1,292 research outputs found

    Super Heavy Dark Matter in light of BICEP2, Planck and Ultra High Energy Cosmic Rays Observations

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    The announcement by BICEP2 of the detection of B-mode polarization consistent with primordial gravitational waves with a tensor-to-scalar ratio, r=0.2−0.05+0.07r=0.2^{+0.07}_{-0.05}, challenged predictions from most inflationary models of a lower value for rr. More recent results by Planck on polarized dust emission show that the observed tensor modes signal is compatible with pure foreground emission. A more significant constraint on rr was then obtained by a joint analysis of Planck, BICEP2 and Keck Array data showing an upper limit to the tensor to scalar ratio r≀0.12r\le 0.12, excluding the case r=0r=0 with low statistical significance. Forthcoming measurements by BICEP3, the Keck Array, and other CMB polarization experiments, open the possibility for making the fundamental measurement of rr. Here we discuss how rr sets the scale for models where the dark matter is created at the inflationary epoch, the generically called super-heavy dark matter models. We also consider the constraints on such scenarios given by recent data from ultrahigh energy cosmic ray observatories which set the limit on super-heavy dark matter particles lifetime. We discuss how super-heavy dark matter can be discovered by a precise measurement of rr combined with future observations of ultra high energy cosmic rays.Comment: 17 pages, 14 eps figures, accepted for publication in JCA

    Tests for primordial non-Gaussianity

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    We investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure (LSS), high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness.Comment: 6 pages, 2 figures, MNRAS in press (minor changes to match the accepted version

    Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X

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    We propose a possible way to detect baryons at low redshifts from the analysis of X-ray absorption spectra of bright AGN pairs. A simple semi-analytical model to simulate the spectra is presented. We model the diffuse warm-hot intergalactic medium (WHIM) component, responsible for the X-ray absorption, using inputs from high-resolution hydro-dynamical simulations and analytical prescriptions. We show that the number of OVII absorbers per unit redshift with column density larger than 1013.510^{13.5} cm−2^{-2} - corresponding to an equivalent width of ∌\sim 1 km/s - which will be possibly detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it Constellation-X} will detect ∌6\sim 6 OVII absorptions per unit redshift with an equivalent width of 10 km/s. Our results show that, in a Λ\LambdaCDM Universe, the characteristic size of these absorbers at z∌0.1z\sim 0.1 is ∌1\sim 1 h−1h^{-1} Mpc. The filamentary structure of WHIM can be probed by finding coincident absorption lines in the spectra of background AGN pairs. We estimate that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to detect this filamentary structure at a 3σ\sigma level. Assuming observations of distant sources using {\it XEUS} for exposure times of 500 ksec, we find that the minimum source flux to probe the filamentary structure is ∌2×10−12\sim 2\times 10^{-12} erg cm−2^{-2} s−1^{-1}, in the 0.1-2.4 keV energy band. Thus, most pairs of these extragalactic X-ray bright sources have already been identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects by future missions could be a powerful way to search for baryons in the low redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded. More optimistic results for Constellation-X. Accepted by MNRA

    Scale Dependence of Halo Bispectrum from Non-Gaussian Initial Conditions in Cosmological N-body Simulations

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    We study the halo bispectrum from non-Gaussian initial conditions. Based on a set of large NN-body simulations starting from initial density fields with local type non-Gaussianity, we find that the halo bispectrum exhibits a strong dependence on the shape and scale of Fourier space triangles near squeezed configurations at large scales. The amplitude of the halo bispectrum roughly scales as fnl2f_nl^2. The resultant scaling on the triangular shape is consistent with that predicted by Jeong & Komatsu based on perturbation theory. We systematically investigate this dependence with varying redshifts and halo mass thresholds. It is shown that the fnlf_nl dependence of the halo bispectrum is stronger for more massive haloes at higher redshifts. This feature can be a useful discriminator of inflation scenarios in future deep and wide galaxy redshift surveys.Comment: 27 pages, 10 figures; revised argument in section 6, added appendix C, JCAP accepted versio

    General Relativistic Dynamics of Irrotational Dust: Cosmological Implications

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    The non--linear dynamics of cosmological perturbations of an irrotational collisionless fluid is analyzed within General Relativity. Relativistic and Newtonian solutions are compared, stressing the different role of boundary conditions in the two theories. Cosmological implications of relativistic effects, already present at second order in perturbation theory, are studied and the dynamical role of the magnetic part of the Weyl tensor is elucidated.Comment: 12 pages , DFPD 93/A/6

    Large-k Limit of Multi-Point Propagators in the RG Formalism

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    Renormalized versions of cosmological perturbation theory have been very successful in recent years in describing the evolution of structure formation in the weakly non-linear regime. The concept of multi-point propagators has been introduced as a tool to quantify the relation between the initial matter distribution and the final one and to push the validity of the approaches to smaller scales. We generalize the n-point propagators that have been considered until now to include a new class of multi-point propagators that are relevant in the framework of the renormalization group formalism. The large-k results obtained for this general class of multi-point propagators match the results obtained earlier both in the case of Gaussian and non-Gaussian initial conditions. We discuss how the large-k results can be used to improve on the accuracy of the calculations of the power spectrum and bispectrum in the presence of initial non-Gaussianities.Comment: 30 page

    The Power Spectrum, Bias Evolution, and the Spatial Three-Point Correlation Function

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    We calculate perturbatively the normalized spatial skewness, S3S_3, and full three-point correlation function (3PCF), ζ\zeta, induced by gravitational instability of Gaussian primordial fluctuations for a biased tracer-mass distribution in flat and open cold-dark-matter (CDM) models. We take into account the dependence on the shape and evolution of the CDM power spectrum, and allow the bias to be nonlinear and/or evolving in time, using an extension of Fry's (1996) bias-evolution model. We derive a scale-dependent, leading-order correction to the standard perturbative expression for S3S_3 in the case of nonlinear biasing, as defined for the unsmoothed galaxy and dark-matter fields, and find that this correction becomes large when probing positive effective power-spectrum indices. This term implies that the inferred nonlinear-bias parameter, as usually defined in terms of the smoothed density fields, might depend on the chosen smoothing scale. In general, we find that the dependence of S3S_3 on the biasing scheme can substantially outweigh that on the adopted cosmology. We demonstrate that the normalized 3PCF, QQ, is an ill-behaved quantity, and instead investigate QVQ_V, the variance-normalized 3PCF. The configuration dependence of QVQ_V shows similarly strong sensitivities to the bias scheme as S3S_3, but also exhibits significant dependence on the form of the CDM power spectrum. Though the degeneracy of S3S_3 with respect to the cosmological parameters and constant linear- and nonlinear-bias parameters can be broken by the full configuration dependence of QVQ_V, neither statistic can distinguish well between evolving and non-evolving bias scenarios. We show that this can be resolved, in principle, by considering the redshift dependence of ζ\zeta.Comment: 41 pages, including 12 Figures. To appear in The Astrophysical Journal, Vol. 521, #

    Perturbations of spacetime: gauge transformations and gauge invariance at second order and beyond

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    We consider in detail the problem of gauge dependence that exists in relativistic perturbation theory, going beyond the linear approximation and treating second and higher order perturbations. We first derive some mathematical results concerning the Taylor expansion of tensor fields under the action of one-parameter families (not necessarily groups) of diffeomorphisms. Second, we define gauge invariance to an arbitrary order nn. Finally, we give a generating formula for the gauge transformation to an arbitrary order and explicit rules to second and third order. This formalism can be used in any field of applied general relativity, such as cosmological and black hole perturbations, as well as in other spacetime theories. As a specific example, we consider here second order perturbations in cosmology, assuming a flat Robertson-Walker background, giving explicit second order transformations between the synchronous and the Poisson (generalized longitudinal) gauges.Comment: slightly revised version, accepted for publication in Classical and Quantum Gravity. 27 pages including 4 figures, latex using 2 CQG style files: ioplppt.sty, iopl10.st

    Signatures of Primordial non-Gaussianities in the Matter Power-Spectrum and Bispectrum: the Time-RG Approach

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    We apply the time-renormalization group approach to study the effect of primordial non-Gaussianities in the non-linear evolution of cosmological dark matter density perturbations. This method improves the standard perturbation approach by solving renormalization group-like equations governing the dynamics of gravitational instability. The primordial bispectra constructed from the dark matter density contrast and the velocity fields represent initial conditions for the renormalization group flow. We consider local, equilateral and folded shapes for the initial non-Gaussianity and analyze as well the case in which the non-linear parameter f_{NL} parametrizing the strength of the non-Gaussianity depends on the momenta in Fourier space through a power-law relation, the so-called running non-Gaussianity. For the local model of non-Gaussianity we compare our findings for the power-spectrum with those of recent N-body simulations and find that they accurately fit the N-body data up to wave-numbers k \sim 0.25 h/Mpc at z=0. We also present predictions for the (reduced) matter bispectra for the various shapes of non-Gaussianity.Comment: 27 pages, 12 figures. Results and discussion for a particular case added. One figure and one reference added. Matches with the version accepted for publication in the JCAP

    Duality invariance and cosmological dynamics

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    A duality transformation that interrelates expanding and contracting cosmological models is shown to single out a duality invariant, interacting two-component description of any irrotational, geodesic and shearfree cosmic medium with vanishing three curvature scalar. We apply this feature to a system of matter and radiation, to a mixture of dark matter and dark energy, to minimal and conformal scalar fields, and to an enlarged Chaplygin gas model of the cosmic substratum. We extend the concept of duality transformations to cosmological perturbations and demonstrate the invariance of adiabatic pressure perturbations under these transformations.Comment: 14 pages, Discussion enlarged, accepted for publication in IJMP
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