Roberto Sussman Livovsky - Profile on Academia.edu (original) (raw)

Papers by Roberto Sussman Livovsky

General Relativity and Gravitation, Dec 1, 1989

The existence of "radial" conformal Killing (RCK) vector fields is discussed for metrics describi... more The existence of "radial" conformal Killing (RCK) vector fields is discussed for metrics describing spherically symmetric, shear-free, perfect fluids. It is shown that conformally flat metrics admit three RCK fields, while nonconformalty flat metrics might admit, at most, one such field. An RCK vector parallel to the 4-velocity of the fluid occurs in a subclass of conformally flat metrics containing the Friedmann-Robertson-Walker space-times as particular cases, A new class of nonconformally flat, self-similarity solutions is found. The necessary conditions for the existence of an RCK field in other nonconformally flat metrics are given in full.

Gravitational entropy of dust sources in General Relativity

Nucleation and Atmospheric Aerosols, 2014

A "gravitational entropy" can be defined for a gravitational field, in the context of G... more A "gravitational entropy" can be defined for a gravitational field, in the context of General Relativity, as an independent concept from the entropy of its sources (thermal fluids or black holes). We probe two proposals of this gravitational entropy in generic spherically symmetric Lemaitre-Tolman-Bondy (LTB) models with a dust source. The conditions for the growth of gravitational entropy in both proposals is directly governed by a negative correlation of fluctuations of the rest-mass density and the Hubble expansion scalar. These conditions hold throughout the time evolution of the models, except near a non-simultaneous Big Bang where density decaying modes are dominant. Perpetually expanding models reach a stable terminal equilibrium characterized by an inhomogeneous entropy maximum in their late time evolution, while regions with decaying modes and collapsing elliptic models exhibit unstable equilibria associated with an entropy minimum. We examine the convergence of the gravitational entropies near ...

Self-gravitating dissipative sources in the “fluid flow” approach

Nucleation and Atmospheric Aerosols, 2010

The application of the second order Extended Irreversible Thermodynamics (EIT) to self‐gravitatin... more The application of the second order Extended Irreversible Thermodynamics (EIT) to self‐gravitating systems can be effectively considered as the coupling of Einstein’s field equations to the transport equations of EIT for the main dissipative fluxes: bulk and shear viscosity and heat flux. A simplified dynamical system approach to this coupling is obtained by assuming spherical symmetry and considering the “fluid flow” evolution equations and constraints instead of the field equations. Given this dynamical framework, we show that choosing (as simplifying assumptions) only one of these fluxes as source of dissipation, together with restricting the fluid kinematics, leads to severe constraints on the physical plausibility of the models.

General Relativity and Gravitation, Apr 1, 2005

We show how an adequate post-Newtonian generalization can be obtained for Newtonian dark matter h... more We show how an adequate post-Newtonian generalization can be obtained for Newtonian dark matter halos associated with an empiric density profile. Applying this approach to halos that follow from the well known numerical simulations of Navarro, Frenk and White (NFW), we derive all dynamical variables and show that NFW halos approximatelly follow an ideal gas type of equation of state which fits very well to a polytropic relation in the region outside the core. This fact suggests that "outer" regions of NFW halos might be related to equilibrium states in the non-extensive Statistical Mechanics formalism proposed by Tsallis.

A Numerical Kinetic Theory Approach for Two-Fluid Cosmological Models

magr, 1996

Ideal Gas Sources for the Tolman-Lemaitre-Bondi Metrics

magr, 1999

Non-Spherical Voids: the Best Alternative to Dark Energy?

Astrophysics and space science proceedings, Nov 28, 2013

Scalar averaging in Szekeres dust models

Nucleation and Atmospheric Aerosols, 2013

We consider a formalism of weighed proper volume scalar averages (the "q-average") for ... more We consider a formalism of weighed proper volume scalar averages (the "q-average") for the study of quasi-spherical Szekeres models. We show that the q-average of the main fluid flow covariant scalars are spherically symmetric and satisfy FLRW evolution laws, so that fluctuations and perturbations with respect to these averages provide a full description of the deviation of the models from homogeneity and spherical symmetry. The main proper tensors of the models are given in terms of these fluctuations, with the averages of scalar invariant contractions expressed as second order statistical moments of the density and Hubble scalar expansion. We discuss a possible application of this formalism in connection to a gravitational entropy functional in which entropy production is directly related to a negative statistical correlation between density and velocity fluctuations.

General analytic results on averaging Lemai^tre-Tolman-Bondi models

Nucleation and Atmospheric Aerosols, 2010

ABSTRACT An effective acceleration, which mimics the effect of dark energy, may arise in the cont... more ABSTRACT An effective acceleration, which mimics the effect of dark energy, may arise in the context of Buchert’s scalar averaging formalism. We examine the conditions for such an acceleration to occur in the asymptotic radial range in generic spherically symmetric Lemaître‐Tolman‐Bondi (LTB) dust models. By looking at the behavior of covariant scalars along space slices orthogonal to the 4‐velocity, we show that this effective acceleration occurs in a class of models with negative spatial curvature that are asymptotically convergent to sections of Minkowski spacetime. As a consequence, the boundary conditions that favor LTB models with an effective acceleration are not a void inhomogeneity embedded in a homogeneous FLRW background (Swiss cheese models), but a local void or clump embedded in a large cosmic void region represented by asymptotically Minkowski conditions.

Environmental Science and Pollution Research, Jun 27, 2022

It is well known that airborne transmission of COVID-19 in indoor spaces occurs through various r... more It is well known that airborne transmission of COVID-19 in indoor spaces occurs through various respiratory activities: breathing, vocalizing, coughing, and sneezing. However, there is a complete lack of knowledge of its possible transmission through exhalations of e-cigarette aerosol (ECA), which is also a respiratory activity. E-cigarettes have become widely popular among smokers seeking a much safer way of nicotine consumption than smoking. Due to restrictive lockdown measures taken during the COVID-19 pandemic, many smokers and vapers (e-cigarette users) were confined to shared indoor spaces, making it necessary to assess the risk of SARS-CoV-2 virus aerial transmission through their exhalations. We summarize inferred knowledge of respiratory particles emission and transport through ECA, as well as a theoretical framework for explaining the visibility of exhaled ECA, which has safety implications and is absent in other respiratory activities (apart from smoking). We also summarize and briefly discuss the effects of new SARS-CoV-2 variants, vaccination rates, and environmental factors that may influence the spread of COVID-19. To estimate the risk of SARS-CoV-2 virus aerial transmission associated with vaping exhalations, we adapt a theoretical risk model that has been used to analyze the risks associated with other respiratory activities in shared indoor spaces. We consider home and restaurant scenarios, with natural and mechanical ventilation, with occupants wearing and not wearing face masks. We consider as "control case" or baseline risk scenario an indoor space (home and restaurant) where respiratory droplets and droplet nuclei are uniformly distributed and aerial contagion risk might originate exclusively from occupants exclusively rest breathing, assuming this to be the only (unavoidable) respiratory activity they all carry on. If an infected occupant uses an e-cigarette in a home or restaurant scenarios, bystanders not wearing face masks exposed to the resulting ECA expirations face a 1 % increase of risk of contagion with respect the control case. This relative added risk with respect to the control case becomes 5 -17% for high-intensity vaping, 44 -176% , and over 260% for speaking for various periods or coughing (all without vaping). Infectious emissions are significantly modified by mechanical ventilation, face mask usage, vaccination, and environmental factors, but given the Resposible editor: Lotfi Aleya • We present the first study modeling of SARS-CoV-2 contagion risk through exposure to exhaled e-cigarette aerosol • Viral content of vaping emissions was inferred from data gathered for other respiratory activities. Exclusive rest breathing was assumed as control state to evaluate relative risks • Increase of relative risks is 1% for low intensity, 5-17% for infrequent high-intensity vaping, and 44%, 88%, 132%, 176%, and 259% for speaking 6, 12, 24, and 30 min and coughing 30 times per hour • Relative risks remain stable with respect to mechanical ventilation, wearing face masks, virus variants, vaccination rates, and environmental factors • Visibility of vaping exhalations allows bystanders to avoid contagion by direct exposure Extended author information available on the last page of the article lack of empiric evidence, we assume as a working hypothesis that all basic parameters of respiratory activities are equally (or roughly equally) affected by these factors. Hence, the relative risk percentages with respect to the control state should remain roughly the same under a wide range of varying conditions. By avoiding direct exposure to the visible exhaled vaping jet, wearers of commonly used face masks are well protected from respiratory droplets and droplet nuclei directly emitted by mask-less vapers. Compared to the control case of an already existing (unavoidable) risk from continuous breathing, vaping emissions in shared indoor spaces pose just a negligible additional risk of COVID-19 contagion. We consider that it is not necessary to take additional preventive measures beyond those already prescribed (1.5 m separation and wearing face masks) in order to protect bystanders from this contagion.

On the spatial volume averaging in Lemaitre-Tolman-Bondi dust models. 1. Back reaction, spatial curvature and binding energy

Classical and Quantum Gravity, Aug 19, 2022

Developing a self-consistent notion of gravitational entropy in the context of cosmological struc... more Developing a self-consistent notion of gravitational entropy in the context of cosmological structure formation has been so far an elusive task. Various theoretical proposals have been presented, initially based on Penrose's Weyl Curvature Hypothesis, and variations of it. A more recent proposal by Clifton, Ellis, and Tavakol (CET) considered a novel approach by defining such entropy from a Gibbs equation constructed from an effective stress-energy tensor that emerges from the 'square root' algebraic decomposition of the Bel-Robinson tensor, the simplest divergence-less tensor related to the Weyl tensor. Since, so far all gravitational entropy proposals have been applied to highly restrictive and symmetric spacetimes, we probe in this paper the CET proposal for a class of much less idealized spactimes (the Szekeres class I models) capable of describing the joint evolution of arrays of arbitrary number of structures: overdensities and voids, all placed on selected spatial locations in an asymptotic ΛCDM backgound. By using suitable covariant variables and their fluctuations, we find the necessary and sufficient conditions for a positive CET entropy production to be a negative sign of the product of the density and Hubble expansion fluctuations. To examine the viability of this theoretical result we examine numerically the CET entropy production for two elongated over dense regions surrounding a central spheroidal void, all evolving jointly from initial linear perturbations at the last scattering era into present day Mpc-size CDM structures. We show that CET entropy production is positive for all times after last scattering at the precise spatial locations where structure growth occurs and where the exact density growing mode is dominant. The present paper provides the least idealized (and most physically robust) probe of a gravitational entropy proposal in the context of structure formation.

She also provides consultancy work on research and development, regulatory affairs support, and f... more She also provides consultancy work on research and development, regulatory affairs support, and formulation to several independent vaping companies in the Pacific Region. In the past she has worked for several pharmaceutical companies, including GlaxoSmithKline and Genomma Lab. She is also a member of the standards committee of the VTANZ and UKVIA. R.P. is a full-time employee of the

Toxics, Aug 29, 2022

The inhalation of metallic compounds in e-cigarette (EC) aerosol emissions presents legitimate co... more The inhalation of metallic compounds in e-cigarette (EC) aerosol emissions presents legitimate concerns of potential harms for users. We provide a critical review of laboratory studies published after 2017 on metal contents in EC aerosol, focusing on the consistency between their experimental design, real life device usage and appropriate evaluation of exposure risks. All experiments reporting levels above toxicological markers for some metals (e.g., nickel, lead, copper, manganese) exhibited the following experimental flaws: (i) high powered sub-ohm tank devices tested by means of puffing protocols whose airflows and puff volumes are conceived and appropriate for low powered devices; this testing necessarily involves overheating conditions that favor the production of toxicants and generate aerosols that are likely repellent to human users; (ii) miscalculation of exposure levels from experimental outcomes; (iii) pods and tank devices acquired months and years before the experiments, so that corrosion effects cannot be ruled out; (iv) failure to disclose important information on the characteristics of pods and tank devices, on the experimental methodology and on the resulting outcomes, thus hindering the interpretation of results and the possibility of replication. In general, low powered devices tested without these shortcomings produced metal exposure levels well below strict reference toxicological markers. We believe this review provides useful guidelines for a more objective risk assessment of EC aerosol emissions and signals the necessity to upgrade current laboratory testing standards.

Monthly Notices of the Royal Astronomical Society, May 17, 2017

Understanding the observations of dynamical tracers and the trajectories of lensed photons at gal... more Understanding the observations of dynamical tracers and the trajectories of lensed photons at galactic scales within the context of General Relativity (GR) requires the introduction of a hypothetical dark matter dominant component. The onset of these gravitational anomalies, where the Schwarzschild solution no longer describes observations, closely corresponds to regions where accelerations drop below the characteristic a 0 acceleration of MOND, which occur at a well-established mass-dependent radial distance, R c ∝ (GM/a 0) 1/2. At cosmological scales, inferred dynamics are also inconsistent with GR and the observed distribution of mass. The current accelerated expansion rate requires the introduction of a hypothetical dark energy dominant component. We here show that for a Schwarzschild metric at galactic scales, the scalar curvature, K, multiplied by (r 4 /M) at the critical MOND transition radius, r = R c , has an invariant value of κ B = K(r 4 /M) = 28Ga 0 /c 4. Further, assuming this condition holds for r > R c , is consistent with the full space-time which under GR corresponds to a dominant isothermal dark matter halo, to within observational precision at galactic level. For an FLRW metric, this same constant bounding curvature condition yields for a spatially flat space-time a cosmic expansion history which agrees with the CDM empirical fit for recent epochs, and which similarly tends asymptotically to a de Sitter solution. Thus, a simple covariant purely geometric condition identifies the low-acceleration regime of observed gravitational anomalies, and can be used to guide the development of extended gravity theories at both galactic and cosmological scales.

International Journal of Environmental Research and Public Health, Feb 3, 2021

We discuss the implications of possible contagion of COVID-19 through e-cigarette aerosol (ECA) f... more We discuss the implications of possible contagion of COVID-19 through e-cigarette aerosol (ECA) for prevention and mitigation strategies during the current pandemic. This is a relevant issue when millions of vapers (and smokers) must remain under indoor confinement and/or share public outdoor spaces with non-users. The fact that the respiratory flow associated with vaping is visible (as opposed to other respiratory activities) clearly delineates a safety distance of 1-2 m along the exhaled jet to prevent direct exposure. Vaping is a relatively infrequent and intermittent respiratory activity for which we infer a mean emission rate of 79.82 droplets per puff (6-200, standard deviation 74.66) comparable to mouth breathing, it adds into shared indoor spaces (home and restaurant scenarios) a 1% extra risk of indirect COVID-19 contagion with respect to a "control case" of existing unavoidable risk from continuous breathing. As a comparative reference, this added relative risk increases to 44-176% for speaking 6-24 min per hour and 260% for coughing every 2 min. Mechanical ventilation decreases absolute emission levels but keeps the same relative risks. As long as direct exposure to the visible exhaled jet is avoided, wearing of face masks effectively protects bystanders and keeps risk estimates very low. As a consequence, protection from possible COVID-19 contagion through vaping emissions does not require extra interventions besides the standard recommendations to the general population: keeping a social separation distance of 2 m and wearing of face masks.

Journal of Mathematical Physics, 1991

Global properties and causal structure are considered for spherically symmetric, perfect fluid so... more Global properties and causal structure are considered for spherically symmetric, perfect fluid solutions admiting a self-similar motion orthogonal to the four-velocity. The fluid admits a stiff equation of state. The momentum-energy tensor is equivalent to that of a free massless scalar field. All solutions have center singularities, some of which are timelike. In the latter cases, there are regions with negative density covered by Cauchy horizons. Regular boundaries at infinity display an asymptotically Minkowski behavior. Models of "vacuum bubbles" arise by performing a C ' matching with a section of Minkowski space-time along the Cauchy horizon. Generalizations associated with a nonzero cosmological constant are briefly examined.

Physical review, Dec 29, 2008

We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data set... more We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit the supernova data almost as well as the LCDM model without requiring a dark energy component. The Szekeres models were originally derived as an exact solution to Einstein's equations with a general metric that has no symmetries and are regarded as good candidates to model the true lumpy universe that we observe. The null geodesics in these models are not radial. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. The first results presented here seem to encourage further investigations of apparent acceleration using various inhomogeneous models and other constraints from CMB and large structure need to be explored next.

Classical and Quantum Gravity, Mar 1, 2012

We obtain an elegant and useful description of the dynamics of Szekeres dust models (in their ful... more We obtain an elegant and useful description of the dynamics of Szekeres dust models (in their full generality) by means of "quasi-local" scalar variables constructed by suitable integral distributions that can be interpreted as weighed proper volume averages of the local covariant scalars. In terms of these variables, the field equations and basic physical and geometric quantities are formally identical to their corresponding expressions in the spherically symmetric LTB dust models. Since we can map every Szekeres model to a unique LTB model, rigorous results valid for the latter models can be readily generalized to a nonspherical Szekeres geometry. The new variables lead naturally to an initial value formulation in which all scalars are expressed as scaling laws in terms of their values at an arbitrary initial space slice. These variables also yield a significant simplification of numerical work, since the fluid flow evolution equations become a system of autonomous ordinary differential equations subjected to algebraic constraints containing the information on the deviations from spherical symmetry. As an example of how this formalism can be applied, we show that spherical symmetry is stable against small dipole-like perturbations. This new approach to the dynamics of the Szekeres solutions has an enormous potential for dealing with a wide variety of theoretical issues and for constructing non-spherical models of cosmological inhomogeneities to fit observational data. * This coordinate choice is not appropriate if the slices 3 T [t] have spherical (S 3 ) or wormhole (S 2 ×R) topologies. In these cases, R 0 (r) must have two zeroes or no zeroes. We look at these cases in Appendix D.

Building Non-Spherical Cosmic Structures

General Relativity and Gravitation, Dec 1, 1989

The existence of "radial" conformal Killing (RCK) vector fields is discussed for metrics describi... more The existence of "radial" conformal Killing (RCK) vector fields is discussed for metrics describing spherically symmetric, shear-free, perfect fluids. It is shown that conformally flat metrics admit three RCK fields, while nonconformalty flat metrics might admit, at most, one such field. An RCK vector parallel to the 4-velocity of the fluid occurs in a subclass of conformally flat metrics containing the Friedmann-Robertson-Walker space-times as particular cases, A new class of nonconformally flat, self-similarity solutions is found. The necessary conditions for the existence of an RCK field in other nonconformally flat metrics are given in full.

Gravitational entropy of dust sources in General Relativity

Nucleation and Atmospheric Aerosols, 2014

A "gravitational entropy" can be defined for a gravitational field, in the context of G... more A "gravitational entropy" can be defined for a gravitational field, in the context of General Relativity, as an independent concept from the entropy of its sources (thermal fluids or black holes). We probe two proposals of this gravitational entropy in generic spherically symmetric Lemaitre-Tolman-Bondy (LTB) models with a dust source. The conditions for the growth of gravitational entropy in both proposals is directly governed by a negative correlation of fluctuations of the rest-mass density and the Hubble expansion scalar. These conditions hold throughout the time evolution of the models, except near a non-simultaneous Big Bang where density decaying modes are dominant. Perpetually expanding models reach a stable terminal equilibrium characterized by an inhomogeneous entropy maximum in their late time evolution, while regions with decaying modes and collapsing elliptic models exhibit unstable equilibria associated with an entropy minimum. We examine the convergence of the gravitational entropies near ...

Self-gravitating dissipative sources in the “fluid flow” approach

Nucleation and Atmospheric Aerosols, 2010

The application of the second order Extended Irreversible Thermodynamics (EIT) to self‐gravitatin... more The application of the second order Extended Irreversible Thermodynamics (EIT) to self‐gravitating systems can be effectively considered as the coupling of Einstein’s field equations to the transport equations of EIT for the main dissipative fluxes: bulk and shear viscosity and heat flux. A simplified dynamical system approach to this coupling is obtained by assuming spherical symmetry and considering the “fluid flow” evolution equations and constraints instead of the field equations. Given this dynamical framework, we show that choosing (as simplifying assumptions) only one of these fluxes as source of dissipation, together with restricting the fluid kinematics, leads to severe constraints on the physical plausibility of the models.

General Relativity and Gravitation, Apr 1, 2005

We show how an adequate post-Newtonian generalization can be obtained for Newtonian dark matter h... more We show how an adequate post-Newtonian generalization can be obtained for Newtonian dark matter halos associated with an empiric density profile. Applying this approach to halos that follow from the well known numerical simulations of Navarro, Frenk and White (NFW), we derive all dynamical variables and show that NFW halos approximatelly follow an ideal gas type of equation of state which fits very well to a polytropic relation in the region outside the core. This fact suggests that "outer" regions of NFW halos might be related to equilibrium states in the non-extensive Statistical Mechanics formalism proposed by Tsallis.

A Numerical Kinetic Theory Approach for Two-Fluid Cosmological Models

magr, 1996

Ideal Gas Sources for the Tolman-Lemaitre-Bondi Metrics

magr, 1999

Non-Spherical Voids: the Best Alternative to Dark Energy?

Astrophysics and space science proceedings, Nov 28, 2013

Scalar averaging in Szekeres dust models

Nucleation and Atmospheric Aerosols, 2013

We consider a formalism of weighed proper volume scalar averages (the "q-average") for ... more We consider a formalism of weighed proper volume scalar averages (the "q-average") for the study of quasi-spherical Szekeres models. We show that the q-average of the main fluid flow covariant scalars are spherically symmetric and satisfy FLRW evolution laws, so that fluctuations and perturbations with respect to these averages provide a full description of the deviation of the models from homogeneity and spherical symmetry. The main proper tensors of the models are given in terms of these fluctuations, with the averages of scalar invariant contractions expressed as second order statistical moments of the density and Hubble scalar expansion. We discuss a possible application of this formalism in connection to a gravitational entropy functional in which entropy production is directly related to a negative statistical correlation between density and velocity fluctuations.

General analytic results on averaging Lemai^tre-Tolman-Bondi models

Nucleation and Atmospheric Aerosols, 2010

ABSTRACT An effective acceleration, which mimics the effect of dark energy, may arise in the cont... more ABSTRACT An effective acceleration, which mimics the effect of dark energy, may arise in the context of Buchert’s scalar averaging formalism. We examine the conditions for such an acceleration to occur in the asymptotic radial range in generic spherically symmetric Lemaître‐Tolman‐Bondi (LTB) dust models. By looking at the behavior of covariant scalars along space slices orthogonal to the 4‐velocity, we show that this effective acceleration occurs in a class of models with negative spatial curvature that are asymptotically convergent to sections of Minkowski spacetime. As a consequence, the boundary conditions that favor LTB models with an effective acceleration are not a void inhomogeneity embedded in a homogeneous FLRW background (Swiss cheese models), but a local void or clump embedded in a large cosmic void region represented by asymptotically Minkowski conditions.

Environmental Science and Pollution Research, Jun 27, 2022

It is well known that airborne transmission of COVID-19 in indoor spaces occurs through various r... more It is well known that airborne transmission of COVID-19 in indoor spaces occurs through various respiratory activities: breathing, vocalizing, coughing, and sneezing. However, there is a complete lack of knowledge of its possible transmission through exhalations of e-cigarette aerosol (ECA), which is also a respiratory activity. E-cigarettes have become widely popular among smokers seeking a much safer way of nicotine consumption than smoking. Due to restrictive lockdown measures taken during the COVID-19 pandemic, many smokers and vapers (e-cigarette users) were confined to shared indoor spaces, making it necessary to assess the risk of SARS-CoV-2 virus aerial transmission through their exhalations. We summarize inferred knowledge of respiratory particles emission and transport through ECA, as well as a theoretical framework for explaining the visibility of exhaled ECA, which has safety implications and is absent in other respiratory activities (apart from smoking). We also summarize and briefly discuss the effects of new SARS-CoV-2 variants, vaccination rates, and environmental factors that may influence the spread of COVID-19. To estimate the risk of SARS-CoV-2 virus aerial transmission associated with vaping exhalations, we adapt a theoretical risk model that has been used to analyze the risks associated with other respiratory activities in shared indoor spaces. We consider home and restaurant scenarios, with natural and mechanical ventilation, with occupants wearing and not wearing face masks. We consider as "control case" or baseline risk scenario an indoor space (home and restaurant) where respiratory droplets and droplet nuclei are uniformly distributed and aerial contagion risk might originate exclusively from occupants exclusively rest breathing, assuming this to be the only (unavoidable) respiratory activity they all carry on. If an infected occupant uses an e-cigarette in a home or restaurant scenarios, bystanders not wearing face masks exposed to the resulting ECA expirations face a 1 % increase of risk of contagion with respect the control case. This relative added risk with respect to the control case becomes 5 -17% for high-intensity vaping, 44 -176% , and over 260% for speaking for various periods or coughing (all without vaping). Infectious emissions are significantly modified by mechanical ventilation, face mask usage, vaccination, and environmental factors, but given the Resposible editor: Lotfi Aleya • We present the first study modeling of SARS-CoV-2 contagion risk through exposure to exhaled e-cigarette aerosol • Viral content of vaping emissions was inferred from data gathered for other respiratory activities. Exclusive rest breathing was assumed as control state to evaluate relative risks • Increase of relative risks is 1% for low intensity, 5-17% for infrequent high-intensity vaping, and 44%, 88%, 132%, 176%, and 259% for speaking 6, 12, 24, and 30 min and coughing 30 times per hour • Relative risks remain stable with respect to mechanical ventilation, wearing face masks, virus variants, vaccination rates, and environmental factors • Visibility of vaping exhalations allows bystanders to avoid contagion by direct exposure Extended author information available on the last page of the article lack of empiric evidence, we assume as a working hypothesis that all basic parameters of respiratory activities are equally (or roughly equally) affected by these factors. Hence, the relative risk percentages with respect to the control state should remain roughly the same under a wide range of varying conditions. By avoiding direct exposure to the visible exhaled vaping jet, wearers of commonly used face masks are well protected from respiratory droplets and droplet nuclei directly emitted by mask-less vapers. Compared to the control case of an already existing (unavoidable) risk from continuous breathing, vaping emissions in shared indoor spaces pose just a negligible additional risk of COVID-19 contagion. We consider that it is not necessary to take additional preventive measures beyond those already prescribed (1.5 m separation and wearing face masks) in order to protect bystanders from this contagion.

On the spatial volume averaging in Lemaitre-Tolman-Bondi dust models. 1. Back reaction, spatial curvature and binding energy

Classical and Quantum Gravity, Aug 19, 2022

Developing a self-consistent notion of gravitational entropy in the context of cosmological struc... more Developing a self-consistent notion of gravitational entropy in the context of cosmological structure formation has been so far an elusive task. Various theoretical proposals have been presented, initially based on Penrose's Weyl Curvature Hypothesis, and variations of it. A more recent proposal by Clifton, Ellis, and Tavakol (CET) considered a novel approach by defining such entropy from a Gibbs equation constructed from an effective stress-energy tensor that emerges from the 'square root' algebraic decomposition of the Bel-Robinson tensor, the simplest divergence-less tensor related to the Weyl tensor. Since, so far all gravitational entropy proposals have been applied to highly restrictive and symmetric spacetimes, we probe in this paper the CET proposal for a class of much less idealized spactimes (the Szekeres class I models) capable of describing the joint evolution of arrays of arbitrary number of structures: overdensities and voids, all placed on selected spatial locations in an asymptotic ΛCDM backgound. By using suitable covariant variables and their fluctuations, we find the necessary and sufficient conditions for a positive CET entropy production to be a negative sign of the product of the density and Hubble expansion fluctuations. To examine the viability of this theoretical result we examine numerically the CET entropy production for two elongated over dense regions surrounding a central spheroidal void, all evolving jointly from initial linear perturbations at the last scattering era into present day Mpc-size CDM structures. We show that CET entropy production is positive for all times after last scattering at the precise spatial locations where structure growth occurs and where the exact density growing mode is dominant. The present paper provides the least idealized (and most physically robust) probe of a gravitational entropy proposal in the context of structure formation.

She also provides consultancy work on research and development, regulatory affairs support, and f... more She also provides consultancy work on research and development, regulatory affairs support, and formulation to several independent vaping companies in the Pacific Region. In the past she has worked for several pharmaceutical companies, including GlaxoSmithKline and Genomma Lab. She is also a member of the standards committee of the VTANZ and UKVIA. R.P. is a full-time employee of the

Toxics, Aug 29, 2022

The inhalation of metallic compounds in e-cigarette (EC) aerosol emissions presents legitimate co... more The inhalation of metallic compounds in e-cigarette (EC) aerosol emissions presents legitimate concerns of potential harms for users. We provide a critical review of laboratory studies published after 2017 on metal contents in EC aerosol, focusing on the consistency between their experimental design, real life device usage and appropriate evaluation of exposure risks. All experiments reporting levels above toxicological markers for some metals (e.g., nickel, lead, copper, manganese) exhibited the following experimental flaws: (i) high powered sub-ohm tank devices tested by means of puffing protocols whose airflows and puff volumes are conceived and appropriate for low powered devices; this testing necessarily involves overheating conditions that favor the production of toxicants and generate aerosols that are likely repellent to human users; (ii) miscalculation of exposure levels from experimental outcomes; (iii) pods and tank devices acquired months and years before the experiments, so that corrosion effects cannot be ruled out; (iv) failure to disclose important information on the characteristics of pods and tank devices, on the experimental methodology and on the resulting outcomes, thus hindering the interpretation of results and the possibility of replication. In general, low powered devices tested without these shortcomings produced metal exposure levels well below strict reference toxicological markers. We believe this review provides useful guidelines for a more objective risk assessment of EC aerosol emissions and signals the necessity to upgrade current laboratory testing standards.

Monthly Notices of the Royal Astronomical Society, May 17, 2017

Understanding the observations of dynamical tracers and the trajectories of lensed photons at gal... more Understanding the observations of dynamical tracers and the trajectories of lensed photons at galactic scales within the context of General Relativity (GR) requires the introduction of a hypothetical dark matter dominant component. The onset of these gravitational anomalies, where the Schwarzschild solution no longer describes observations, closely corresponds to regions where accelerations drop below the characteristic a 0 acceleration of MOND, which occur at a well-established mass-dependent radial distance, R c ∝ (GM/a 0) 1/2. At cosmological scales, inferred dynamics are also inconsistent with GR and the observed distribution of mass. The current accelerated expansion rate requires the introduction of a hypothetical dark energy dominant component. We here show that for a Schwarzschild metric at galactic scales, the scalar curvature, K, multiplied by (r 4 /M) at the critical MOND transition radius, r = R c , has an invariant value of κ B = K(r 4 /M) = 28Ga 0 /c 4. Further, assuming this condition holds for r > R c , is consistent with the full space-time which under GR corresponds to a dominant isothermal dark matter halo, to within observational precision at galactic level. For an FLRW metric, this same constant bounding curvature condition yields for a spatially flat space-time a cosmic expansion history which agrees with the CDM empirical fit for recent epochs, and which similarly tends asymptotically to a de Sitter solution. Thus, a simple covariant purely geometric condition identifies the low-acceleration regime of observed gravitational anomalies, and can be used to guide the development of extended gravity theories at both galactic and cosmological scales.

International Journal of Environmental Research and Public Health, Feb 3, 2021

We discuss the implications of possible contagion of COVID-19 through e-cigarette aerosol (ECA) f... more We discuss the implications of possible contagion of COVID-19 through e-cigarette aerosol (ECA) for prevention and mitigation strategies during the current pandemic. This is a relevant issue when millions of vapers (and smokers) must remain under indoor confinement and/or share public outdoor spaces with non-users. The fact that the respiratory flow associated with vaping is visible (as opposed to other respiratory activities) clearly delineates a safety distance of 1-2 m along the exhaled jet to prevent direct exposure. Vaping is a relatively infrequent and intermittent respiratory activity for which we infer a mean emission rate of 79.82 droplets per puff (6-200, standard deviation 74.66) comparable to mouth breathing, it adds into shared indoor spaces (home and restaurant scenarios) a 1% extra risk of indirect COVID-19 contagion with respect to a "control case" of existing unavoidable risk from continuous breathing. As a comparative reference, this added relative risk increases to 44-176% for speaking 6-24 min per hour and 260% for coughing every 2 min. Mechanical ventilation decreases absolute emission levels but keeps the same relative risks. As long as direct exposure to the visible exhaled jet is avoided, wearing of face masks effectively protects bystanders and keeps risk estimates very low. As a consequence, protection from possible COVID-19 contagion through vaping emissions does not require extra interventions besides the standard recommendations to the general population: keeping a social separation distance of 2 m and wearing of face masks.

Journal of Mathematical Physics, 1991

Global properties and causal structure are considered for spherically symmetric, perfect fluid so... more Global properties and causal structure are considered for spherically symmetric, perfect fluid solutions admiting a self-similar motion orthogonal to the four-velocity. The fluid admits a stiff equation of state. The momentum-energy tensor is equivalent to that of a free massless scalar field. All solutions have center singularities, some of which are timelike. In the latter cases, there are regions with negative density covered by Cauchy horizons. Regular boundaries at infinity display an asymptotically Minkowski behavior. Models of "vacuum bubbles" arise by performing a C ' matching with a section of Minkowski space-time along the Cauchy horizon. Generalizations associated with a nonzero cosmological constant are briefly examined.

Physical review, Dec 29, 2008

We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data set... more We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit the supernova data almost as well as the LCDM model without requiring a dark energy component. The Szekeres models were originally derived as an exact solution to Einstein's equations with a general metric that has no symmetries and are regarded as good candidates to model the true lumpy universe that we observe. The null geodesics in these models are not radial. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. The first results presented here seem to encourage further investigations of apparent acceleration using various inhomogeneous models and other constraints from CMB and large structure need to be explored next.

Classical and Quantum Gravity, Mar 1, 2012

We obtain an elegant and useful description of the dynamics of Szekeres dust models (in their ful... more We obtain an elegant and useful description of the dynamics of Szekeres dust models (in their full generality) by means of "quasi-local" scalar variables constructed by suitable integral distributions that can be interpreted as weighed proper volume averages of the local covariant scalars. In terms of these variables, the field equations and basic physical and geometric quantities are formally identical to their corresponding expressions in the spherically symmetric LTB dust models. Since we can map every Szekeres model to a unique LTB model, rigorous results valid for the latter models can be readily generalized to a nonspherical Szekeres geometry. The new variables lead naturally to an initial value formulation in which all scalars are expressed as scaling laws in terms of their values at an arbitrary initial space slice. These variables also yield a significant simplification of numerical work, since the fluid flow evolution equations become a system of autonomous ordinary differential equations subjected to algebraic constraints containing the information on the deviations from spherical symmetry. As an example of how this formalism can be applied, we show that spherical symmetry is stable against small dipole-like perturbations. This new approach to the dynamics of the Szekeres solutions has an enormous potential for dealing with a wide variety of theoretical issues and for constructing non-spherical models of cosmological inhomogeneities to fit observational data. * This coordinate choice is not appropriate if the slices 3 T [t] have spherical (S 3 ) or wormhole (S 2 ×R) topologies. In these cases, R 0 (r) must have two zeroes or no zeroes. We look at these cases in Appendix D.

Building Non-Spherical Cosmic Structures