The present cosmic accelerating stage is discussed through a new kinematic method based on the Sunyaev- Zel'dovich effect (SZE) and X-ray surface brightness data from galaxy clusters. By using the SZE/X-ray data from 38 galaxy clusters in the redshift range $0.14 \leq z \leq 0.89 $ [Bonamente et al., Astrop. J. {\bf 647}, 25 (2006)] it is found that the present Universe is accelerating and that the transition from an earlier decelerating to a late time accelerating regime is relatively recent. The ability of the ongoing Planck satellite mission to obtain tighter constraints on the expansion history through SZE/X-ray angular diameters is also discussed. Our results are fully independent on the validity of any metric gravity theory, the possible matter- energy contents filling the Universe, as well as on the SNe Ia Hubble diagram from which the presenting accelerating stage was inferred.
We study Kaluza-Klein cosmology in cuscuton gravity and find an exact solution describing an accelerating 4-dimensional universe with a stable extra dimension. A cuscuton which is a non-dynamical scalar field is responsible for the accelerating expansion and a vector field makes the extra dimensional space stable. Remarkably, the accelerating universe in our model is not exactly de Sitter.
It has been suggested recently to study the dynamics of a gravitating gluon condensate q in the context of a spatially flat Friedmann-Robertson-Walker universe. The expansion of the Universe (or, more generally, the presence of a nonvanishing Ricci curvature scalar R) perturbs the gluon condensate and may induce a nonanalytic term \tilde{h}(R,q) in the effective gravitational action. The aim of this article is to explore the cosmological implications of a particular nonanalytic term \tilde{h} \propto η|R|^{1/2} |q|^{3/4}. With a quadratic approximation of the gravitating gluon-condensate vacuu…
Among various phenomenological $Λ$ models, a time-dependent model $\dot Λ\sim H^3$ is selected here to investigate the $Λ$-CDM cosmology. Using this model the expressions for the time-dependent equation of state parameter $ω$ and other physical parameters are derived. It is shown that in $H^3$ model accelerated expansion of the Universe takes place at negative energy density, but with a positive pressure. It has also been possible to obtain the change of sign of the deceleration parameter $q$ during cosmic evolution.
In accordance with current models of the accelerating Universe as a spacetime with a positive cosmological constant, new results about a cosmological upper bound for the area of stable marginally outer trapped surfaces are found taking into account angular momentum, gravitational waves and matter. Compared to previous results which take into account only some of the aforementioned variables, the bound is found to be tighter, giving a concrete limit to the size of black holes especially relevant in the early Universe.
It is shown that the acceleration of the universe can be understood by considering a F(T) gravity models. For these F(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. Some explicit examples of F(T) are reconstructed from the background FRW expansion history.
A higher value of Hubble constant has been obtained from measurements with nearby Type Ia supernovae, than that obtained at much higher redshift. With the peculiar motions of their hosts, we find that the matter content at such low redshift is only about 10% of that at much higher redshifts; such a low matter density cannot be produced from density perturbations in the background of the ΛCDM expansion. Recently the Planck team has reported a lower Hubble constant and a higher matter content. We find that the dark energy density increases with cosmic time, so that its equation-of-state paramete…
We extend investigations of infrared dynamics in accelerating universes. In the presence of massless and minimally coupled scalar fields, physical quantities may acquire growing time dependences through quantum fluctuations at super-horizon scales. From a semiclassical viewpoint, it was proposed that such infrared effects are described by a Langevin equation. In de Sitter space, the stochastic approach has been proved to be equivalent to resummation of the growing time dependences at the leading power. In this paper, we make the resummation derivation of the Langevin equation in a general acce…
The Nobel Prize in Physics 2011 has just been awarded to three astronomers: Saul Perlmutter, Brian Schmidt, and Adam Riess, for their amazing discovery of the accelerating expansion of the Universe. Without diminishing the achievement of our community's laureates, here I elaborate on the role of the C&T project in this discovery.
The accelerating universe is closely related to today's version of the cosmological constant problem; fine-tuning and coincidence problems. We show how successfully the scalar-tensor theory, a rather rigid theoretical idea, provides us with a simple and natural way to understand why today's observed cosmological constant is small only because we are old cosmologically, without fine-tuning theoretical parameters extremely.
Inflation generically produces primordial gravitational waves with a red spectral tilt. In this paper we calculate the backreaction produced by these gravitational waves on the expansion of the universe. We find that in radiation domination the backreaction acts as a relativistic fluid, while in matter domination a small dark energy emerges with an equation of state w=-8/9.
Theoretical background of our proposed relation between the accelerating universe and the time-variability of the fine-structure constant is discussed, based on the scalar-tensor theory, with emphases on the intuitive aspects of underlying physical principles. An important comment is added on the successful understanding of the size of the effective cosmological constant responsible for the acceleration, without appealing to fine-tuning parameters.
In this article we review the discovery of the accelerating universe using type Ia supernovae. We then outline ways in which dark energy - component that causes the acceleration - is phenomenologically described. We finally describe principal cosmological techniques to measure large-scale properties of dark energy. This chapter complements other articles in this book that describe theoretical understanding (or lack thereof) of the cause for the accelerating universe.
We proposed the generalized holographic dark energy model where infrared cutoff is identified with the combination of the FRW universe parameters: the Hubble rate, particle and future horizons, cosmological constant, the universe life-time (if finite) and their derivatives. It is demonstrated that with the corresponding choice of the cutoff one can map such holographic dark energy to modified gravity or gravity with general fluid. Explicitly, F(R) gravity and general perfect fluid are worked out in detail and corresponding infrared cutoff is found. Using this correspondence, we get realistic i…
We study linear perturbations around time dependent spherically symmetric solutions in the Lambda_3 massive gravity theory, which self-accelerate in the vacuum. We find that the dynamics of the scalar perturbations depend on the coordinate choice for the background solutions. For particular choices of coordinates there is a symmetry enhancement, leaving no propagating scalar degrees of freedom at linear order in perturbations. In contrast, any other coordinate choice propagates a single scalar mode. We find that the Hamiltonian of this scalar mode is unbounded from below for all self-accelerat…
We investigate the effect of backreaction due to inhomogeneities on the evolution of the present universe within the Buchert framework. Our analysis shows how backreaction from inhomogeneities in the presence of the cosmic event horizon causes the current acceleration of the Universe to slow down in the future and even lead in certain cases to the emergence of a future decelerating epoch.
The main sources of information energy in the universe are shown to be stellar heated gas and dust and black holes. Information energy has properties similar to dark energy with a significant energy density that has remained nearly constant for at least the last half of cosmic time, corresponding to an equation of state parameter, wi~-1, the value unique to dark energy. Changes in universe information content during star formation require an accelerating universe expansion in order to ensure dI>0. The size of this required acceleration is in good agreement with the observed extra doubling in u…
We continue the study of covariant power-counting renormalizable gravity constrained by scalar Lagrange multiplier. Lorentz symmetry breaking is investigated in such a theory in comparison with the one in ghost condensation model. Covariant power-counting renormalizable vector gravity which is invariant under $U(1)$ gauge symmetry is proposed. Several forms of vector Lagrange multiplier in this theory are discussed. It is shown that covariant scalar/vector gravity under consideration may have power-law or de Sitter accelerating cosmological solution corresponding to inflationary era. Simplest …
The recent simultaneous detection of gravitational waves and a gamma ray burst from a neutron star merger significantly shrank the space of viable scalar-tensor theories by demanding that the speed of gravity is equal to that of light. The survived theories belong to the class of degenerate higher order scalar-tensor theories. We study whether these theories are suitable as dark energy candidates. We find scaling solutions in the matter dominated universe that lead to de Sitter solutions at late times without the cosmological constant, realising self-acceleration. We evaluate quasi-static pert…
A kinematic method to access cosmic acceleration based exclusively on the Sunyaev-Zel'dovich effect (SZE) and X-ray surface brightness data from galaxy clusters is proposed. By using the SZE/X-ray data from 38 galaxy clusters [Bonament et al., Astrop. J. 647, 25 (2006)], we find that the present Universe is accelerating and that the transition from an earlier decelerating to a late time accelerating regime occurred relatively recent. Such results are fully independent on the validity of any metric gravity theory, the possible matter-energy contents filling the Universe, as well as on the SNe t…
We study cosmologies in modified theories of gravity considering Lagrangian density $f(R)$ which is a polynomial function of scalar curvature ($R$) in the Einstein-Hilbert action in vacuum. The field equation obtained from the modified action corresponding to a Robertson-Walker metric is highly non-linear and not simple enough to obtain analytic solution. Consequently we adopt a numerical technique to study the evolution of the FRW universe. A number of evolutionary phases of the universe including the present accelerating phase are found to exist in the higher derivative theories of gravity. …
Accelerating universe or the existence of a small and positive cosmological constant is probably the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories of gravity, including string theory. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter brane embedded in a higher-dimensional spacetime described by warped geometry. In this scenario, the four-dimensional cosmological constant may be uniquely determined in terms of two length sca…
We consider generalizations of the entropic accelerating universe recently proposed in Ref. [4,5] and show that their background equations can be made equivalent to a model with a dark energy component with constant parameter of state $w_{X} = -1 + 2\, γ/3$, where $γ$ is related to the coefficients of the new terms in the Friedman equations. After discussing all the Friedman equations for an arbitrary $γ$, we show how to recover the standard scalings for dust and radiation. The acoustic scale $\ell_A$, related to the peak positions in the pattern of the angular power spectrum of the Cosmic Mic…
In this essay we propose that the theory of gravity's vacuum is described by a de Sitter geometry. Under this assumption we consider an adjustment mechanism able to screen any value of the vacuum energy of the matter fields. We discuss the most general scalar-tensor cosmological models with second order equations of motion that have a fixed de Sitter critical point for any kind of material content. These models give rise to interesting cosmological evolutions that we shall discuss.
We investigate the effect of backreaction due to inhomogeneities on the evolution of the present Universe by considering the Universe to be partitioned into multiple domains within the Buchert framework. Taking the observed present acceleration of the universe as an essential input, we study the effect of inhomogeneities on the future evolution. We find that the backreaction from inhomogeneities causes the acceleration to slow down in the future for a range of initial configurations and model parameters, and even lead in certain cases to the emergence of a future decelerating epoch. We conside…
We derive the escape velocity profile for an Einasto density field in an accelerating universe and demonstrate its physical viability by comparing theoretical expectations to both light-cone data generated from N-body simulations and archival data on 20 galaxy clusters. We demonstrate that the projection function ($g(β)$) is deemed physically viable only for the theoretical expectation that includes a cosmology-dependent term. Using simulations, we show that the inferred velocity anisotropy is more than 6σ away from the expected value for the theoretical profile that ignores the acceleration o…
We study cosmological perturbations of self-accelerating universe solutions in the recently proposed nonlinear theory of massive gravity, with general matter content. While the broken diffeomorphism invariance implies that there generically are 2 tensor, 2 vector and 2 scalar degrees of freedom in the gravity sector, we find that the scalar and vector degrees have vanishing kinetic terms and nonzero mass terms. Depending on their nonlinear behavior, this indicates either nondynamical nature of these degrees or strong couplings. Assuming the former, we integrate out the 2 vector and 2 scalar de…
We explore the possibility of realising self-accelerated expansion of the Universe taking into account the vector components of a massive graviton. The effective action in the decoupling limit contains an infinite number of terms, once the vector degrees of freedom are included. These can be re-summed in physically interesting situations, which result in non-polynomial couplings between the scalar and vector modes. We show there are self-accelerating background solutions for this effective action, with the possibility of having a non-trivial profile for the vector fields. We then study fluctua…
To accommodate the observed accelerated expansion of the universe, one popular idea is to invoke a driving term in the Friedmann-Lemaitre equation of dark energy which must then comprise 70% of the present cosmological energy density. We propose an alternative interpretation which takes into account the entropy and temperature intrinsic to the horizon of the universe due to the information holographically stored there. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on the horizon surface screen. We consider an add…
We investigate the effect of backreaction due to inhomogeneities on the evolution of the present universe by considering a two-scale model within the Buchert framework. Taking the observed present acceleration of the universe as an essential input, we study the effect of inhomogeneities in the future evolution. We find that the backreaction from inhomogeneities causes the acceleration to slow down in the future for a range of initial configurations and model parameters. The present acceleration ensures formation of the cosmic event horizon, and our analysis brings out how the effect of the eve…
We present a cosmological model with a solution that self-accelerates at late-times without signs of ghost instabilities on small scales. The model is a natural extension of the Brans-Dicke (BD) theory including a non-linear derivative interaction, which appears in a theory with the Galilean shift symmetry. The existence of the self-accelerating universe requires a negative BD parameter but, thanks to the non-linear term, small fluctuations around the solution are stable on small scales. General relativity is recovered at early times and on small scales by this non-linear interaction via the V…
In this paper we show that the flat space Galilean theories with up to three scalars in the equation of motion (the quartic Galileons) are recovered in the decoupling limit of certain scalar theories non-minimally coupled to gravity, the so-called "Slotheonic" theories. These theories are also invariant under the generalized Galilean shifts in curved spacetime. While Galilean self-(derivative)couplings are not explicit in the action, they appear after integrating out gravity. We then argue that Galilean supersymmetric theories may only be found in the context of supergravity. Finally, we discu…
Exact inhomogeneous solutions of Einstein's equations have been used in the literature to build models reproducing the cosmological data without dark energy. However, owing to the degrees of freedom pertaining to these models, it is necessary to get rid of the degeneracy often exhibited by the problem of distinguishing between them and accelerating universe models. We give an overview of redshift drift in inhomogeneous cosmologies, and explain how it serves to this purpose. One class of models which fits the data is the Szekeres Swiss-cheese class where non-spherically symmetric voids exhibit …
A recent analysis of the Supernova Ia data claims a 'marginal' ($\sim3σ$) evidence for a cosmic acceleration. This result has been complemented with a non-accelerating $R_{h}=ct$ cosmology, which was presented as a valid alternative to the $Λ$CDM model. In this paper, we use the same analysis to show that a non-marginal evidence for acceleration is actually found. We compare the standard Friedmann models to the $R_{h}=ct$ cosmology by complementing SN Ia data with the Baryon Acoustic Oscillations, Gamma Ray Bursts and Observational Hubble datasets. We also study the power-law model which is a …
A recently published paper by Nielsen, Guffanti and Sarkar argues that the evidence for cosmic acceleration is marginal and that a coasting universe - the Milne Universe - fits the same supernovae data in a Hubble diagram nearly as well. The Milne Universe has negative spatial curvature. Nevertheless, we confirm that the Milne model fits the data just as well as LCDM. We show that this unexpected result points to a weakness in the Hubble diagram rather than to a flaw in LCDM. It seems the Hubble diagram is insensitive to spatial curvature. Here we present the data and both models in a scale fa…
The influence of the shear stress and angular momentum on the nonlinear spherical collapse model is discussed in the framework of the Einstein-de Sitter (EdS) and $Λ$CDM models. By assuming that the vacuum component is not clustering within the homogeneous nonspherical overdensities, we show how the local rotation and shear affects the linear density threshold for collapse of the non-relativistic component ($δ_\mathrm{c}$) and its virial overdensity ($Δ_\mathrm{V}$). It is also found that the net effect of shear and rotation in galactic scale is responsible for higher values of the linear over…
