Precision measurements of anisotropies in the cosmic microwave background and of the clustering of large-scale structure have supposedly established that the universe is presently dominated by “dark energy” which has negative pressure and behaves similarly to a cosmological constant. This is based on the assumption that the primordial density perturbation has a nearly scale-invariant power-law spectrum and that the dark matter consists of “cold” particles. However there are theoretical and observational indications that the spectrum is not scale-free and it is known that sub-eV mass neutrinos contribute a small component of hot dark matter. This would be sufficient to fit the same observational data without requiring any dark energy.

The Nearby Supernova Factory aims at discovering and stud- ying a large sample of nearby (0.03 < z < 0.08) thermonuclear supernovae. Potential targets are extracted from the unbiased Palomar-QUEST survey, and follow-up spectro-photometric observations are performed using the dedicated Supernovae Integral-Field Spectrograph. The current sample comprises more than 2700 flux-calibrated optical spectra (320-1000 nm) from 181 supernovæ followed over their full life-time. Specific operation and data-reduction issues are discussed, and first scientific results from this unprecedented dataset are presented.

We model the cosmic medium as the mixture of a generalized Chaplygin gas and a pressureless matter component. Within a neo-Newtonian approach we compute the matter power spectrum. The 2dFGRS data are used to discriminate between unified models of the dark sector and different models, for which there is separate dark matter, in addition to that accounted for by the generalized Chaplygin gas. Leaving the corresponding density parameters free, we find that the unified models are strongly disfavored. On the other hand, using unified model priors, the observational data are also well described, in particular for small and large values of the generalized Chaplygin gas parameter α.

Gamma–Ray Bursts (GRBs) are potentially a very powerful tool for the measure of cosmological parameters. Indeed, they are the brightest sources in the universe (up to more than 1054 erg/s released in a few tens of s), their redshift distribution extends from about 0.1 to at least 6.7, and they radiate mostly in the hard X–ray band, thus avoiding, e.g., the dust extinction problems affecting type Ia SNe. However, the luminosities of GRBs span several orders of magnitude, thus a way to “standardize” them in a way similar, e.g., to what is done with type Ia SNe has to be found. Under this respect, the most promising and discussed tool is the correlation between the photon energy, Ep,i, at which the νFν spectrum peaks and the GRB radiated energy (Eiso) or luminosity (Liso, Lp,iso). By studying the scatter of these spectrum–energy correlations as a function of the adopted cosmology it is possible, with the present sample of ~80 GRBs with known redshift and spectral parameters, to derive, for a flat universe, significant constraints on ΩM consistent with the “concordance” cosmology value. Moreover, simulations show that, with the enlargement of the sample expected by present and future missions (Swift, GLAST/GBM, Konus–WIND, SVOM) it will be possible to simultaneously constrain both ΩM and ΩΛ, and, possibly, obtain information on the equation of state of dark energy and its evolution.

This review surveys the signatures of dark energy on large-scale structures (LSS) at low redshift, focusing on observables relying on the evolution of matter perturbations, i.e. on the growth of structures. We discuss in particular galaxy clustering, weak-gravitational lensing by LSS, also called cosmic-shear, and Lyman-α forest, which respectively describe the 3D, 2D, and 1D power spectrum of matter density fluctuations. We illustrate some example of data analysis for each these observables.

During the last ten years astrophysical cosmology has brou- ght a remarkable result of deep impact for fundamental physics: the domination of the density of the acceleration of the expansion of universe univers. This last is probably the most revolutionizing one in modern physics: the scientific review Sciences has considered twice results on this question as Breakthrough of the Year (for 1998 and 2003). However, direct evidence of acceleration are still rather weak, and the strength of the standard scenario relies more on the “concordance” argument rather than on the robustness of direct evidences. Supernovae evolution could mimic an accelerating expansion. Furthermore, recent data on distant x-ray clusters obtained from XMM and Chandra indicate that the observed abundances of clusters at high redshift taken at face value favors an Einstein de Sitter model and are hard to reconcile with the concordance model. However, recent results on large scale structure has brought decisive pieces of information on this question: galaxy distribution on large scales cannot be reproduced by matter power in Einstein de Sitter model.

The dimming of the SN Ia apparent luminosity is generally ascribed to the influence of a “dark energy” component, which is supposed to yield a late-time acceleration of the Universe expansion rate. However, this interpretation assumes we are leaving in an homogeneous Universe where the influence of the inhomogeneities is negligible at all scales. Now, the last years have experienced an increase in the papers devoted to the study of such an influence and two main types of methods have been developed to deal with this problem: one is directed at evaluating backreaction effects with averaging strategies, the other makes use of models which are exact solutions of Einstein's equations and which, as they become more and more sophisticated, are able to reproduce the largest possible data sets (not only the supernovae). We focus here our interest on the later method and review the most achieved among the latest contributions available in the literature.

Observations are inconsistent with a homogeneous and iso-tropic universe with ordinary matter and gravity. The universe is far from exact homogeneity and isotropy at late times, and the effect of the non-linear structures has to be quantified before concluding that new physics is needed. We explain how structure formation can lead to accelerated expansion, and discuss a semi-realistic model where the timescale in the change in the expansion rate emerges from the physics of structure formation.

There is an ongoing debate in the literature concerning the effects of averaging out inhomogeneities (“backreaction”) in cosmology. In particular, some simple models of structure formation studied in the literature seem to indicate that the backreaction can play a significant role at late times, and it has also been suggested that the standard perturbed FLRW framework is no longer a good approximation during structure formation, when the density contrast becomes nonlinear. In this work we use Zalaletdinov's covariant averaging scheme (macroscopic gravity or MG) to show that as long as the metric of the Universe can be described by the perturbed FLRW form, the corrections due to averaging remain negligibly small. Further, using a fully relativistic and reasonably generic model of pressureless spherical collapse, we show that as long as matter velocities remain small (which is true in our model), the perturbed FLRW form of the metric can be explicitly recovered. Together, these results imply that the backreaction remains small even during nonlinear structure formation, and we confirm this within the toy model with a numerical calculation.

Due to the non-commutation of spatial averaging and temporal evolution, inhomogeneities and anisotropies (cosmic structures) influence the evolution of the averaged Universe via the cosmological backreaction mechanism. We study the backreaction effect as a function of averaging scale in a perturbative approach up to higher orders. We calculate the hierarchy of the critical scales, at which 10% effects show up from averaging at different orders, observable up to scales of 200 Mpc. The cosmic variance of the local Hubble rate is 10% (5%) for spherical regions of radius 40 (60) Mpc. We compare our results to the Hubble Space Telescope (HST) Key Project data.

I briefly outline a new physical interpretation to the average cosmological parameters for an inhomogeneous universe with backreaction. The variance in local geometry and gravitational energy between ideal isotropic observers in bound structures and isotropic observers at the volume average location in voids plays a crucial role. Fits of a model universe to observational data suggest the possibility of a new concordance cosmology, in which dark energy is revealed as a mis-identification of gravitational energy gradients that become important when voids grow at late epochs.

We investigate the spacetime of a slowly rotating black hole in Chern-Simons modified gravity. The frame-dragging effect under the Chern-Simon gravity gives a similar feature to that of galaxy rotation curves.

In this paper, we have constructed some cosmological models which satisfy the present day observational data and the initial conditions as proposed by Sivaram et al. (Brans & Dicke 1961) with additional requirements G ∝ tn & Λ∝ tn, Gρ/Λ as a constant along with study of big-rip scenerio.

In chameleon theories where the field couples to matter much more strongly than gravity does, the fifth force between two bodies with thin-shell is independent of their coupling to the field. As a consequence present days bounds on these models can be exponentially relaxed.

For a long time, the Lagrangian linear perturbation (Zel'dovich approximation, ZA) is used for initial conditions for cosmological N-body simulations. Recently, Crocce et al. (2006) proposed the improvement of the initial conditions. They applied Lagrangian second-order perturbation (2LPT) for the initial condition. Then they calculated the Lagrangian linear perturbation and showed that 2LPT initial conditions improve the evolution of non-Gaussianity for the density fluctuation in the Universe. Here we have one question. Although 2LPT initial conditions improve the cosmological N-body simulation, is 2LPT initial conditions sufficient? To answer this question, in addition to the ZA and 2LPT, we also calculate the non-Gaussianity with the initial conditions based on third-order Lagrangian perturbation theory (3LPT). Then we show reasonable order of perturbation and redshift for the initial conditions.

Over the past decade, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. The observational evidence for the existence of dark matter is reviewed: rotation curves of galaxies, weak lensing measurements, hot gas in clusters, primordial nucleosynthesis and microwave background experiments. In addition, a new line of research on Dark Stars is presented, which suggests that the first stars to exist in the universe were powered by dark matter heating rather than by fusion: the observational possibilities of discovering dark matter in this way are discussed.

We discuss the influence of the cosmological constant Λ on the solar system dynamics. Observational constraints are derived from measurements of the periastron advance of the planets. Moreover, we consider the change in the mean motion due to Λ. Up to now, Earth and Mars data give the best constraint, Λ ~ 10-36 km-2. If properly accounting for the gravito-magnetic effect, this upper limit on Λ could greatly improve in the near future thanks to new data from planned or already operating space-missions. Dark matter or modifications of the Newtonian inverse-square law in the solar system are discussed as well. Variations in the 1/r2 behavior are considered in the form of either a possible Yukawa-like interaction or a modification of gravity of MOND type.

In the past years a wealth of observations has unraveled the structural properties of the Dark and Luminous mass distribution in spirals. These have pointed out to an intriguing scenario not easily explained by present theories of galaxy formation. The investigation of individual and coadded objects has shown that the spiral rotation curves follow, from their centers out to their virial radii, a Universal profile (URC) that arises from the tuned combination of a stellar disk and of a dark halo. The importance of the latter component decreases with galaxy mass. Individual objects, on the other hand, have clearly revealed that the dark halos encompassing the luminous discs have a constant density core. This resulting observational scenario poses important challenges to presently favored theoretical ΛCDM Cosmology.

Many scaling relations have been observed for self-gravitating systems (SGS) in the universe. We explore a consistent understanding of them from a simple principle based on the proposal that the collision-less dark matter (DM) fluid terns into a turbulent state, i.e. dark turbulence, after crossing the caustic surface in the non-linear stage. After deriving Kolmogorov scaling laws from Navier-Stokes and Jeans equations by the method used in solving the Smoluchowski coagulation equation, we apply this to several observations such as the scale-dependent velocity dispersion, mass-luminosity ratio, and mass-angular momentum relation. They all point the concordant value for the constant energy flow per mass: 0.3 cm2/s3, which may be understood as the speed of the hierarchical coalescence process in the cosmic structure formation.