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Added by
Swagat Saurav Mishra
Publication date
2017
fields
Physics
and research's language is
English
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We describe a new class of dark energy (DE) models which behave like cosmological trackers at early times. These models are based on the $alpha$-attractor set of potentials, originally discussed in the context of inflation. The new models allow the current acceleration of the universe to be reached from a wide class of initial conditions. Prominent examples of this class of models are the potentials $cothvarphi$ and $coshvarphi$. A remarkable feature of this new class of models is that they lead to large enough negative values of the equation of state at the present epoch, consistent with the observations of accelerated expansion of the universe, from a very large initial basin of attraction. They therefore avoid the fine tuning problem which afflicts many models of DE.
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The origin of accelerating expansion of the Universe is one the biggest conundrum of fundamental physics. In this paper we review vacuum energy issues as the origin of accelerating expansion - generally called dark energy - and give an overview of alternatives, which a large number of them can be classified as interacting scalar field models. We review properties of these models both as classical field and as quantum condensates in the framework of non-equilibrium quantum field theory. Finally, we review phenomenology of models with the goal of discriminating between them.
Studying the effects of dark energy and modified gravity on cosmological scales has led to a great number of physical models being developed. The effective field theory (EFT) of cosmic acceleration allows an efficient exploration of this large model space, usually carried out on a phenomenological basis. However, constraints on such parametrized EFT coefficients cannot be trivially connected to fundamental covariant theories. In this paper we reconstruct the class of covariant Horndeski scalar-tensor theories that reproduce the same background dynamics and linear perturbations as a given EFT action. One can use this reconstruction to interpret constraints on parametrized EFT coefficients in terms of viable covariant Horndeski theories. We demonstrate this method with a number of well-known models and discuss a range of future applications.
We provide a general framework for studying the evolution of background and cosmological perturbations in the presence of a vector field $A_{mu}$ coupled to cold dark matter (CDM). We consider an interacting Lagrangian of the form $Q f(X) T_c$, where $Q$ is a coupling constant, $f$ is an arbitrary function of $X=-A_{mu}A^{mu}/2$, and $T_c$ is a trace of the CDM energy-momentum tensor. The matter coupling affects the no-ghost condition and sound speed of linear scalar perturbations deep inside the sound horizon, while those of tensor and vector perturbations are not subject to modifications. The existence of interactions also modifies the no-ghost condition of CDM density perturbations. We propose a concrete model of coupled vector dark energy with the tensor propagation speed equivalent to that of light. In comparison to the $Q=0$ case, we show that the decay of CDM to the vector field leads to the phantom dark energy equation of state $w_{rm DE}$ closer to $-1$. This alleviates the problem of observational incompatibility of uncoupled models in which $w_{rm DE}$ significantly deviates from $-1$. The maximum values of $w_{rm DE}$ reached during the matter era are bounded from the CDM no-ghost condition of future de Sitter solutions.
We consider cosmological models with a dynamical dark energy field, and study the presence of three types of commonly found instabilities, namely ghost (when fields have negative kinetic energy), gradient (negative momentum squared) and tachyon (negative mass squared). In particular, we study the linear scalar perturbations of theories with two interacting scalar fields as a proxy for a dark energy and matter fields, and explicitly show how canonical transformations relate these three types of instabilities with each other. We generically show that low-energy ghosts are equivalent to tachyonic instabilities, and that high-energy ghosts are equivalent to gradient instabilities. Via examples we make evident the fact that whenever one of these fields exhibits an instability then the entire physical system becomes unstable, with an unbounded Hamiltonian. Finally, we discuss the role of interactions between the two fields, and show that whereas most of the time interactions will not determine whether an instability is present or not, they may affect the timescale of the instability. We also find exceptional cases in which the two fields are ghosts and hence the physical system is seemingly unstable, but the presence of interactions actually lead to stable solutions. These results are very important for assessing the viability of dark energy models that may exhibit ghost, gradient or tachyonic modes.
We study the phase space dynamics of cosmological models in the theoretical formulations of non-minimal metric-torsion couplings with a scalar field, and investigate in particular the critical points which yield stable solutions exhibiting cosmic acceleration driven by the {em dark energy}. The latter is defined in a way that it effectively has no direct interaction with the cosmological fluid, although in an equivalent scalar-tensor cosmological setup the scalar field interacts with the fluid (which we consider to be the pressureless dust). Determining the conditions for the existence of the stable critical points we check their physical viability, in both Einstein and Jordan frames. We also verify that in either of these frames, the evolution of the universe at the corresponding stable points matches with that given by the respective exact solutions we have found in an earlier work (arXiv: 1611.00654 [gr-qc]). We not only examine the regions of physical relevance for the trajectories in the phase space when the coupling parameter is varied, but also demonstrate the evolution profiles of the cosmological parameters of interest along fiducial trajectories in the effectively non-interacting scenarios, in both Einstein and Jordan frames.
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