1,721,283 research outputs found
ISW effect in Unified Dark Matter Scalar Field Cosmologies: An analytical approach.
No full textWe perform an analytical study of the integrated Sachs–Wolfe (ISW) effect within the framework of unified dark matter models based on a scalar field which aim at a unified description of dark energy and dark matter. Computing the temperature power spectrum of the cosmic microwave background anisotropies we are able to isolate those contributions that can potentially lead to strong deviations from the usual ISW effect occurring in a ΛCDM universe. This helps to highlight the crucial role played by the sound speed in the unified dark matter models. Our treatment is completely general in that all the results depend only on the speed of sound of the dark component and thus it can be applied to a variety of unified models, including those which are not described by a scalar field but rely on a single dark fluid
One-loop graviton corrections to the curvature perturbation from inflation
No full textWe compute one-loop corrections to the power spectrum of the curvature perturbation in single-field slow-roll inflation arising from gravitons and inflaton interactions. The quantum corrections due to gravitons to the power spectrum of the inflaton field are computed around the time of horizon crossing and their effect on the curvature perturbation is obtained on superhorizon scales through the δN formalism. We point out that one-loop corrections from the tensor modes are of the same magnitude as those coming from scalar self-interactions; therefore they cannot be neglected in a self-consistent calculation
Scalar tensor gravity and quintessence
No full textScalar fields with inverse power-law effective potentials may provide a negative pressure component to the energy density of the universe today, as required by cosmological observations. In order to be cosmologically relevant today, the scalar field should have a mass mφ=O(10-33eV), thus potentially inducing sizable violations of the equivalence principle and space-time variations of the coupling constants. Scalar-tensor theories of gravity provide a framework for accommodating phenomenologically acceptable ultralight scalar fields. We discuss nonminimally coupled scalar-tensor theories in which the scalar-matter coupling is a dynamical quantity. Two attractor mechanisms are operative at the same time: one towards the tracker solution, which accounts for the accelerated expansion of the Universe, and one towards general relativity, which makes the ultralight scalar field phenomenologically safe today. As in usual tracker-field models, the late-time behavior is largely independent of the initial conditions. Strong distortions in the cosmic microwave background anisotropy spectra as well as in the matter power spectrum are expected
The effect of quantum decoherence on inflationary gravitational waves
Full text linkThe theory of inflation provides a mechanism to explain the structures we observe today in the Universe, starting from quantum-mechanically generated fluctuations. However, this leaves the question of: how did the quantum-to-classical transition, occur? During inflation, tensor perturbations interact (at least gravitationally) with other fields, meaning that we need to view these perturbations as an open system that interacts with an environment. In this paper, the evolution of the system is described using a Lindblad equation, which describes the quantum decoherence of the system. This is a possible mechanism for explaining the quantum-to-classical transition. We show that this quantum decoherence during a de Sitter phase leads to a scale-dependent increase of the gravitational wave power spectrum, depending on the strength and time dependence of the interaction between the system and the environment. By using current upper bounds on the gravitational wave power spectrum from inflation, obtained from CMB and the LIGO-Virgo-KAGRA constraints, we find an upper bound on the interaction strength. Furthermore, we compute the decoherence criterion, which indicates the minimal interaction strength needed for a specific scale to have successfully decohered by the end of inflation. Assuming that the CMB modes have completely decohered, we indicate a lower bound on the interaction strength. In addition, this decoherence criterion allows us to look at which scales might not have fully decohered and could still show some relic quantum signatures. Lastly, we use sensitivity forecasts to study how future gravitational-wave detectors, such as LISA and ET, could constrain the decoherence parameter space. Due to the scale-dependence of the power spectrum, LISA could only have a very small impact. However, ET will be able to significantly improve our current constraints for specific decoherence scenarios
On the non-Gaussianity from Recombination
No full textThe non-linear effects operating at the recombination epoch generate a non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant because it represents a major part of the second-order radiation transfer function which must be determined in order to have a complete control of both the primordial and non-primordial part of non-Gaussianity in the CMB anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB arising from the recombination epoch which shows up mainly in the equilateral configuration. We find that it causes a contamination to the possible measurement of the equilateral primordial bispectrum shifting the minimum detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL= O(10) for an experiment like Planck
Gauge-Invariant Temperature Anisotropies and Primordial Non-Gaussianity
Full text linkWe provide the gauge-invariant expression for large-scale cosmic microwave background temperature fluctuations at second-order perturbation theory. This enables us to define unambiguously the nonlinearity parameter fNL, which is used by experimental collaborations to pin down the level of non-Gaussianity in the temperature fluctuations. Furthermore, it contains a primordial term encoding all the information about the non-Gaussianity generated at primordial epochs and about the mechanism which gave rise to cosmological perturbations, thus neatly disentangling the primordial contribution to non-Gaussianity from the one caused by the postinflationary evolution
Antisymmetric galaxy cross-correlations in and beyond ΛCDM
No full textMany different techniques to analyze galaxy clustering data and obtain cosmological constraints have been proposed, tested, and used. Given the large amount of data that will be available soon, it is worth investigating new observables and ways to extract information from such datasets. In this paper, we focus on antisymmetric correlations that arise in the cross-correlation of different galaxy populations when the small-scale power spectrum is modulated by a long-wavelength field. In Lambda cold dark matter this happens because of nonlinear clustering of sources that trace the underlying matter distribution in different ways. Beyond the standard model, this observable is sourced naturally in various new physics scenarios. We derive, for the first time, its complete expression up to second order in redshift space and show that this improves detectability compared to previous evaluations at first order in real space. Moreover, we explore a few potential applications to use this observable to detect models with vector modes, or where different types of sources respond in different ways to the underlying modulating long mode, and anisotropic models with privileged directions in the sky. This shows how antisymmetric correlations can be a useful tool for testing exotic cosmological models
Adiabatic and Isocurvature Perturbations from Inflation: Power Spectra and Consistency Relation
No full textWe study adiabatic and isocurvature perturbations produced during a period of cosmological inflation. We compute the power spectra and cross spectra of the curvature and isocurvature modes, as well as the tensor perturbation spectrum in terms of the slow-roll parameters. We provide two consistency relations for the amplitudes and spectral indices of the corresponding power spectra. These relations represent a definite prediction and a test of inflationary models which should be adopted when studying cosmological perturbations through the cosmic microwave background in forthcoming satellite experiments
Towards a viable effective field theory of mimetic gravity
No full textWe discuss mimetic gravity theories with direct couplings between the curvature and higher derivatives of the scalar field, up to the quintic order, which were proposed to solve the instability problem for linear perturbations around the FLRW background for this kind of models. Restricting to homogeneous scalar field configurations in the action, we derive degeneracy conditions to obtain an effective field theory with three degrees of freedom. However, performing the Hamiltonian analysis for a generic scalar field we show that there are in general four or more degrees of freedom. The discrepancy is resolved because, for a homogeneous scalar field profile, ∂iφ ≈ 0, the Dirac matrix becomes singular, resulting in further constraints, which reduces the number of degrees of freedom to three. Similarly, in linear perturbation theory the additional scalar degree of freedom can only be seen by considering a non-homogeneous background profile of the scalar field. Therefore, restricting to homogeneous scalar fields these kinds of models provide viable explicitly Lorentz violating effective field theories of mimetic gravity
Spherical collapse in covariant Galileon theory
No full textIn this paper we study the evolution of a spherical matter overdensity in the context of the recently introduced Galileon field theory. Our analysis considers the complete covariant Lagrangian in four dimensions. This theory is composed by a potential and a standard kinetic term, a cubic kinetic term and two additional terms that include the coupling between the Galileon and the metric, to preserve the original properties of Galileons also in curved space-times. Here we extend previous studies, which considered both the quintessence and the cubic terms, by focussing on the role of the last two terms. The background evolution we consider is driven by a tracker solution. Studying scalar perturbations in the non-linear regime, we find constraints on the parameter of the model. We will show how the new terms contribute to the collapse phase and how they modify physical parameters, such as the linearized density contrast and the virial overdensity. The results show that the Galileon modifies substantially the dynamics of the collapse, thus making it possible to observationally constrain the parameters of this theory
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