arXiv subjects

Christine C. Dantas

Publications and source records attributed to Christine C. Dantas.

At least 19 recordsLinked to original sources

arXiv2026

The Radial Acceleration Relation in Galaxies and Clusters from a Two-Component, Virial-Motivated Framework

We present a comparative analysis of acceleration data for gravitational systems drawn from multiple observational sources, including: (i) early-type galaxies (ETGs) (Lelli et al. 2017); (ii) Brightest Cluster Galaxies (BCGs) and galaxy clusters (Tian et al. 2020, 2024); and (iii) weak gravitational lensing of isolated galaxies (Brouwer et al. 2021; Mistele et al. 2024). These data are interpreted within a framework motivated by the Two-component Virial Theorem (2VT), which defines a global baryon-dark matter (DM) coupling and sets a characteristic acceleration scale. This baseline is complemented by two models that account for the main empirical features of the Radial Acceleration Relation (RAR) over a broad range of masses and accelerations. The Constant-Interaction Approximation Model (CIA) reproduces the observed RAR trends for ETGs, BCGs, and galaxy clusters. It extends earlier results (Dantas et al. 2000, 2018), and accounts for both the small intrinsic scatter and the emergence of a characteristic acceleration scale. At the very low accelerations probed by weak-lensing data (below accelerations of order $10^{-14}~\mathrm{m\,s^{-2}}$), however, this model breaks down. In this regime, the Virial-Motivated Interaction Model (VIM) incorporates the radial structure of the baryon-DM interaction through a local, radius-dependent contribution to the acceleration. Taken together, the 2VT (global scale), the CIA and the VIM provide a physically motivated framework that captures the main empirical features of the RAR.

astro-ph.GA↗

arXiv2026

A No-Go Theorem for Quantum Cosmologies with Non-natural Hamiltonians

The Eisenhart-Duval lift (ED) geometrizes classical dynamics by embedding their trajectories into null geodesics of a higher-dimensional Lorentzian spacetime. However, such a construction requires a natural Hamiltonian, that is, quadratic in the canonical momenta. As a consequence, mini-superspace cosmological models governed by non-natural Hamiltonians cannot admit an ED lift. Effective models in Loop Quantum Cosmology provide a concrete example: polymer-modified Hamiltonians become non-polynomial in the momenta and therefore fall outside the metric framework of the ED lift. We thus establish a kinematical no-go theorem: non-quadratic cosmological dynamics cannot be geometrized via ED constructions. Quantum-corrected bounce models therefore illustrate a structural limitation of metric geometrization within the ED framework. A few qualitative comments on the possibility of Stäckel-lifting a single-degree-of-freedom non-natural Hamiltonian in quantum cosmology is also presented.

gr-qc↗

arXiv2024

Ergodic Concepts for a Self-Organizing Trivalent Spin Network: A Path to $(2+1)$-dimensional Black Hole Entropy

We consider, from a dynamical systems point of view, a trivalent spin network model in Loop Quantum Gravity presenting self-organized criticality (SOC), arising from a spin propagation dynamics. We obtain a partition function for the domains of stability connecting gauge non-invariant avalanches, leading to an entropy formula for the asymptotic SOC state. The microscopic origin of this SOC entropy is therefore given by the excitation-relaxation spin dynamics in the avalanche cycle. The puncturing of TSN edges participating in the avalanche are counted in terms of an ensemble perimeter over the implicit avalanches. By identifying this perimeter with that of an isolated $(2+1)$-dim. black hole horizon, we conjecture that the SOC entropy reduces to the Bekenstein-Hawking perimeter-entropy law for the Bañados, Teitelboim, and Zanelli (BTZ) black hole, by an appropriate adjustment of a potential function based on the thermodynamical formalism of Sinai, Ruelle and Bowen.

gr-qc↗

arXiv2024

A Note on Norton's Dome

"Norton's Dome" is an example of a Newtonian system that violates the Lipschitz condition at a single point, leading to non-unique solutions (indeterminism). Here we reformulate this problem into a "weak" form (in the sense of distributions). In our description the indeterminism manifests through the problematic interpretation of initial conditions, since distributions (as linear functionals on the space of test functions) do not have values at individual points.

physics.class-ph↗

arXiv2024

The Kormendy relation of cluster galaxies in PPS regions

We study a sample of 936 early-type galaxies located in 48 low-z regular galaxy clusters with $M_{200}\geq 10^{14}~ M_\odot$ at $z< 0.1$. We examine variations in the Kormendy relation (KR) according to their location in the projected phase space (PPS) of the clusters. We have used a combination of Bayesian statistical methods to identify possible differences between the fitted relations. Our results indicate that the overall KR is better fitted when we take into account the information about PPS regions. We also find that objects with time since infall $\geq 6.5$ Gyr have a significant statistical difference of the KR coefficients relative to objects that are more recent in the cluster environment. We show that giant central ellipticals are responsible for tilting the KR relation towards smaller slopes. These galaxies present a late growth probably due to cumulative preprocessing during infall, plus cannibalism and accretion of smaller stripped objects near the center of the clusters.

astro-ph.GA↗

arXiv2022

Mesoscopic Energy Ranking Constraints in the IllustrisTNG Simulations

We revisited the problem of mixing in a gravitational N-body system from the point of view of the ordering of coarse-grained cells in the one-particle energy space, here denoted {\it energy ranking preservation} (ERP). This effect has been noted for some time in simulations, although individual particle energies and their phase-space variables mix considerably. The present investigation aimed to map ERP in terms of parameters involving the collective range in which it is effective, as well as in terms of global and historical characterisations of gravitational systems evolving towards equilibrium. We examined a subset of the IllustrisTNG cosmological magnetohydrodynamical simulations (TNG50-4 and TNG100-3), considering both their full and dark-only versions. For each simulation, we selected the $20$ most massive haloes at redshift $z=0$, tracing their ERP fractions back at selected redshift markers ({\tt z} $= \{1.0, 5.0, 10.0 \}$), and for a coarse-graining set ranging from $5$ to $30$ energy bins. At the redshift marker {\tt z} $= 1$, we found high ERP fractions (above $\sim 80 \%$) in both simulations, regardless of the coarse-graining level. The {\it decline} in ERP fractions with redshift was roughly a function of mass and fractional mass increase in the analysed TNG50-4 haloes, but not in the TNG100-3 ones, indicating a possible relative susceptibility of the ERP effect to mass accretion for haloes less massive than $\sim 10^{14} ~ M_{\odot}$. We confirmed earlier indications in the literature concerning a possible "mesoscopic" constraint operative in a time span of at least several Gyr.

astro-ph.CO↗

arXiv2021

Expanding and Self-Organizing 2D Universe Models Emerging from Frozen Trivalent Spin Networks

We revisit the topic of self-organized criticality (SOC) in simple statistical graph models, with the purpose of capturing essential processes leading to the emergence of macroscopic spacetime from the microscopic dynamics in loop quantum gravity (LQG). We performed a large set of simulations based on extensions of the frozen trivalent spin network (TSN) model explored previously by Ansari and Smolin. Their model mimicked the sandpile dynamics by the application of random vertex propagation rules in the TSN, leading to a SOC behavior in the distribution of the avalanche sizes, as well as a slowly expanding, $2$-dimensional dual (triangulated) space. Here we show that a growth scheme for the stochastic, slow external driving force, differing from the classical sandpile model, also resulted in power-law distributed avalanche sizes. Our simulations also produced expanding dual spaces, with two basic classes of evolution: one with power-law correlations in "space" and "time", and the other with "loitering" and exponential phases. Our work expands the range of models in which critical states in the TSN may lead to expansion effects in the dual space, without fine-tuning.

gr-qc↗

arXiv2020

Counts-in-Cells of subhaloes in the IllustrisTNG simulations: the role of baryonic physics

We present an analysis of the Counts-in-Cells (CiC) statistics of subhaloes in the publicly available IllustrisTNG cosmological simulations (TNG100-1, TNG100-3 and TNG300-3), considering their full and dark-only versions, in redshifts ranging from $z = 0$ to $z=5$, and different cell sizes. We evaluated two CiC models: the gravitational quasi-equilibrium distribution (GQED) and the negative binomial distribution (NBD), both presenting good fits, with small detectable differences in the presence of baryons. Scaling and time dependencies of the best-fit parameters showed similar trends compared with the literature. We derived a matter density-in-cells probability distribution function (PDF), associated with the GQED, which was compared to the PDF given in Uhlemann et al. (2016), for the IllustrisTNG 100-3-Dark run at $z=0$. Our results indicate that the simplest gravithermodynamical assumptions of the GQED model hold in the presence of baryonic dissipation. Interestingly, the smoothed (density-in-cells) version of the GQED is also adequate for describing the dark matter one-point statistics of subhaloes and converges, to subpercentage levels (for an interval of parameters), to the Uhlemann et al. PDF in the high density range.

astro-ph.GA↗

arXiv2017

The two-component virial theorem and the acceleration-discrepancy relation

We revisit the "two-component virial theorem" (2VT) in the light of recent theoretical and observational results related to the "dark matter"(DM) problem. This modification of the virial theorem offers a physically meaningful framework to investigate possible dynamical couplings between the baryonic and DM components of extragalactic systems. In particular, we examine the predictions of the 2VT with respect to the "acceleration-discrepancy relation" (ADR). Considering the combined data (composed of systems supported by rotation and by velocity dispersion), we find that: (i) the overall behavior of the 2VT is consistent with the ADR; and (ii) the 2VT predicts a nearly constant behavior in the lower acceleration regime, as suggested in recent data on dwarf spheroidals. We also briefly comment on possible differentiations between the 2VT and some modified gravity theories.

astro-ph.GA↗

arXiv2016

An Inhomogeneous Space-Time Patching Model Based on a Nonlocal and Nonlinear Schrodinger Equation

We consider an integrable, nonlocal and nonlinear, Schrödinger equation (NNSE) as a model for building space-time patchings in inhomogeneous loop quantum cosmology (LQC). We briefly review exact solutions of the NNSE, specially those obtained through "geometric equivalence" methods. Furthemore, we argue that the integrability of the NNSE could be linked to consistency conditions derived from LQC, under the assumption that the patchwork dynamics behaves as an integrable many-body system.

gr-qc↗

arXiv2015

Stochastic quintessence models: jerk and fine-structure variability constraints

We report on constraints to the cosmological jerk parameter ($j$) and to possible variability of the fine-structure constant ($Δα/α$) based on stochastic quintessence models of dark energy, discussed by Chongchitnan and Efstathiou (2007). We confirm the results by these authors in the sense that many viable solutions can be obtained, obeying current observational constraints in low redshifts. We add the observables $j$ and $Δα/α$ to this conclusion. However, we find peculiarities that may produce, in the nearby Universe, potential observational imprints in future cosmological data. We conclude, for redshifts $z \lesssim 3$, that: {\it (i) } $j(z)$ fluctuates due to the stochasticity of the models, reaching an amplitude of up to $5\%$ relatively to the $Λ$CDM model value ($j_{Λ{\rm CDM}}=1$); and {\it (ii)} by contrasting two distinct ("extreme") types of solutions, variabilities in $α(z)$, linked to a linear coupling ($ζ$) between the dark energy and electromagnetic sectors, are weakly dependent on redshift, for couplings of the order $|ζ| \sim 10 ^{-4}$, even for large variations in the equation of state parameter at relatively low redshifts. Nonlinear couplings produce an earlier and steeper onset of the evolution in $Δα/α(z)$, but can still accommodate the data for weak enough couplings.

astro-ph.CO↗

arXiv2014

A Self-Consistent Extrapolation Method for the Complex Permittivity and Permeability Based on Finite Frequency Data

We describe a method of extrapolation based on a "truncated" Kramers-Kronig relation for the complex permittivity ($ε$) and permeability ($μ$) parameters of a material, based on finite frequency data. Considering a few assumptions, such as the behavior of the loss tangent and the overall nature of corrections, the method is robust within a small relative error, if the assumed hypotheses hold at the extrapolated frequency range.

cond-mat.mtrl-sci↗

arXiv2012

An Approach to Loop Quantum Cosmology Through Integrable Discrete Heisenberg Spin Chains

The quantum evolution equation of Loop Quantum Cosmology (LQC) -- the quantum Hamiltonian constraint -- is a difference equation. We relate the LQC constraint equation in vacuum Bianchi I separable (locally rotationally symmetric) models with an integrable differential-difference nonlinear Schrödinger type equation, which in turn is known to be associated with integrable, discrete Heisenberg spin chain models in condensed matter physics. We illustrate the similarity between both systems with a simple constraint in the linear regime.

gr-qc↗

arXiv2012

Non-linear (loop) quantum cosmology

Inhomogeneous quantum cosmology is modeled as a dynamical system of discrete patches, whose interacting many-body equations can be mapped to a non-linear minisuperspace equation by methods analogous to Bose-Einstein condensation. Complicated gravitational dynamics can therefore be described by more-manageable equations for finitely many degrees of freedom, for which powerful solution procedures are available, including effective equations. The specific form of non-linear and non-local equations suggests new questions for mathematical and computational investigations, and general properties of non-linear wave equations lead to several new options for physical effects and tests of the consistency of loop quantum gravity. In particular, our quantum cosmological methods show how sizeable quantum corrections in a low-curvature universe can arise from tiny local contributions adding up coherently in large regions.

gr-qc↗

arXiv2012

Technical Notes on Classical Electromagnetism, with exercises

The present technical notes offer a brief summary of the essential points of electromagnetism at the undergraduate physics level. Some problems are presented at the end of each section; those with solutions are marked with an asterisk.

physics.class-ph↗

arXiv2011

Analysis of the Collective Behavior of a 10 by 10 Array of Fe3O4 Dots in a Large Micromagnetic Simulation

We report a full (3D) micromagnetic simulation of a set of 100 ferrite (Fe$_3$O$_4$) cylindrical dots, arranged in a 10 by 10 square (planar) array of side 3.27 $μ$m, excited by an external in-plane magnetic field. The resulting power spectrum of magnetic excitations and the dynamical magnetization field at the resulting resonance modes were investigated. The absorption spectrum deviates considerably from that of a single particle reference simulation, presenting a mode-shifting and splitting effect. We found an inversion symmetry through the center of the array, in the sense that each particle and its inversion counterpart share approximately the same magnetization mode behavior. Magnonic designs aiming at synchronous or coherent tunings of spin-wave excitations at given spatially separated points within a regular square array may benefit from the new effects here described.

cond-mat.mes-hall↗

arXiv2011

Dependence of microwave absorption properties on ferrite volume fraction in MnZn ferrite/rubber radar absorbing materials

We report the analysis of measurements of the complex magnetic permeability ($μ_r$) and dielectric permittivity ($ε_r$) spectra of a rubber radar absorbing material (RAM) with various MnZn ferrite volume fractions. The transmission/reflection measurements were carried out in a vector network analyzer. Optimum conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the impedance matching equation. Both the MnZn ferrite content and the RAM thickness effects on the microwave absorption properties, in the frequency range of 2 to 18 GHz, were evaluated. The results show that the complex permeability and permittivity spectra of the RAM increase directly with the ferrite volume fraction. Reflection loss calculations by the impedance matching degree (reflection coefficient) show the dependence of this parameter on both thickness and composition of RAM.

cond-mat.mtrl-sci↗

arXiv2010

Fundamental plane: dark matter and dissipation contributions

Stellar and galactic systems are objects in dynamical equilibrium that are composed of ordinary baryonic matter hypothetically embedded in extended dominant dark matter halos. Our aim is to investigate the scaling relations and dissipational features of these objects over a wide range of their properties, taking the dynamical influence of the dark matter component into account. We study the physical properties of these self-gravitating systems using the two-component virial theorem in conjunction with data that embrace a wide range of astrophysical systems. We find that the scaling relations defined by the properties of these objects admit a dark-to-luminous density ratio parameter as a natural requirement in this framework. We also probe dissipational effects on the fundamental surface defined by the two-component virial theorem and discuss their relations with respect to the region devoid of objects in the data distribution. Our results indicate complementary contributions of dissipation and dark matter to the orign of scaling relations in astrophysical systems.

astro-ph.CO↗