arXiv subjects

C. Ortiz

Publications and source records attributed to C. Ortiz.

At least 19 recordsLinked to original sources

arXiv2026

Covariant virtual work and the d'Alembert-Lagrange formulation of general relativity

We develop a covariant virtual work structure and a corresponding d'Alembert-Lagrange principle for spacetime geometry. Within this framework, General Relativity arises as a particular realization of the principle, leading to a covariant d'Alembert-Lagrange formulation in which the Einstein field equations arise from the vanishing of total covariant virtual work on admissible metric variations, rather than from action extremality. The covariant virtual work structure provides a covariant classification of constraint-induced contributions, distinguishing ideal reactions, which perform no virtual work, from non-ideal sectors contribute explicitly to it. The structure extends naturally to one-sided admissibility conditions, yielding a covariant inequality structure. Constraints generate reaction terms. In particular, an isoperimetric constraint produces a cosmological term as an ideal reaction fixed by spacetime averages, so that the cosmological constant emerges as a global parameter determined by admissibility, reflecting an intrinsically nonlocal geometric origin.

gr-qc↗

arXiv2026

Signatures of Extended Dark Energy Parametrisations in Structure Formation under Background Constraints

We study structure formation in alternative cosmological models constrained by background observations, including $\Lambda$CDM, wCDM, the Chevallier-Polarski-Linder parametrisation and a flexible Chebyshev expansion of the dark energy equation of state. The models are constrained using baryon acoustic oscillations, cosmic microwave background, cosmic chronometers and strong lensing measurements. Using the best-fitting parameters, we generate cosmology-dependent initial conditions and perform N-body simulations to analyse the matter power spectrum, halo mass function and halo density profiles. Although all models remain broadly consistent with $\Lambda$CDM at the background level, differences in the physical matter density $\Omega_{0m}h^2$ and in the expansion history $H(z)$ lead to distinct growth histories that are amplified by non-linear evolution. We find a clear hierarchy in the power spectrum amplitude and in $\sigma_8$, with the Chebyshev and CPL models exhibiting enhanced small-scale power, earlier halo formation at $z\gtrsim2$ and a migration of excess toward higher masses at late times. The wCDM model displays milder and partially compensating effects driven by its different expansion history. When expressed in terms of the scaled radius $r/R_{200c}$, halo density profiles show a high degree of universality across cosmologies, indicating that internal halo structure is largely governed by the same gravitational dynamics. These results demonstrate that even modest background-level variations in $w(z)$ can translate into coherent non-linear signatures, highlighting the constraining power of large-scale structure observables in extended dark energy models.

astro-ph.CO↗

arXiv2025

Flavor Enhanced Chromomagnetic Dipole Moment in the Bestest Little Higgs Framework

We investigate the anomalous Chromomagnetic Dipole Moment (CMDM), $\hat{\mu}_t^{\mathrm{BLHM}}$, of the top quark within the Bestest Little Higgs Model (BLHM). Our study incorporates novel interactions arising from the extended CKM matrix in the BLHM and explores a broad region of the experimentally allowed parameter space, yielding CMDM values on the order of $10^{-3}$. This result represents an improvement over previous CMDM calculations within the BLHM and makes it competitive with other beyond the Standard Model scenarios. Experimental and model parameter uncertainties are considered and propagated through our calculations, using a Monte Carlo method.

hep-ph↗

arXiv2025

Boundary terms in cosmology

In the derivation of the Einstein field equations via Hamilton's principle, the inclusion of a boundary term is essential to render the variational problem well-posed, as it addresses variations that do not vanish at the boundary of the spacetime manifold. Typically, this term is chosen as the Gibbons-Hawking-York boundary term. In this work, we propose an alternative treatment of the boundary term within a cosmological framework by employing the Lagrange multiplier method. This approach enforces the vanishing of the boundary term throughout the evolution of the Universe, leading to the prediction of a fluid component that decays as the sixth power of the scale factor. This type of fluid has been studied in the context of the early universe under the name of stiff matter, and it can be related to a scalar field known as kination.

gr-qc↗

arXiv2025

Shadows of black holes in dynamical Chern-Simons modified gravity

We revisit and extend the study of null geodesics around a slowly rotating black hole in Chern-Simons modified gravity. We employ the Hamilton-Jacobi formalism to derive the equations for the shadow profile and determine its shape. We compare our results with numerical ray tracing, finding good agreement within the validity of our approximations for slow rotation and small Chern-Simons coupling. We forecast constraints on the model parameters using the uncertainty in EHT data for the observed shadow of SgrA* as a reference.

gr-qc↗

arXiv2025

Gravitational friction from d'Alembert's principle

The least action principle played a central role in the development of modern physics. A major drawback of the principle is that its applicability is limited to holonomic constraints. In the present work, we investigate the energy lost by particles as a result of the gravitational interaction in a homogeneous low-density medium subject to non-holonomic constraints. We perform the calculation for an arbitrary particle and outline the specific result for photons. The energy lost is calculated from first principles based on the principle of virtual work and the d'Alembert principle. Under the formalism mentioned above, the dissipative nature of the effect is established. Furthermore, we show that the results agree with an alternative derivation based on continuum mechanics and the Euler-Cauchy stress principle.

physics.class-ph↗

arXiv2025

Constraining modified gravity models through strong lensing cosmography

We analyze cosmography as a tool to constrain modified gravity theories. We take four distinct models and obtain their parameters in terms of the cosmographic parameters favored by observational data of strong gravitational lensing. We contrast with the values obtained by direct comparison between each model and the observational data. In general, we find consistency between the two approaches at 2$σ$ for all models considered in this work. Our study bridges the gap between theoretical predictions of modified gravity and empirical observations of strong gravitational lensing, providing a simple methodology to test the validity of these models.

gr-qc↗

arXiv2024

Nonlinear Dissipative Forces in Celestial Motion Using the Method of Multiple Scales

This paper investigates the influence of nonlinear dissipative forces, specifically Gravitational Friction (GF), on the precession of celestial bodies within the framework of general relativity. We derive a modified line element by introducing a density-dependent term to model interactions between planetary bodies and the low-density interplanetary medium, providing a covariant description of dissipative forces in planetary motion. The resulting metric modification leads to corrections in the perihelion precession of Mercury, also reproducing the classical relativistic predictions. Utilizing the method of multiple scales, we analyze perturbative effects induced by GF. Using this model, we successfully constrain the medium density near Mercury to approximately $ρ_0 \approx 1.12 \times 10^{-10} \, \text{kg/m}^3$. These findings offer a new approach for incorporating dissipative mechanisms into general relativity, with potential applications in other astrophysical systems.

gr-qc↗

arXiv2024

Dissipative Forces in Photon-Medium Interactions Using Perturbation Theory

This study examines dissipative forces in photon-medium interactions through time-independent perturbation theory, with a specific focus on single Helium-4 atoms. Utilizing a Hamiltonian framework, energy corrections induced by dissipative gravitational frictional effects in low-density systems are derived and analyzed as a function of inter-atomic distance. The calculations reveal an energy correction peak at $r_1 = 0.1 nm$, followed by rapid exponential decay, highlighting the dominance of nonlinear dissipative effects at nanoscale separations. These findings emphasize the critical role of short-range interactions, governed by the de-Broglie wavelength of Helium-4, and provide a rigorous theoretical basis for understanding photon-medium interactions at quantum scales. This novel single-particle approach departs from macroscopic mean-field models, offering unique insights into the microscopic mechanisms underlying energy dissipation. The results have potential implications for advancing quantum information processing, nonlinear optics, and the study of dissipative mechanisms in quantum fluids. Experimental validation of the theoretical predictions is proposed using state-of-the-art techniques in optical media, levitated nanoparticle systems, and integrated photonic circuits.

cond-mat.other↗

arXiv2023

On phenomenological parametrizations for the luminosity distance of gravitational waves

The propagation of gravitational waves offers new possibilities for testing the theory of gravity. Amongst these possibilities there is the luminosity distance of gravitational waves, $d_{gw}$. It has been proposed to study this property by means of phenomenological parametrizations, which in this work we confront to the actual predictions of Einstein-scalar-Gauss-Bonnet gravity, finding that the simplest parametrization performs better. We propose a novel parametrization that covers a wider range of models, in particular, within degenerate higher order scalar-tensor theories of gravity. Also, regarding model selection from best-fit parameters, we find that even quantities derived from $d_{gw}$ can lead to inconsistent model selection if they are treated independently. This highlights that it is essential to perform simultaneous analysis and include other types of data. We expect our findings to be relevant for future constraints on modified gravity based on the properties of standard sirens.

gr-qc↗

arXiv2023

Cosmological fluids in the equivalence between Rastall and Einstein gravity

Rastall gravity is a modified gravity proposal that incorporates a non-conserved energy momentum tensor (EMT). We study the equivalence between Rastall gravity and general relativity, analyzing its consequences for an EMT of dark matter and dark energy. We find that the translation between the Rastall and Einstein interpretations modifies the equation of state for each component. For instance, cold dark matter can translate into warm dark matter. If the EMT components are allowed to interact, the translation also changes the type of interaction between the components.

gr-qc↗

arXiv2023

Large Low Background kTon-Scale Liquid Argon Time Projection Chambers

We find that it is possible to increase sensitivity to low energy physics in a third or fourth DUNE-like module with careful controls over radiopurity and targeted modifications to a detector similar to the DUNE Far Detector design. In particular, sensitivity to supernova and solar neutrinos can be enhanced with improved MeV-scale reach. A neutrinoless double beta decay search with $^{136}$Xe loading appears feasible. Furthermore, sensitivity to Weakly-Interacting Massive Particle (WIMP) Dark Matter (DM) becomes competitive with the planned world program in such a detector, offering a unique seasonal variation detection that is characteristic for the nature of WIMPs.

hep-ex↗

arXiv2022

Cosmological Boundary Flux Parameter

The {\it{Cosmological Boundary Flux Parameter}} is a novel proposal that attempts to explain the origin of the cosmological parameter $Λ$ purely by geometric nature. Then we implement this new approach to a flat FLRW universe along with a barotropic fluid. We present an ansatz in which $Λ$ is straightforwardly coupled to the matter sector; therefore, only one additional parameter was introduced: $λ$. Also, through a statistical analysis, using late-time data of observational Hubble and type Ia Supernovae, we computed the joint best-fit value of the free parameters by means of the affine-invariant MCMC. We want to emphasise that the joint analysis produces a smaller $H_{0}^{\rm CBFP}=69.80\rm\,\, Km \,s^{-1}\,Mpc^{-1}$ in contrast to the flat $Λ$CDM result $H_{0}^{Λ\rm CDM}=70.53\rm\,\, Km \,s^{-1}\,Mpc^{-1}$. The work presented here seeks to contribute to the discussion of the possible explanation for the cosmos' acceleration, together with tackling other important questions in modern cosmology.

gr-qc↗

arXiv2022

Low Background kTon-Scale Liquid Argon Time Projection Chambers

We find that it is possible to increase sensitivity to low energy physics in a third or fourth DUNE-like module with careful controls over radiopurity and some modifications to a detector similar to the DUNE Far Detector design. In particular, sensitivity to supernova and solar neutrinos can be enhanced with improved MeV-scale reach. A neutrinoless double beta decay search with $^{136}$Xe loading appears feasible. Furthermore, sensitivity to Weakly-Interacting Massive Particle (WIMP) Dark Matter (DM) becomes competitive with the planned world program in such a detector, offering a unique seasonal variation detection that is characteristic for the nature of WIMPs.

physics.ins-det↗

arXiv2020

Surface Tension: Accelerated Expansion, Coincidence Problem & Hubble Tension

In this paper we give a physical explanation to the accelerated expansion of the Universe, alleviating the tension between the discrepancy of Hubble constant measurements. By the Euler Cauchy stress principle, we identify a controversy on the lack of consideration of the surface forces contemplated in the study of the expansion of the Universe. We distinguish a new effect that modifies the spacetime fabric by means of the energy conservation equation. The resulting dynamical equations from the proposed hypothesis are contrasted to several testable astrophysical predictions. This paper also explains why we have not found any particle or fluid responsible for dark energy and clarifies the Cosmological Coincidence Problem. These explanations are achieved without assuming the existence of exotic matter of unphysical meaning or having to modify the Einstein's Field Equations.

gr-qc↗

arXiv2020

Strong lensing by DHOST black holes

The deflection of light in the strong field limit is an important test for alternative theories of gravity. However, solutions for the metric that allow for analytic computations are not always available. We implement a hybrid analytic-numerical approximation to determine the deflection angle in static, spherically symmetrics pacetimes. We apply this to a set of numerical black hole solutions within the class of theories known as Degenerate Higher Order Scalar-tensor Theories. Comparing our results to a more time consuming full numerical integration, we find that we can accurately describe the deflection angle for light rays passing at arbitrary distances from the photon sphere with a combination of two analytic-numerical approximations. Furthermore, we find a range of parameters where our DHOST black holes predict strong lensing effects whose size is comparable with the uncertainty in the properties of the supermassive black hole in M87 reported by the Event Horizon Telescope, showing that strong lensing is a viable alternative to put constraints on these models.

gr-qc↗

arXiv2017

Energy density profile inspired by noncommutativity

An important consequence which comes from noncommutativity (NC) is undoubtedly the energy density characterized by a microscopic free parameter; indeed a Trans-Planckian parameter. However, its functional form is an interesting and useful equation which can be analyzed in astrophysical scenarios giving now astrophysical constraints. In this sense, this paper is devoted to explore the astrophysical consequences of an energy density with the same functional form of NC; mainly in stellar dynamics and rotation curves of galaxies. We start exploring toy models of stars with incompressible and polytropic fluids respectively, with the addition and coexistence with this new fluid. In both cases, we propose an appropriate constriction based on the difference between a correct and an anomalous behavior. As a complement, we explore the rotation curves of galaxies assuming that the halo is a fluid with the same characteristic of a NC equation, obtaining the range of values for the free parameter through the analysis of eighteen LSB galaxies. Our results are compared with traditional models studied in literature like Pseudoisothermal (PISO), Navarro-Frenk-White (NFW), Burkert and WaveDM dark matter models. Finally, we have computed the surface density, $ρ_i r_i$ for each dark matter model, where $i$ is for PISO, NFW, Burkert, WaveDM and NC macroscopic version. In the later case, following the results found using SPARC galaxy catalog, we have found a theoretical value of $116.97 M_\odot$ pc$^{-2}$ while the data analysis gives us a value of $144.21 M_\odot$ pc$^{-2}$.

gr-qc↗

arXiv2016

Gravitational collapse in brane-worlds revisited

This paper is dedicated to revisit the modifications caused by branes in the collapse of a stellar structure under the Snyder-Oppenheimer scheme. Due to the homogeneity and isotropy of the model, we choose study the case of a closed geometry described by $k=1$, through the tool of dynamical systems. We revisit the different components of the star and its evolution during the stellar collapse, paying particular attention to the non-local effects and the quadratic terms of the energy momentum tensor that come from branes corrections. In the same vein we realize a phase portrait together with a stability analysis with the aim of obtain information about the attractors or saddle points of the dynamical system under different initial conditions in the density parameters, remarking the parameters that come from branes contributions.

gr-qc↗