Detecting dark energy in orbit: The cosmological chameleon

Abstract

We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a large class of scalar potentials that are also consistent with local tests of gravity. These provide explicit realizations of a quintessence model where the quintessence scalar field couples directly to baryons and dark matter with gravitational strength. We analyze the cosmological evolution of the chameleon field and show the existence of an attractor solution with the chameleon following the minimum of its effective potential. For a wide range of initial conditions, spanning many orders of magnitude in initial chameleon energy density, the attractor is reached before nucleosynthesis.Surprisingly, the range of allowed initial conditions leading to a successful cosmology is wider than in normal quintessence. We discuss applications to the cyclic model of the universe and show how the chameleon mechanism weakens some of the constraints on cyclic potentials. I. INTRODUCTIONA host of observations concord with the existence of a dark energy component with negative pressure, accounting for more than two thirds of the current energy budget. The evidence comes, for instance, from measurements of the cosmic microwave background temperature anisotropy [1] and Type Ia supernovae [2]. While the data is so far consistent with the dark fluid being a cosmological constant, it is nevertheless interesting to consider the possibility that near future observations will reveal that w differs from −1.Having w = −1 implies that a parameter of the effective Lagrangian, namely the vacuum energy, is time-dependent.It follows from general covariance and locality that it must also be a function of space; in other words, the vacuum energy is a field, assumed for simplicity to be a fundamental scalar φ. Scalar field models of dark energy generally come under the label of quintessence [3]. Of course, this argument assumes that gravity is described by General Relativity (GR) for all relevant scales, and it is conceivable that the observed acceleration could result from a break down of GR on large scales [4,5,6]. However, we focus on the former possibility.

Beyond the cosmological standard model

“…This possibility was first explored in [184,185,273]. Heuristically, the aim is to suitably choose V (φ) and A(φ) such that the mass coming from the effective potential…”

Stability analysis and observational measurement in chameleonic generalised Brans-Dicke cosmology

“…The cosmological models with such exponential functions have been known lead to interesting physics in a variety of context, ranging from existence of accelerated expansions [35] to cosmological scaling solutions [36]- [37]. In particular, the exponential forms of f (φ) and V (φ) are motivated by chameleon models [32] and also from stability considerations [38].…”

Dark energy and extending the geodesic equations of motion: its construction and experimental constraints

“…Indeed, connection to Modified Newtonian Dynamics has been made for logarithmic f (R) terms [31,32], while with other choices of f (R) connection with quintessence has been made [33][34][35][36][37][38] as well. Importantly, issues with the introduction of a "fifth force", and compatibility with terrestrial experiments have begun to be addressed through the Chameleon Effect (see [39][40][41][42] and an overview in [33]), which is used to hide the effects of field with a small mass that would otherwise be seen. It is also important to note that while f (R) theories change the action for gravity, in our approach we do not; we still take the action for gravity to be the Hilbert action with the addition of a cosmological constant.…”