1,721,012 research outputs found
The Effect of Non-Gaussian Statistics on the Mass Multiplicity of Cosmic Structures
No full textThe mass function of cosmic structures is computed in the framework of the hierarchical clustering picture for a general statistics of density perturbations. 'Hierarchical' distributions are extensively analyzed; it is found that the multiplicity function preserves the Press-Schechter functional form with enhanced power on large scales compared to the Gaussian case. A class of scale-invariant non-Gaussian statistics, among which are a model due to Peebles and the lognormal distribution, are also analyzed. All these predict a mass function which is a decreasing power law at low mass followed by an exponential decay at high mass; none of them, however, yields a mass function of the Press-Schechter type. The effect of a statistical bias on the origin of condensations is also discussed. The comparison of these theoretical formulae with the observed mass multiplicity of galaxies, groups, and clusters may represent a powerful tool to test the statistics of cosmological perturbations
Kinematical Properties of Generalized Inflation
No full textThe kinematical properties of Robertson-Walker models which allow a solution of the cosmological horizon and flatness problems are studied: these are called models of generalized inflation. A useful illustration of such inflation is considered in which the background equation of state assumes the form p/ρ = const. < - 1/3. In this context we also discuss a possible solution of the Ω-problem. The general properties of the spectrum of tensor and scalar perturbations generated during the inflationary phase are analyzed. Finally we discuss some examples of generalized inflation which occur mainly in the frame of some recently proposed models of Kaluza-Klein cosmology
The Quark-Hadron Phase Transition and the Temperature of the MW Background Radiation
No full textIt is shown that the temperature of the MW background radiation might be directly connected with the details of the quark-hadron phase transition. In fact, at temperatures just above 10 GeV the universe has no room to contain the present baryon number in hadron form. This temperature might be smaller, even by a large factor, if a part of the present entropy originated at a redshift of from 10 to 100
The Mass Function of Galaxy Systems from Local Density Maxima: Groups and Clusters of Galaxies
No full textThe mass function of galaxy systems in a cold dark matter cosmogony is considered with the peak constraint that proto-objects form around local density maxima. Different possible definitions of peak mass are analyzed to test their reliability down to the low-mass tail. The effects of non-Gaussian features on the distribution of primordial fluctuations are also discussed. A prediction of the model is the time evolution of the luminosity function and the characteristic mass: a self-similar evolution provides a fairly accurate description even at moderate redshifts. The resulting mass multiplicity, within a biased cold dark matter model with Gaussian, scale-invariant (n = 1), adiabatic primordial perturbations, provides a fit of the existing data on the optical luminosity function for groups and clusters of galaxies
A Path-Integral Approach to Large-Scale Matter Distribution Originated by Non-Gaussian Fluctuations
No full textThe possibility that, in the framework of a biased theory of galaxy clustering, the underlying matter distribution be non-Gaussian itself, because of the very mechanisms generating its present status, is explored. It is shown that a number of contradictory results, seemingly present in large-scale data, in principle can recover full coherence, once the requirement that the underlying matter distribution be Gaussian is dropped. For example, in the present framework, the requirement that the two-point correlation functions vanish at the same scale (for different kinds of objects) is overcome. A general formula, showing the effects of a non-Gaussian background on the expression of three-point correlations in terms of two-point correlations, is given
Velocity Dipoles in Cold Dark Matter Cosmologies
No full textGalaxy velocity dipoles are calculated in the frame of a cold dark matter scenario for galaxy formation, allowing the primordial spectral index n and the density parameter Omega0 to take values smaller than the standard ones n = 1, Omega = 1. Different values for the bias parameter b are also considered. Constraints on n, Omega0, and b are obtained by applying a maximum likelihood test to velocity dipoles deduced from a sample of elliptical galaxies, a sample of spiral galaxies, and the motion of the Local Group. The observed galaxy bulk flows favor a flat, or nearly flat, model with a low value for the spectral index and a modest amount of bias
Cold Dark Matter Dominated, Inflationary Universe with Omega_0<1 and n<1
No full textThe theoretical prejudice for a flat universe with an initially scale-invariant power spectrum has restricted the number of cosmological scenarios investigated for studying the formation of structure in the universe. A cold dark matter-dominated universe with a density parameter Omega(0) and a primordial spectral index n different from unity is considered, and its possible consistency with the inflationary model is discussed. It is shown that some of the difficulties of a flat cold dark matter scenario can be avoided by having Omega(0) less than 1 and n less than 1. For Omega(0) roughly 0.4 and n roughly 0.75 a good agreement is obtained with the large-scale drifts, the bounds on the cosmic microwave background smoothness, the Abell cluster abundance, and their correlation function
The gravitational wave contribution to cosmic microwave background anisotropies and the amplitude of mass fluctuations from COBE results
No full textA stochastic background of primordial gravitational waves may substantially contribute, via the SachsWolfe effect, to the large-scale cosmic microwave background (CMB) anisotropies recently detected by COBE. This implies a bias in any resulting determination of the primordial amplitude of density fluctuations. We consider the constraints imposed on n < 1 ("tilted") power-law fluctuation spectra, taking into account the contribution from both scalar and tensor waves, as predicted by power-law inflation. The gravitational wave contribution to CMB anisotropies generally reduces the required rms level of mass fluctuation, thereby increasing the linear bias parameter, even in models where the spectral index is close to the Harrison-Zel'dovich value n = 1. This "gravitational wave bias" helps to reconcile the predictions of CDM models with observations on pairwise galaxy velocity dispersion on small scales
Primordial Spectrum and Omega_0-Problem in a Power-Law Inflation
No full textThe possibility of defining a valid power-law inflationary model of an open universe with a primordial density fluctuation spectrum flatter than a scale-invariant spectrum is examined. The model is developed by relaxing the Big Bang requirements of exponential growth and no divergence from the perturbation specific binding energy. The accelerated expansion of the power-law model satisfies the horizon conditions of a Friedmann universe. An expression is found for relating the observed density parameter (about 10 percent) to the parameter at the moment of the inflationary phase. A natural large-scale cutoff is found to arise for the perturbations during inflation. The analysis supports a model of the universe as dominated by cold dark matter, and identifies problems inherent in a purely baryonic universe and in interpretation of clustering on the basis of analysis of the cosmic microwave background radiation
Multipole Anisotropies of the Cosmic Background Radiation and Inflationary Models
No full textThe effect of stochastic backgrounds of density and gravitational waves (such as those originated by quantum processes in an inflationary stage of the early universe) on the anisotropy of the cosmic background radiation is investigated at large and intermediate angular scales. In particular, Gaussian perturbation spectra whose variance has a power-law shape at horizon crossing are considered. Simple expressions are obtained for the harmonic amplitudes and correlation function of the anisotropy. The experimental data on the dipole and quadrupole harmonics as well as the anisotropies at 2-6 deg are used to constrain parameters of many inflationary cosmologies, namely standard de Sitter, power-law, polelike, and induced gravity-inflation models
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