arXiv subjects

Andrea Giusti

Publications and source records attributed to Andrea Giusti.

At least 19 recordsLinked to original sources

arXiv2026

Thermomechanics of Horndeski Scalar Hair with Spacelike Gradients

We develop a thermomechanical framework for viable Horndeski scalar hair with spacelike scalar-field gradients. Adapting the $(1{+}1{+}2)$ formalism to the scalar configuration, we apply the effective imperfect-fluid decomposition to the model and derive general results governing its thermomechanical properties. Specifically, we infer the constitutive relations for the effective medium, in both observer-dependent and observer-independent approaches, thereby identifying the conditions under which the scalar hair admits thermomechanical interpretations. The observer-dependent approach leads to an analogy with anisotropic media characterized by a preferred spatial direction, whereas the observer-independent view shows similarities with layered media. The general results are then specialized to static, spherically symmetric configurations with scalar hair depending solely on the areal radius. Interestingly, this scenario allows us to connect the thermomechanical framework with the Tolman--Ehrenfest criterion for thermal equilibrium.

gr-qc↗

arXiv2026

Coherent quantum geometry: de Sitter spacetime in different foliations

In any theory of quantum gravity, an interesting question to address is to what extent known solutions of the Einstein field equations can be obtained as expectation values of metric operators on suitable quantum states. In this work, we consider coherent states (to ensure minimum uncertainty) for different foliations of the de~Sitter spacetime and study temporal evolution and coordinate invariance. A general framework will first be introduced for metrics that can be diagonalised globally. We will then find that the normalisability of coherent states in this framework requires using reference frames without coordinate singularities.

gr-qc↗

arXiv2026

Six-Field Rational Extended Thermodynamics of Polyatomic Gases in Curved Spacetime

We formulate a generally covariant six-field Rational Extended Thermodynamics model (RET$_6$) for relativistic polyatomic gases, with the dynamical pressure as the only non-equilibrium variable. The model is based on a polyatomic extension of the Boltzmann-Chernikov kinetic equation, where the one-particle distribution depends also on an internal-energy variable, and on the Maximum Entropy closure of the associated relativistic moment hierarchy. The resulting field equations, closure relations, and production term are therefore fixed by the underlying kinetic structure rather than postulated phenomenologically. We extend the RET$_6$ model from Minkowski spacetime to a general curved spacetime by the minimal coupling prescription and couple it to the Einstein equations. As a first structural result, we prove a kinetic-theory no-go theorem in this polyatomic RET setting stating that any stress-energy tensor induced by a non-negative relativistic one-particle distribution function satisfies the strong energy condition. We then specialize the theory to a homogeneous and isotropic Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) spacetime. In this setting the dynamical pressure modifies the expansion dynamics with respect to the perfect-fluid Euler case, but the no-go theorem excludes acceleration driven by the RET$_6$ gas alone. Finally, we reintroduce a cosmological constant and study the combined $\Lambda$RET$_6$ model. For the diatomic equation of state and a constant positive relaxation time, we prove the existence and local stability of a de Sitter attractor at late times. Numerical integrations show that, for representative post-recombination initial data and constant relaxation times, the expansion history rapidly approaches that of $\Lambda$CDM, with small non-equilibrium corrections controlled by the relaxation time and by the initial value of the dynamical pressure.

gr-qc↗

arXiv2026

Effect of a magnetostatic field on laminar premixed hydrogen-air flames

Magnetic fields have shown potential to affect flame characteristics; however, the mechanisms of interaction are not fully understood. This paper investigates the effect of magnetic fields on premixed hydrogen-air flames that are prone to intrinsic instabilities, with a focus on the role of magnetic forces on the flame behaviour. The study is conducted using direct numerical simulations. Two flame conditions, both with an equivalence ratio of 0.5, are studied, one with the reactants at atmospheric conditions and the other at high pressure and high temperature. Different configurations of the magnetic field are investigated, each characterised by a different gradient of the square of the magnitude of the magnetic field, oriented in the direction opposite to the velocity of the incoming reactants. Results show that the investigated configurations of the magnetic field can reduce the flame consumption speed, an effect that is substantial in the lower pressure case, while it becomes negligible at high pressure. The effect of the magnetic forces increases with increasing gradient of the magnetic field and is mainly due to the reduction of the flame area. Results also show that the effects of magnetic fields on the reactivity of the flame and on the small cell structures developed along the flame front are negligible. Analysis of the force contributions demonstrates that the change in the flame area is caused by the rotational component of the magnetic forces, which alter the vorticity of the flow such that the finger-like structures formed by hydrodynamic instabilities tend to close. These forces are significant at low pressure, while they become negligible compared to the pressure gradient at high pressure. Ultimately, the results of this work indicate that magnetic forces have the potential to change the flame behaviour, a mechanism that could be used for active control of flames.

physics.flu-dyn↗

arXiv2026

Towards a causal effective thermodynamics of scalar-tensor gravity

The thermal analogy between the effective fluid of scalar-tensor gravity and Eckart's irreversible thermodynamics is extended to the causal Israel-Stewart model, adopting the minimal ansatz of promoting the heat flux density to a timelike vector. This choice yields analytically manageable constitutive equations, allowing for the first consistent decoupling of the effective temperature $\mathcal{T}$ and the effective thermal conductivity $\mathcal{K}$ of scalar-tensor gravity. Crucially, this new framework preserves the interpretation of general relativity as the equilibrium state approached via a dynamical relaxation process in the vanishing-$\mathcal {KT}$ limit. This new causal formalism is applied to cosmology.

gr-qc↗

arXiv2026

On gravitational collapse and integrable singularities

Schwarzschild black holes are expected to emerge as the end states of the classical gravitational collapse from non-singular configurations. After integrable curvature singularities appear, the interior geometry can be modelled to exhibit a transition, called ``Minkowski breaking'', when the inner horizon disappears, before all matter collapses into the central singularity. This picture implies a quantum framework to describe the final stages of the gravitational collapse, and here we will provide more insights from the semiclassical approximation for the energy-momentum tensor and the Madelung approximation for collapsing matter. In particular, we will show that the quantum potential in the Raychaudhuri equation starts to strongly oppose the collapse towards the Schwarzschild singularity precisely after the Minkowski breaking.

gr-qc↗

arXiv2026

Investigation of Differential Diffusion and Strain Coupling in Large Eddy Simulations of Hydrogen-Air Flames

Large Eddy Simulations with flamelet-based thermochemistry are used to investigate the behaviour of a premixed hydrogen-air flame stabilised by a bluff-body. Validation against experimental data is carried out first to demonstrate the model's ability to predict both velocity field and flame structure. The capability of the model in predicting differential diffusion effects is then assessed, in particular regarding the coupling between differential diffusion, tangential strain and curvature, and their effect on mixture fraction redistribution and reaction rate variation. Results indicate that unstretched flamelet thermochemistry is capable of capturing the increase in mixture fraction caused by positive resolved strain, as well as negative variations of mixture fraction due to negative curvature. Furthermore, the model is observed to mimic the effects of negative Markstein length to a certain extent, so that positive tangential strain causes reaction rate increase. The interplay between resolved stretch and preferential diffusion is also shown to lead to a shorter flame length which is in better agreement with experimental observations as compared to simulations under unity Lewis number assumption. These findings highlight that the macroscopic effects of differential diffusion and stretch on the premixed hydrogen flame, characterised by significant strain levels, can be predicted using a flamelet-based approach and without recurring to strained flamelets database, which implies important simplifications in the combustion modelling of turbulent hydrogen-premixed flames and offers valuable insights for the design of novel combustors.

physics.flu-dyn↗

arXiv2026

Revisiting induced gravity in scalar-tensor thermodynamics

Induced gravity, defined as a globally scale-invariant ``first-generation'' scalar-tensor theory, is investigated within the framework of the thermodynamics of modified gravity theories. The ``temperature of gravity'' and its evolution equation are derived for this model, and the resulting expressions are used to analyse General-Relativity equilibrium states and to investigate the possible existence of an attractor mechanism toward Einstein's theory with a cosmological constant.

gr-qc↗

arXiv2026

Probing Cosmic Expansion and Early Universe with Einstein Telescope

Over the next two decades, gravitational-wave (GW) observations are expected to evolve from a discovery-driven endeavour into a precision tool for astrophysics, cosmology, and fundamental physics. Current second-generation ground-based detectors have established the existence of compact-binary mergers and enabled GW multi-messenger astronomy, but they remain limited in sensitivity, redshift reach, frequency coverage, and duty cycle. These limitations prevent them from addressing many fundamental open questions in cosmology. By the 2040s, wide-field electromagnetic surveys will have mapped the luminous Universe with unprecedented depth and accuracy. Nevertheless, key problems including the nature of dark matter, the physical origin of cosmic acceleration, the properties of gravity on cosmological scales, and the physical conditions of the earliest moments after the Big Bang will remain only partially constrained by electromagnetic observations alone. Progress on these fronts requires access to physical processes and epochs that do not emit light. Gravitational waves provide a unique and complementary observational channel: they propagate over cosmological distances largely unaffected by intervening matter, probe extreme astrophysical environments, and respond directly to the geometry of spacetime. In this context, next-generation GW observatories such as the Einstein Telescope (ET) will be transformative for European astronomy. Operating at sensitivities and frequencies beyond existing detectors, ET will observe binary black holes and neutron stars out to previously inaccessible redshifts, enable continuous high signal-to-noise monitoring of compact sources, and detect gravitational-wave backgrounds of astrophysical and cosmological origin. Together with space-based detectors, ET will play a central role in advancing our understanding of cosmic evolution and fundamental physics.

astro-ph.CO↗

arXiv2026

Numerical Investigation of the Effect of a Magnetic Field on the Transport of Oxygen in Air

The effects of magnetisation forces in a binary mixture of gases characterised by large differences in magnetic susceptibility are studied using numerical simulations, with a focus on the differential diffusion of the species and the role of the gradient of mixture composition on the flow field resulting from magnetically-induced forces. A quiescent binary mixture of nitrogen and oxygen, representative of air, confined between two parallel plates is considered. In all simulations, a gradient of $\mathbf{B}^2$, the square of the magnetic flux density magnitude, uniform and directed normal to the walls is imposed. Cases characterised by different pressures, different strengths of $\nabla(\mathbf{B}^2)$, and different initial gradients of species composition are investigated, while the same initial temperature is used in all cases. Non-dimensional groups related to the examined configuration are proposed. In cases characterised by an initially uniform mixture composition, species tend to separate and accumulate at opposite walls, due to differential magnetic forces arising from the differences in magnetic susceptibility. For a given strength of $\nabla(\mathbf{B}^2)$, the effect of the magnetic field on the separation of species increases with decreasing pressure.In addition to species separation, it is shown that an initial gradient in the mixture composition perpendicular to $\nabla(\mathbf{B}^2)$ induces a significant change in the velocity field, which enhances the transport of species. This effect is due to a lack of alignment between the gradient of averaged magnetic susceptibility and $\nabla(\mathbf{B}^2)$ and could be exploited to achieve targeted mixing using engineered magnetic fields.

physics.flu-dyn↗

arXiv2025

Quantum dust cores of black holes and their quasi-normal modes

The quantum description of a gravitationally collapsed ball of dust proposed in Ref.~\cite{Casadio:2023ymt} is characterised by a linear effective Misner-Sharp-Hernandez mass function describing a matter core hidden by the event horizon. After reviewing the original model and some of its refinements, we investigate the quasi-normal mode spectrum of the resulting spacetime and compare it with the Schwarzschild case. Computations are performed within the WKB approximation, based on the Pad\'e approximants up to thirteenth order. Our analysis shows that deviations from the Schwarzschild spectrum are sensitive to the quantum nature of the core surface.

gr-qc↗

arXiv2025

Black Hole Solutions in Quantum Gravity with Vilkovisky-DeWitt Effective Action

We study new black hole solutions in quantum gravity. We use the Vilkovisky-DeWitt unique effective action to obtain quantum gravitational corrections to Einstein's equations. In full analogy to previous work done for quadratic gravity, we find new black hole like solutions. We show that these new solutions exist close to the horizon and in the far-field limit.

hep-th↗

arXiv2025

Coherent electrically-charged quantum black holes

We improve upon the results presented in [R. Casadio, et al., Phys. Rev. D 105 (2022) 124026] deriving a quantum-corrected Reissner-Nordstr\"om geometry containing an integrable singularity at its center while being devoid of spurious oscillations around the classical configuration. We further investigate some relevant physical observables, related to geodesics and quasinormal modes of scalar perturbations, associated with this geometry to complement our theoretical analysis.

gr-qc↗

arXiv2025

Quasinormal modes for coherent quantum black holes

Coherent quantum black holes are quantum geometries obtained by means of a mean-field-like approach to the gravitational interaction. This procedure attenuates the classical spacetime singularities of general relativity by replacing them with integrable singularities in the quantum-corrected geometry. After discussing some relevant observables for a novel geometry for spherically symmetric black holes, we investigate the quasinormal modes spectrum of scalar, electromagnetic, and gravitational fields for the proposed model. The results indicate potential deviations from general relativity, the magnitude of which is gauged by the value of the ultraviolet regulator of the model (physically identifiable as a matter core). Observations of the ringdown phase in black hole mergers could help detect such deviations.

gr-qc↗

arXiv2025

A computational approach for the study of electromagnetic interactions in reacting flows

A computational fluid dynamics methodology for the simulation of electromagnetic interactions in compressible reacting flows has been formulated. The developed code, named EMI, is based on the SENGA Direct Numerical Simulation (DNS) software. Static electric and magnetic fields are solved using Gauss's laws of Maxwell's equations. Electromagnetic wave propagation is solved by discretizing Ampere's and Faraday's equations using the explicit Finite-Difference Time-Domain (FDTD) method. The equations for the electromagnetic fields are fully coupled with the Navier-Stokes equations, such that interactions between the electromagnetic fields and the fluid are included in the formulation. The interaction terms include the Lorentz, polarization, and magnetization forces. These forces determine volume forces that affect the transport of momentum, the diffusion velocity, and the energy conservation equations. In addition, the medium's properties affect the propagation of the electromagnetic fields via electrical permittivity and conductivity, charge density, and magnetic permeability. The solution of electromagnetic fields is validated against analytical and numerical solutions. The implementation of the coupling between electromagnetic fields and conservation equations for species, energy, and momentum is validated with laminar reacting flow numerical solutions from the literature. The capabilities of the formulation are investigated for a range of laminar methane-air computations under electrostatic, magnetostatic, and high-frequency electromagnetic waves. The validity of the electrostatic formulation in the presence of currents related to the movement of charged species is also assessed. Results demonstrate that EMI-SENGA can capture the fundamental effects of electromagnetic fields on reacting flows and the dynamics of charged species and their effect on flame shape and reactivity.

physics.flu-dyn↗

arXiv2025

CONCERT: a Modular Reconfigurable Robot for Construction

This paper presents CONCERT, a fully reconfigurable modular collaborative robot (cobot) for multiple on-site operations in a construction site. CONCERT has been designed to support human activities in construction sites by leveraging two main characteristics: high-power density motors and modularity. In this way, the robot is able to perform a wide range of highly demanding tasks by acting as a co-worker of the human operator or by autonomously executing them following user instructions. Most of its versatility comes from the possibility of rapidly changing its kinematic structure by adding or removing passive or active modules. In this way, the robot can be set up in a vast set of morphologies, consequently changing its workspace and capabilities depending on the task to be executed. In the same way, distal end-effectors can be replaced for the execution of different operations. This paper also includes a full description of the software pipeline employed to automatically discover and deploy the robot morphology. Specifically, depending on the modules installed, the robot updates the kinematic, dynamic, and geometric parameters, taking into account the information embedded in each module. In this way, we demonstrate how the robot can be fully reassembled and made operational in less than ten minutes. We validated the CONCERT robot across different use cases, including drilling, sanding, plastering, and collaborative transportation with obstacle avoidance, all performed in a real construction site scenario. We demonstrated the robot's adaptivity and performance in multiple scenarios characterized by different requirements in terms of power and workspace. CONCERT has been designed and built by the Humanoid and Human-Centered Mechatronics Laboratory (HHCM) at the Istituto Italiano di Tecnologia in the context of the European Project Horizon 2020 CONCERT.

cs.RO↗

arXiv2025

Quantum maximally symmetric space-times

We show that 4-dimensional maximally symmetric spacetimes can be obtained from a coherent state quantisation of gravity, always resulting in geometries that approach the Minkowski vacuum exponentially away from the radius of curvature. A possible connection with the central charge in the AdS/CFT correspondence is also noted.

gr-qc↗

arXiv2025

Thermal origin of the attractor-to-general-relativity in scalar-tensor gravity

The convergence of scalar-tensor gravity to general relativity, or the departure from it, are described in a new analogy with heat dissipation in a viscous fluid. This new thermal picture is applied to cosmology, shedding light on whether gravity deviates from general relativity early on and approaches it later in the cosmic history.

gr-qc↗