Accelerating Universe from Constraints

TL;DR

This paper demonstrates that a single constrained scalar field with non-minimal coupling can produce an accelerating universe, with analytical solutions and well-behaved perturbations, regardless of the presence of matter or the frame used.

Contribution

It introduces a novel scalar field model with non-minimal coupling that naturally leads to cosmic acceleration and explores its behavior in different frames and matter couplings.

Findings

Analytical solutions show transition from radiation-like to exponential expansion.

Scalar perturbations are well-behaved in the Jordan frame.

Matter minimally coupled does not affect the acceleration, but non-minimal coupling allows for phantom-like equations of state.

Abstract

We show that a single constrained scalar field with non-minimal coupling to gravity can give rise to an accelerating Universe. First, we consider this model in the absence of external matter. In the original Jordan frame, we show that the dynamics of space-time is independent of the value of the cosmological constant. We find analytical solutions, which include evolution from a radiation dominated-like universe to an exponential expansion, as well as evolution that starts with super-Hubble expansion and then relaxes towards an exponential expansion of the Universe. We also show that the scalar perturbations of this theory are well-behaved in the Jordan frame. Furthermore, we perform the formulation of the corresponding theory in the Einstein frame, and analyse the corresponding accelerating solutions. Second, we consider the model in the presence of matter. We find that the matter does not affect the evolution of the universe if minimally coupled to the scalar field. In other words, in the Jordan frame, in order to influence the evolution of the space-time, the matter should be non-minimally coupled to the constrained scalar, while it may be minimally coupled to gravity. We show that in this case, the solutions are similar to the free case, and, in addition, allow for the phantom-like equation of state. Finally, we consider the theory in the minimal frame, in which the matter is minimally coupled to both gravity and the constrained scalar, and show that among other possibilities, the phantom-like equation of state can still be satisfied in this minimal frame.