Dark-energy evolution across the cosmological-constant boundary

Abstract

We explore the properties of dark energy models for which the equation-of-state, w, defined as the ratio of pressure to energy density, crosses the cosmological-constant boundary w = −1. We adopt an empirical approach, treating the dark energy as an uncoupled fluid or a generalized scalar field. We describe the requirements for a viable model, in terms of the equation-of-state and sound speed. A generalized scalar field cannot safely traverse w = −1, although a pair of scalars with w > −1 and w < −1 will work. A fluid description with a well-defined sound speed can also cross the boundary. Contrary to expectations, such a crossing model does not instantaneously resemble a cosmological constant at the moment w = −1 since the density and pressure perturbations do not necessarily vanish. But because a dark energy with w < −1 dominates only at very late times, and because the dark energy is not generally prone to gravitational clustering, then crossing the cosmological-constant boundary leaves no distinct imprint.Numerous observations and experiments indicate that our universe has low matter-density, negligible spatial curvature, and is currently undergoing accelerated cosmic expansion [1,2,3,4,5,6,7]. The remarkable implication is that approximately two-thirds of the cosmic energy is due to some form of as-yet unidentified dark energy. While the leading interpretation is that the dark energy is due to a cosmological constant, the physical origin of such a constant remains a mystery, and it is widely regarded as a placeholder until a deeper understanding of the dark energy can be established.In an effort to characterize the nature of the dark energy, attention has focused on its presumed equation-of-state, w, defined as the ratio of its mean pressure to energy density, w ≡ p/ρ. A cosmological constant corresponds to w = −1. Another conjecture is that the dark energy is due to quintessence, a dynamical, ultra-light scalar field with negative pressure, for which w > −1 [8,9,10]. A separate class of models with w < −1, representing an exotic field or perhaps new gravitational phenomena, are also under investigation (e.g. [11]). Extensive analysis of the cosmological predictions for all these cases finds that the current data favors dark energy models with an equation-of-state in the vicinity of w = −1, straddling the cosmological-constant boundary.If indeed dark energy with w < −1 is within the realm of possibilities, then it would seem inevitable to inquire about a transition from w > −1 to w < −1. This question has been taken up recently [12,13]; here we contribute our results and perspective on the issue. This article examines possible mechanisms by which dark energy can cross w = −1. We assume that Einstein's gravitation is valid and that the dark energy system interacts only gravitationally with the rest of the world -all of our ignorance is captured in w. The sticking point is the stability of such a "crossing component," which brings into question the physics of the dark energy.Let's start from the observations:...

The phantom divide in string gas cosmology

“…This is in fact the kind of crossing discussed specifically in [41] and [42]. The question, motivated by persistent observational suggestions that w may indeed currently be less than −1 [and also by analyses showing that better fits to the data [47] are obtained by models which cross the phantom divide in recent times], is whether string gas cosmologies can sustain NEC violation not just briefly, in the extremely early Universe, but also over cosmologically significant periods.…”

Kinetic k-essence and quintessence

“…If a DE evolves from one regime to another, this is called phantom crossing, but [27], e.g., showed that this is impossible for a purely kinetic k-essence. We refer to [28,29] for a discussion of phantom crossing in the context of other DE models.…”

Dynamics of cosmological scalar fields

“…Extended models where the scalar * Electronic address: n.tamanini.11@ucl.ac.uk field Lagrangian is a general function of both the scalar field φ and its kinetic term, are known as k-essence theories [11]. Within this framework it is possible to obtain not only the standard dark energy evolution but also the so called phantom regime and quintessence to phantom transition, though fatal problems always arise at the level of perturbations [12][13][14].…”

“…However in this model the scalar field EoS parameter is never greater than −1 creating problems at early times where dark matter with vanishing pressure must dominate. Scalar fields which can cross the phantom barrier at w φ = −1 are usually dubbed quintom models and imply either the use of extended Lagrangians, generally instable, or of two different scalar fields [14,18].…”