Papers by Giovanni Acquaviva
Cornell University - arXiv, May 16, 2018
Dissipative features of dark matter affect its clustering properties and could lead to observable... more Dissipative features of dark matter affect its clustering properties and could lead to observable consequences for the evolution of large-scale structure. We analyse the evolution of cold dark matter density perturbations allowing for the possibility of bulk viscous pressure in a causal dissipative theory. Our analysis employs a Newtonian approximation for cosmological dynamics and the transport properties of bulk viscosity are described by the Israel-Stewart theory. We obtain a third order evolution equation for density perturbations. For some parameter values the density contrast can be suppressed compared to results obtained in the ΛCDM scenario. For other values causal bulk viscous dark matter can exhibit an enhancement of clustering.
arXiv (Cornell University), Feb 28, 2019
We demonstrate that the thermodynamics of a perfect fluid describing baryonic matter can, in cert... more We demonstrate that the thermodynamics of a perfect fluid describing baryonic matter can, in certain limits, lead to an equation of state similar to that of dark energy. We keep the cosmic fluid equation of state quite general by just demanding that the speed of sound is positive and less than the speed of light. In this framework, we discuss some propositions by looking at the asymptotic behaviour of the cosmic fluid.
Universe
Analogs of fundamental physical phenomena can be used in two ways. One way consists in reproducin... more Analogs of fundamental physical phenomena can be used in two ways. One way consists in reproducing specific aspects of the classical or quantum gravity of quantum fields in curved space or of other high-energy scenarios on lower-energy corresponding systems. The “reverse way” consists in building fundamental physical theories, for instance, quantum gravity models, inspired by the lower-energy corresponding systems. Here, we present the case of graphene and other Dirac materials.
SSRN Electronic Journal
We perform the dynamical system analysis of a cosmological model in the energy-momentum squared g... more We perform the dynamical system analysis of a cosmological model in the energy-momentum squared gravity (EMSG) of the form f (Tµν T µν) = α ln(λTµν T µν), which is known as energymomentum log gravity (EMLG). In particular, we show that the analytical cosmological solution of EMLG presented by Akarsu et al. (Eur. Phys. J. C 79:846, 2019) is a future attractor. It includes new terms in the right-hand side of the Einstein field equations, which yield constant inertial mass density and provide a dynamical dark energy with a density passing below zero at large redshifts, accommodating a mechanism for screening Λ in the past, suggested for alleviating some cosmological tensions. We show that the model gives rise to an entire class of new stable late-time solutions with H → (Λ + 2α)/3 as a → ∞, where the new term is due to the constant effective inertial mass density that arises from EMLG contribution of dust, whereas H → Λ/3 as a → ∞ in the ΛCDM model. We also show existence of new interesting features and trajectories that are absent in the ΛCDM model.
In this contribution we describe some interesting interplay between quantum theory, general relat... more In this contribution we describe some interesting interplay between quantum theory, general relativity and thermodynamics. In order to highlight the connection between these theories, we describe two approaches that allow to calculate thermal features as perceived by different observers in curved spacetimes. the tunnelling method and the Unruh-DeWitt detector. In this context, the semi-classical tunnelling approach is applied to the issue of Hawking radiation and allows the calculation of the horizon temperature in a wide variety of scenarios. The Unruh-DeWitt model is instead a quantum field-theoretical approach that should give a more exact answer in terms of transition rates between energy levels of an idealized detector.
Modern Physics Letters A, 2021
In Polymer Quantum Mechanics, a quantization scheme that naturally emerges from Loop Quantum Grav... more In Polymer Quantum Mechanics, a quantization scheme that naturally emerges from Loop Quantum Gravity, position and momentum operators cannot be both well defined on the Hilbert space [Formula: see text]. It is henceforth deemed impossible to define standard creation and annihilation operators. In this paper, we show that a [Formula: see text]-oscillator structure, and hence [Formula: see text]-deformed creation/annihilation operators, can be naturally defined on [Formula: see text], which is then mapped into the sum of many copies of the [Formula: see text]-oscillator Hilbert space. This shows that the [Formula: see text]-calculus is a natural calculus for Polymer Quantum Mechanics. Moreover, we show that the inequivalence of different superselected sectors of [Formula: see text] is of topological nature.
Proceedings of 40th International Conference on High Energy physics — PoS(ICHEP2020), 2021
Here we briefly resume the idea, originally introduced in Phys. Rev. D 102, 106002 (2020), that t... more Here we briefly resume the idea, originally introduced in Phys. Rev. D 102, 106002 (2020), that the Bekenstein bound on entropy is a consequence of the fermionic nature of fundamental degrees of freedom, which arrange themselves to form matter and spacetime. The main point is discussed by means of a toy-model of black hole evaporation, which describes the dynamics of such degrees of freedom, called Xons. An intrinsic notion of interior/exterior of the black hole during the evaporation process is given and both von Neumann and black hole/environment entropies are computed.
arXiv: General Relativity and Quantum Cosmology, 2015
We use the formulation of thermodynamics of gravity as proposed by Clifton, Ellis and Tavakol on ... more We use the formulation of thermodynamics of gravity as proposed by Clifton, Ellis and Tavakol on the gravitational collapse of dustlike matter, that violates the strong or weak cosmic censorship conjecture depending on the initial data. We transparently demonstrate that the gravitational entropy prefers the scenario where the stronger version is violated but the weak censorship conjecture is satisfied. This is a novel result, showing the weak cosmic censorship and hence the future asymptotically simple structure of spacetime, is being validated by the nature of gravity, without imposing any extra constraint on the form of matter.
Physical Review D, 2021
In this work, we first discuss the possibility that dark energy models with negative energy densi... more In this work, we first discuss the possibility that dark energy models with negative energy density values in the past can alleviate the H0 tension, as well as the discrepancy with the baryon acoustic oscillation (BAO) Lyman-α data, both which prevail within the ΛCDM model. We then investigate whether two minimal extensions of the ΛCDM model, together or separately, can successfully realize such a scenario: (i) the spatial curvature, which, in the case of spatially closed universe, mimics a negative density source and (ii) simple-graduated dark energy (gDE), which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant-if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. We find that, when the Planck data are not included in the observational analysis, the models with simple-gDE predict interesting and some significant deviations from the ΛCDM model. In particular, a spatially closed universe along with a simple-gDE of positive inertial mass density, which work in contrast to each other, results in minor improvement to the H0 tension. The joint dataset, including the Planck data, presents no evidence for a deviation from spatial flatness but almost the same evidence for a cosmological constant and the simple-gDE with an inertial mass density of order O(10 −12) eV 4. The latter case predicts almost no deviation from the ΛCDM model up until today-so that it results in no improvement regarding the BAO Ly-α data-except that it slightly aggravates the H0 tension. We also study via dynamical analysis the history of the Universe in the models, as the simple-gDE results in futures different than the de Sitter future of the ΛCDM model.
Annals of Physics, 2021
We discuss the representations of the algebra of quantization, the canonical commutation relation... more We discuss the representations of the algebra of quantization, the canonical commutation relations, in a scalar quantum field theory with spontaneously broken U (1) internal symmetry, when a topological defect of the vortex type is formed via the condensation of Nambu-Goldstone particles. We find that the usual thermodynamic limit is not necessary in order to have the inequivalent representations needed for the existence of physically disjoint phases of the system. This is a new type of inequivalence, due to the nontrivial topological structure of the phase space, that appears at finite volume. We regard this as a first step towards a unifying view of topological and thermodynamic phases, and offer here comments on the possible application of this scenario to quantum gravity.
Physical Review D, 2020
Assuming that the degrees of freedom of a black hole are finite in number and of fermionic nature... more Assuming that the degrees of freedom of a black hole are finite in number and of fermionic nature, we naturally obtain, within a second-quantized toy model of the evaporation, that the Bekenstein bound is a consequence of the Pauli exclusion principle for these fundamental degrees of freedom. We show that entanglement, Bekenstein and thermodynamic entropies of the black hole all stem from the same approach, based on the entropy operator whose structure is the one typical of Takahashi and Umezawa's Thermofield Dynamics. We then evaluate the von Neumann black hole-environment entropy and noticeably obtain a Page-like evolution. We finally show that this is a consequence of a duality between our model and a quantum dissipative-like fermionic system.
Physical Review D, 2019
In this work we perform a dynamical analysis of a broad class of non-minimally coupled real scala... more In this work we perform a dynamical analysis of a broad class of non-minimally coupled real scalar fields in the Friedmann-Robertson-Walker (FRW) spacetime framework. The first part of our study concerns the dynamics of an unspecified positive potential in a spatially curved FRW spacetime, for which we define a new set of dimensionless variables and a new evolution parameter. In the framework of this general setup we have recognized several general features of the system, like symmetries, invariant subsets and critical points, and provide their cosmological interpretation. The second part of our work focuses on flat FRW cases for which the tracker parameter is constant, i.e. we examine specific classes of potentials. After analyzing these cases dynamically, we discuss their physical interpretation.
Physical Review D, 2020
In this article we perform dynamical analysis of a broad class of barotropic fluids in the spatia... more In this article we perform dynamical analysis of a broad class of barotropic fluids in the spatially curved Friedmann-Robertson-Walker (FRW) spacetime background without considering the cosmological constant. The first part of our study concerns the dynamics of a fluid with an unspecified barotropic equation of state (EoS) having as the only assumption the non-negativity of the fluid's energy density. After defining a new set of dimensionless variables and a new evolution parameter, we introduce the function Γ that encodes the EoS. In this general setup several features of the system are identified: critical points, invariant subsets and the characteristics of the function Γ, along with their cosmological interpretations. The second part of our work provides two examples with specific Γ functions. In the first example we provide a Γ function and then we exhibit how it can be trimmed down to a specific class of EoS through physical arguments, while in the second example we discuss the quadratic EoS studied in Phys.Rev. D 74, 023523 (2006) by comparing our approach with their analysis.
Classical and Quantum Gravity, 2018
We perform a global analysis of curved Friedmann-Robertson-Walker cosmologies in the presence of ... more We perform a global analysis of curved Friedmann-Robertson-Walker cosmologies in the presence of a viscous fluid. The fluid's bulk viscosity is governed by a first order theory recently proposed in [18], and the analysis is carried out in a compactified parameter space with dimensionless coordinates. We provide stability properties, cosmological interpretation and thermodynamic features of the critical points.
Proceedings of Corfu Summer Institute 2017 "Schools and Workshops on Elementary Particle Physics and Gravity" — PoS(CORFU2017), 2018
We provide general arguments regarding the connection between low-energy theories (gravity and qu... more We provide general arguments regarding the connection between low-energy theories (gravity and quantum field theory) and a hypothetical fundamental theory of quantum gravity, under the assumptions of (i) validity of the holographic bound and (ii) preservation of unitary evolution at the level of the fundamental theory. In particular, the appeal to the holographic bound imposed on generic physical systems by the Bekenstein-Hawking entropy implies that both classical geometry and quantum fields propagating on it should be regarded as phenomena emergent from the dynamics of the fundamental theory. The reshuffling of the fundamental degrees of freedom during the unitary evolution then leads to an entanglement between geometry and quantum fields. The consequences of such scenario are considered in the context of black hole evaporation and the related information-loss issue: we provide a simplistic toy model in which an average loss of information is obtained as a consequence of the geometry-field entanglement.
Classical and Quantum Gravity, 2018
A proposal for the gravitational energy-momentum tensor, known in the literature as the square ro... more A proposal for the gravitational energy-momentum tensor, known in the literature as the square root of Bel-Robinson tensor, is analyzed in detail. Being constructed exclusively from the Weyl part of the Riemann tensor, such tensor encapsulates the geometric properties of free gravitational fields in terms of optical scalars of null congruences: making use of the general decomposition of any energymomentum tensor, we explore the thermodynamic interpretation of such geometric quantities. While the matter energy-momentum is identically conserved due to Einstein's field equations, the SQBR is not necessarily conserved and dissipative terms could arise in its vacuum continuity equation. We discuss the possible physical interpretations of such mathematical properties.
Physical Review D, 2016
The inclusion of dissipative effects in cosmic fluids modifies their clustering properties and co... more The inclusion of dissipative effects in cosmic fluids modifies their clustering properties and could have observable effects on the formation of large scale structures. We analyse the evolution of density perturbations of cold dark matter endowed with causal bulk viscosity. The perturbative analysis is carried out in the Newtonian approximation and the bulk viscosity is described by the causal Israel-Stewart (IS) theory. In contrast to the non-causal Eckart theory, we obtain a third order evolution equation for the density contrast that depends on three free parameters. For certain parameter values, the density contrast and growth factor in IS mimic their behaviour in ΛCDM when z ≥ 1. Interestingly, and contrary to intuition, certain sets of parameters lead to an increase of the clustering.
We consider a Friedmann-Robertson-Walker spacetime filled with both viscous radiation and nonvisc... more We consider a Friedmann-Robertson-Walker spacetime filled with both viscous radiation and nonviscous dust. The former has a bulk viscosity which is proportional to an arbitrary power of the energy density, i.e. ζ ∝ ρ ν v , and viscous pressure satisfying a nonlinear evolution equation. The analysis is carried out in the context of dynamical systems and the properties of solutions corresponding to the fixed points are discussed. For some ranges of the relevant parameter ν we find that the trajectories in the phase space evolve from a FRW singularity towards an asymptotic de Sitter attractor, confirming and extending previous analysis in the literature. For other values of the parameter, instead, the behaviour differs from previous works.
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Papers by Giovanni Acquaviva