We propose a novel parameterization of the dark energy density. It is particularly well suited to describe a non-negligible contribution of dark energy at early times and contains only three parameters, which are all physically meaningful: the fractional dark energy density today, the equation of state today and the fractional dark energy density at early times. As we parameterize Ω d (a) directly instead of the equation of state, we can give analytic expressions for the Hubble parameter, the conformal horizon today and at last scattering, the sound horizon at last scattering, the acoustic scale as well as the luminosity distance. For an equation of state today w0 < −1, our model crosses the cosmological constant boundary. We perform numerical studies to constrain the parameters of our model by using Cosmic Microwave Background, Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that the fractional dark energy density at early times Ω e d < 0.06. This bound tightens considerably to Ω e d < 0.04 when the latest Boomerang data is included. We find that both the gold sample of Riess et. al. and the SNLS data of Astier et. al. when combined with CMB and LSS data mildly prefer w0 < −1, but are well compatible with a cosmological constant.

We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as ∼ 3 × 10 −16 e cm (roughly 1.6 × 10 −5 µB) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope. PACS numbers: 95.35+d, 13.40.Em, 95.30.cq, 98.80.Cq

We consider the consequences of a neutral dark-matter particle with a nonzero electric and/or magnetic dipole moment. Theoretical constraints, as well as constraints from direct searches, precision tests of the standard model, the cosmic microwave background and matter power spectra, and cosmic gamma rays, are included. We find that a relatively light particle with mass between an MeV and a few GeV and an electric or magnetic dipole as large as ∼ 3 × 10 −16 e cm (roughly 1.6 × 10 −5 µB) satisfies all experimental and observational bounds. Some of the remaining parameter space may be probed with forthcoming more sensitive direct searches and with the Gamma-Ray Large Area Space Telescope. PACS numbers: 95.35+d, 13.40.Em, 95.30.cq, 98.80.Cq

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In a companion paper [1], we have introduced a model of scalar field dark energy, Cuscuton, which can be realized as the incompressible (or infinite speed of sound) limit of a k-essence fluid. In this paper, we study how Cuscuton modifies the constraint sector of Einstein gravity. In particular, we study Cuscuton cosmology and show that even though Cuscuton can have an arbitrary equation of state, or time dependence, and is thus inhomogeneous, its perturbations do not introduce any additional dynamical degree of freedom and only satisfy a constraint equation, amounting to an effective modification of gravity on large scales. Therefore, Cuscuton can be considered to be a minimal theory of evolving dark energy, or a minimal modification of a cosmological constant, as it has no internal dynamics. Moreover, this is the only modification of Einstein gravity to our knowledge, that does not introduce any additional degrees freedom (and is not conformally equivalent to the Einstein gravity). We then study two simple Cuscuton models, with quadratic and exponential potentials. The quadratic model has the exact same expansion history as ΛCDM, and yet contains an early dark energy component with constant energy fraction, which is constrained to ΩQ < ∼ 2%, mainly from WMAP Cosmic Microwave Background (CMB) and SDSS Lyman-α forest observations. The exponential model has the same expansion history as the DGP self-accelerating braneworld model, but generates a much smaller integrated Sachs-Wolfe (ISW) effect, and is thus consistent with the CMB observations. Finally, we show that the evolution is local on super-horizon scales, implying that there is no gross violation of causality, despite Cuscuton's infinite speed of sound.

We argue that a few per cent of "Early Dark Energy" can be detected by the statistics of nonlinear structures. The presence of Dark Energy during linear structure formation is natural in models where the current low Dark-Energy density is related to the age of the Universe rather than a new fundamental small parameter. Generalisation of the spherical collapse model shows that the linear collapse parameter δ c is lowered. The corresponding relative enhancement of weak gravitational lensing on arc-minute scales lowers the value of σ 8 inferred from a given lensing amplitude, as compared to ΛCDM. In the presence of Early Dark Energy, structures grow slower, such that for a given σ 8 , the number of galaxies and galaxy clusters is substantially increased at moderate and high redshift. For realistic models, the number of clusters detectable through their thermal Sunyaev-Zel'dovich effect at redshift unity and above, e.g. with the Planck satellite, can be an order of magnitude larger than for ΛCDM.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The stability of scalar quintessence potentials under quantum fluctuations is investigated for both uncoupled models and models with a coupling to fermions. We find that uncoupled models are usually stable in the late universe. However, a coupling to fermions is severely restricted. We check whether a graviton induced fermion-quintessence coupling is compatible with this restriction.

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:...