1,721,056 research outputs found
Observed galaxy number counts on the light cone up to second order: III. Magnification bias
No full textWe study up to second order the galaxy number over-density that depends on magnification in redshift space on cosmological scales for a concordance model. The result contains all general relativistic effects up to second order which arise from observations of the past light cone, including all redshift and lensing distortions, contributions from velocities, Sachs–Wolfe, integrated SW, and time-delay terms. We find several new terms and contributions that could be potentially important for an accurate calculation of the bias on estimates of non-Gaussianity and on precision parameter estimates
Unified Dark Matter in Scalar Field Cosmologies
No full textIn this thesis I have investigated the possibility that the
dynamics of a single scalar field can account for a unified description of the Dark Matter and Dark Energy sectors: Unified Dark Matter (UDM).
In particular considering the general Lagrangian of k-essence models, I study and classify them through variables
connected to the fluid equation of state parameter w_kappa. This allows to find solutions around which the scalar field describes a mixture of dark matter and cosmological constant-like dark energy (UDM) (Bertacca, Matarrese, Pietroni 2007).
Subsequently I also 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 I am able to isolate
those contributions that can potentially lead to strong deviations from the usual ISW effect occurring in a Lambda CDM Universe.
This helps to highlight the crucial role played by the sound speed in the unified dark matter models. This 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 relies on a single dark fluid (Bertacca and Bartolo 2007).
Finally I also investigated the static and spherically symmetric
solutions of Einstein's equations for a scalar field with non-canonical kinetic term (Bertacca, Bartolo, Matarrese 2007)
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
Halos of Unified Dark Matter Scalar Field
No full textWe investigate the static and spherically symmetric solutions of Einstein's equations for a scalar field with a non-canonical kinetic term, assumed to provide both the dark matter and dark energy components of the Universe. In particular, we give a prescription to obtain solutions (dark halos) whose rotation curve vc(r) is in good agreement with observational data. We show that there exist suitable scalar field Lagrangians that allow us to describe the cosmological background evolution and the static solutions with a single dark fluid
Degeneracy between primordial non-Gaussianity and interaction in the dark sector
Full text linkIf dark energy and dark matter interact via exchange of energy and momentum, then this may affect the galaxy power spectrum on large scales. When this happens, it may be degenerate with the signal from primordial non-Gaussianity via a scale-dependent bias. We consider a class of interacting dark energy models and show that the matter overdensity is scale dependent on large scales. We estimate the effective non-Gaussianity arising from the large-scale effects of interaction in the dark sector. The signal of dark sector interaction can be disentangled from a primordial non-Gaussian signal by measuring the power at two redshifts
Observed galaxy number counts on the lightcone up to second order:I. main result
Full text linkWe present the galaxy number overdensity up to second order in redshift space on cosmological scales for a concordance model. The result contains all general relativistic effects up to second order that arise from observing on the past light cone, including all redshift effects, lensing distortions from convergence and shear, and contributions from velocities, Sachs-Wolfe, integrated SW and time-delay terms. This result will be important for accurate calculation of the bias on estimates of non-Gaussianity and on precision parameter estimates, introduced by nonlinear projection effects
A new approach to cosmological perturbations in f(R) models
No full textWe propose an analytic procedure that allows to determine quantitatively the deviation in the behavior of cosmological perturbations between a given f(R) modified gravity model and a ΛCDM reference model. Our method allows to study structure formation in these models from the largest scales, of the order of the Hubble horizon, down to scales deeply inside the Hubble radius, without employing the so-called "quasi-static" approximation. Although we restrict our analysis here to linear perturbations, our technique is completely general and can be extended to any perturbative order
Measuring our peculiar velocity from spectroscopic redshift surveys
Full text linkOur peculiar velocity imprints a dipole on galaxy density maps derived from redshift surveys. The dipole gives rise to an oscillatory signal in the multipole moments of the observed power spectrum which we indicate as the finger-of-the-observer (FOTO) effect. Using a suite of large mock catalogues mimicking ongoing and future Hα- and Hi-selected surveys, we demonstrate that the oscillatory features can be measured with a signal-to-noise ratio of up to 7 (depending on the sky area coverage and provided that observational systematics are kept under control on large scales). We also show that the FOTO effect cannot be erased by correcting the individual galaxy redshifts. On the contrary, by leveraging the power of the redshift corrections, we propose a novel method to determine both the magnitude and the direction of our peculiar velocity. After applying this technique to our mock catalogues, we conclude that it can be used to either test the kinematic interpretation of the temperature dipole in the cosmic microwave background or to extract cosmological information such as the matter density parameter and the equation of state of dark energy
The full-sky Spherical Fourier-Bessel power spectrum in general relativity
Full text linkWe present a formalism for analyzing galaxy clustering on the lightcone with
the 2-point correlation in the Spherical Fourier-Bessel (SFB) formalism, which
is a natural choice to account for all wide-angle and relativistic (GR)
effects. We extend previous studies by including all projection and GR effects,
developing an efficient numerical implementation that avoids the use of the
Limber approximation, includes multi-bins correlations and a full non-diagonal
covariance.
Using this formalism, we investigate the impact of neglecting GR corrections,
and in particular how much this could bias measurements of the non-Gaussianity
parameter .
Our results show that not including relativistic projection terms can
systematically and non-negligibly bias estimates of . The exact
results depend on survey specifications and galaxy population properties, but
we stress that a bias will generally be present.
Finally, we develop a novel prescription for cross-bin correlations that
allow to search for a clean signal of relativistic corrections, and show that
this requires the use of the 3D full-sky formalism
Observed galaxy number counts on the lightcone up to second order:II. Derivation
Full text linkWe present a detailed derivation of the observed galaxy number over-density on cosmological scales up to second order in perturbation theory. We include all relativistic effects that arise from observing on the past lightcone. The derivation is in a general gauge, and applies to all dark energy models (including interacting dark energy) and to metric theories of modified gravity. The result will be important for accurate cosmological parameter estimation, including non-Gaussianity, since all projection effects need to be taken into account. It also offers the potential for new probes of General Relativity, dark energy and modified gravity. This paper accompanies Paper I which presents the key results for the concordance model in Poisson gauge
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