Showing papers by "Francesca Perrotta published in 2000"

Tracking extended quintessence

Abstract: We study the cosmological role of a Tracking Field $\phi$ in Extended Quintessence scenarios (TEQ), where the dynamical vacuum energy driving the acceleration of the universe today is coupled with the Ricci scalar, $R$, with a term of the form $F(\phi)R/2$, where $F(\phi) = 1/8\pi G+\xi(\phi^{2}-\phi_{0}^{2})$. Tracker solutions for these NMC models, with inverse power-law potentials, possess an initial enhancement of the scalar field dynamics, named $R$-boost, caused by the Ricci scalar in the Klein-Gordon equation. During this phase the field performs a "gravitational" slow rolling which we model analytically, with energy density scaling as $(1+z)^{2}$. We evolve linear perturbations in TEQ models assuming Gaussian scale-invariant initial spectrum. We obtain significant changes in the Integrated Sachs Wolfe effect and in the acoustic peaks locations on the Cosmic Microwave Background, as well as in the turnover on the matter power spectrum. All these corrections may assume positive as well as negative values, depending on the sign of the NMC parameter $\xi$. We give analytical formulas describing all these effects. We show that they can be as large as $10 - 30%$ with respect to equivalent cosmological constant and ordinary tracking Quintessence models, respecting all the existing experimental constraints on scalar-tensor theories of gravity. These results demonstrate that the next decade data will provide deep constraints on the nature of the dark energy in the Universe, as well as the structure of the theory of gravity.

Neural networks and the separation of cosmic microwave background and astrophysical signals in sky maps

Abstract: We implement an independent component analysis (ICA) algorithm to separate signals of different origin in sky maps at several frequencies. Owing to its self-organizing capability, it works without prior assumptions on either the frequency dependence or the angular power spectrum of the various signals; rather, it learns directly from the input data how to identify the statistically independent components, on the assumption that all but, at most, one of the components have non-Gaussian distributions. We have applied the ICA algorithm to simulated patches of the sky at the four frequencies (30, 44, 70 and 100GHz) used by the Low Frequency Instrument of the European Space Agency's Planck satellite. Simulations include the cosmic microwave background (CMB), the synchrotron and thermal dust emissions, and extragalactic radio sources. The effects of the angular response functions of the detectors and of instrumental noise have been ignored in this first exploratory study. The ICA algorithm reconstructs the spatial distribution of each component with rms errors of about 1per cent for the CMB, and 10per cent for the much weaker Galactic components. Radio sources are almost completely recovered down to a flux limit corresponding to .0.7sCMB, where sCMB is the rms level of the CMB fluctuations. The signal recovered has equal quality on all scales larger than the pixel size. In addition, we show that for the strongest components (CMB and radio sources) the frequency scaling is recovered with per cent precision. Thus, algorithms of the type presented here appear to be very promising tools for component separation. On the other hand, we have been dealing here with a highly idealized situation. Work to include instrumental noise, the effect of different resolving powers at different frequencies and a more complete and realistic characterization of astrophysical foregrounds is in progress.

Implications for quintessence models from MAXIMA-1 and BOOMERANG-98

Abstract: Prompted by the recent MAXIMA-1 and BOOMERANG-98 measurements of the cosmic microwave background (CMB) anisotropy power spectrum, and motivated by the results from the observation of high-redshift Type Ia supernovae, we investigate CMB anisotropies in quintessence models in order to characterize the nature of the dark energy today We perform a Bayesian likelihood analysis, using the MAXIMA-1 and BOOMERANG-98 published bandpowers, in combination with COBE/DMR, to explore the space of quintessence parameters: the quintessence energy density \Omega_\phi and equation of state w_\phi We restrict our analysis to flat, scale-invariant, inflationary adiabatic models We find that this simple class of inflationary models, with a quintessence component \Omega_\phi < ~07, -1 < = w_\phi < ~-05, is in good agreement with the data Within the assumptions of our analysis, pure quintessence models seem to be slightly favored, although the simple cosmological constant scenario is consistent with the data

Effects of inflationary bubbles on the polarization and temperature anisotropies of the cosmic microwave background

Abstract: We predict the imprint of linear bubbly perturbations on the polarization and temperature anisotropies of the cosmic microwave background (CMB). We model analytically a bubbly density perturbation at the beginning of the radiation-dominated era and we apply the linear theory of cosmological perturbations to compute its time evolution. At decoupling, it uniquely marks the CMB polarization and temperature anisotropy sky. As predicted by recent general work regarding spatially limited cosmological seeds, during evolution the perturbation propagates beyond the size of the bubble and it reaches the CMB sound horizon in the time considered. Therefore, its signal appears as a series of concentric rings, each characterized by its own amplitude and sign, on the scale of the sound horizon at decoupling (1° on the sky). Polarization and temperature rings are strictly correlated; photons coming from the centre of the bubble are not polarized, because of the spherical symmetry of the present problem. As expected for linear perturbations with size L and density contrast δ at decoupling, δTT is roughly δ(LH−1)2; the polarization is about 10 per cent of the temperature anisotropy. We predict the impact of a distribution of bubbles on the CMB polarization and temperature power spectra. Considering models containing both cold dark matter (CDM) Gaussian and bubbly non-Gaussian fluctuations, we simulate and analyse 10°×10° sky patches with angular resolution of about 3.5 arcmin. The CMB power associated with the bubbles is entirely on subdegree angular scales (200l1000), which will be explored by the forthcoming high-resolution CMB experiments with per cent precision. Depending on the parameters of the bubbly distribution, we find extra power with respect to the ordinary CDM Gaussian fluctuations; we infer simple analytical scalings of the power induced by bubbly perturbations and we constrain our parameters with the existing data.

Extended Quintessence: imprints on the cosmic microwave background spectra

Abstract: We describe the observable features of the recently proposed Extended Quintessence scenarios on the Cosmic Microwave Background (CMB) anisotropy spectra. In this class of models a scalar field $\phi$, assumed to provide most of the cosmic energy density today, is non-minimally coupled to the Ricci curvature scalar $R$. We implement the linear theory of cosmological perturbations in scalar tensor gravitational theories to compute CMB temperature and polarization spectra. All the interesting spectral features are affected: on sub-degree angular scales, the acoustic peaks change both in amplitude and position; on larger scales the low redshift dynamics enhances the Integrated Sachs Wolfe effect. These results show how the future CMB experiments could give information on the vacuum energy as well as on the structure of the gravitational Lagrangian term.