arXiv subjects

Nicola Bartolo

Publications and source records attributed to Nicola Bartolo.

At least 109 records · Page 6Linked to original sources

arXiv2016

Cosmology and Fundamental Physics with the Euclid Satellite

Euclid is a European Space Agency medium class mission selected for launch in 2020 within the Cosmic Vision 2015 2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

astro-ph.CO↗

arXiv2016

A relativistic signature in large-scale structure

In General Relativity, the constraint equation relating metric and density perturbations is inherently nonlinear, leading to an effective non-Gaussianity in the dark matter density field on large scales - even if the primordial metric perturbation is Gaussian. Intrinsic non-Gaussianity in the large-scale dark matter overdensity in GR is real and physical. However, the variance smoothed on a local physical scale is not correlated with the large-scale curvature perturbation, so that there is no relativistic signature in the galaxy bias when using the simplest model of bias. It is an open question whether the observable mass proxies such as luminosity or weak lensing correspond directly to the physical mass in the simple halo bias model. If not, there may be observables that encode this relativistic signature.

astro-ph.CO↗

arXiv2016

Nonequilibrium Casimir-Polder plasmonic interactions

We investigate how the combination of nonequilibrium effects and material properties impacts on the Casimir-Polder interaction between an atom and a surface. By addressing systems with temperature inhomogeneities and laser interactions, we show that nonmonotonous energetic landscapes can be produced where barriers and minima appear. Our treatment provides a self-consistent quantum theoretical framework for investigating the properties of a class of nonequilibrium atom-surface interactions.

quant-ph↗

arXiv2016

Primordial trispectra and CMB spectral distortions

We study the $TTμ$ bispectrum, generated by correlations between Cosmic Microwave Background temperature (T) anisotropies and chemical potential ($μ$) distortions, and we analyze its dependence on primordial local trispectrum parameters $g_{\rm NL}$ and $τ_{\rm NL}$. We cross-check our results by comparing the full bispectrum calculation with the expectations from a general physical argument, based on predicting the shape of $μ$-T correlations from the couplings between short and long perturbation modes induced by primordial non-Gaussianity. We show that $both$ $g_{\rm NL}$ and $τ_{\rm NL}$-parts of the primordial trispectrum source a non-vanishing $TTμ$ signal, contrary to the $μμ$ auto-correlation function, which is sensitive only to the $τ_{\rm NL}$-component. A simple Fisher matrix-based forecast shows that a futuristic, cosmic-variance dominated experiment could in principle detect $g_{\rm NL} \sim 0.4$ and $τ_{\rm NL} \sim 40$ using $TTμ$.

astro-ph.CO↗

arXiv2015

The two faces of mimetic Horndeski gravity: disformal transformations and Lagrange multiplier

We show that very general scalar-tensor theories of gravity (including, e.g., Horndeski models) are generically invariant under disformal transformations. However there is a special subset, when the transformation is not invertible, that yields new equations of motion which are a generalization of the so-called "mimetic" dark matter theory recently introduced by Chamsedinne and Mukhanov. These conclusions hold true irrespective of whether the scalar field in the action of the assumed scalar-tensor theory of gravity is the same or different than the scalar field involved in the transformation. The new equations of motion for our general mimetic theory can also be derived from an action containing an additional Lagrange multiplier field. The general mimetic scalar-tensor theory has the same number of derivatives in the equations of motion as the original scalar-tensor theory. As an application we show that the simplest mimetic scalar-tensor model is able to mimic the cosmological background of a flat FLRW model with a barotropic perfect fluid with any constant equation of state.

gr-qc↗

arXiv2015

Distinctive signatures of space-time diffeomorphism breaking in EFT of inflation

The effective field theory of inflation is a powerful tool for obtaining model independent predictions common to large classes of inflationary models. It requires only information about the symmetries broken during the inflationary era, and on the number and nature of fields that drive inflation. In this paper, we consider the case for scenarios that simultaneously break time reparameterization and spatial diffeomorphisms during inflation. We examine how to analyse such systems using an effective field theory approach, and we discuss several observational consequences for the statistics of scalar and tensor modes. For example, examining the three point functions, we show that this symmetry breaking pattern can lead to an enhanced amplitude for the squeezed bispectra, and to a distinctive angular dependence between their three wavevectors. We also discuss how our results indicate prospects for constraining the level of spatial diffeomorphism breaking during inflation.

astro-ph.CO↗

arXiv2015

Cosmology and fundamental physics with the Euclid satellite

Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

astro-ph.CO↗

arXiv2015

Galaxy bias and gauges at second order in General Relativity

We discuss the question of gauge choice when analysing relativistic density perturbations at second order. We compare Newtonian and General Relativistic approaches. Some misconceptions in the recent literature are addressed. We show that the comoving-synchronous gauge is the unique gauge in General Relativity that corresponds to the Lagrangian frame and is entirely appropriate to describe the matter overdensity at second order. The comoving-synchronous gauge is the simplest gauge in which to describe Lagrangian bias at second order.

astro-ph.CO↗

arXiv2015

Measuring primordial anisotropic correlators with CMB spectral distortions

We show that inflationary models with broken rotational invariance generate testable off-diagonal signatures in the correlation between the $μ$-type distortion and temperature fluctuations of the cosmic microwave background. More precisely, scenarios with a quadrupolar bispectrum asymmetry, usually generated by fluctuations of primordial vector fields, produce a nonvanishing $μ$-$T$ correlation when $|\ell_1-\ell_2|=2$. Since spectral distortions are sensitive to primordial fluctuations up to very small scales, a cosmic variance limited spectral distortion experiment can detect such effects with a high signal-to-noise ratio.

astro-ph.CO↗

arXiv2015

Parity-violating CMB correlators with non-decaying statistical anisotropy

We examine the cosmological correlators induced by the simultaneous breaking of parity and of statistical isotropy, e.g., in presence of the coupling ${\cal L} = f(ϕ) ( - \frac{1}{4} F^2 + \fracγ{4} F \tilde{F} )$ between the inflaton $ϕ$ and a vector field with vacuum expectation value ${\bf A}$. For a suitably chosen function $f$, the energy in the vector field $ρ_{\rm A}$ does not decay during inflation. This results in nearly scale-invariant signatures of broken statistical isotropy and parity. Specifically, we find that the scalar-scalar correlator of primordial curvature perturbations includes a quadrupolar anisotropy, $P_ζ( {\bf k}) = P(k)[ 1 + g_* ( \hat{\bf k} \cdot \hat{\bf A})^2]$, and a (angle-averaged) scalar bispectrum that is a linear combination of the first $3$ Legendre polynomials, $B_ζ(k_1, k_2, k_3) = \sum_L c_L P_L (\hat{\bf k}_1 \cdot \hat{\bf k}_2) P(k_1) P(k_2) + 2~{\rm perms} $, with $c_0 : c_1 : c_2 = 2 : -3 : 1$ ($c_1 \neq 0$ is a consequence of parity violation, corresponding to the constant $γ\neq 0$). The latter is one of the main results of this paper, which provides for the first time a clear example of an inflationary model where a non-negligible $c_1$ contribution to the bispectrum is generated. The scalar-tensor and tensor-tensor correlators induce characteristic signatures in the Cosmic Microwave Background temperature anisotropies (T) and polarization (E/B modes); namely, non-diagonal contributions to $\langle a_{\ell_1 m_1} a_{\ell_2 m_2}^* \rangle$, with $|\ell_1 - \ell_2| = 1$ in TT, TE, EE and BB, and $|\ell_1 - \ell_2| = 2$ in TB and EB. The latest CMB bounds on the scalar observables ($g_*$, $c_0$, $c_1$ and $c_2$), translate into the upper limit $ρ_{\rm A} / ρ_ϕ\lesssim 10^{-9}$ at $γ=0$. We find that the upper limit on the vector energy density becomes much more stringent as $γ$ grows.

astro-ph.CO↗

arXiv2015

Future Constraints on Angle-Dependent Non-Gaussianity from Large Radio Surveys

We investigate how well future large-scale radio surveys could measure different shapes of primordial non-Gaussianity; in particular we focus on angle-dependent non-Gaussianity arising from primordial anisotropic sources, whose bispectrum has an angle dependence between the three wavevectors that is characterized by Legendre polynomials $\mathcal{P}_L$ and expansion coefficients $c_L$. We provide forecasts for measurements of galaxy power spectrum, finding that Large-Scale Structure (LSS) data could allow measurements of primordial non-Gaussianity competitive or improving upon current constraints set by CMB experiments, for all the shapes considered. We argue that the best constraints will come from the possibility to assign redshift information to radio galaxy surveys, and investigate a few possible scenarios for the EMU and SKA surveys. A realistic (futuristic) modeling could provide constraints of $f_{\rm NL}^{\rm loc} \approx 1 (0.5)$ for the local shape, $f_{\rm NL}$ of $\mathcal{O}(10) (\mathcal{O}(1))$ for the orthogonal, equilateral and folded shapes, and $c_{L=1} \approx 80 (2)$, $c_{L=2} \approx 400 (10)$ for angle-dependent non-Gaussianity. The more futuristic forecasts show the potential of LSS analyses to considerably improve current constraints on non-Gaussianity, and so on models of the primordial Universe. Finally, we find the minimum requirements that would be needed to reach $σ(c_{L=1})=10$, which can be considered as a typical (lower) value predicted by some (inflationary) models.

astro-ph.CO↗

arXiv2015

Vacuum Casimir energy densities and field divergences at boundaries

We consider and review the emergence of singular energy densities and field fluctuations at sharp boundaries or point-like field sources in the vacuum. The presence of singular energy densities of a field may be relevant from a conceptual point of view, because they contribute to the self-energy of the system. They should also generate significant gravitational effects. We first consider the case of the interface between a metallic boundary and the vacuum, and obtain the structure of the singular electric and magnetic energy densities at the interface through an appropriate limit from a dielectric to an ideal conductor. Then, we consider the case of a point-like source of the electromagnetic field, and show that also in this case the electric and magnetic energy densities show a singular structure at the source position. We discuss how, in both cases, these singularities give an essential contribution to the electromagnetic self-energy of the system; moreover, they solve an apparent inconsistency between the space integral of the field energy density and the average value of the field Hamiltonian. The singular behavior we find is softened, or even eliminated, for boundaries fluctuating in space and for extended field sources. We discuss in detail the case in which a reflecting boundary is not fixed in space but is allowed to move around an equilibrium position, under the effect of quantum fluctuations of its position. Specifically, we consider the simple case of a one-dimensional massless scalar field in a cavity with one fixed and one mobile wall described quantum-mechanically. We investigate how the possible motion of the wall changes the vacuum fluctuations and the energy density of the field, compared with the fixed-wall case. Also, we explicitly show how the fluctuating motion of the wall smears out the singular behaviour of the field energy density at the boundary.

quant-ph↗

arXiv2015

A toy model for the dipolar-induced resonance in quasi-one-dimensional systems

We discuss the properties of the effective dipolar interaction for two particles tightly confined along a one-dimensional tube, stressing the emergence of a single dipolar-induced resonance in a regime for which two classical dipoles would just repel each other. We present a toy-model potential reproducing the main features of the effective interaction: a non-zero-range repulsive potential competing with an attractive contact term. The existence of a single resonance is confirmed analytically. The toy model is than generalized to investigate the interplay between dipolar and contact interaction, giving an intuitive interpretation of the resonance mechanism.

cond-mat.quant-gas↗

arXiv2015

Parity-violating and anisotropic correlations in pseudoscalar inflation

A pseudo-scalar inflaton field can have interesting phenomenological signatures associated with parity violation. The existing analyses of these signatures typically assume statistical isotropy. In the present work we instead investigate the possibility that a pseudo-scalar inflaton is coupled to a vector field carrying a small but non-negligible vacuum expectation value (vev) coherent over our Hubble patch. We show that, in such case, correlators involving the primordial curvature perturbations and gravitational waves violate both statistical isotropy and parity symmetry. We compute the Cosmic Microwave Background (CMB) temperature anisotropies (T) and polarization (E/B) generated by these primordial modes. The CMB two-point correlation functions present distinct signals of broken rotational and parity invariance. Specifically, we find non-vanishing TT, TE, EE and BB correlators between $\ell_1$ and $\ell_2 = \ell_1 \pm 1$ multipoles, and non-vanishing TB and EB correlators between $\ell_1$ and $\ell_2 = \ell_1 \pm 2$ multipoles. Such signatures are specific of the models under consideration and they cannot be generated if one of parity and isotropy is preserved. As a specific example we consider the simple case in which the vector field has just an "electric" background component decaying in the standard way as $a^{-2}$. In this case a strong scale-dependent quadrupolar modulation of the primordial power spectra is generated and we find that almost noiseless data of the large-scale temperature and E-mode polarization anisotropies (like, e.g., the ones provided by WMAP or $Planck$) should be able to constrain the quadrupolar amplitude coefficients $g_{2M}$ of the primordial scalar power spectrum (normalized at the pivot scale comparable to the present horizon size $k_0^{-1} = 14~{\rm Gpc}$) down to $g_{2M} = 30$ (68%CL).

astro-ph.CO↗

arXiv2014

Matter waves in two-dimensional arbitrary atomic crystals

We present a general scheme to realize a cold-atom quantum simulator of bidimensional atomic crystals. Our model is based on the use of two independently trapped atomic species: the first one, subject to a strong in-plane confinement, constitutes a two-dimensional matter wave which interacts only with atoms of the second species, deeply trapped around the nodes of a two-dimensional optical lattice. By introducing a general analytic approach we show that the system Green function can be exactly determined, allowing for the investigation of the matter-wave transport properties. We propose some illustrative applications to both Bravais (square, triangular) and non-Bravais (graphene, kagomé) lattices, studying both ideal periodic systems and experimental-size and disordered ones. Some remarkable spectral properties of these atomic artificial lattices are pointed out, such as the emergence of single and multiple gaps, flat bands, and Dirac cones. All these features can be manipulated via the interspecies interaction, which proves to be widely tunable due to the interplay between scattering length and confinements.

cond-mat.quant-gas↗

arXiv2014

Matter Waves in Atomic Artificial Graphene

We present a new model to realize artificial 2D lattices with cold atoms investigating the atomic artificial graphene: a 2D-confined matter wave is scattered by atoms of a second species trapped around the nodes of a honeycomb optical lattice. The system allows an exact determination of the Green function, hence of the transport properties. The inter-species interaction can be tuned via the interplay between scattering length and confinements. Band structure and density of states of a periodic lattice are derived for different values of the interaction strength. Emergence and features of Dirac cones are pointed out, together with the appearance of multiple gaps and a non-dispersive and isolated flat band. Robustness against finite-size and vacancies effects is numerically investigated.

cond-mat.quant-gas↗

arXiv2014

The expected anisotropy in solid inflation

Solid inflation is an effective field theory of inflation in which isotropy and homogeneity are accomplished via a specific combination of anisotropic sources (three scalar fields that individually break isotropy). This results in specific observational signatures that are not found in standard models of inflation: a non-trivial angular dependence for the squeezed bispectrum, and a possibly long period of anisotropic inflation (to drive inflation, the "solid" must be very insensitive to any deformation, and thus background anisotropies are very slowly erased). In this paper we compute the expected level of statistical anisotropy in the power spectrum of the curvature perturbations of this model. To do so, we account for the classical background values of the three scalar fields that are generated on large (superhorizon) scales during inflation via a random walk sum, as the perturbation modes leave the horizon. Such an anisotropy is unavoidably generated, even starting from perfectly isotropic classical initial conditions. The expected level of anisotropy is related to the duration of inflation and to the amplitude of the squeezed bispectrum. If this amplitude is close to its current observational limit (so that one of the most interesting predictions of the model can be observed in the near future), we find that a level of statistical anisotropy $\gtrsim 3\%$ in the power spectrum is to be expected, if inflation lasted $\gtrsim 20-30$ e-folds more than the final $50-60$ efolds required to generare the CMB modes. We also comment and point out various similarities between solid inflation and models of inflation where a suitable coupling of the inflaton to a vector kinetic term $F^{2}$ gives frozen and scale invariant vector perturbations on superhorizon scales.

astro-ph.CO↗

arXiv2014

Mild quasi-local non-Gaussianity as a signature of modified gravity during inflation

We show that modifications of Einstein gravity during inflation could leave potentially measurable imprints on cosmological observables in the form of non-Gaussian perturbations. This is due to the fact that these modifications appear in the form of an extra field that could have non-trivial interactions with the inflaton. We show it explicitly for the case $R+αR^2$, where nearly scale-invariant non-Gaussianity at the level of $f_{\rm NL} \approx -(1$ to $30)$ can be obtained, in a quasi-local configuration.

astro-ph.CO↗