1,721,001 research outputs found
Flowing with time: A new approach to non-linear cosmological perturbations
No full textNon-linear effects are crucial for computing the cosmological matter power spectrum to the accuracy required by future generation surveys. Here, a new approach is presented, in which the power spectrum, the bispectrum and higher order correlations are obtained - at any redshift and for any momentum scale - by integrating a system of differential equations. The method is similar to that of the familiar BBGKY (Bogoliubov-Born-Green-Kirkwood-Yvon) hierarchy. Truncating at the level of the trispectrum, the solution of the equations corresponds to the summation of an infinite class of perturbative corrections. Compared to other resummation frameworks, the scheme discussed here is particularly suited to cosmologies other than ΛCDM (CDM: cold dark matter), such as those based on modifications of gravity and those containing massive neutrinos. As a first application, we compute the baryonic acoustic oscillation feature of the power spectrum, and compare the results with perturbation theory, the halo model, and N-body simulations. The density-velocity and velocity-velocity power spectra are also computed, revealing that they are much less contaminated by non-linearities than the density-density one. The approach can be seen as a particular formulation of the renormalization group, in which time is the flow parameter. © 2008 IOP Publishing Ltd
Dark energy condensation
No full textThe two most popular candidates for dark energy, i.e. a cosmological constant and quintessence, are very difficult to distinguish observationally, mostly because the quintessence field does not have sizable fluctuations. We study a scalar field model for dark energy in which the scalar field is invariant under reflection symmetry â. Under general assumptions, there is a phase transition at late times (0.5). Before the phase transition, the field behaves as a cosmological constant. After the phase transition, a time-dependent -condensate forms, the field couples with dark matter and develops sizable perturbations tracking those of dark matter. The background cosmological evolution is in agreement with existing observations, but might be clearly distinguished from that of a cosmological constant by future Supernovae surveys. The growth of cosmological perturbations carries the imprint of the phase transition, however a nonlinear approach has to be developed in order to study it quantitatively. © 2005 The American Physical Society
Brane worlds and the cosmic coincidence problem
No full textBrane world models with âlargeâ extra dimensions with radii in the [Formula Presented] range and smaller ones at [Formula Presented] have the potential to solve the cosmic coincidence problem, i.e., the apparently fortuitous equality between dark matter and dark energy components today. The main ingredient is the assumption of a stabilization mechanism fixing the total volume of the compact submanifold, but allowing for shape deformations. The latter are associated with phenomenologically safe ultralight scalar fields. The bulk field Casimir energy naturally plays the role of dark energy, which decreases in time because of the expanding [Formula Presented] Stable Kaluza-Klein states may play the role of dark matter with an increasing, [Formula Presented] mass. The cosmological equations exhibit attractor solutions in which the global equation of state is negative, the ratio between dark energy and dark matter is constant, and the observed value of the ratio is obtained for two large extra dimensions. Experimental searches for large extra dimensions should take into account that, due to the strong coupling between dark matter and radii dynamics, the size of the large extra dimensions inside the galactic halo may be smaller than the average value. © 2003 The American Physical Society
Non-equilibrium in cosmology
No full textAll the non-trivial features of the Universe we see around us, such as particles, stars, galaxies, and clusters of galaxies, are the result of non-equilibrium processes in the cosmic evolution. These lectures aim to provide some general background in cosmology and to examine specific, and notable, examples of departures from thermal equilibrium. They are organized as follows: 1) Overview of the thermal history of the Universe after the Big Bang: the relevant time-scales and the mechanism of particle decoupling from the themal bath; 2) Explicit examples of cosmic relics: nucleosynthesis, photons and the cosmic microwave background, neutrinos, and cold dark matter; 3) Baryogenesis: the generation of the baryon asymmetry of the Universe; 4) The formation of cosmic structures (galaxies, clusters of galaxies): from the Vlasov equation to the renormalization group. © EDP Sciences and Springer 2009
Structure formation beyond shell-crossing: Nonperturbative expansions and late-time attractors
No full textStructure formation in 1+1 dimensions is considered, with emphasis on the effects of shell-crossing. The breakdown of the perturbative expansion beyond shell-crossing is discussed, and it is shown, in a simple example, that the perturbative series can be extended to a transseries including nonperturbative terms. The latter converges to the exact result well beyond the range of validity of perturbation theory. The crucial role of the divergences induced by shell-crossing is discussed. They provide constraints on the structure of the transseries and act as a bridge between the perturbative and the nonperturbative sectors. Then, we show that the dynamics in the deep multistreaming regime is governed by attractors. In the case of simple initial conditions, these attractors coincide with the asymptotic configurations of the adhesion model, but in general they may differ. These results are applied to a cosmological setting, and an algorithm to build the attractor solution starting from the Zel'dovich approximation is developed. Finally, this algorithm is applied to the search of 'haloes' and the results are compared with those obtained from the exact dynamical equations
Einstein and Jordan frames reconciled: A frame-invariant approach to scalar-tensor cosmology
No full textScalar-tensor theories of gravity can be formulated in different frames, most notably, the Einstein and the Jordan one. While some debate still persists in the literature on the physical status of the different frames, a frame transformation in scalar-tensor theories amounts to a local redefinition of the metric, and then should not affect physical results. We analyze the issue in a cosmological context. In particular, we define all the relevant observables (redshift, distances, cross sections,...) in terms of frame-independent quantities. Then, we give a frame-independent formulation of the Boltzmann equation, and outline its use in relevant examples such as particle freeze-out and the evolution of the cosmic microwave background photon distribution function. Finally, we derive the gravitational equations for the frame-independent quantities at first order in perturbation theory. From a practical point of view, the present approach allows the simultaneous implementation of the good aspects of the two frames in a clear and straightforward way. © 2007 The American Physical Society
Galilean invariance and the consistency relation for the nonlinear squeezed bispectrum of large scale structure
No full textWe discuss the constraints imposed on the nonlinear evolution of the Large Scale Structure (LSS) of the universe by galilean invariance, the symmetry relevant on subhorizon scales. Using Ward identities associated to the invariance, we derive fully nonlinear consistency relations between statistical correlators of the density and velocity perturbations, such as the power spectrum and the bispectrum. These relations are valid up to O(f NL2) corrections. We then show that most of the semi-analytic methods proposed so far to resum the perturbative expansion of the LSS dynamics fail to fulfill the constraints imposed by galilean invariance, and are therefore susceptible to non-physical infrared effects. Finally, we identify and discuss a nonperturbative semi-analytical scheme which is manifestly galilean invariant at any order of its expansion. © 2013 IOP Publishing Ltd and Sissa Medialab srl
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