A new kind of accelerating flat model with no dark energy that is fully dominated by cold dark matter (CDM) is investigated. The number of CDM particles is not conserved and the present accelerating stage is a consequence of the negative pressure describing the irreversible process of gravitational particle creation. A related work involving accelerating CDM cosmology has been discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53, 4287 (1996)]. However, in order to have a transition from a decelerating to an accelerating regime at low redshifts, the matter creation rate proposed here includes a constant term of the order of the Hubble parameter. In this case, H0 does not need to be small in order to solve the age problem and the transition happens even if the matter creation is negligible during the radiation and part of the matter dominated phase. Therefore, instead of the vacuum dominance at redshifts of the order of a few, the present accelerating stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the gravitational particle creation process. As an extra bonus, in the present scenario does not exist the coincidence problem that plagues models with dominance of dark energy. The model is able to harmonize a CDM picture with the present age of the universe, the latest measurements of the Hubble parameter and the Supernovae observations. PACS numbers: 95.35.+d,95.30.Tg

The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996 [1]) without requiring the presence of dark energy or a cosmological constant. In a recent study Lima et al. 2008 [2] (LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations [3] of the recent transition from a decelerating to an accelerating Universe. Here we test the evolution of such models at high redshift using the constraint on zeq, the redshift of the epoch of matter -radiation equality, provided by the WMAP constraints on the early Integrated Sachs-Wolfe effect (ISW) [4]. Since the contribution of baryons and radiation was ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM is tested and constrained at widely-separated epochs (z ≈ zeq and z ≈ 0) in the evolution of the Universe. This comparison reveals a tension between the high redshift CMB constraint on zeq and that which follows from the low redshift SNIa data, challenging the viability of this class of models. 95.35.+d,95.30.Tg

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $\epsilon$, whereas the dark energy satisfies the equation of state $p_x=\omega \rho_x$ ($\omega < 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.Comment: 7 pages, 7 figures. Minor corrections to match published versio

By means of a triple master action we deduce here a linearized version of the "New Massive Gravity" (NMG) in arbitrary dimensions. The theory contains a 4th-order and a 2nd-order term in derivatives. The 4th-order term is invariant under a generalized Weyl symmetry. The action is formulated in terms of a traceless η µν Ω µνρ = 0 mixed symmetry tensor Ω µνρ = −Ω µρν and corresponds to the massive Fierz-Pauli action with the replacement e µν = ∂ ρ Ω µνρ . The linearized 3D and 4D NMG theories are recovered via the invertible maps Ω µνρ = ǫ

We investigate some observational constraints on decaying vacuum cosmologies based on the recently discovered old high redshift quasar APM 08279+5255. This object is located at z = 3.91 and has an estimated age of 2-3 Gyr. The class of Λ(t) cosmologies is characterized by a positive β parameter smaller than unity which quantifies the ratio between the vacuum and the total energy density. Assuming the lower limit age (2 Gyr) and that the cold dark matter contributes with Ω M = 0.2 we show that β is constrained to be ≥ 0.07 while for an age of 3 Gyr and Ω M = 0.4 the β parameter must be greater than 0.32. Our analysis includes closed, flat and hyperbolic scenarios, and it strongly suggests that there is no age crisis for this kind of Λ(t) cosmologies. Lower limits to the redshift quasar formation are also briefly discussed to the flat case. For Ω M = 0.4 we found that the redshift formation is constrained by z f ≥ 8.0. * jvital@dfte.ufrn.br † rose@dfte.ufrn.br Recent observations from Supernovae (SNe) type Ia strongly suggest that the bulk of energy in the Universe is repulsive and appears like a dark component; an unknown form of energy with negative pressure [in addition to the ordinary dark matter] which is probably of primordial origin [1]. The most natural candidate for dark energy is the cosmological constant (Λ), or equivalently, a perfect fluid obeying the equation of state, p v = −ρ v , which is usually interpreted as the constant vacuum energy density of all fields existing in the Universe. The Λ-term is the simplest but not the unique possibility. Other candidates appearing in the literature are: a relic scalar field component (SFC) which is slowly rolling down its potencial [2], a decaying vacuum energy density, or a time varying Λ-term [3], the so-called "X-matter", an extra component [4] characterized by an equation of state p x = ωρ x(XCDM), and the Chaplygin type gas whose equation of state is p = −A/ρ α , where A and α are positive constants [5] (see also Lima [6] for a quick review). On the other hand, the existence of old high-redshift objects is one of the best methods for constraining the age of the Universe, as well the basic cosmological parameters [7]. Such objects also provide an important key for determining the first epoch of galaxy formation. In this connection, quasars are among the most luminous objects known in the universe and their prominent emission-lines contains valuable information to estimate their ages. The recently reported age estimates of the APM 08279+5255 quasar with a lower limit of 2-Gyr-old at redshift z = 3.91 is therefore a particularly interesting event [8]. In this article we investigate some cosmological implications from the existence of this quasar to a large class of decaying vacuum cosmologies proposed by Lima and collaborators [9]. Some constraints on the first epoch of quasar formation are also discussed.

The existence of inhomogeneities in the observed Universe modifies the distance-redshift relations thereby affecting the results of cosmological tests in comparison to the ones derived assuming spatially uniform models. By modeling the inhomogeneities through a Zeldovich-Kantowski-DyerRoeder (ZKDR) approach which is phenomenologically characterized by a smoothness parameter α, we rediscuss the constraints on the cosmic parameters based on type Ia Supernovae (SNe Ia) and Gamma-Ray Bursts (GRBs) data. The present analysis is restricted to a flat ΛCDM model with the reasonable assumption that Λ does not clump. A χ 2 -analysis using 557 SNe Ia data from and α ≥ 0.52. As a general result, although considering that current GRB data alone cannot constrain the smoothness α parameter our analysis provides an interesting cosmological probe for dark energy even in the presence of inhomogeneities.PACS numbers: 95.35.+d, 95.36.+x,98.70.Rz,97.60.Bw

The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter α(z) and a power index γ, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the α parameter is totally unconstrained by this sample of angular diameter data. 95.36.+x; 95.35.+d

The presence of inhomogeneities modifies the cosmic distances through the gravitational lensing effect, and, indirectly, must affect the main cosmological tests. Assuming that the dark energy is a smooth component, the simplest way to account for the influence of clustering is to suppose that the average evolution of the expanding Universe is governed by the total matter-energy density whereas the focusing of light is only affected by a fraction of the total matter density quantified by the α DyerRoeder parameter. By using two different samples of SNe type Ia data, the Ωm and α parameters are constrained by applying the Zeldovich-Kantowski-Dyer-Roeder (ZKDR) luminosity distance redshift relation for a flat (ΛCDM) model. A χ 2 -analysis using the 115 SNe Ia data of Astier et al. sample (2006) constrains the density parameter to be Ωm = 0.26 +0.17 −0.07 (2σ) while the α parameter is weakly limited (all the values ∈ [0, 1] are allowed even at 1σ). However, a similar analysis based the 182 SNe Ia data of Riess et al. (2007) constrains the pair of parameters to be Ωm = 0.33 +0.09 −0.07 and α ≥ 0.42 (2σ). Basically, this occurs because the Riess et al. sample extends to appreciably higher redshifts. As a general result, even considering the existence of inhomogeneities as described by the smoothness α parameter, the Einstein-de Sitter model is ruled out by the two samples with a high degree of statistical confidence (11.5σ and 9.9σ, respectively). The inhomogeneous Hubble-Sandage diagram discussed here highlight the necessity of the dark energy, and a transition deceleration/accelerating phase at z ∼ 0.5 is also required.