arXiv subjects

Michele Maggiore

Publications and source records attributed to Michele Maggiore.

At least 55 records · Page 3Linked to original sources

arXiv2020

Gravity in the infrared and effective nonlocal models

We provide a systematic and updated discussion of a research line carried out by our group over the last few years, in which gravity is modified at cosmological distances by the introduction of nonlocal terms, assumed to emerge at an effective level from the infrared behavior of the quantum theory. The requirement of producing a viable cosmology turns out to be very stringent and basically selects a unique model, in which the nonlocal term describes an effective mass for the conformal mode. We discuss how such a specific structure could emerge from a fundamental local theory of gravity, and we perform a detailed comparison of this model with the most recent cosmological datasets, confirming that it fits current data at the same level as $Λ$CDM. Most notably, the model has striking predictions in the sector of tensor perturbations, leading to a very large effect in the propagation of gravitational wave (GWs) over cosmological distances. At the redshifts relevant for the next generation of GW detectors such as Einstein Telescope, Cosmic Explorer and LISA, this leads to deviations from GR that could be as large as $80\%$, and could be verified with the detection of just a single coalescing binary with electromagnetic counterpart. This would also have potentially important consequences for the search of the counterpart since, for a given luminosity distance to the source, as inferred through the GW signal, the actual source redshift could be significantly different from that predicted by $Λ$CDM. At the redshifts relevant for advanced LIGO/Virgo/Kagra the effect is smaller, but still potentially observable over a few years of runs at target sensitivity.

astro-ph.CO↗

arXiv2019

Gaussian processes reconstruction of modified gravitational wave propagation

Recent work has shown that modified gravitational wave (GW) propagation can be a powerful probe of dark energy and modified gravity, specific to GW observations. We use the technique of Gaussian processes, that allows the reconstruction of a function from the data without assuming any parametrization, to measurements of the GW luminosity distance from simulated joint GW-GRB detections, combined with measurements of the electromagnetic luminosity distance by simulated DES data. For the GW events we consider both a second-generation LIGO/Virgo/Kagra (HVLKI) network, and a third-generation detector such as the Einstein Telescope. We find that the HVLKI network at target sensitivity, with $O(15)$ neutron star binaries with electromagnetic counterpart, could already detect deviations from GR at a level predicted by some modified gravity models, and a third-generation detector such as ET would have a remarkable discovery potential. We discuss the complementarity of the Gaussian processes technique to the $(Ξ_0,n)$ parametrization of modified GW propagation.

astro-ph.CO↗

arXiv2019

Nonlocal gravity and gravitational-wave observations

We discuss a modified gravity model which fits cosmological observations at a level statistically indistinguishable from $Λ$CDM and at the same time predicts very large deviations from General Relativity (GR) in the propagation of gravitational waves (GWs) across cosmological distances. The model is a variant of the RT nonlocal model proposed and developed by our group, with initial conditions set during inflation, and predicts a GW luminosity distance that, at the redshifts accessible to LISA or to a third-generation GW detector such as the Einstein Telescope (ET), can differ from that in GR by as much as $60\%$. An effect of this size could be detected with just a single standard siren with counterpart by LISA or ET. At the redshifts accessible to a LIGO/Virgo/Kagra network at target sensitivity the effect is smaller but still potentially detectable. Indeed, for the recently announced LIGO/Virgo NS-BH candidate S190814bv, the RT model predicts that, given the measured GW luminosity distance, the actual luminosity distance, and the redshift of an electromagnetic counterpart, would be smaller by as much as $7\%$ with respect to the value inferred from $Λ$CDM.

astro-ph.CO↗

arXiv2019

Cosmology and dark energy from joint gravitational wave-GRB observations

Gravitational-wave (GW) detectors can contribute to the measurement of cosmological parameters and to testing the dark-energy sector of alternatives to $Λ$CDM, by using standard sirens. In this paper we focus on binary neutron stars with a counterpart detected through a gamma-ray burst (GRB), both at a second-generation network made by advanced LIGO+advanced Virgo+LIGO India+Kagra, and at third-generation (3G) detectors, discussing in particular the cases of a single Einstein Telescope (ET), and of a network of ET plus two Cosmic Explorer (CE). We construct mock catalogs of standard sirens, using different scenarios for the local merger rate and for the detection of the electromagnetic counterpart. For 3G detectors we estimate the coincidences with a GRB detector with the characteristics of the proposed THESEUS mission. We discuss how these standard sirens with a GRB counterpart can improve the determination of cosmological parameters (and particularly of $H_0$) in $Λ$CDM, and we then study how to extract information on dark energy, considering both a non-trivial dark energy equation of state and modified GW propagation. We find that a 2G detector network can already reach, over several years of data taking, an interesting sensitivity to modified GW propagation, while a single ET detector would have a remarkable potential for discovery. We also find that, to fully exploit the potential of a ET+CE+CE network, it is necessary a much stronger program of search for electromagnetic counterparts (or else to resort to statistical methods for standard sirens), and furthermore gravitational lensing can become a limiting factor.

astro-ph.CO↗

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↗

arXiv2018

Testing nonlocal gravity with Lunar Laser Ranging

We study the impact of the limit on $|\dot{G}|/G$ from Lunar Laser Ranging on "nonlocal gravity", i.e. on models of the quantum effective action of gravity that include nonlocal terms relevant in the infrared, such as the "RR" and "RT" models proposed by our group, and the Deser-Woodard (DW) model. We elaborate on the analysis of Barreira et al. [1] and we confirm their findings that (under plausible assumptions such as the absence of strong backreaction from non-linear structures), the RR model is ruled out. We also show that the mechanism of "perfect screening for free" suggested for the DW model actually does not work and the DW model is also ruled out. In contrast, the RT model passes all phenomenological consistency tests and is still a viable candidate.

gr-qc↗

arXiv2018

Stability issues of nonlocal gravity during primordial inflation

We study the cosmological evolution of some nonlocal gravity models, when the initial conditions are set during a phase of primordial inflation. We examine in particular three models, the so-called RT, RR and $Δ_4$ models, previously introduced by our group. We find that the RR and $Δ_4$ models have a stable evolution also during inflation. The RT model has an apparent instability, but we show that, because of the smallness of the scale associated to the nonlocal term compared to the inflationary scale, this instability is innocuous and also the RT model has a viable evolution even when its initial conditions are set during a phase of primordial inflation.

hep-th↗

arXiv2018

Modified gravitational-wave propagation and standard sirens

Studies of dark energy at advanced gravitational-wave (GW) interferometers normally focus on the dark energy equation of state $w_{\rm DE}(z)$. However, modified gravity theories that predict a non-trivial dark energy equation of state generically also predict deviations from general relativity in the propagation of GWs across cosmological distances, even in theories where the speed of gravity is equal to $c$. We find that, in generic modified gravity models, the effect of modified GW propagation dominates over that of $w_{\rm DE}(z)$, making modified GW propagation a crucial observable for dark energy studies with standard sirens. We present a convenient parametrization of the effect in terms of two parameters $(Ξ_0,n)$, analogue to the $(w_0,w_a)$ parametrization of the dark energy equation of state, and we give a limit from the LIGO/Virgo measurement of $H_0$ with the neutron star binary GW170817. We then perform a Markov Chain Monte Carlo analysis to estimate the sensitivity of the Einstein Telescope (ET) to the cosmological parameters, including $(Ξ_0,n)$, both using only standard sirens, and combining them with other cosmological datasets. In particular, the Hubble parameter can be measured with an accuracy better than $1\%$ already using only standard sirens while, when combining ET with current CMB+BAO+SNe data, $Ξ_0$ can be measured to $0.8\%$ . We discuss the predictions for modified GW propagation of a specific nonlocal modification of gravity, recently developed by our group, and we show that they are within the reach of ET. Modified GW propagation also affects the GW transfer function, and therefore the tensor contribution to the ISW effect.

gr-qc↗

arXiv2018

The gravitational-wave luminosity distance in modified gravity theories

In modified gravity the propagation of gravitational waves (GWs) is in general different from that in general relativity. As a result, the luminosity distance for GWs can differ from that for electromagnetic signals, and is affected both by the dark energy equation of state $w_{\rm DE}(z)$ and by a function $δ(z)$ describing modified propagation. We show that the effect of modified propagation in general dominates over the effect of the dark energy equation of state, making it easier to distinguish a modified gravity model from $Λ$CDM. We illustrate this using a nonlocal modification of gravity, that has been shown to fit remarkably well CMB, SNe, BAO and structure formation data, and we discuss the prospects for distinguishing nonlocal gravity from $Λ$CDM with the Einstein Telescope. We find that, depending on the exact sensitivity, a few tens of standard sirens with measured redshift at $z\sim 0.4$, or a few hundreds at $1 < z < 2$, could suffice.

astro-ph.CO↗

arXiv2018

Nonlocal gravity. Conceptual aspects and cosmological predictions

Even if the fundamental action of gravity is local, the corresponding quantum effective action, that includes the effect of quantum fluctuations, is a nonlocal object. These nonlocalities are well understood in the ultraviolet regime but much less in the infrared, where they could in principle give rise to important cosmological effects. Here we systematize and extend previous work of our group, in which it is assumed that a mass scale $Λ$ is dynamically generated in the infrared, giving rise to nonlocal terms in the quantum effective action of gravity. We give a detailed discussion of conceptual aspects related to nonlocal gravity and of the cosmological consequences of these models. The requirement of providing a viable cosmological evolution severely restricts the form of the nonlocal terms, and selects a model (the so-called RR model) that corresponds to a dynamical mass generation for the conformal mode. For such a model: (1) there is a FRW background evolution, where the nonlocal term acts as an effective dark energy with a phantom equation of state, providing accelerated expansion without a cosmological constant. (2) Cosmological perturbations are well behaved. (3) Implementing the model in a Boltzmann code and comparing with observations we find that the RR model fits the CMB, BAO, SNe, structure formation data and local $H_0$ measurements at a level statistically equivalent to $Λ$CDM. (4) Bayesian parameter estimation shows that the value of $H_0$ obtained in the RR model is higher than in $Λ$CDM, reducing to $2.0σ$ the tension with the value from local measurements. (5) The RR model provides a prediction for the sum of neutrino masses that falls within the limits set by oscillation and terrestrial experiments. (6) Gravitational waves propagate at the speed of light, complying with the limit from GW170817/GRB 170817A.

hep-th↗

arXiv2016

Nonlocal Infrared Modifications of Gravity. A Review

We review an approach developed in the last few years by our group in which GR is modified in the infrared, at an effective level, by nonlocal terms associated to a mass scale. We begin by recalling the notion of quantum effective action and its associated nonlocalities, illustrating some of their features with the anomaly-induced effective actions in $D=2$ and $D=4$. We examine conceptual issues of nonlocal theories such as causality, degrees of freedoms and ghosts, stressing the importance of the fact that these nonlocalities only emerge at the effective level. We discuss a particular class of nonlocal theories where the nonlocal operator is associated to a mass scale, and we show that they perform very well in the comparison with cosmological observations, to the extent that they fit CMB, supernovae, BAO and structure formation data at a level fully competitive with $Λ$CDM, with the same number of free parameters. We explore some extensions of these `minimal' models, and we finally discuss some directions of investigation for deriving the required effective nonlocality from a fundamental local QFT.

hep-th↗

arXiv2016

Non-local gravity and comparison with observational datasets. II. Updated results and Bayesian model comparison with $Λ$CDM

We present a comprehensive and updated comparison with cosmological observations of two non-local modifications of gravity previously introduced by our group, the so called RR and RT models. We implement the background evolution and the cosmological perturbations of the models in a modified Boltzmann code, using CLASS. We then test the non-local models against the {\em Planck} 2015 TT, TE, EE and Cosmic Microwave Background (CMB) lensing data, isotropic and anisotropic Baryonic Acoustic Oscillations (BAO) data, JLA supernovae, $H_0$ measurements and growth rate data, and we perform Bayesian parameter estimation. We then compare the RR, RT and $Λ$CDM models, using the Savage-Dickey method. We find that the RT model and $Λ$CDM perform equally well, while the performance of the RR model with respect to $Λ$CDM depends on whether or not we include a prior on $H_0$ based on local measurements.

astro-ph.CO↗

arXiv2016

Perturbative loop corrections and nonlocal gravity

Nonlocal gravity has been shown to provide a phenomenologically viable infrared modification of GR. A natural question is whether the required nonlocality can emerge from perturbative quantum loop corrections due to light particles. We show that this is not the case. For the value of the mass scale of the non-local models required by cosmology, the perturbative form factors obtained from the loop corrections, in the present cosmological epoch, are in the regime where they are local. The mechanism behind the generation of the required nonlocality must be more complex, possibly related to strong infrared effects and non-perturbative mass generation for the conformal mode.

hep-th↗

arXiv2016

Conformal symmetry and nonlinear extensions of nonlocal gravity

We study two nonlinear extensions of the nonlocal $R\,\Box^{-2}R$ gravity theory. We extend this theory in two different ways suggested by conformal symmetry, either replacing $\Box^{-2}$ with $(-\Box + R/6)^{-2}$, which is the operator that enters the action for a conformally-coupled scalar field, or replacing $\Box^{-2}$ with the inverse of the Paneitz operator, which is a four-derivative operator that enters in the effective action induced by the conformal anomaly. We show that the former modification gives an interesting and viable cosmological model, with a dark energy equation of state today $w_{\rm DE}\simeq -1.01$, which very closely mimics $Λ$CDM and evolves asymptotically into a de Sitter solution. The model based on the Paneitz operator seems instead excluded by the comparison with observations. We also review some issues about the causality of nonlocal theories, and we point out that these nonlocal models can be modified so to nicely interpolate between Starobinski inflation in the primordial universe and accelerated expansion in the recent epoch.

hep-th↗

arXiv2016

Non-local gravity with a Weyl-square term

Recent work has shown that modifications of General Relativity based on the addition to the action of a non-local term $R\,\Box^{-2}R$, or on the addition to the equations of motion of a term involving $(g_{μν}\Box^{-1} R)$, produce dynamical models of dark energy which are cosmologically viable both at the background level and at the level of cosmological perturbations. We explore a more general class of models based on the addition to the action of terms proportional to $R_{μν}\,\Box^{-2}R^{μν}$ and $C_{μνρσ}\, \Box^{-2}C^{μνρσ}$, where $C_{μνρσ}$ is the Weyl tensor. We find that the term $R_{μν}\,\Box^{-2}R^{μν}$ does not give a viable background evolution. The non-local Weyl-square term, in contrast, does not contribute to the background evolution but we find that, at the level of cosmological perturbations, it gives instabilities in the tensor sector. Thus, only non-local terms which depend just on the Ricci scalar $R$ appear to be cosmologically viable. We discuss how these results can provide a hint for the mechanism that might generate these effective non-local terms from a fundamental local theory.

hep-th↗

arXiv2015

Dark energy and dimensional transmutation in $R^2$ gravity

Recent work has shown that non-local modifications of gravity involving terms such as $m^2R\Box^{-2}R$ (and no cosmological constant) provide a phenomenologically viable alternative to $Λ$CDM. We first discuss the possibility that such non-local terms emerge in the far infrared from the running of a coupling constant associated to the $R^2$ term in higher-derivative gravity, which, depending on the UV completion of the theory, can be asymptotically free in the ultraviolet and strongly coupled in the infrared. In this scenario the mass scale $m$ of the non-local model emerges from dimensional transmutation, similarly to $Λ_{\rm QCD}$ for strong interactions, leading to a technically natural value and to a novel understanding of the scale associated to dark energy. Motivated by these findings, we then explore the possibility of generating strong infrared effects in Einstein gravity, with no $R^2$ terms, as a consequence of the higher-derivative term generated by the conformal anomaly.

hep-th↗

arXiv2014

Non-local gravity and comparison with observational datasets

We study the cosmological predictions of two recently proposed non-local modifications of General Relativity. Both models have the same number of parameters as $Λ$CDM, with a mass parameter $m$ replacing the cosmological constant. We implement the cosmological perturbations of the non-local models into a modification of the CLASS Boltzmann code, and we make a full comparison to CMB, BAO and supernova data. We find that the non-local models fit these datasets as well as $Λ$CDM. For both non-local models parameter estimation using Planck+JLA+BAO data gives a value of $H_0$ higher than in $Λ$CDM, and in better agreement with the values obtained from local measurements.

astro-ph.CO↗

arXiv2014

Non-local formulation of ghost-free bigravity theory

We study the ghost-free bimetric theory of Hassan and Rosen, with parameters $β_i$ such that a flat Minkowski solution exists for both metrics. We show that, expanding around this solution and eliminating one of the two metrics with its own equation of motion, the remaining metric is governed by the Einstein-Hilbert action plus a non-local term proportional to $W_{μνρσ} (\Box-m^2)^{-1}W^{μνρσ}$, where $W_{μνρσ}$ is the Weyl tensor. The result is valid to quadratic order in the metric perturbation and to all orders in the derivative expansion. This example shows, in a simple setting, how such non-local extensions of GR can emerge from an underlying consistent theory, at the purely classical level.

hep-th↗