Constraints on holographic dark energy models using the differential ages of passively evolving galaxies

Using the absolute ages of passively evolving galaxies observed at different redshifts, one can obtain the differential ages, the derivative of redshift $z$ with respect to the cosmic time $t$ (i.e. ${\rm d} z/{\rm d}t$). Thus, the Hubble parameter $H(z)$ can be measured through the relation $H(z)=-({\rm d} z/{\rm d}t)/(1+z)$. By comparing the measured Hubble parameter at different redshifts with the theoretical one containing free cosmological parameters, one can constrain current cosmological models. In this paper, we use this method to present the constraint on a spatially flat Friedman-Robert-Walker Universe with a matter component and a holographic dark energy component, in which the parameter $c$ plays a significant role in this dark energy model. Firstly we consider three fixed values of $c$=0.6, 1.0 and 1.4 in the fitting of data. If we set $c$ free, the best fitting values are $c=0.26$, $\Omega_{\rm m0}=0.16$, $h=0.9998$. It is shown that the holographic dark energy behaves like a quintom-type at the $1\sigma$ level. This result is consistent with some other independent cosmological constrains, which imply that $c<1.0$ is favored. We also test the results derived from the differential ages using another independent method based on the lookback time to galaxy clusters and the age of the universe. It shows that our results are reliable.Comment: 18 pages including 7 figures and 1 tables. Final version for publication in Modern Physics Letters A (MPLA)[minor revision to match the appear version

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernova (SNe Ia) and anisotropy of cosmic microwave background (CMB).The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$, which is consistent with Riess et al. The transition redshift is $z_{T}=0.60_{-0.08}^{+0.06}$. We also discuss several dark energy models that define the $w(z)$ of the parameterized equation of state of dark energy including one parameter and two parameters ($w(z)$ being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from $z_{T}=0.29_{-0.06}^{+0.07}$ to $z_{T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum redshift $z_{c}$ at which the current observations need the universe to accelerate.Comment: 16 pages, 5 figures, 1 tabl

Anisotropic Dark Energy and the Generalized Second Law of Thermodynamics

We consider a Bianchi type $I$ model in which anisotropic dark energy is interacting with dark matter and anisotropic radiation. With this scenario, we investigate the validity of the generalized second law of thermodynamics. It is concluded that the validity of this law depends on different parameters like shear, skewness and equation of state.Comment: 12 pages, accepted for publication in Phys. Scr. arXiv admin note: text overlap with arXiv:1008.0692 and arXiv:1106.241

The Case for an Accelerating Universe from Supernovae

The unexpected faintness of high-redshift Type Ia supernovae (SNe Ia), as measured by two teams, has been interpreted as evidence that the expansion of the Universe is accelerating. We review the current challenges to this interpretation and seek to answer whether the cosmological implications are compelling. We discuss future observations of SNe Ia which could offer extraordinary evidence to test acceleration.Comment: To appear as an Invited Review for PASP 20 pages, 13 figure

Transition between phantom and non-phantom phases with time dependent cosmological constant and Cardy-Verlinde formula

We investigate the transition phenomenon of the universe between a phantom and a non-phantom phases. Particular attention is devoted to the case in which the cosmological constant depends on time and is proportional to the square of the Hubble parameter. Inhomogeneous equations of state are used and the equation of motion is solved. We find that, depending on the choice of the input parameters, the universe can transit from the non-phantom to the phantom phase leading to the appearance of singularities. In particular, we find that the phantom universe ends in the singularity of type III, unlike the case without variable cosmological constant in which the phantom phase ends exclusively in the big rip (singularity of type I). The Cardy-Verlinde formula is also introduced for inhomogeneous equation of state and we find that its equivalence with the total entropy of the universe, coming from the Friedmann equations, occurs only for special choice of the input parameter $m$ at the present time.Comment: 12 pages, 2 figure

Higher Dimensional Cosmology with Some Dark Energy Models in Emergent, Logamediate and Intermediate Scenarios of the Universe

We have considered N-dimensional Einstein field equations in which four-dimensional space-time is described by a FRW metric and that of extra dimensions by an Euclidean metric. We have chosen the exponential forms of scale factors a and d numbers of b in such a way that there is no singularity for evolution of the higher dimensional Universe. We have supposed that the Universe is filled with K-essence, Tachyonic, Normal Scalar Field and DBI-essence. Here we have found the nature of potential of different scalar field and graphically analyzed the potentials and the fields for three scenario namely Emergent Scenario, Logamediate Scenario and Intermediate Scenario. Also graphically we have depicted the geometrical parameters named statefinder parameters and slow-roll parameters in the higher dimensional cosmology with the above mentioned scenarios.Comment: 21 pages, 36 figure

Detection of a redshift 3.04 filament

The filamentary structure of the early universe has until now only been seen in numerical simulations. Despite this lack of direct observational evidence, the prediction of early filamentary structure formation in a Cold Dark Matter dominated universe has become a paradigm for our understanding of galaxy assembly at high redshifts. Clearly observational confirmation is required. Lyman Break galaxies are too rare to be used as tracers of filaments and we argue that to map out filaments in the high z universe, one will need to identify classes of objects fainter than those currently accessible via the Lyman Break technique. Objects selected via their Ly-alpha emission, and/or as DLA absorbers, populate the faintest accessible part of the high redshift galaxy luminosity function, and as such make up good candidates for objects which will map out high redshift filaments. Here we present the first direct detection of a filament (at z=3.04) mapped by those classes of objects. The observations are the deepest yet to have been done in Ly-alpha imaging at high redshift, and they reveal a single string of proto-galaxies spanning about 5 Mpc (20 Mpc comoving). Expanding the cosmological test proposed by Alcock & Paczynski (1979), we outline how observations of this type can be used to determine Omega_Lambda at z=3.Comment: 5 pages, LaTeX, 3 PostScript figures; Accepted for publication in A&A-Letter

Dynamics of Logamediate and Intermediate Scenarios in the Dark Energy Filled Universe

We have considered a model of two component mixture i.e., mixture of Chaplygin gas and barotropic fluid with tachyonic field. In the case, when they have no interaction then both of them retain their own properties. Let us consider an energy flow between barotropic and tachyonic fluids. In both the cases we find the exact solutions for the tachyonic field and the tachyonic potential and show that the tachyonic potential follows the asymptotic behavior. We have considered an interaction between these two fluids by introducing a coupling term. Finally, we have considered a model of three component mixture i.e., mixture of tachyonic field, Chaplygin gas and barotropic fluid with or without interaction. The coupling functions decays with time indicating a strong energy flow at the initial period and weak stable interaction at later stage. To keep the observational support of recent acceleration we have considered two particular forms (i) Logamediate Scenario and (ii) Intermediate Scenario, of evolution of the Universe. We have examined the natures of the recent developed statefinder parameters and slow-roll parameters in both scenarios with and without interactions in whole evolution of the universe.Comment: 28 pages, 20 figure

No realistic wormholes from ghost-free scalar-tensor phantom dark energy

It is proved that no wormholes can be formed in viable scalar-tensor models of dark energy admitting its phantom-like ($w < -1$) behaviour in cosmology, even in the presence of electric or magnetic fields, if the non-minimal coupling function $f(\Phi)$ is everywhere positive and the scalar field $\Phi$ itself is not a ghost. Some special static, spherically symmetric wormhole solutions may exist if $f(\Phi)$ is allowed to reach zero or to become negative, so that the effective gravitational constant becomes negative in some region making the graviton a ghost. If $f$ remains non-negative, such solutions require severe fine tuning and a very peculiar kind of model. If $f < 0$ is allowed, it is argued (and confirmed by previous investigations) that such solutions are generically unstable under non-static perturbations, the instability appearing right near transition surfaces to negative $f$.Comment: 8 pages, late