arXiv subjects

Nicola Bartolo

Publications and source records attributed to Nicola Bartolo.

At least 55 records · Page 3Linked to original sources

arXiv2023

Updated constraints on amplitude and tilt of the tensor primordial spectrum

We have taken a comprehensive approach to update the limits on the tensor-to-scalar ratio ($r$) and the tensor spectral index ($n_t$), using 10 datasets from the BICEP/Keck Array 2015 and 2018, Planck releases 3 and 4, and LIGO-Virgo-KAGRA Collaboration. By fitting the complete $Λ$CDM+$r$+$n_t$ model with two different approaches for the tensor sector, we have not only established which method is the most reliable, but have also achieved the strongest constraint on the tensor-to-scalar ratio in current literature: $r<0.028$ and $-1.37 < n_t < 0.42$ at 95% confidence level. Furthermore, our examination of the common signal detected by the NANOGrav Collaboration further confirms that a simple power-law cannot reconcile the constraints from different datasets if the NANOGrav detection is due to a primordial inflationary gravitational wave background, as previously shown in the literature.

astro-ph.CO↗

arXiv2023

Cosmic decoherence: primordial power spectra and non-Gaussianities

We study the effect of quantum decoherence on the inflationary cosmological perturbations. This process might imprint specific observational signatures revealing the quantum nature of the inflationary mechanism being related to the longstanding issue of the quantum-to-classical transition of inflationary fluctuations. Several works have investigated the effect of quantum decoherence on the statistical properties of primordial fluctuations. In particular, it has been shown that cosmic decoherence leads to corrections to the curvature power spectrum predicted by standard slow-roll inflation. Equally interesting, a non zero curvature trispectrum has been shown to be purely induced by cosmic decoherence, but surprisingly, decoherence seems not to generate any bispectrum. We further develop such an analysis by adopting a generalized form of the pointer observable, showing that decoherence does induce a non vanishing curvature bispectrum and providing a specific underlying concrete physical process. Present constraints on primordial bispectra allow to put an upper bound on the strength of the environment-system interaction. In full generality, the decoherence-induced bispectrum can be scale dependent provided one imposes the corresponding correction to the power spectrum to be scale independent. Such scale dependence on the largest cosmological scales might represent a distinctive imprint of the quantum decoherence process taking place during inflation. We also provide a criterion that allows to understand when cosmic decoherence induces scale independent corrections, independently of the type of environment considered. As a final result, we study the effect of cosmic decoherence on tensor perturbations and we derive the decoherence corrected tensor-to-scalar perturbation ratio. In specific cases, decoherence induces a blue tilted correction to the standard tensor power spectrum.

astro-ph.CO↗

arXiv2022

Test of the Statistical Isotropy of the Universe using Gravitational Waves

Since WMAP and Planck some anomalous features appeared in the Cosmic Microwave Background (CMB) large-angle anisotropy, the so-called anomalies. One of these is the hemispherical power asymmetry, i.e. a difference in the average power on the two hemispheres centered around (l, b) = (221, -20), which shows a relatively high level of significance. Such an anomaly could be the signature of a departure from statistical isotropy on large scales. Here we investigate the physical origin of this anomaly using the Cosmological Gravitational Wave Background (CGWB) detectable by future GW detectors. Indeed, the CGWB offers a unique window to explore the early universe and we show that it can be used in combination with CMB data to shed light on the statistical isotropy of our universe. Specifically, we study the evolution of gravitons in the presence of a modulating field in the scalar gravitational potentials accounting for the hemispherical power asymmetry and we infer the amplitude of this modulating field through a minimal variance estimator exploiting both constrained and unconstrained realizations of the CGWB. We show that the addition of the CGWB will allow an improvement in the assessment of the physical origin of the CMB power asymmetry. Accounting for the expected performances of LISA and BBO, we also show that the latter is expected to be signal-dominated on large-scales, proving that the CGWB could be the keystone to assess the significance of this anomaly.

astro-ph.CO↗

arXiv2022

Signatures of Primordial Gravitational Waves on the Large-Scale Structure of the Universe

We study the generation and evolution of second-order energy-density perturbations arising from primordial gravitational waves. Such "tensor-induced scalar modes" approximately evolve as standard linear matter perturbations and may leave observable signatures in the Large-Scale Structure of the Universe. We study the imprint on the matter power-spectrum of some primordial models which predict a large gravitational-wave signal at high frequencies. This novel mechanism in principle allows us to constrain or detect primordial gravitational waves by looking at specific features in the matter or galaxy power-spectrum, thereby allowing to probe them on a range of scales unexplored so far.

astro-ph.CO↗

arXiv2022

The Physical Content of Long Tensor Modes in Cosmology

We analyze the physical content of squeezed bispectra involving long-wavelength tensor perturbations, showing that these modes cannot be gauged away, except for the exact (unphysical) limit of infinite wavelength, $k = 0$. This result has a direct implication on the validity of the Maldacena consistency relation, respected by a subclass of inflationary models. Consequently, in the squeezed limit, as in the case of the scalar-scalar-scalar bispectrum, squeezed mixed correlators could be observed by future experiments, remaining a key channel to study Early Universe physics and discriminate among different models of inflation.

astro-ph.CO↗

arXiv2022

Cosmic Birefringence: Cross-Spectra and Cross-Bispectra with CMB Anisotropies

Parity-violating extensions of Maxwell electromagnetism induce a rotation of the linear polarization plane of photons during propagation. This effect, known as cosmic birefringence, impacts on the Cosmic Microwave Background (CMB) observations producing a mixing of $E$ and $B$ polarization modes which is otherwise null in the standard scenario. Such an effect is naturally parametrized by a rotation angle which can be written as the sum of an isotropic component $α_0$ and an anisotropic one $δα(\hat{\mathbf{n}})$. In this paper we compute angular power spectra and bispectra involving $δα$ and the CMB temperature and polarization maps. In particular, contrarily to what happens for the cross-spectra, we show that even in absence of primordial cross-correlations between the anisotropic birefringence angle and the CMB maps, there exist non-vanishing three-point correlation functions carrying signatures of parity-breaking physics. Furthermore, we find that such angular bispectra still survive in a regime of purely anisotropic cosmic birefringence, which corresponds to the conservative case of having $α_0=0$. These bispectra represent an additional observable aimed at studying cosmic birefringence and its parity-violating nature beyond power spectrum analyses. They provide also a way to perform consistency checks for specific models of cosmic birefringence. Moreover, we estimate that among all the possible birefringent bispectra, $\langleδα\, TB\rangle$ and $\langleδα\,EB\rangle$ are the ones which contain the largest signal-to-noise ratio. Once the cosmic birefringence signal is taken to be at the level of current constraints, we show that these bispectra are within reach of future CMB experiments, as LiteBIRD.

astro-ph.CO↗

arXiv2022

Cuscuton Inflation

We study the impact of (generalized) cuscuton models on standard single scalar field inflation. Generalized cuscuton models are characterized by spatial covariant gravity where a scalar degree of freedom is made non dynamical, and there are just two tensor degrees of freedom. The presence of the non-dynamical scalar field does not spoil inflation but instead the modifications are, in general, slow-roll suppressed leading to almost scale-invariant power spectra. However, the extra free parameters, which can be tuned relatively independently, lead to a larger parameter range for observable quantities, such as the tensor-to-scalar ratio. For the (generalized) cuscuton model the non-Gaussianties of the curvature bispectrum are suppressed by the slow-roll parameters, and, therefore, outside the reach of current experiments. However, generalized cuscuton models can lead to a different shape for the bispectrum which might be constrained by future experiments.

gr-qc↗

arXiv2022

New Horizons for Fundamental Physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of GWs can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.

gr-qc↗

arXiv2022

Cosmology with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.

astro-ph.CO↗

arXiv2022

Sensitivity of third-generation interferometers to extra polarizations in the Stochastic Gravitational Wave Background

When modified theories of gravity are considered, at most six gravitational wave polarization modes are allowed and classified in tensor modes, the only ones predicted by General Relativity (GR), along with additional vector and scalar modes. Therefore, gravitational waves represent a powerful tool to test alternative theories of gravitation. In this paper, we forecast the sensitivity of third-generation ground-based interferometers, Einstein Telescope and Cosmic Explorer, to non-GR polarization modes focusing on the stochastic gravitational wave background. We consider the latest technical specifications of the two independent detectors and the full network in order to estimate both the optimal signal-to-noise ratio and the detectable energy density limits relative to all polarization modes in the stochastic background for several locations on Earth and orientations of the two observatories. By considering optimal detector configurations, we find that in 5 years of observation the detection limit for tensor and extra polarization modes could reach $h_0^2Ω^{T,V,S}_{GW} \approx 10^{-12}-10^{-11}$, depending on the network configuration and the stochastic background (i.e., if only one among vector and scalar modes exists or both are present). This means that the network sensitivity to different polarization modes can be approximately improved by a factor $10^3$ with respect to second-generation interferometers. We finally discuss the possibility of breaking the scalar modes degeneracy by considering both detectors angular responses to sufficiently high gravitational wave frequencies.

astro-ph.CO↗

arXiv2022

Clustering of Gravitational Wave and Supernovae events: a multitracer analysis in Luminosity Distance Space

We study the clustering of Gravitational Wave (GW) merger events and Supernovae IA (SN), as cosmic tracers in Luminosity Distance Space. We modify the publicly available CAMB code to numerically evaluate auto- and cross- power spectra for the different sources, including Luminosity Distance Space distortion effects generated by peculiar velocities and lensing convergence. We perform a multitracer Fisher analysis to forecast expected constraints on cosmological and GW bias coefficients, using outputs from hydrodinamical N-body simulations to determine the bias fiducial model and considering future observations from the Vera Rubin Observatory and Einstein Telescope (ET), both single and in a 3 detector network configuration. We find that adding SN to the GW merger dataset considerably improves the forecast, mostly by breaking significant parameter degeneracies, with final constraints comparable to those obtainable from a Euclid-like survey. GW merger bias is forecasted to be detectable with good significance even in the single ET case.

astro-ph.CO↗

arXiv2022

In-flight polarization angle calibration for LiteBIRD: blind challenge and cosmological implications

We present a demonstration of the in-flight polarization angle calibration for the JAXA/ISAS second strategic large class mission, LiteBIRD, and estimate its impact on the measurement of the tensor-to-scalar ratio parameter, r, using simulated data. We generate a set of simulated sky maps with CMB and polarized foreground emission, and inject instrumental noise and polarization angle offsets to the 22 (partially overlapping) LiteBIRD frequency channels. Our in-flight angle calibration relies on nulling the EB cross correlation of the polarized signal in each channel. This calibration step has been carried out by two independent groups with a blind analysis, allowing an accuracy of the order of a few arc-minutes to be reached on the estimate of the angle offsets. Both the corrected and uncorrected multi-frequency maps are propagated through the foreground cleaning step, with the goal of computing clean CMB maps. We employ two component separation algorithms, the Bayesian-Separation of Components and Residuals Estimate Tool (B-SeCRET), and the Needlet Internal Linear Combination (NILC). We find that the recovered CMB maps obtained with algorithms that do not make any assumptions about the foreground properties, such as NILC, are only mildly affected by the angle miscalibration. However, polarization angle offsets strongly bias results obtained with the parametric fitting method. Once the miscalibration angles are corrected by EB nulling prior to the component separation, both component separation algorithms result in an unbiased estimation of the r parameter. While this work is motivated by the conceptual design study for LiteBIRD, its framework can be broadly applied to any CMB polarization experiment. In particular, the combination of simulation plus blind analysis provides a robust forecast by taking into account not only detector sensitivity but also systematic effects.

astro-ph.CO↗

arXiv2022

Probing Anisotropies of the Stochastic Gravitational Wave Background with LISA

We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discuss the main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We then perform a Fisher matrix analysis of the prospects of detectability of anisotropic features with LISA for individual multipoles, focusing on a SGWB with a power-law frequency profile. We compute the noise angular spectrum taking into account the specific scan strategy of the LISA detector. We analyze the case of the kinematic dipole and quadrupole generated by Doppler boosting an isotropic SGWB. We find that $β\, Ω_{\rm GW}\sim 2\times 10^{-11}$ is required to observe a dipolar signal with LISA. The detector response to the quadrupole has a factor $\sim 10^3 \,β$ relative to that of the dipole. The characterization of the anisotropies, both from a theoretical perspective and from a map-making point of view, allows us to extract information that can be used to understand the origin of the SGWB, and to discriminate among distinct superimposed SGWB sources.

astro-ph.CO↗

arXiv2021

Cross-correlating Astrophysical and Cosmological Gravitational Wave Backgrounds with the Cosmic Microwave Background

General Relativity provides us with an extremely powerful tool to extract at the same time astrophysical and cosmological information from the Stochastic Gravitational Wave Backgrounds (SGWBs): the cross-correlation with other cosmological tracers, since their anisotropies share a common origin and the same perturbed geodesics. In this letter we explore the cross-correlation of the cosmological and astrophysical SGWBs with Cosmic Microwave Background (CMB) anisotropies, showing that future GW detectors, such as LISA or BBO, have the ability to measure such cross-correlation signals. We also present, as a new tool in this context, constrained realization maps of the SGWBs extracted from the high-resolution CMB {\it Planck} maps. This technique allows, in the low-noise regime, to faithfully reconstruct the expected SGWB map by starting from CMB measurements.

astro-ph.CO↗

arXiv2021

Non-Markovian open quantum system approach to the early universe: I. Damping of gravitational waves by matter

By revising the application of the open quantum system approach to the early universe and extending it to the conditions beyond the Markovian approximation, we obtain a new non-Markovian quantum Boltzmann equation. Throughout the paper, we also develop an extension of the quantum Boltzmann equation to describe the processes that are irreversible at the macroscopic level. This new kinetic equation is, in principle, applicable to a wide variety of processes in the early universe. For instance, using this equation one can accurately study the microscopic influence of a cosmic environment on a system of cosmic background photons or stochastic gravitational waves. In this paper, we apply the non-Markovian quantum Boltzmann equation to study the damping of gravitational waves propagating in a medium consisting of decoupled ultra-relativistic neutrinos. For such a system, we study the time evolution of the intensity and the polarization of the gravitational waves. It is shown that, in contrast to intensity and linear polarization which are damped, the circular polarization (V-mode) of the gravitational wave (if present) is amplified by propagating through such a medium.

astro-ph.CO↗

arXiv2021

Tensor non-Gaussianity in chiral scalar-tensor theories of gravity

Violation of parity symmetry in the gravitational sector, which manifests into unequal left and right circular polarization states of primordial gravitational waves, represents a way to test high-energy modifications to general relativity. In this paper we study inflation within recently proposed chiral scalar-tensor theories of gravity, that extend Chern-Simons gravity by including parity-violating operators containing first and second derivatives of the non-minimally coupled scalar (inflaton) field. Given the degeneracy between different parity-violating theories at the level of the power spectrum statistics, we make a detailed analysis of the parity violation on primordial tensor non-Gaussianity. We show, with an explicit computation, that no new contributions arise in the graviton bispectra if the couplings in the new operators are constant in a pure de Sitter phase. On the other hand, if the coupling functions are time-dependent during inflation, the tensor bispectra acquire non-vanishing contributions from the parity-breaking operators even in the exact de Sitter limit, with maximal signal in the squeezed and equilateral configurations. We also comment on the consistency relation of the three-point function of tensor modes in this class of models and discuss prospects of detecting parity-breaking signatures through Cosmic Microwave Background $B$-mode bispectra.

astro-ph.CO↗

arXiv2020

Let Effective Field Theory of Inflation flow: stochastic generation of models with red/blue tensor tilt

We extend the method of Flow Equations to the Effective Field Theory framework of inflation, in order to investigate the observable predictions of a very broad class of inflationary models. Focusing our attention on the gravitational-wave sector, we derive a general expression for the consistency relation for effective models and provide a numerical implementation which allows to study how the generated models populate the $(r,nt)$ plane. We analyse $5 \times 10^{4}$ realizations of inflationary scenarios that respect the Null-Energy Condition ($ε> 0$) and $5 \times 10^{4}$ realizations that violate it ($ε> 0$). In both cases, 90% of the viable models are below the most recent upper bound on the tensor-to-scalar ratio from Planck and BICEP2/Keck Array BK15 data: $r_{0.002} < 0.056$ at 95 % CL. We find that general EFT inflationary models with $ε> 0$ are typically characterized by $n_{t} < 0$, whereas the vast majority of NEC-violating models presents a blue-tilted spectrum ($n_{t} > 0$). Since a blue tensor spectral index implies more power on small scales, this result is of considerable interest in view of a possible direct detection of the primordial gravitational-wave background.

astro-ph.CO↗

arXiv2020

CMB $V$ modes from photon-photon forward scattering revisited

Recent literature has shown that photon-photon forward scattering mediated by Euler-Heisenberg interactions may generate some amount of the circular polarization ($V$ modes) in the cosmic microwave background (CMB) photons. However, there is an apparent contradiction among the different references about the predicted level of the amplitude of this circular polarization. In this work, we will resolve this discrepancy by showing that with a quantum Boltzmann equation formalism we obtain the same amount of circular polarization as using a geometrical approach that is based on the index of refraction of the cosmological medium. We will show that the expected amplitude of $V$ modes is expected to be $\approx$ 8 orders of magnitude smaller than the amplitude of $E$-polarization modes that we actually observe in the CMB, thus confirming that it is going to be challenging to observe such a signature. Throughout the paper, we also develop a general method to study the generation of $V$ modes from photon-photon and photon-spin-1-massive-particle forward scatterings without relying on a specific interaction, which thus represent possible new signatures of physics beyond the Standard Model.

hep-ph↗