free parameters of the model have been derived in Ref.…”