Assisted dark energy

TL;DR

This paper investigates multi-field scalar dark energy models with scaling solutions, showing they can produce cosmic acceleration consistent with observations and analyzing their dynamics and constraints.

Contribution

It provides a comprehensive analysis of multi-field dark energy models with general Lagrangians, highlighting conditions for cosmic acceleration and observational compatibility.

Findings

Cosmic acceleration can be transient or eternal after the matter era.

Thawing behavior of scalar fields influences the evolution of dark energy.

Models can satisfy observational bounds on the equation of state w_{phi} < -0.8.

Abstract

Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale problem of dark energy. In the presence of multiple scalar fields the scaling solution can exit to the epoch of cosmic acceleration through the so-called assisted inflation mechanism. We study cosmological dynamics of a multi-field system in details with a general Lagrangian density p=sum_{i=1}^n X_i g(X_i e^{lambda_i phi_i}), where X_i=-(nabla phi_i)^2/2 is the kinetic energy of the i-th field phi_i, lambda_i is a constant, and g is an arbitrary function in terms of Y_i=X_i e^{lambda_i phi_i}. This covers most of the scalar-field models of dark energy proposed in literature that possess scaling solutions. Using the bound coming from Big-Bang-Nucleosynthesis and the condition under which the each field cannot drive inflation as a single component of the universe, we find the following features: (i) a transient or eternal cosmic acceleration can be realized after the scaling matter era, (ii) a "thawing" property of assisting scalar fields is crucial to determine the evolution of the field equation of state w_{phi}, and (iii) the field equation of state today can be consistent with the observational bound w_{phi}<-0.8 in the presence of multiple scalar fields.