Effective equation of state for dark energy: Mimicking quintessence and phantom energy through a variable Λ

Abstract

While there is mounting evidence in all fronts of experimental cosmology for a non-vanishing dark energy component in the Universe, we are still far away from understanding its ultimate nature. A fundamental cosmological constant, Λ, is the most natural candidate, but many dynamical mechanisms to generate an effective Λ have been devised which postulate the existence of a peculiar scalar field (so-called quintessence, and generalizations thereof). These models are essentially ad hoc, but they lead to the attractive possibility of a time-evolving dark energy with a non-trivial equation of state (EOS). Most, if not all, future experimental studies on precision cosmology (e.g. the SNAP and PLANCK projects) address very carefully the determination of an EOS parametrized a la quintessence. Here we show that by fitting cosmological data to an EOS of that kind can also be interpreted as a hint of a fundamental, but time-evolving, cosmological term: Λ = Λ(t). We exemplify this possibility by studying the effective EOS associated to a renormalization group (RG) model for Λ. We find that the effective EOS can correspond to both normal quintessence and phantom dark energy, depending on the value of a single parameter of the RG model. We conclude that behind a non-trivial EOS of a purported quintessence or phantom scalar field there can actually be a running cosmological term Λ of a fundamental quantum field theory.

Starobinsky-Like Inflation and Running Vacuum in the Context of Supergravity

“…appearing in (4) are dimensionless and receive contributions from loop corrections of boson and fermion matter fields with different masses M i . It must be stressed that the general covariance of the action [13,[16][17][18][19] necessitates the appearance of only even powers of the (cosmic-time t dependent) Hubble parameter H(t) on the right-hand-side of (4). For a specific framework where the above RG is concretely realized and the β-function coefficients can be computed, see [13].…”

“…When one computes the effective action in a fixed de Sitter background, it is tempting to identify a Λ term with the Ricci scalar, which eventually will be allowed to depend on time. Thus, naively, the presence of logarithms would imply non-polynomial terms of the form R lnR, which would be problematic for any RVM interpretation of the exit from the inflationary phase, as it would contradict the spirit of the approach where only integer powers of the curvature terms would be allowed in the respective flow equations [13][14][15][17][18][19][20][21][22]. Fortunately, this is not the case.…”

“…Also very important for our considerations is the framework of the running vacuum model (RVM) [13][14][15][16][17][18][19]; see [20][21][22] and the references therein for a recent detailed exposition. The implications of these dynamical vacuum models have recently been analysed both for the early Universe [22][23][24][25][26][27][28], as well as for the phenomenology of the current Universe [29][30][31]; see also [32][33][34][35][36][37][38][39][40] for previous analyses.…”

Phenomenological Fluids from Interacting Tachyonic Scalar Fields

“…Alternative models of dark energy suggest a dynamical form of dark energy, which at least in an effective level, can originate from a variable cosmological constant [24,25], or from various fields, such as a canonical scalar field [26][27][28][29][30] (quintessence), a phantom field [31][32][33][34][35][36][37][38] or the quintom [39][40][41][42][43][44][45][46][47][48][49][50][51]. By using some basic of quantum gravitational principles, we can formulate several other models for dark energy and in literature they are known as holographic dark energy paradigm [52][53][54][55][56][57][58][59][60][61][62][63] and agegraphic dark energy models [64][65][66].…”

Galaxy clusters and structure formation in quintessence versus phantom dark energy universe

“…These extensions are under investigation and are going to be presented in a future publication. Finally, note that in reality, the dark energy sector may have an effective nature, and thus its equation of state too, not corresponding to fundamental fields or degrees of freedom [85][86][87]. In this case its implication on the galaxy cluster might change too, and thus it might offer a way to distinguish amongst the various dark energy scenarios.…”