1,902 research outputs found
Testing the Frozen Flow Approximation
No full textWe investigate the accuracy of the frozen-flow approximation (FFA), recently proposed by Matarrese et al., for tracing of the non-linear evolution of cosmological density fluctuations under gravitational instability. We compare a number of statistics between results of the FFA and N-body simulations, including those used by Melon, Pellman & Shandarin to test the Zel'dovich approximation. The FFA performs reasonably well in a statistical sense (for example, in reproducing the counts-in-cells distribution) at small scales, but it does poorly in the cross-correlation with N-body simulations, which means that it is generally not moving mass to the right place, especially in models with high small-scale power
Evolution of massive haloes in non-Gaussian scenarios
No full textWe have performed high-resolution cosmological N-body simulations of a concordance ΛCDM model to study the evolution of virialized, dark matter haloes in the presence of primordial non-Gaussianity. Following a standard procedure, departures from Gaussianity are modelled through a quadratic Gaussian term in the primordial gravitational potential, characterized by a dimensionless non-linearity strength parameter fNL. We find that the halo mass function and its redshift evolution closely follow the analytic predictions of Matarrese, Verde & Jimenez. The existence of precise analytic predictions makes the observation of rare, massive objects at large redshift an even more attractive test to detect primordial non-Gaussian features in the large-scale structure of the Universe
Cosmic Microwave Background Anisotropies up to Second Order
No full textThese lecture notes present the computation of the full system of Boltzmann equations describing the evolution of the photon, baryon and cold dark matter fluids up to second order in perturbation theory, as recently studied in (Bartolo, Matarrese & Riotto 2006, 2007). These equations allow to follow the time evolution of the cosmic microwave background anisotropies at all angular scales from the early epoch, when the cosmological perturbations were generated, to the present, through the recombination era. The inclusion of second-order contributions is mandatory when one is interested in studying possible deviations from Gaussianity of cosmological perturbations, either of primordial (e.g. inflationary) origin or due to their subsequent evolution. Most of the emphasis in these lectures notes will be given to the derivation of the relevant equations for the study of cosmic microwave background anisotropies and to their analytical solutions.These lecture notes present the computation of the full system of Boltzmann equations describing the evolution of the photon, baryon and cold dark matter fluids up to second order in perturbation theory, as recently studied in (Bartolo, Matarrese & Riotto 2006, 2007). These equations allow to follow the time evolution of the cosmic microwave background anisotropies at all angular scales from the early epoch, when the cosmological perturbations were generated, to the present, through the recombination era. The inclusion of second-order contributions is mandatory when one is interested in studying possible deviations from Gaussianity of cosmological perturbations, either of primordial (e.g. inflationary) origin or due to their subsequent evolution. Most of the emphasis in these lectures notes will be given to the derivation of the relevant equations for the study of cosmic microwave background anisotropies and to their analytical solutions
Large-Scale Bias in the Universe: II Redshift Space Bispectrum
No full textThe determination of the density parameter Omega_0 from the large-scale distribution of galaxies is one of the major goals of modern cosmology. However, if galaxies are biased tracers of the underlying mass distribution, linear perturbation theory leads to a degeneracy between Omega_0 and the linear bias parameter b, and the density parameter cannot be estimated. In Matarrese, Verde & Heavens we developed a method based on second-order perturbation theory to use the bispectrum to lift this degeneracy by measuring the bias parameter in an Omega_0-independent way. The formalism was developed assuming that one has perfect information on the positions of galaxies in three dimensions. In galaxy redshift surveys, the three-dimensional information is imperfect, because of the contaminating effects of peculiar velocities, and the resulting clustering pattern in redshift space is distorted. In this paper we combine second-order perturbation theory with a model for collapsed, virialized structures, to extend the method to redshift space, and demonstrate that the method should be successful in determining with reasonable accuracy the bias parameter from state-of-the-art surveys such as the Anglo-Australian 2 degree Field Survey and the Sloan Digital Sky Survey
The Effect of Inhomogeneities on the Luminosity Distance-Redshift Relation: is Dark Energy Necessary in a Perturbed Universe?
No full textThe luminosity distance-redshift relation is one of the fundamental tools of modern cosmology. We compute the luminosity distance-redshift relation in a perturbed flat matter-dominated Universe, taking into account the presence of cosmological inhomogeneities up to second order in perturbation theory. Cosmological observations implementing the luminosity distance-redshift relation tell us that the Universe is presently undergoing a phase of accelerated expansion. This seems to call for a mysterious Dark Energy component with negative pressure. Our findings suggest that the need of a Dark Energy fluid may be challenged once a realistic inhomogeneous Universe is considered and that an accelerated expansion may be consistent with a matter-dominated Universe
Vector Particle Creation and Annihilation in a Friedmann Expansion
No full textA vector field is considered in a Friedmann metric. At the level of classical field theory, one can show that the field comprises a spin-one part and a spin-zero part, whose mass has to be zero, and which is not the usual spin-zero part of vector fields in flat space-time. At the quantum level, because of the Friedmann expansion, the particle number varies. This is fully analogous to a well-known phenomenon already thoroughly studied for scalar and spinor fields. However, because of the presence of the new spin-zero part, in the initial stages of the universal expansion the number of particles must decrease, necessarily implying the existence of a large number of vector quanta since the Planck time. Later on, the expansion will cause particle creation in the same way as in the scalar and spinor theories
Large-scale Curvature Perturbations with Spatial and Time Variations of the Inflaton Decay Rate
No full textWe present a gauge-invariant formalism to study the evolution of the curvature and entropy perturbations in the case in which spatial and time variations of the inflaton decay rate into ordinary matter are present. During the reheating stage after inflation, curvature perturbations can vary with time on super-horizon scales sourced by a gauge-invariant inflaton decay rate perturbation. We show that the latter is a function not only of the spatial variations of the decay rate generated during inflation, as envisaged in a recently proposed scenario, but also of the time variation of the inflaton decay rate during reheating. If only the second source is present, the final curvature perturbation at the end of the reheating stage is proportional to the curvature perturbation at the beginning of reheating, with a coefficient of proportionality which can be either smaller or larger than unity depending upon the underlying physics governing the time variation of the inflaton decay rate. As a consequence, we show that the standard consistency relation between the amplitude of curvature perturbations, the amplitude of tensor perturbations and the tensor spectral index of one-single-field models of inflation is violated and there is the possibility that the tensor-to-curvature amplitude ratio is larger than in the standard case
On the Classical and Quantum Irrotational Motions of a Relativistic Perfect Fluid: I. Classical Theory
No full textSome aspects of perfect fluid general-relativistic hydrodynamics under the assumption of irrotationality and isentropicity are analyzed. A new derivation of the known fact that the Lagrangian for these fluids is just the pressure is given. Then we study the fluctuations around a given background configuration, extracting a rule that connects the order at which a Taylor expansion of the action functional possibly stops with the fluid equation of state. From a classical invariance of the action we deduce the conserved Noether current. Because of the spontaneous breaking of such an invariance of the vacuum state Goldstone bosons arise, which turn out to be just phonons (quantized sound waves). Some useful results concerning the linear theory of sound waves are also given
Non-Gaussianity and the Cosmic Microwave Background Anisotropies
No full textWe review in a pedagogical way the present status of the impact of non-Gaussianity (NG) on the cosmic
microwave background (CMB) anisotropies. We first show how to set the initial conditions at second order for the CMB anisotropies when some primordial NG is present. However, there are many sources of NG in CMB anisotropies, beyond the primordial one, which can contaminate the primordial signal. We mainly focus on the NG generated from the post inflationary evolution of the CMB anisotropies at second order in perturbation theory at large and small angular scales, such as the ones generated at the recombination epoch. We show how to derive the equations to study the second-order CMB anisotropies and provide analytical computations to evaluate their contamination to primordial NG (complemented with numerical examples). We also offer a brief summary of other secondary effects. This paper requires basic knowledge of the theory of cosmological perturbations at the linear level
Computational cosmology: A general relativistic approach
No full textNumerical simulations of the large-scale formation of the Universe, which is largely governed by gravity,
are traditionally based on Newton’s lawof gravitation. But general relativistic effects should be included to achieve a realistic cosmological framework that can be compared with the increasingly high-quality data from large cosmological surveys ). In Nature Physics, Julian Adamek et al. discuss the results of computer simulations of structure formation in the Universe. Their numerical code aims to solve Einstein’s field equations relative to the dynamics of cold collisionless matter, with a minimum set of physically motivated simplifying assumptions. Here we comment on this paper and discuss how to progress further in this direction
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