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Register a new userThe effects of primordial non-Gaussianity on giant-arc statistics
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For over a decade, it has been debated whether the concordance LCDM model is consistent with the observed abundance of giant arcs in clusters. While previous theoretical studies have focused on properties of the lens and source populations, as well as cosmological effects such as dark energy, the impact of initial conditions on the giant-arc abundance is relatively unexplored. Here, we quantify the impact of non-Gaussian initial conditions with the local bispectrum shape on the predicted frequency of giant arcs. Using a path-integral formulation of the excursion set formalism, we extend a semi-analytic model for calculating halo concentrations to the case of primordial non-Gaussianity, which may be useful for applications outside of this work. We find that massive halos tend to collapse earlier in models with positive f_NL, relative to the Gaussian case, leading to enhanced concentration parameters. The converse is true for f_NL < 0. In addition to these effects, which change the lensing cross sections, non-Gaussianity also modifies the abundance of supercritical clusters available for lensing. These combined effects work together to either enhance (f_NL > 0) or suppress (f_NL < 0) the probability of giant-arc formation. Using the best value and 95% confidence levels currently available from the Wilkinson Microwave Anisotropy Probe, we find that the giant-arc optical depth for sources at z_s~2 is enhanced by ~20% and ~45% for f_NL = 32 and 74 respectively. In contrast, we calculate a suppression of ~5% for f_NL = -10. These differences translate to similar relative changes in the predicted all-sky number of giant arcs.
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In a recently published article, we quantified the impact of primordial non-Gaussianity on the probability of giant-arc formation. In that work, we focused on the local form of non-Gaussianity and found that it can have only a modest effect given the most recent constraints from Cosmic Microwave Background (CMB) measurements. Here, we present new calculations using a parameterization of scale-dependent non-Gaussianity in which the primordial bispectrum has the equilateral shape and the effective f_NL parameter depends on scale. We find that non-Gaussianity of this type can yield a larger effect on the giant-arc abundance compared to the local form due to both the scale dependence and the relatively weaker constraints on the equilateral shape from CMB measurements. In contrast to the maximum ~40% effect (within the latest CMB constraints) previously found for the local form, we find that the predicted giant-arc abundance for the scale-dependent equilateral form can differ by a factor of a few with respect to the Gaussian case.
Here we review the present status of modelling of and searching for primordial non-Gaussianity of cosmological perturbations. After introducing the models for non-Gaussianity generation during inflation, we discuss the search for non-Gaussian signatures in the Cosmic Microwave Background and in the Large-Scale Structure of the Universe.
Our current understanding of the Universe is established through the pristine measurements of structure in the cosmic microwave background (CMB) and the distribution and shapes of galaxies tracing the large scale structure (LSS) of the Universe. One key ingredient that underlies cosmological observables is that the field that sources the observed structure is assumed to be initially Gaussian with high precision. Nevertheless, a minimal deviation from Gaussianityis perhaps the most robust theoretical prediction of models that explain the observed Universe; itis necessarily present even in the simplest scenarios. In addition, most inflationary models produce far higher levels of non-Gaussianity. Since non-Gaussianity directly probes the dynamics in the early Universe, a detection would present a monumental discovery in cosmology, providing clues about physics at energy scales as high as the GUT scale.
We present the third-order analytic solution of the matter density fluctuation in the proper-time hypersurface of nonrelativistic matter flows by solving the nonlinear general relativistic equations. The proper-time hypersurface provides a coordinate system that a local observer can set up without knowledge beyond its neighborhood, along with physical connections to the local Newtonian descriptions in the relativistic context. The initial condition of our analytic solution is set up by the curvature perturbation in the comoving gauge, clarifying its impact on the nonlinear evolution. We compute the effective non-Gaussian parameters due to the nonlinearity in the relativistic equations. With proper coordinate rescaling, we show that the equivalence principle is respected and the relativistic effect vanishes in the large-scale limit.
We investigate the effects of non-Gaussianity in the primordial density field on the reionization history. We rely on a semi-analytic method to describe the processes acting on the intergalactic medium (IGM), relating the distribution of the ionizing sources to that of dark matter haloes. Extending previous work in the literature, we consider models in which the primordial non-Gaussianity is measured by the dimensionless non-linearity parameter f_NL, using the constraints recently obtained from cosmic microwave background data. We predict the ionized fraction and the optical depth at different cosmological epochs assuming two different kinds of non-Gaussianity, characterized by a scale-independent and a scale-dependent f_NL and comparing the results to those for the standard Gaussian scenario. We find that a positive f_NL enhances the formation of high-mass haloes at early epochs, when reionization begins, and, as a consequence, the IGM ionized fraction can grow by a factor up to 5 with respect to the corresponding Gaussian model. The increase of the filling factor has a small impact on the reionization optical depth and is of order ~ 10 per cent if a scale-dependent non-Gaussianity is assumed. Our predictions for non-Gaussian models are in agreement with the latest WMAP results within the error bars, but a higher precision is required to constrain the scale dependence of non-Gaussianity.
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