Weak Gravitational Lensing by Voids

We consider the prospects for detecting weak gravitational lensing by underdensities (voids) in the large-scale matter distribution. We derive the basic expressions for magnification and distortion by spherical voids. Clustering of the background sources and cosmic variance are the main factors which limit in principle the detection of lensing by voids. We conclude that only voids with radii larger than $\sim 100$ \hm have lensing signal to noise larger than unity.Comment: 12 pages, 7 figures, uses mn-1_4.sty file, submitted to MNRA

Viable Singularity-Free f(R) Gravity Without a Cosmological Constant

Several authors have argued that self-consistent $f(R)$ gravity models distinct from $\Lambda$CDM are almost ruled out. Confronting such claims, we present a particular two-parameter $f(R)$ model that: (a) is cosmologically viable and distinguishable from $\Lambda$CDM; (b) is compatible with the existence of relativistic stars; (c) is free of singularities of the Ricci scalar during the cosmological evolution and (d) allows the addition of high curvature corrections that could be relevant for inflation.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let

When did cosmic acceleration start? How fast was the transition?

Cosmic acceleration is investigated through a kink-like expression for the deceleration parameter (q). The new parametrization depends on the initial (q_i) and final (q_f) values of q, on the redshift of the transition from deceleration to acceleration (z_{t}) and the width of such transition (\tau). We show that although supernovae (SN) observations (Gold182 and SNLS data samples) indicate, at high confidence, that a transition occurred in the past (z_{t}>0) they do not, by themselves, impose strong constraints on the maximum value of z_{t}. However, when we combine SN with the measurements of the ratio between the comoving distance to the last scattering surface and the SDSS+2dfGRS BAO distance scale (S_{k}/D_{v}) we obtain, at 95.4% confidence level, z_{t}=0.84+{0.17}-{0.13} and \tau =0.51-{0.17}+{0.23} for (S_{k}/D_{v}+Gold182), and z_{t}=0.88-{0.10}+{0.12} and \tau =0.35-{0.10}+{0.12} for (S_{k}/D_{v} + SNLS), assuming q_i=0.5 and q_f=-1. We also analyze the general case, q_f\in(-\infty,0) finding the constraints that the combined tests (S_{k}/D_{v} + SNLS) impose on the present value of the deceleration parameter (q_0).Comment: 7 pages, 3 figures. Replaced to match the published versio

Dark Interactions and Cosmological Fine-Tuning

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the $\Lambda$CDM model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light as to whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.Comment: 13 pages, 9 figures, accepted for publication in JCAP. Minor corrections, one figure replaced, references adde

Cosmological constraints from lensing statistics and supernovae on the cosmic equation of state

We investigate observational constraints from lensing statistics and high-z type Ia supernovae on flat cosmological models with nonrelativistic matter and an exotic fluid with equation of state, $p_x=(m/3 -1)\rho_x$. We show that agreement with both tests at the 68% confidence level is possible if the parameter $m$ is low ($m \lesssim 0.85$) and $0.24 \lesssim \Omega_{m0} \lesssim 0.38$ with lower values of $\Omega_{m0}$ corresponding to higher $m$. We find that a conventional cosmological constant model with $\Omega_{m0}\simeq 0.33$ is the best fit model of the combined likelihood.Comment: 7 pages, 4 postscript figures, revtex, submitted to Phys. Rev.

Observational Constraints on Chaplygin Quartessence: Background Results

We derive the constraints set by several experiments on the quartessence Chaplygin model (QCM). In this scenario, a single fluid component drives the Universe from a nonrelativistic matter-dominated phase to an accelerated expansion phase behaving, first, like dark matter and in a more recent epoch like dark energy. We consider current data from SNIa experiments, statistics of gravitational lensing, FR IIb radio galaxies, and x-ray gas mass fraction in galaxy clusters. We investigate the constraints from this data set on flat Chaplygin quartessence cosmologies. The observables considered here are dependent essentially on the background geometry, and not on the specific form of the QCM fluctuations. We obtain the confidence region on the two parameters of the model from a combined analysis of all the above tests. We find that the best-fit occurs close to the $\Lambda$CDM limit ($\alpha=0$). The standard Chaplygin quartessence ($\alpha=1$) is also allowed by the data, but only at the $\sim2\sigma$ level.Comment: Replaced to match the published version, references update