arXiv subjects

Luca Amendola

Publications and source records attributed to Luca Amendola.

At least 55 records · Page 3Linked to original sources

arXiv2020

Boosting MonteCarlo sampling with a non-Gaussian fit

We propose a new method, called MonteCarlo Posterior Fit, to boost the MonteCarlo sampling of likelihood (posterior) functions. The idea is to approximate the posterior function by an analytical multidimensional non-Gaussian fit. The many free parameters of this fit can be obtained by a smaller sampling than is needed to derive the full numerical posterior. In the examples that we consider, based on supernovae and cosmic microwave background data, we find that one needs an order of magnitude smaller sampling than in the standard algorithms to achieve comparable precision. This method can be applied to a variety of situations and is expected to significantly improve the performance of the MonteCarlo routines in all the cases in which sampling is very time-consuming. Finally, it can also be applied to Fisher matrix forecasts, and can help solve various limitations of the standard approach.

astro-ph.CO↗

arXiv2020

Ricci-inverse gravity: a novel alternative gravity, its flaws, and how to cure them

We introduce a novel theory of gravity based on the inverse of the Ricci tensor, that we call the anticurvature tensor. We derive the general equations of motion for any Lagrangian function of the curvature and anticurvature scalars. We then demonstrate a no-go theorem: no Lagrangian that contains terms linear in any positive or negative power of the anticurvature scalar can drive an evolution from deceleration to acceleration, as required by observations. This effectively rules out many realizations of this theory, as we illustrate in detail in a particular case. Finally, we speculate on how to circumvent the no-go theorem.

astro-ph.CO↗

arXiv2020

Update on Coupled Dark Energy and the $H_0$ tension

In this work we provide updated constraints on coupled dark energy (CDE) cosmology with Peebles-Ratra (PR) potential and constant coupling strength $β$. This modified gravity scenario introduces a fifth force between dark matter particles, mediated by a scalar field that plays the role of dark energy. The mass of the dark matter particles does not remain constant, but changes with time as a function of the scalar field. Here we focus on the phenomenological behavior of the model, and assess its ability to describe updated cosmological data sets that include the Planck 2018 cosmic microwave background (CMB) temperature, polarization and lensing, baryon acoustic oscillations, the Pantheon compilation of supernovae of Type Ia, data on $H(z)$ from cosmic chronometers, and redshift-space distortions. We also study which is the impact of the local measurement of $H_0$ from SH0ES and the strong-lensing time delay data from the H0LICOW collaboration on the parameter that controls the strength of the interaction in the dark sector. We find a peak corresponding to a coupling $β> 0$ and to a potential parameter $α> 0$, more or less evident depending on the data set combination. We show separately the impact of each data set and remark that it is especially CMB lensing the one data set that shifts the peak the most towards $Λ$CDM. When a model selection criterion based on the full Bayesian evidence is applied, however, $Λ$CDM is still preferred in all cases, due to the additional parameters introduced in the CDE model.

astro-ph.CO↗

arXiv2020

Scaling solutions and weak gravity in dark energy with energy and momentum couplings

We argue that the $Λ$CDM tensions of the Hubble-Lemaitre expansion rate $H_0$ and the clustering normalization $σ_8$ can be eased, at least in principle, by considering an interaction between dark energy and dark matter in such a way to induce a small and positive early effective equation of state and a weaker gravity. For a dark energy scalar field $ϕ$ interacting with dark matter through an exchange of both energy and momentum, we derive a general form of the Lagrangian allowing for the presence of scaling solutions. In a subclass of such interacting theories, we show the existence of a scaling $ϕ$-matter-dominated-era ($ϕ$MDE) which can potentially alleviate the $H_0$ tension by generating an effective high-redshift equation of state. We also study the evolution of perturbations for a model with $ϕ$MDE followed by cosmic acceleration and find that the effective gravitational coupling relevant to the linear growth of large-scale structures can be smaller than the Newton gravitational constant $G$ at low redshifts. The momentum exchange between dark energy and dark matter plays a crucial role for realizing weak gravity, while the energy transfer is also required for the existence of $ϕ$MDE.

gr-qc↗

arXiv2020

Constraining coupled quintessence with the 21cm signal

The 21cm line probes the evolution of matter perturbations over a wide range of redshifts, from the dark ages down to the completion of reionization. Observing the 21cm cosmological signal will extend our understanding of the evolution of the Universe and it is thus important to investigate the predictions of different cosmological models. In this paper we focus on the prospect of constraining coupled quintessence models during the Epoch of Reionization both for global signal experiments and for intensity mapping surveys. To derive the all-sky 21cm signal and fluctuations in coupled quintessence, we simulate cosmological volumes of the 21cm signal including the coupling between dark matter and the quintessence field, where the strength of the coupling is labeled by the parameter $Q$. We show that the coupling between dark matter and quintessence modifies structure formation and expedites the process of reionization. For the upcoming 21cm line surveys like SKA and a fiducial global 21cm signal experiment, we perform a Fisher matrix analysis to constrain the coupling $Q$ and the dark matter density parameter $Ω_\mathrm{dm}$. The results indicate that SKA will be able to place a 68% upper limit $0.04$ to $|Q|$. At the same time, such a global 21cm detector provides a constraint on the dark matter density parameter $Ω_\mathrm{dm}$ around $ΔΩ_{\rm dm}\approx 0.005$, whereas SKA sets a quite weaker constraint around $ΔΩ_{\rm dm}\approx0.1$. These constraints are comparable to those already obtained from the cosmic microwave background, but explore an entirely different redshift range.

astro-ph.CO↗

arXiv2020

Transient weak gravity in scalar-tensor theories

Sub-horizon perturbations in scalar-tensor theories have been shown to grow generically faster than in uncoupled models due to a positive, additive Yukawa force. In such cases, the amount of clustering becomes larger than in the standard cosmological model, exacerbating the observed tension in the $σ_8$ parameter. Here we show instead that in some simple cases of conformally coupled dark energy one can obtain a transient regime of negative Yukawa force, without introducing ghosts or other instabilities.

gr-qc↗

arXiv2020

Linear cosmological perturbations in Scalar-tensor-vector gravity

We investigate the cosmological perturbations in the context of a Scalar-Tensor-Vector theory of Gravity known as MOG in the literature. Recent investigations show that MOG reproduces a viable background cosmological evolution comparable to $Λ$CDM. However, the matter-dominated era is slightly different. In this paper, we study the linear matter perturbations and estimate the relevant modified gravity parameters. We show that MOG reduces the growth rate of the perturbations and comparing with the RSD data reveals that MOG suggests a higher value for $σ_8$, compare to $Λ$CDM. This point, constitutes a powerful challenge to the cosmological viability of MOG.

gr-qc↗

arXiv2020

Breaking the spell of Gaussianity: forecasting with higher order Fisher matrices

We present the new method DALI (Derivative Approximation for LIkelihoods) for reconstructing and forecasting posteriors. DALI extends the Fisher Matrix formalism but allows for a much wider range of posterior shapes. While the Fisher Matrix formalism is limited to yield ellipsoidal confidence contours, our method can reproduce the often observed flexed, deformed or curved shapes of known posteriors. This gain in shape fidelity is obtained by expanding the posterior to higher order in derivatives with respect to parameters, such that non-Gaussianity in the parameter space is taken into account. The resulting expansion is positive definite and normalizable at every order. Here, we present the new technique, highlight its advantages and limitations, and show a representative application to a posterior of dark energy parameters from supernovae measurements.

astro-ph.CO↗

arXiv2019

Primordial dark matter halos from fifth forces

We argue that primordial dark matter halos could be generated during radiation domination by long range attractive forces stronger than gravity. In this paper we derive the conditions under which these structures could dominate the dark matter content of the Universe while passing microlensing constraints and cosmic microwave background energy injection bounds. The dark matter particles would be clumped in objects in the solar mass range with typical sizes of the order of the solar system. Consequences for direct dark matter searches are important.

astro-ph.CO↗

arXiv2019

Galaxy rotation curves in modified gravity models

In this work, we investigate the possibility that the galaxy rotation curves can be explained in the framework of modified gravity models that introduce a Yukawa term in the gravitational potential. We include dark matter and assume that the fifth-force couples differently to dark matter and to baryons. We aim at constraining the modified gravity parameters $β$ and $λ$, that is, the strength and the range of the Yukawa fifth force, respectively, using a set of 40 galaxy rotation curves data from the SPARC catalogue. We include baryonic gas, disk and bulge components, along with a NFW halo of dark matter. Each galaxy rotation curve is modeled with three free parameters, beside the two global Yukawa parameter. We find that the inclusion of the Yukawa term improves the $χ^2$ from $680.75$ to $536.23$ for $655$ degrees of freedom. As global best-fit we obtain $β= 0.34\pm0.04$ and $λ= 5.61\pm0.91$kpc and a dark matter content on average 20\% smaller than without the Yukawa term. The Bayesian evidence in favor of a NFW profile plus Yukawa term is higher than 8$σ$ with respect to the standard gravity parametrization.

astro-ph.GA↗

arXiv2019

Beyond self-acceleration: force- and fluid-acceleration

The notion of self-acceleration has been introduced as a convenient way to theoretically distinguish cosmological models in which acceleration is due to modified gravity from those in which it is due to the properties of matter or fields. In this paper we review the concept of self-acceleration as given, for example, by [1], and highlight two problems. First, that it applies only to universal couplings, and second, that it is too narrow, i.e. it excludes models in which the acceleration can be shown to be induced by a genuine modification of gravity, for instance coupled dark energy with a universal coupling, the Hu-Sawicki f(R) model or, in the context of inflation, the Starobinski model. We then propose two new, more general, concepts in its place: force-acceleration and field-acceleration, which are also applicable in presence of non universal cosmologies. We illustrate their concrete application with two examples, among the modified gravity classes which are still in agreement with current data, i.e. f(R) models and coupled dark energy.

astro-ph.CO↗

arXiv2019

Fisher matrix for multiple tracers: model independent constraints on the redshift distortion parameter

We show how to obtain constraints on $β=f/b$, the ratio of the matter growth rate and the bias that quantifies the linear redshift-space distortions, that are independent of the cosmological model, using multiple tracers of large-scale structure. For a single tracer the uncertainties on $β$ are constrained by the uncertainties in the amplitude and shape of the power spectrum, which is limited by cosmic variance. However, for two or more tracers this limit does not apply, since taking the ratio of power spectra cosmic variance cancels out, and in the linear (Kaiser) approximation one measures directly the quantity $(1+ β_1 μ^2)^2/(1+ β_2 μ^2)^2$, where $μ$ is the angle of a given mode with the line of sight. We provide analytic formulae for the Fisher matrix for one and two tracers, and quantify the signal-to-noise ratio needed to make effective use of the multiple-tracer technique. We also forecast the errors on $β$ for a survey like Euclid.

astro-ph.CO↗

arXiv2019

$H_0$ from cosmic chronometers and Type Ia supernovae, with Gaussian processes and the weighted polynomial regression method

We derive new constraints on the Hubble parameter $H_0$ using the available data on $H(z)$ from cosmic chronometers (CCH), and the Hubble rate data points from the supernovae of Type Ia (SnIa) of the Pantheon compilation and the Hubble Space Telescope (HST) CANDELS and CLASH Multy-Cycle Treasury (MCT) programs. We employ two alternative techniques, Gaussian Processes (GPs) and the Weighted Polynomial Regression (WPR) method, to reconstruct the Hubble function, determine the derived values of $H_0$, and compare them with the local HST measurement provided by Riess {\it et al.} (2018), $H_0^{\rm HST}=(73.48\pm1.66)$ km/s/Mpc, and with the Planck+$Λ$CDM value, $H_0^{\rm P18}=(66.88\pm 0.92)$ km/s/Mpc. With GPs we obtain $H_0=(67.99\pm 1.94)$ km/s/Mpc, and with the WPR method $H_0=(68.90\pm 1.96)$ km/s/Mpc. Both are fully compatible at $<1σ$ c.l., and also with $H_0^{\rm P18}$. In contrast, they are in $\sim 2σ$ tension with $H_0^{\rm HST}$.

astro-ph.CO↗

arXiv2019

Measuring gravity at cosmological scales

This paper is a pedagogical introduction to models of gravity and how to constrain them through cosmological observations. We focus on the Horndeski scalar-tensor theory and on the quantities that can be measured with a minimum of assumptions. Alternatives or extensions of General Relativity have been proposed ever since its early years. Because of Lovelock theorem, modifying gravity in four dimensions typically means adding new degrees of freedom. The simplest way is to include a scalar field coupled to the curvature tensor terms. The most general way of doing so without incurring in the Ostrogradski instability is the Horndeski Lagrangian and its extensions. Testing gravity means therefore, in its simplest term, testing the Horndeski Lagrangian. Since local gravity experiments can always be evaded by assuming some screening mechanism or that baryons are decoupled, or even that the effects of modified gravity are visible only at early times, we need to test gravity with cosmological observations in the late universe (large-scale structure) and in the early universe (cosmic microwave background). In this work we review the basic tools to test gravity at cosmological scales, focusing on model-independent measurements.

astro-ph.CO↗

arXiv2019

Observational Constraints in Nonlocal Gravity: the Deser-Woodard Case

We study the cosmology of a specific class of nonlocal model of modified gravity, the so-called Deser-Woodard (DW) model, modifying the Einstein-Hilbert action by a term $\sim R f(\Box^{-1}R)$, where $f$ is a free function. Choosing $f$ so as to reproduce the $Λ{\rm CDM}$ cosmological background expansion history within the nonlocal model, we implement the model in a cosmological linear Einstein--Boltzmann solver and study the deviations to GR the model induces in the scalar and tensor perturbations. We observe that the DW nonlocal model describes a modified propagation for the gravitational waves, as well as a lower linear growth rate and a stronger lensing power as compared to $Λ{\rm CDM}$, up to several percents. Such prominent growth and lensing features lead to the inference of a significantly smaller value of $σ_8$ with respect to the one in $Λ{\rm CDM}$, given \textit{Planck} CMB+lensing data. The prediction for the linear growth rate $f σ_8$ within the DW model is therefore significantly smaller than the one in $Λ{\rm CDM}$ and the addition of growth rate data $f σ_8$ from Redshift-space distortion measurements to \textit{Planck} CMB+lensing, opens a (dominant) tension between Redshift-space distortion data and the reconstructed \textit{Planck} CMB lensing potential. However, model selection issues only result in "weak" evidences for $Λ{\rm CDM}$ against the DW model given the data. Such a fact shows that the datasets we consider are not constraining enough for distinguishing between the models. As we discuss, the addition of galaxy WL data or cosmological constraints from future galaxy clustering, weak lensing surveys, but also third generation gravitational wave interferometers, prove to be useful for discriminating modified gravity models such as the DW one from $Λ{\rm CDM}$, within the close future.

astro-ph.CO↗

arXiv2019

Doppelgänger dark energy: modified gravity with non-universal couplings after GW170817

Gravitational Wave (GW) astronomy severely narrowed down the theoretical space for scalar-tensor theories. We propose a new class of attractor models for Horndeski action in which GWs propagate at the speed of light in the nearby universe but not in the past. To do so we derive new solutions to the interacting dark sector in which the ratio of dark energy and dark matter remains constant, which we refer to as doppelgänger dark energy (DDE). We then remove the interaction between dark matter and dark energy by a suitable change of variables. The accelerated expansion that (we) baryons observe is due to a conformal coupling to the dark energy scalar field. We show how in this context it is possible to find a non trivial subset of solutions in which GWs propagate at the speed of light only at low red-shifts. The model is an attractor, thus reaching the limit $c_{T}\to1$ relatively fast. However, the effect of baryons turns out to be non-negligible and severely constrains the form of the Lagrangian. In passing, we found that in the simplest DDE models the no-ghost conditions for perturbations require a non-universal coupling to gravity. In the end, we comment on possible ways to solve the lack of matter domination stage for DDE models.

gr-qc↗

arXiv2019

Future constraints on the gravitational slip with the mass profiles of galaxy clusters

The gravitational slip parameter is an important discriminator between large classes of gravity theories at cosmological and astrophysical scales. In this work we use a combination of simulated information of galaxy cluster mass profiles, inferred by Strong+Weak lensing analyses and by the study of the dynamics of the cluster member galaxies, to reconstruct the gravitational slip parameter $η$ and predict the accuracy with which it can be constrained with current and future galaxy cluster surveys. Performing a full-likelihood statistical analysis, we show that galaxy cluster observations can constrain $η$ down to the percent level already with a few tens of clusters. We discuss the significance of possible systematics, and show that the cluster masses and numbers of galaxy members used to reconstruct the dynamics mass profile have a mild effect on the predicted constraints.

astro-ph.CO↗

arXiv2019

Coupled quintessence with a $Λ$CDM background: removing the $σ_8$ tension

A well-known problem of the $Λ$CDM model is the tension between the relatively high level of clustering, as quantified by the parameter $σ_8$, found in cosmic microwave background experiments and the smaller one obtained from large-scale observations in the late Universe. In this paper we show that coupled quintessence, i.e. a single dark energy scalar field conformally coupled to dark matter through a constant coupling, can solve this problem if the background is taken to be identical to the $Λ$CDM one. We show that two competing effects arise. On one hand, the additional scalar force is attractive, and is therefore expected to increase the clustering. On the other, in order to obtain the same background as $Λ$CDM, coupled quintessence must have a smaller amount of dark matter near the present epoch. We show that the second effect is dominating today and leads to an overall slower growth. Comparing to redshift distortion data, we find that coupled quintessence with $Λ$CDM background solves the tension between early and late clustering. We find for the coupling $β$ and for $σ_8$ the best fit values $|β| = 0.079^{+ 0.059}_{- 0.067}$ and $σ_8 = 0.818^{+0.115}_{-0.088}$. These values also fit the lensing data from the KiDS-450 survey. We also estimate that the future missions SKA and Euclid will constrain $β$ with an error of $\pm\, 1.5\times10^{-3}$ and for $σ_8$ of $\pm \,1.8\times10^{-3}$ at $1σ$ level.

astro-ph.CO↗