Phantom Dark Energy Models with Negative Kinetic Term

We examine phantom dark energy models derived from a scalar field with a negative kinetic term for which V(phi) approaches infinity asymptotically. All such models can be divided into three classes, corresponding to an equation of state parameter w with asymptotic behavior w -> -1, w -> w_0 infinity. We derive the conditions on the potential V(phi) which lead to each of these three types of behavior. For models with w -> -1, we derive the conditions on V(phi) which determine whether or not such models produce a future big rip. Observational constraints are derived on two classes of these models: power-law potentials with V(phi) = lambda phi^alpha (with alpha positive or negative) and exponential potentials of the form V(phi) = beta e^{lambda phi^alpha}. It is shown that these models spend more time in a state with Omega_m ~ Omega_phi than do corresponding models with a constant value of w, thus providing a more satisfactory solution to the coincidence problem.Comment: 9 pages, 11 figures, minor clarifications added, to appear in Phys. Rev.

The Effect of Time Variation in the Higgs Vacuum Expectation Value on the Cosmic Microwave Background

A time variation in the Higgs vacuum expectation value alters the electron mass and thereby changes the ionization history of the universe. This change produces a measurable imprint on the pattern of cosmic microwave background (CMB) fluctuations. The nuclear masses and nuclear binding energies, as well as the Fermi coupling constant, are also altered, with negligible impact on the CMB. We calculate the changes in the spectrum of the CMB fluctuations as a function of the change in the electron mass. We find that future CMB experiments could be sensitive to |\Delta m_e/m_e| \sim |\Delta G_F/G_F| \sim 10^{-2} - 10^{-3}. However, we also show that a change in the electron mass is nearly, but not exactly, degenerate with a change in the fine-structure constant. If both the electron mass and the fine-structure constant are time-varying, the corresponding CMB limits are much weaker, particularly for l < 1000.Comment: 6 pages, 3 figures, Fig. 3 modified, other minor correction

Models for Little Rip Dark Energy

We examine in more detail specific models which yield a little rip cosmology, i.e., a universe in which the dark energy density increases without bound but the universe never reaches a finite-time singularity. We derive the conditions for the little rip in terms of the inertial force in the expanding universe and present two representative models to illustrate in more detail the difference between little rip models and those which are asymptotically de Sitter. We derive conditions on the equation of state parameter of the dark energy to distinguish between the two types of models. We show that coupling between dark matter and dark energy with a little rip equation of state can alter the evolution, changing the little rip into an asymptotic de Sitter expansion. We give conditions on minimally-coupled phantom scalar field models and on scalar-tensor models that indicate whether or not they correspond to a little rip expansion. We show that, counterintuitively, despite local instability, a little-rip cosmology has an infinite lifetime.Comment: LaTeX, 10 pages, no figure, version to appear in Phys.Lett

Is the Universe Inflating? Dark Energy and the Future of the Universe

We consider the fate of the observable universe in the light of the discovery of a dark energy component to the cosmic energy budget. We extend results for a cosmological constant to a general dark energy component and examine the constraints on phenomena that may prevent the eternal acceleration of our patch of the universe. We find that the period of accelerated cosmic expansion has not lasted long enough for observations to confirm that we are undergoing inflation; such an observation will be possible when the dark energy density has risen to between 90% and 95% of the critical. The best we can do is make cosmological observations in order to verify the continued presence of dark energy to some high redshift. Having done that, the only possibility that could spoil the conclusion that we are inflating would be the existence of a disturbance (the surface of a true vacuum bubble, for example) that is moving toward us with sufficiently high velocity, but is too far away to be currently observable. Such a disturbance would have to move toward us with speed greater than about 0.8c in order to spoil the late-time inflation of our patch of the universe and yet avoid being detectable.Comment: 7 pages, 7 figure

Alternatives to Quintessence Model Building

We discuss the issue of toy model building for the dark energy component of the universe. Specifically, we consider two generic toy models recently proposed as alternatives to quintessence models, known as Cardassian expansion and the Chaplygin gas. We show that the former is enteriely equivalent to a class of quintessence models. We determine the observational constraints on the latter, coming from recent supernovae results and from the shape of the matter power spectrum. As expected, these restrict the model to a behaviour that closely matches that of a standard cosmological constant $\Lambda$.Comment: RevTex4; 7 pages, 4 figures. v2: Improved discussion of constraints on Chaplygin gas models. Other clarifications added. Phys Rev. D (in press

Transition Redshift: New Kinematic Constraints from Supernovae

The transition redshift (deceleration/acceleration) is discussed by expanding the deceleration parameter to first order around its present value. A detailed study is carried out by considering two different parameterizations: $q=q_0 + q_1z$ and $q=q_0 + q_1 z(1+z)^{-1}$, and the associated free parameters ($q_o, q_1$) are constrained by 3 different supernova samples. The previous analysis by Riess {\it{et al.}} [ApJ 607, 665, 2004] using the first expansion is slightly improved and confirmed in light of their recent data ({\emph{Gold}}07 sample). However, by fitting the model with the Supernova Legacy Survey (SNLS) type Ia sample we find that the best fit to the redshift transition is $z_t = 0.61$ instead of $z_t = 0.46$ as derived by the High-z Supernovae Search (HZSNS) team. This result based in the SNLS sample is also in good agreement with the Davis {\it{et al.}} sample, $z_t=0.60^{+0.28}_{-0.11}$ ($1\sigma$). Such results are in line with some independent analyzes and accommodates more easily the concordance flat model ($\Lambda$CDM). For both parameterizations, the three SNe type Ia samples considered favor recent acceleration and past deceleration with a high degree of statistical confidence level. All the kinematic results presented here depend neither on the validity of general relativity nor the matter-energy contents of the Universe.Comment: 19 pages, 15 figures, 1 table, revised version accepted for publication in MNRA

Further Evidence for Cosmological Evolution of the Fine Structure Constant

We describe the results of a search for time variability of the fine structure constant, alpha, using absorption systems in the spectra of distant quasars. Three large optical datasets and two 21cm/mm absorption systems provide four independent samples, spanning 23% to 87% of the age of the universe. Each sample yields a smaller alpha in the past and the optical sample shows a 4-sigma deviation: da/a = -0.72 +/- 0.18 x 10^{-5} over the redshift range 0.5 < z < 3.5. We find no systematic effects which can explain our results. The only potentially significant systematic effects push da/a towards positive values, i.e. our results would become more significant were we to correct for them.Comment: 5 pages, 1 figure. Published in Phys. Rev. Lett. Small changes to discussion, added an acknowledgement and a referenc

Linear and non-linear perturbations in dark energy models

I review the linear and second-order perturbation theory in dark energy models with explicit interaction to matter in view of applications to N-body simulations and non-linear phenomena. Several new or generalized results are obtained: the general equations for the linear perturbation growth; an analytical expression for the bias induced by a species-dependent interaction; the Yukawa correction to the gravitational potential due to dark energy interaction; the second-order perturbation equations in coupled dark energy and their Newtonian limit. I also show that a density-dependent effective dark energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a typ

Cosmic Mimicry: Is LCDM a Braneworld in Disguise ?

For a broad range of parameter values, braneworld models display a remarkable property which we call cosmic mimicry. Cosmic mimicry is characterized by the fact that, at low redshifts, the Hubble parameter in the braneworld model is virtually indistinguishable from that in the LCDM cosmology. An important point to note is that the \Omega_m parameters in the braneworld model and in the LCDM cosmology can nevertheless be quite different. Thus, at high redshifts (early times), the braneworld asymptotically expands like a matter-dominated universe with the value of \Omega_m inferred from the observations of the local matter density. At low redshifts (late times), the braneworld model behaves almost exactly like the LCDM model but with a renormalized value of the cosmological density parameter \Omega_m^{LCDM}. The redshift which characterizes cosmic mimicry is related to the parameters in the higher-dimensional braneworld Lagrangian. Cosmic mimicry is a natural consequence of the scale-dependence of gravity in braneworld models. The change in the value of the cosmological density parameter is shown to be related to the spatial dependence of the effective gravitational constant in braneworld theory. A subclass of mimicry models lead to an older age of the universe and also predict a redshift of reionization which is lower than z_{reion} \simeq 17 in the LCDM cosmology. These models might therefore provide a background cosmology which is in better agreement both with the observed quasar abundance at z \gsim 4 and with the large optical depth to reionization measured by the Wilkinson Microwave Anisotropy Probe.Comment: 22 pages, 4 figures. A subsection and references added; main results remain unchanged. Accepted for publication in JCA

Bianchi Type III Anisotropic Dark Energy Models with Constant Deceleration Parameter

The Bianchi type III dark energy models with constant deceleration parameter are investigated. The equation of state parameter $\omega$ is found to be time dependent and its existing range for this model is consistent with the recent observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy clustering statistics. The physical aspect of the dark energy models are discussed.Comment: 12 pages, 2 figures, Accepted version of IJT