arXiv subjects

Daniele Bertacca

Publications and source records attributed to Daniele Bertacca.

At least 55 records · Page 3Linked to original sources

arXiv2021

Gravitational-wave cosmological distances in scalar-tensor theories of gravity

We analyze the propagation of high-frequency gravitational waves (GW) in scalar-tensor theories of gravity, with the aim of examining properties of cosmological distances as inferred from GW measurements. By using symmetry principles, we first determine the most general structure of the GW linearized equations and of the GW energy momentum tensor, assuming that GW move with the speed of light. Modified gravity effects are encoded in a small number of parameters, and we study the conditions for ensuring graviton number conservation in our covariant set-up. We then apply our general findings to the case of GW propagating through a perturbed cosmological space-time, deriving the expressions for the GW luminosity distance $d_L^{({\rm GW})}$ and the GW angular distance $d_A^{({\rm GW})}$. We prove for the first time the validity of Etherington reciprocity law $d_L^{({\rm GW})}\,=\,(1+z)^2\,d_A^{({\rm GW})}$ for a perturbed universe within a scalar-tensor framework. We find that besides the GW luminosity distance, also the GW angular distance can be modified with respect to General Relativity. We discuss implications of this result for gravitational lensing, focussing on time-delays of lensed GW and lensed photons emitted simultaneously during a multimessenger event. We explicitly show how modified gravity effects compensate between different coefficients in the GW time-delay formula: lensed GW arrive at the same time as their lensed electromagnetic counterparts, in agreement with causality constraints.

gr-qc↗

arXiv2021

Detecting Dark Energy Fluctuations with Gravitational Waves

Luminosity distance estimates from electromagnetic and gravitational wave sources are generally different in models of dynamical dark energy and gravity beyond the standard cosmological scenario. We show that this leaves a unique imprint on the angular power-spectrum of fluctuations of the luminosity distance of gravitational-wave observations which tracks inhomogeneities in the dark energy field. Exploiting the synergy in supernovae and gravitational wave distance measurements, we build a joint estimator that directly probes dark energy fluctuations, providing a conclusive evidence for their existence in case of detection. Moreover, such measurement would also allow to probe the running of the Planck mass. We discuss experimental requirements to detect these signals.

astro-ph.CO↗

arXiv2020

Gravitational waves and geometrical optics in scalar-tensor theories

The detection of gravitational waves (GWs) propagating through cosmic structures can provide invaluable information on the geometry and content of our Universe, as well as on the fundamental theory of gravity. In order to test possible departures from General Relativity, it is essential to analyse, in a modified gravity setting, how GWs propagate through a perturbed cosmological space-time. Working within the framework of geometrical optics, we develop tools to address this topic for a broad class of scalar-tensor theories, including scenarios with non-minimal, derivative couplings between scalar and tensor modes. We determine the corresponding evolution equations for the GW amplitude and polarization tensor. The former satisfies a generalised evolution equation that includes possible effects due to a variation of the effective Planck scale; the latter can fail to be parallely transported along GW geodesics unless certain conditions are satisfied. We apply our general formulas to specific scalar-tensor theories with unit tensor speed, and then focus on GW propagation on a perturbed space-time. We determine corrections to standard formulas for the GW luminosity distance and for the evolution of the polarization tensor, which depend both on modified gravity and on the effects of cosmological perturbations. Our results can constitute a starting point to disentangle among degeneracies from different sectors that can influence GW propagation through cosmological space-times.

gr-qc↗

arXiv2020

Generalisation of the Kaiser Rocket effect in general relativity in the wide-angle galaxy 2-point correlation function

We study wide-angle correlations in the galaxy power spectrum in redshift space, including all general relativistic effects and the Kaiser Rocket effect in general relativity. We find that the Kaiser Rocket effect becomes important on large scales and at high redshifts, and leads to new contributions in wide-angle correlations. We believe this effect might be very important for future large volume surveys.

gr-qc↗

arXiv2020

The Imprint of Relativistic Particles on the Anisotropies of the Stochastic Gravitational-Wave Background

The Stochastic Gravitational-Wave Background (SGWB) is expected to be a key observable for Gravitational-Wave (GW) interferometry. Its detection will open a new window on early Universe cosmology, on the astrophysics of compact objects and, as shown in this Letter, on the particle physics content of the Universe. In this Letter we show that, besides their effects on the Cosmic Microwave Background (CMB) and on Large Scale Structure (LSS), relativistic particles in the early Universe leave a clear imprint on the anisotropies of the SGWB. In particular we show that a change in the number of decoupled relativistic particles shifts the angular power spectrum of the SGWB, as both the Sachs-Wolfe (SW) and the Integrated Sachs-Wolfe (ISW) terms are affected. Being very large-angle effects, these lead to new testable predictions for future GW interferometers.

astro-ph.CO↗

arXiv2020

Science Case for the Einstein Telescope

The Einstein Telescope (ET), a proposed European ground-based gravitational-wave detector of third-generation, is an evolution of second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA which could be operating in the mid 2030s. ET will explore the universe with gravitational waves up to cosmological distances. We discuss its main scientific objectives and its potential for discoveries in astrophysics, cosmology and fundamental physics.

astro-ph.CO↗

arXiv2019

From Primordial Black Holes Abundance to Primordial Curvature Power Spectrum (and back)

In the model where Primordial Black Holes (PBHs) form from large primordial curvature (C) perturbations, i.e., CPBHs, constraints on PBH abundance provide in principle constraints on the primordial curvature power spectrum. This connection however depends necessarily on the details of PBH formation mechanism. In this paper we provide, for the first time, constraints on the primordial curvature power spectrum from the latest limits on PBH abundance, taking into account all the steps from gravitational collapse in real space to PBH formation. In particular, we use results from numerical relativity simulations and peak theory to study the conditions for PBH formation for a range of perturbation shapes, including non-linearities, perturbation profile and a careful treatment of smoothing and filtering scales. We then obtain updated PBH formation conditions and translate that into primordial spectrum constraints for a wide range of shapes and abundances. These updated constraints cover a range of scales not probed by other cosmological observables. Our results show that the correct and accurate modelling of non-linearities, filtering and typical perturbation profile, is crucial for deriving meaningful cosmological implications.

astro-ph.CO↗

arXiv2019

Cosmological perturbation effects on gravitational-wave luminosity distance estimates

Waveforms of gravitational waves provide information about a variety of parameters for the binary system merging. However, standard calculations have been performed assuming a FLRW universe with no perturbations. In reality this assumption should be dropped: we show that the inclusion of cosmological perturbations translates into corrections to the estimate of astrophysical parameters derived for the merging binary systems. We compute corrections to the estimate of the luminosity distance due to velocity, volume, lensing and gravitational potential effects. Our results show that the amplitude of the corrections will be negligible for current instruments, mildly important for experiments like the planned DECIGO, and very important for future ones such as the Big Bang Observer.

gr-qc↗

arXiv2019

On systematic and GR effects on muon $g-2$ experiments

We derive in full generality the equations that govern the time dependence of the energy ${\mathcal E}$ of the decay electrons in a muon $g-2$ experiment. We include both electromagnetic and gravitational effects and we estimate possible systematics on the measurements of $g-2\equiv 2(1+a)$, whose experimental uncertainty will soon reach $Δa/a\approx 10^{-7}$. In addition to the standard modulation of ${\mathcal E}$ when the motion is orthogonal to a constant magnetic field $B$, with angular frequency $ω_a=e a |B|/m$, we study effects due to: (1) a non constant muon $γ$ factor, in presence of electric fields $E$, (2) a correction due to a component of the muon velocity along $B$ (the `pitch correction'), (3) corrections to the precession rate due to $E$ fields, (4) non-trivial spacetime metrics. Oscillations along the radial and vertical directions of the muon lead to oscillations in ${\mathcal E}$ with a relative size of order $10^{-6}$, for the BNL $g-2$ experiment. We then find a subleading effect in the `pitch' correction, leading to a frequency shift of $Δω_a/ω_a \approx {\cal O}(10^{-9})$ and subleading effects of about $Δω_a/ω_a \approx {\rm few} \times {\cal O}(10^{-8}-10^{-9})$ due to $E$ fields. Finally we show that GR effects are dominated by the Coriolis force, due to the Earth rotation with angular frequency $ω_T$, leading to a correction of about $Δω_a/ω_a \approx ω_T/(γω_a) \approx {\cal O}(10^{-12})$. A similar correction might be more appreciable for future electron $g-2$ experiments, being of order $Δω_a/ω_{a, {\rm el}} \approx ω_T/(ω_{a, {\rm el}}) \approx 7\times 10^{-13}$, compared to the present experimental uncertainty, $Δa_{\rm el}/a_{\rm el}\approx 10^{-10}$, and forecasted to reach soon $Δa_{\rm el}/a_{\rm el}\approx 10^{-11}$.

hep-ph↗

arXiv2019

Cosmology and the Early Universe

This Astro-2020 White Paper deals with what we might learn from future gravitational wave observations about the early universe phase transitions and their energy scales, primordial black holes, Hubble parameter, dark matter and dark energy, modified theories of gravity and extra dimensions.

astro-ph.HE↗

arXiv2019

Does Quartessence Ease Tensions?

Tensions between cosmic microwave background observations and the growth of the large-scale structure inferred from late-time probes pose a serious challenge to the concordance $Λ$CDM cosmological model. State-of-the-art data from the Planck satellite predicts a higher rate of structure growth than what preferred by low-redshift observables. Such tension has hitherto eluded conclusive explanations in terms of straightforward modifications to $Λ$CDM, e.g. the inclusion of massive neutrinos or a dynamical dark energy component. Here, we investigate models of 'quartessence' -- a single dark component mimicking both dark matter and dark energy -- whose non-vanishing sound speed inhibits structure growth at late times on scales smaller than its corresponding Jeans' length. In principle, this could reconcile high- and low-redshift observations. We put this hypothesis to test against temperature and polarisation spectra from the latest Planck release, SDSS DR12 measurements of baryon acoustic oscillations and redshift-space distortions, and cosmic shear correlation functions from KiDS. This the first time that any specific model of quartessence is applied to actual data. We show that, if we naively apply $Λ$CDM nonlinear prescription to quartessence, the combined data sets allow for tight constraints on the model parameters. Apparently, quartessence alleviates the tension between the total matter fraction and late-time structure clustering, although in fact the tension is transferred from the latter to the quartessence sound speed parameter. However, we found that this strongly depends upon information from nonlinear scales. Indeed, if we relax this assumption, quartessence models appear still viable. For this reason, we argue that the nonlinear behaviour of quartessence deserves further investigation and may lead to a deeper understanding of the physics of the dark Universe.

astro-ph.CO↗

arXiv2018

Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

We present a detailed overview of the cosmological surveys that will be carried out with Phase 1 of the Square Kilometre Array (SKA1), and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5,000 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 3 with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to z = 6. These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical surveys like LSST and Euclid, leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys, and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

astro-ph.CO↗

arXiv2018

The observed galaxy bispectrum from single-field inflation in the squeezed limit

Using the consistency relation in Fourier space, we derive the observed galaxy bispectrum from single-field inflation in the squeezed limit, in which one of the three modes has a wavelength much longer than the other two. This provides a non-trivial check of the full computation of the bispectrum based on second-order cosmological perturbation theory in this limit. We show that gauge modes need to be carefully removed in the second-order cosmological perturbations in order to calculate the observed galaxy bispectrum in the squeezed limit. We then give an estimate of the effective non-Gaussianity due to general relativistic lightcone effects that could mimic a primordial non-Gaussian signal.

astro-ph.CO↗

arXiv2018

Relativistic wide-angle galaxy bispectrum on the light-cone

Given the important role that the galaxy bispectrum has recently acquired in cosmology and the scale and precision of forthcoming galaxy clustering observations, it is timely to derive the full expression of the large-scale bispectrum going beyond approximated treatments which neglect integrated terms or higher-order bias terms or use the Limber approximation. On cosmological scales, relativistic effects that arise from observing on the past light-cone alter the observed galaxy number counts, therefore leaving their imprints on N-point correlators at all orders. In this paper we compute for the first time the bispectrum including all general relativistic, local and integrated, effects at second order, the tracers' bias at second order, geometric effects as well as the primordial non-Gaussianity contribution. This is timely considering that future surveys will probe scales comparable to the horizon where approximations widely used currently may not hold; neglecting these effects may introduce biases in estimation of cosmological parameters as well as primordial non-Gaussianity.

astro-ph.CO↗

arXiv2017

Observed galaxy number counts on the lightcone up to second order: III. Magnification Bias

We 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 observing on 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.

astro-ph.CO↗

arXiv2017

Disantangling the effects of Doppler velocity and primordial non-Gaussianity in galaxy power spectra

We study the detectability of large-scale velocity effects on galaxy clustering, by simulating galaxy surveys and combining the clustering of different types of tracers of large-scale structure. We employ a set of lognormal mocks that simulate a $20.000$ deg$^2$ near-complete survey up to $z=0.8$, in which each galaxy mock traces the spatial distribution of dark matter of that mock with a realistic bias prescription. We find that the ratios of the monopoles of the power spectra of different types of tracers carry most of the information that can be extracted from a multi-tracer analysis. In particular, we show that with a multi-tracer technique it will be possible to detect velocity effects with $\gtrsim 3 σ$. Finally, we investigate the degeneracy of these effects with the (local) non-Gaussianity parameter $f_{\rm NL}$, and how large-scale velocity contributions could be mistaken for the signatures of primordial non-Gaussianity.

astro-ph.CO↗

arXiv2017

LIGER: mock relativistic light-cones from Newtonian simulations

We introduce a method to create mock galaxy catalogues in redshift space including general relativistic effects to linear order in the cosmological perturbations. We dub our method LIGER, short for `light cones with general relativity'. LIGER takes a (N-body or hydrodynamic) Newtonian simulation as an input and outputs the distribution of galaxies in comoving redshift space. This result is achieved making use of a coordinate transformation and simultaneously accounting for lensing magnification. The calculation includes both local corrections and terms that have been integrated along the line of sight. Our fast implementation allows the production of many realizations that can be used to forecast the performance of forthcoming wide-angle surveys and to estimate the covariance matrix of the observables. To facilitate this use, we also present a variant of LIGER designed for large-volume simulations with low mass resolution. In this case, the galaxy distribution on large scales is obtained by biasing the matter-density field. Finally, we present two sample applications of LIGER. First, we discuss the impact of weak gravitational lensing onto the angular clustering of galaxies in a Euclid-like survey. In agreement with previous analytical studies, we find that magnification bias can be measured with high confidence. Second, we focus on two generally neglected Doppler-induced effects: magnification and the change of number counts with redshift. We show that the corresponding redshift-space distortions can be detected at 5.5$σ$ significance with the completed Square Kilometre Array.

astro-ph.CO↗

arXiv2016

Lagrangian theory for cosmic structure formation with vorticity: Newtonian and post-Friedmann approximations

We study the nonlinear gravitational dynamics of a universe filled with a pressureless fluid and a cosmological constant $Λ$ in the context of Newtonian gravity, and in the relativistic post-Friedmann approach proposed in paper I [I. Milillo et al., Phys. Rev. D 92, 023519 (2015).]. The post-Friedmann approximation scheme is based on the $1/c$ expansion of the space-time metric and the energy-momentum tensor, and includes nonlinear Newtonian cosmology. Here we establish the nonlinear post-Friedmann framework in the Lagrangian-coordinates approach for structure formation. For this we first identify a Lagrangian gauge which is suitable for incorporating nonzero vorticity. We analyze our results in two limits: at the leading order we recover the fully nonlinear Newtonian cosmological equations in the Lagrangian formulation, and we provide a space-time metric consistent from the perspective of general relativity. We then linearize our expressions and recover the relativistic results at first order in cosmological perturbation theory. Therefore, the introduced approximation scheme provides a unified treatment for the two leading-order regimes, from the small scales described by Newtonian gravity to the large linear scale, where first-order relativistic cosmological perturbation theory gives a very good description of structure formation.

gr-qc↗