arXiv subjects

Michele Mancarella

Publications and source records attributed to Michele Mancarella.

48 records · Page 3Linked to original sources

arXiv2021

Probing modified gravitational wave propagation with strongly lensed coalescing binaries

It has been recently shown that quadruply lensed gravitational-wave (GW) events due to coalescing binaries can be localized to one or just a few galaxies, even in the absence of an electromagnetic counterpart. We discuss how this can be used to extract information on modified GW propagation, which is a crucial signature of modifications of gravity at cosmological scales. We show that, using quadruply lensed systems, it is possible to constrain the parameter $Ξ_0$ that characterizes modified GW propagation, without the need of imposing a prior on $H_0$. A LIGO/Virgo/Kagra network at target sensitivity might already get a significant measurement of $Ξ_0$, while a third generation GW detector such as the Einstein Telescope could reach a very interesting accuracy.

gr-qc↗

arXiv2020

(Non)-neutrality of science and algorithms: Machine Learning between fundamental physics and society

The impact of Machine Learning (ML) algorithms in the age of big data and platform capitalism has not spared scientific research in academia. In this work, we will analyse the use of ML in fundamental physics and its relationship to other cases that directly affect society. We will deal with different aspects of the issue, from a bibliometric analysis of the publications, to a detailed discussion of the literature, to an overview on the productive and working context inside and outside academia. The analysis will be conducted on the basis of three key elements: the non-neutrality of science, understood as its intrinsic relationship with history and society; the non-neutrality of the algorithms, in the sense of the presence of elements that depend on the choices of the programmer, which cannot be eliminated whatever the technological progress is; the problematic nature of a paradigm shift in favour of a data-driven science (and society). The deconstruction of the presumed universality of scientific thought from the inside becomes in this perspective a necessary first step also for any social and political discussion. This is the subject of this work in the case study of ML.

physics.soc-ph↗

arXiv2019

Mimetic gravity as DHOST theories

We show that theories of mimetic gravity can be viewed as degenerate higher-order scalar-tensor (DHOST) theories that admit an extra local (gauge) symmetry in addition to the usual diffeomorphism invariance. We reformulate and classify mimetic theories in this perspective. Using the effective theory of dark energy, recently extended to include DHOST theories, we then investigate the linear perturbations about a homogeneous and isotropic background for all mimetic theories. We also include matter, in the form of a $k$-essence scalar field, and we derive the quadratic action for linear perturbations in this case.

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↗

arXiv2017

An effective description of dark energy: from theory to phenomenology

In the last decades, a cosmological model that fits observations through a vast range of scales emerged. It goes under the name of $Λ$CDM. However, there are still challenging questions that remain unanswered by this model, such as what causes the observed accelerated expansion of the universe, and many alternatives have been proposed. This thesis concerns an approach to test such models known as "Effective Theory of Dark Energy" . It applies to all models where general relativity is modified by adding a single scalar degree of freedom, called "scalar-tensor theories". In Chapter 1 I summarise the most general class of such theories currently known, called "Degenerate higher-Order Scalar-Tensor" (DHOST) theories. In Chapter 2, I introduce the effective theory of dark energy. The inclusion of a general coupling between matter and the gravitational sector is the subject of Chapter 3. Chapter 4 analyses in details the stability of different classes of theories. Notably, I show that the most general class of theories free from instabilities reduces to the so-called Horndeski and beyond-Horndeski theories, up to a non minimal coupling to matter. Another goal of the thesis is to study the observable effects of deviations from $Λ$CDM. In Chapter 5, I consider the possibility of an interaction between dark matter and dark energy and I analyse the constraining power of future surveys on the free parameters of the theory. Chapter 6 focuses on the observational effects of theories where a kinetic mixing between matter and the scalar field exists. This gives a peculiar and potentially observable effect, namely the weakening of gravity at large scale structure scales.

astro-ph.CO↗

arXiv2017

Effective Description of Higher-Order Scalar-Tensor Theories

Most existing theories of dark energy and/or modified gravity, involving a scalar degree of freedom, can be conveniently described within the framework of the Effective Theory of Dark Energy, based on the unitary gauge where the scalar field is uniform. We extend this effective approach by allowing the Lagrangian in unitary gauge to depend on the time derivative of the lapse function. Although this dependence generically signals the presence of an extra scalar degree of freedom, theories that contain only one propagating scalar degree of freedom, in addition to the usual tensor modes, can be constructed by requiring the initial Lagrangian to be degenerate. Starting from a general quadratic action, we derive the dispersion relations for the linear perturbations around Minkowski and a cosmological background. Our analysis directly applies to the recently introduced Degenerate Higher-Order Scalar-Tensor (DHOST) theories. For these theories, we find that one cannot recover a Poisson-like equation in the static linear regime except for the subclass that includes the Horndeski and so-called "beyond Horndeski" theories. We also discuss Lorentz-breaking models inspired by Horava gravity.

hep-th↗

arXiv2016

Weakening Gravity on Redshift-Survey Scales with Kinetic Matter Mixing

We explore general scalar-tensor models in the presence of a kinetic mixing between matter and the scalar field, which we call Kinetic Matter Mixing. In the frame where gravity is de-mixed from the scalar this is due to disformal couplings of matter species to the gravitational sector, with disformal coefficients that depend on the gradient of the scalar field. In the frame where matter is minimally coupled, it originates from the so-called beyond Horndeski quadratic Lagrangian. We extend the Effective Theory of Interacting Dark Energy by allowing disformal coupling coefficients to depend on the gradient of the scalar field as well. In this very general approach, we derive the conditions to avoid ghost and gradient instabilities and we define Kinetic Matter Mixing independently of the frame metric used to described the action. We study its phenomenological consequences for a $Λ$CDM background evolution, first analytically on small scales. Then, we compute the matter power spectrum and the angular spectra of the CMB anisotropies and the CMB lensing potential, on all scales. We employ the public version of COOP, a numerical Einstein-Boltzmann solver that implements very general scalar-tensor modifications of gravity. Rather uniquely, Kinetic Matter Mixing weakens gravity on short scales, predicting a lower $σ_8$ with respect to the $Λ$CDM case. We propose this as a possible solution to the tension between the CMB best-fit model and low-redshift observables.

astro-ph.CO↗

arXiv2016

Effective Theory of Dark Energy at Redshift Survey Scales

We explore the phenomenological consequences of general late-time modifications of gravity in the quasi-static approximation, in the case where cold dark matter is non-minimally coupled to the gravitational sector. Assuming spectroscopic and photometric surveys with configuration parameters similar to those of the Euclid mission, we derive constraints on our effective description from three observables: the galaxy power spectrum in redshift space, tomographic weak-lensing shear power spectrum and the correlation spectrum between the integrated Sachs-Wolfe effect and the galaxy distribution. In particular, with $Λ$CDM as fiducial model and a specific choice for the time dependence of our effective functions, we perform a Fisher matrix analysis and find that the unmarginalized $68\%$ CL errors on the parameters describing the modifications of gravity are of order $σ\sim10^{-2}$--$10^{-3}$. We also consider two other fiducial models. A nonminimal coupling of CDM enhances the effects of modified gravity and reduces the above statistical errors accordingly. In all cases, we find that the parameters are highly degenerate, which prevents the inversion of the Fisher matrices. Some of these degeneracies can be broken by combining all three observational probes.

astro-ph.CO↗

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

Effective Theory of Interacting Dark Energy

We present a unifying treatment of dark energy and modified gravity that allows distinct conformal-disformal couplings of matter species to the gravitational sector. In this very general approach, we derive the conditions to avoid ghost and gradient instabilities. We compute the equations of motion for background quantities and linear perturbations. We illustrate our formalism with two simple scenarios, where either cold dark matter or a relativistic fluid is nonminimally coupled. This extends previous studies of coupled dark energy to a much broader spectrum of gravitational theories.

astro-ph.CO↗

arXiv2014

Non-local gravity and dark energy

We discuss a nonlocal modification of gravity obtained adding a term $m^2 R\,\Box^{-2}R$ to the Einstein-Hilbert action. We find that the mass parameter $m$ only affects the non-radiative sector of the theory, while the graviton remains massless, there is no propagating ghost-like degree of freedom, no vDVZ discontinuity, and no Vainshtein radius below which the theory becomes strongly coupled. For $m={\cal O}(H_0)$ the theory therefore recovers all successes of GR at solar system and lab scales, and only deviates from it at cosmological scales. We examine the cosmological consequences of the model and we find that it automatically generates a dynamical dark energy and a self-accelerating evolution. After fixing our only free parameter $m$ so to reproduce the observed value of the dark energy density today, we get a pure prediction for the dark energy equation of state, $w_ {\rm DE}\simeq-1.14$. This value is consistent with the existing data, and could also resolve the possible tension between the Planck data and local measurements of the Hubble parameter.

hep-th↗