arXiv subjects

Carlo Baccigalupi

Publications and source records attributed to Carlo Baccigalupi.

At least 145 records · Page 8Linked to original sources

arXiv2002

Halo Concentrations and Weak-Lensing Number Counts in Dark Energy Cosmologies

We study the effects of a dark energy component with equation of state p=w rho with constant w >= -1 on the formation of Cold Dark Matter (CDM) haloes. We find two main effects: first, haloes form earlier as w increases, and second, the amplitude of the dark-matter power spectrum gets reduced in order to remain compatible with the large scale Cosmic Microwave Background (CMB) anisotropies. These effects counteract. Using recipes derived from numerical simulations, we show that haloes are expected to be up to ~ 50% more concentrated in CDM models with quintessence compared to Lambda-CDM models, the maximum increase being reached for w ~ -0.6. For larger w, the amplitude of the power spectrum decreases rapidly and makes expected halo concentrations drop. Halo detections through weak gravitational lensing are highly sensitive to halo concentrations. We show that weak-lensing halo counts with the aperture-mass technique can increase by a factor of ~ 2 as w is increased from -1 to -0.6, offering a new method for constraining the nature of dark energy.

astro-ph↗

arXiv2002

On the dark energy clustering properties

We highlight a viable mechanism leading to the formation of dark energy structures on sub-horizon cosmological scales, starting from linear perturbations in scalar-tensor cosmologies. We show that the coupling of the dark energy scalar field, or Quintessence, to the Ricci scalar induces a "dragging" of its density perturbations through the general relativistic gravitational potentials. We discuss, in particular, how this process forces dark energy to behave as a pressureless component if the cosmic evolution is dominated by non-relativistic matter. This property is also analyzed in terms of the effective sound speed of the dark energy, which correspondingly approaches the behavior of the dominant cosmological component, being effectively vanishing after matter-radiation equality. To illustrate this effect, we consider Extended Quintessence scenarios involving a quadratic coupling between the field and the Ricci scalar. We show that Quintessence density perturbations reach non-linearity at scales and redshifts relevant for the structure formation process, respecting all the existing constraints on scalar-tensor theories of Gravity. This study opens new perspectives on the standard picture of structure formation in dark energy cosmologies, since the Quintessence field itself, if non-minimally coupled to Gravity, may undergo clustering processes, eventually forming density perturbations exiting from the linear regime. A non-linear approach is then required to further investigate the evolution of these structures, and in particular their role in the dark haloes surrounding galaxies and clusters.

astro-ph↗

arXiv2002

Millimeter-band Surveys of Extragalactic Sources

Surveys at mm wavelengths emphasize rare classes of extragalactic radio sources characterized by spectra keeping flat or inverted up to high frequencies (such as blazars, GPS sources, advection dominated sources, or even gamma-ray burst afterglows), which are are difficult to single out at lower frequencies where the counts are dominated by far more numerous populations. Such surveys may lead to the discovery of populations with extreme properties, such as sources of the general GPS or blazar type but peaking at mm wavelengths, whose existence is hinted by currently available data. At relatively lower flux levels mm counts are dominated by high-redshift dusty galaxies, such as those detected by SCUBA and MAMBO surveys. These data have profound implications for our understanding of the formation and early evolution of galaxies. High sensitivity surveys at cm/mm wavelengths may also detect galaxy-scale Sunyaev-Zeldovich effects due to gas heated by central active nuclei.

astro-ph↗

arXiv2002

Constraints on flat cosmologies with tracking Quintessence from Cosmic Microwave Background observations

We constrain cosmological parameters in flat cosmologies with tracking dark energy (or Quintessence) using the existing data on Cosmic Microwave Background (CMB) anisotropies. We perform a maximum likelihood analysis using combined data from COBE/DMR, BOOMERanG, DASI and MAXIMA, obtaining estimates for the dark energy density $Ω_{Q}$ and equation of state $w_{Q}$, the physical baryon density $Ω_{b}h^{2}$, the scalar perturbation spectral index $n_{S}$, the ratio $R$ between the tensor and scalar perturbation amplitude (or the tensor spectral index $n_{T}$). Dark energy is found to be the dominant cosmological component $Ω_{Q}=0.71^{+0.05}_{-0.04}$, with equation of state $w_{Q}=-0.82^{+0.14}_{-0.11}$ (68% C.L.). Our best fit value of the physical baryon density is in good agreement with the primordial nucleosynthesis bound. We find no significant evidence for deviations from scale-invariance, although a scalar spectral index slightly smaller than unity is marginally preferred. Finally, we find that the contribution of cosmological gravitational waves is negligible. These results confirm that Quintessence is slightly preferred with respect to ordinary cosmological constant by the present CMB data.

astro-ph↗

arXiv2001

Workshop summary and the future

We present a tentative summary of the many very interesting issues that have been addresses at this workshop, focussing in particular on the perspectives for measuring the polarization power spectra of the Cosmic Microwave Background produced by scalar and tensor perturbations, in the presence of foregrounds.

astro-ph↗

arXiv2001

Polarized Foregrounds Power Spectra vs CMB

We briefly review our work about the polarized foreground contamination of the Cosmic Microwave Background maps. We start by summarizing the main properties of the polarized cosmological signal, resulting in "electric" (E) and "magnetic" (B) components of the polarization tensor field on the sky. Then we describe our present understanding of sub-degree anisotropies from Galactic synchrotron and from extra-Galactic point sources. We discuss their contamination of the cosmological E and B modes.

astro-ph↗

arXiv2001

Gravitational lensing: effects of cosmology and of lens and source profiles

We present detailed calculations of the magnification distribution, including both weak and strong lensing, using very recent solutions of the Dyer-Roeder (1973) equation for light propagation in a inhomogeneous universe with a cosmological constant and up-to-date models for the evolving cosmological distribution of dark matter halos. We address in particular the effect of a non-zero cosmological constant, of different density profiles of lenses, and of finite sizes of lensed sources. We show that, if dark matter fluctuations are normalized to the local cluster abundance, in the presence of a cosmological constant the optical depth for lensing {\em decreases} compared to the case of an Einstein-de Sitter universe, because halos in the relevant mass range are less abundant over a large redshift interval. We also discuss the differences in the magnification probability distributions produced by Navarro, Frenk & White (NFW) and by Singular Isothermal Sphere (SIS) density profiles of lenses. We find that the NFW lens is more efficient for moderate magnifications ($2\lsim A\lsim 4$), and less efficient for larger magnifications. Moreover, we discuss quantitatively the maximum magnification, $A_{\rm max}$, that can be achieved in the case of extended sources (galaxies) with realistic luminosity profiles, taking into account the possible ellipticity of the lens potential. Finally, we apply our results to a class of sources following the luminosity evolution typical for a unified scheme of QSO formation. We find that the lensed source counts at $850 μ$m can be larger than the unlensed ones by several orders of magnitude at flux densities $\gta100 $mJy.

astro-ph↗

arXiv2000

Tracking Extended Quintessence

We study the cosmological role of a Tracking Field $ϕ$ 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 $F(ϕ)R/2$, where $F(ϕ) = 1/8πG+ξ(ϕ^{2}-ϕ_{0}^{2})$. 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 $ξ$. 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.

astro-ph↗

arXiv2000

Extended Quintessence: imprints on the cosmic microwave background spectra

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 $ϕ$, 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.

astro-ph↗

arXiv1999

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

We predict the imprint of linear bubbly perturbations on the polarization and temperature anisotropies of the cosmic microwave background (CMB). We analytically model 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 signs the CMB polarization and temperature anisotropy sky. During evolution the perturbation propagates beyond the size of the bubble and reaches the CMB sound horizon at 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 1^{o} on the sky, even if the real seed size is much smaller. Polarization and temperature rings are strictly correlated. As expected for linear perturbations with size L and density contrast δat decoupling, δT/T is roughly δ(L/H^{-1})^{2}; the polarization is about 10% 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 CDM Gaussian and bubbly non-Gaussian fluctuations, we simulate and analyze 10^{o} x 10^{o} sky patches with angular resolution of about 3.5^{'}. The CMB power associated with the bubbles is entirely on sub-degree angular scales (200<= l<=1000), that will be explored by the forthcoming high resolution CMB experiments with the percent 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.

astro-ph↗

arXiv1999

Extended Quintessence

We study Quintessence cosmologies in the context of scalar-tensor theories of gravity, where a scalar field $ϕ$, assumed to provide most of the cosmic energy density today, is non-minimally coupled to the Ricci curvature scalar $R$. Such `Extended Quintessence' cosmologies have the appealing feature that the same field causing the time (and space) variation of the cosmological constant is the source of a varying Newton's constant à la Jordan-Brans-Dicke. We investigate here two classes of models, where the gravitational sector of the Lagrangian is $F(ϕ)R$ with $F(ϕ)=ξϕ^{2}$ (Induced Gravity, IG) and $F(ϕ)=1+ξϕ^{2}$ (Non-Minimal Coupling, NMC). As a first application of this idea we consider a specific model, where the Quintessence field, $ϕ$, obeying the simplest inverse power potential, has $Ω_ϕ=0.6$ today, in the context of the Cold Dark Matter scenario, with scale-invariant adiabatic initial perturbations. We find that, if $ξ\lesssim 5\times 10^{-4}$ for IG and $ξ\lesssim 5\times 10^{-3}(\sqrt{G}ϕ_{0})^{-1}$ for NMC ($ϕ_{0}$ is the present Quintessence value) our Quintessence field satisfies the existing solar system experimental constraints. Using linear perturbation theory we then obtain the polarization and temperature anisotropy spectra of the Cosmic Microwave Background (CMB) as well as the matter power-spectrum. The perturbation behavior possesses distinctive features, that we name `QR-effects', regarding acoustic peak location and height, late time integrated Sachs-Wolfe effect, as well as turnover and amplitude in the matter power spectrum. These features could be detected in the upcoming observations on CMB and large-scale structure.

astro-ph↗

arXiv1999

Cosmic microwave background: polarization and temperature anisotropies from symmetric structures

I consider the case of anisotropies in the Cosmic Microwave Background (CMB) from one single ordered perturbation source, or seed, existing well before decoupling between matter and radiation. Such structures could have been left by high energy symmetries breaking in the early universe. I focus on the cases of spherical and cylindrical symmetry of the seed. I give general analytic expressions for the polarization and temperature linear perturbations, factoring out of the Fourier integral the dependence on the photon propagation direction and on the geometric coordinates describing the seed. I show how the CMB perturbations manifestly reflect the symmetries of their seeds. CMB anisotropies are obtained with a line of sight integration. This treatment highlights the undulatory properties of the CMB. I show with numerical examples how the polarization and temperature perturbations propagate beyond the size of their seeds, reaching the CMB sound horizon at the time considered. Just like the waves from a pebble thrown in a pond, CMB anisotropy from a seed intersecting the last scattering surface appears as a series of temperature and polarization waves surrounding the seed, extending on the scale of the CMB sound horizon at decoupling, roughly $1^{o}$ in the sky. Each wave is characterized by its own value of the CMB perturbation, with the same mean amplitude of the signal coming from the seed interior. These waves could allow to distinguish relics from high energy processes of the early universe from point-like astrophysical sources, because of their angular extension and amplitude. Also, the marked analogy between polarization and temperature signals offers cross correlation possibilities for the future Planck Surveyor observations.

astro-ph↗

arXiv1999

Early time perturbations behaviour in scalar field cosmologies

We consider the problem of the initial conditions and behaviour of the perturbations in scalar field cosmology with general potential. We use the general definition of adiabatic and isocurvature conditions to set the appropriate initial values for the perturbation in the scalar field and in the ordinary matter and radiation components. In both the cases of initial adiabaticity and isocurvature, we solve the Einstein and fluid equation at early times and on superhorizon scales to find the initial behaviour of the relevant quantities. In particular, in the isocurvature case, we consider models in which the initial perturbation arises from the matter as well as from the scalar field itself, provided that the initial value of the gauge invariant curvature is zero. We extend the standard code to include all these cases, and we show some results concerning the power spectrum of the cosmic microwave background temperature and polarization anisotropies. In particular, it turns out that the acoustic peaks follow opposite behaviours in the adiabatic and isocurvature regimes: in the first case their amplitude is higher than in the corresponding pure cold dark matter model, while they make the opposite thing for pure isocurvature initial perturbations.

astro-ph↗

arXiv1999

Throwing pebbles in the primordial pond

We consider anisotropies in the Cosmic Microwave Background (CMB) generated by spatially limited seeds; these objects could correspond to relics of high energy symmetry breaking in the early universe. It is shown how the CMB perturbation propagate beyond the size of the seed in the form of waves traveling with the CMB sound velocity. Moreover, these waves are the substantial part of the signal, both for polarization and temperature. The explanation of this phenomenology in terms of the CMB equations is given. Observationally, this effect is threefold promising. First, it enlarges the signal from a seed intersecting the last scattering surface to the scale of the CMB sound horizon at decoupling, that is roughly one degree in the sky. Second, it offers cross correlation possibilities between the polarization and temperature signals. Third, it allows to unambiguously distinguish these structures from point-like astrophysical sources.

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arXiv1998

Perturbations in Quintessential Inflation

Recently Peebles and Vilenkin proposed and quantitatively analyzed the fascinating idea that a substantial fraction of the present cosmic energy density could reside in the vacuum potential energy of the scalar field responsible for inflation (quintessential inflation). Here we compute the signature of this model in the cosmic microwave background polarization and temperature anisotropies and in the large scale structure.

astro-ph↗

arXiv1998

Non-Gaussian Dark Matter perturbations and their imprint on the microwave background

The phases of the Fourier transform of any linear cosmological perturbation may be random (the Gaussian case) or not (the non-Gaussian case). If a non-Gaussian inhomogeneity was generated during the inflationary era by some process of very high energy physics, its evolution may be computed using the usual linear theory. I focus on the perturbations induced on the microwave background (CMB) by an underlying non-Gaussian distribution of dark matter (DM). Giving the specific example of an inflationary bubble, I show how non-Gaussian structures lying on the last scattering surface (LSS) may give rise to ordered patterns in the CMB anisotropy field. The next high resolution observations of the CMB sky may eventually detect these signals.

astro-ph↗

arXiv1997

Sub-degree CMB anisotropies from inflationary bubbles

It is well known that processes of first order phase transitions may have occurred in the inflationary era. If one or more occurred well before the end of inflation, the nucleated bubbles are stretched to large scales and the primordial power spectrum contains a scale dependent non-Gaussian component provided by the remnants of the bubbles. We predict the anisotropies in the cosmic microwave background (CMB) induced by inflationary bubbles. We build a general analytic model for describing a bubbly perturbation; we evolve each Fourier mode using the linear theory of perturbations from reheating until decoupling; we get the CMB anisotropies by considering the bubbly perturbation intersecting the last scattering surface. The CMB image of an inflationary bubble is a series of concentric isothermal rings of different color (sign of $δT/T$) on the scale of the sound horizon at decoupling ($\le 1^{o}$ in the sky); the resulting anisotropy is therefore strongly non-Gaussian. The mean amplitude of $δT/T$ for a bubble of size $L$ follows the known estimates for linear perturbations, $δT/T\simeqδρ/ρ\cdot (L/H^{-1})^{2}$. In particular, bubbles with size corresponding to the seeds of the observed large scale voids (tens of comoving Mpc) induce an interesting pattern of CMB anisotropies on the sub-degree angular scale, to be further investigated and compared with the forthcoming high resolution CMB maps provided by the MAP and the Planck experiments.

astro-ph↗

arXiv1997

Stochastic gravitational background from inflationary phase transitions

We consider true vacuum bubbles generated in a first order phase transition occurring during the slow rolling era of a two field inflation: it is known that gravitational waves are produced by the collision of such bubbles. We find that the epoch of the phase transition strongly affects the characteristic peak frequency of the gravitational waves, causing an observationally interesting redshift in addition to the post-inflationary expansion. In particular it is found that a phase transition occurring typically 10$÷$20 $e-$foldings before the reheating at $kT\simeq 10^{15}$ GeV may be detected by the next Ligo gravity waves interferometers. Moreover, for recently proposed models capable of generating the observed large scale voids as remnants of the primordial bubbles (for which the characteristic wave lengths are several tens of Mpc), it is found that the level of anisotropy of the cosmic microwave background provides a deep insight upon the physical parameters of the effective Lagrangian.

gr-qc↗