arXiv subjects

Felipe A. Asenjo

Publications and source records attributed to Felipe A. Asenjo.

At least 19 recordsLinked to original sources

arXiv2026

Dark energy and negative compressibility of the Universe: A study based on cosmic equations of state

All cosmological models are defined through energy and matter, which describe the evolution of the targeted Universe. Whether the content is simple or complex, it can be defined through an effective equation of state (EoS). We study different cosmological models by analyzing the properties of the complete EoS of their fluid contents. We obtained general expressions for the ratio between pressure $p$ and energy density $ρ$, and for their derivative $dp/dρ$ in terms of deceleration and jerk parameters. We show that, in general, a cosmology whose energy density is constructed from multiple ideal gases cannot be described by the EoS of a single, ideal gas. This is directly related to a variation in the deceleration parameter during the evolution of the Universe. Furthermore, we studied the effective compressibility properties of the EoS, demonstrating that dark energy features of the fluid should be related to the negative compressibility of the contents. These features depend on the dynamics of the jerk parameter for each cosmological system, implying that the knowledge of dark energy evolution is encoded in the third-order derivatives of the scale factor. This approach to understanding the properties of cosmological models indicates that the accelerating evolution of some models, such as the lambda-cold dark matter model ($Λ$CDM) or the Chaplygin gas model, is due to the EoS of their nonideal content fluids rather than an associated anomalous (dark energy) nature. On the contrary, models such as $w$-cold dark matter ($w$CDM), $w_0$$w_a$-cold dark matter ($w_0$$w_a$CDM), or Dvali-Gabadadze-Porrati models, include by construction fluids with anomalous content, implying a negative compressibility and an effective dark energy behavior. Our work highlights the importance of treating all contents of the Universe as a nonideal fluid system, with a unique EoS.

astro-ph.CO↗

arXiv2026

Reconnection of Gravitational Fields

In a tetrad formulation of general relativity, an ideal Ohm-type condition for the gravitational field implies the conservation of gravitational fluxes and the preservation of the connectivity of gravitational field sheets. We investigate the consequences of departures from ideal gravitational evolution by developing a theory of gravitational reconnection, defined as a change in the connectivity of gravitational field sheets arising from the breakdown of the ideal frozen-in evolution of the gravitational field. We derive quantitative measures of the gravitational reconnection rate, show that gravitational reconnection alters the energy-momentum exchange between the gravitational and matter sectors, and determine how it modifies the evolution of gravitational helicity. Our results provide a quantitative framework for describing changes in gravitational connectivity and their consequences in dynamical spacetimes.

gr-qc↗

arXiv2026

Revisiting Aristarchus's method to estimate the ratio of the distances from Earth to the Sun and the Moon

In this article we revisit Aristarchus' ancient experiment to determine the ratios between the Earth-Moon and Earth-Sun distances. We present here the construction of a simple instrument, basically a tube that, by projecting its shadow, that allows one to estimate the Sun-Earth-Moon angle at the moment of first (or last) quarter. The simplicity of its construction and the accuracy of the measurements obtained may come as a surprise. We discuss the complexities involved in determining the exact moment of first quarter, based on the shape of the elliptical shadow projected on the Moon. Finally, we analyze and implement a strategy that allows us to approximate the measurement of this angle and estimate its value using observations that can be performed twice a month, and more accurately a couple of times during the year. We place Aristarchus' problem in its historical context and detail the physical and mathematical topics that can be developed and motivated through this experiment, in addition to carrying out a very thorough analysis of the errors associated with this measurement.

physics.pop-ph↗

arXiv2026

Frozen-In Gravitational Fields

Spacetime can undergo complex nonlinear evolution, as governed by the Einstein field equations, and a central challenge is to understand the geometric structures that arise, persist, and interact throughout its evolution. Using a formulation of Einstein equations that parallels nonlinear electrodynamics of continuous media, we show that general relativity admits gravitational field connections---two-surfaces and associated field lines whose connectivity is maintained by the spacetime dynamics. This gravitational frozen-in behavior is enabled by an ideal Ohm-type condition for the gravitational field. We further show that the same framework naturally leads to a conserved ``gravitational magnetic'' flux. A conserved gravitational helicity also emerges, with a clear topological interpretation in terms of gravitational field-line structures. These results identify well-defined topological constraints on admissible spacetime evolution and provide an organizing principle underlying the nonlinear dynamics of spacetime.

gr-qc↗

arXiv2026

First Principles Magnetohydrodynamical Theory for the Expanding Box Model: Effects on Expansion-Induced Alfvén Wave Reflection

The Expanding Box Model (EBM) has been widely employed to simulate multiscale plasma phenomena in the expanding solar wind by transforming the MHD equations to a co-moving, non-inertial frame. However, traditional formulations have suffered from historical ambiguity regarding the physical separation between the co-moving and inertial reference frames, primarily arising from a classical approximation of an invariant magnetic field between them. To resolve this inconsistency, we reformulate the EBM from first principles using a fully covariant approach. Here, we model the expanding solar wind frame as an anisotropic expanding spacetime metric, allowing us to incorporate radial acceleration profiles and differential transverse expansion, ensuring that all physical fields are correctly transformed by expansion. We demonstrate that asymmetries identified in previous EBM-MHD literature are direct consequences of neglecting the tensorial scaling of the magnetic field. Our covariant treatment eliminates these residues, restoring symmetry in the co-moving frame. Projecting our system back into the inertial frame clarifies the distinction between local plasma dynamics and plasma expansion, revealing the anisotropy of the Parker spiral as a geometric projection. Furthermore, linear wave analysis using Elsässer variables reveals that plasma expansion induces a low-frequency cutoff and geometric damping. Numerical integration demonstrates that expansion drives the reflection of Alfvén waves, generating counter-propagating modes primarily at low frequencies relative to the expansion rate, while high frequencies converge to the WKB approximation. This provides a consistent foundation for simulations, establishing that expansion can serve as a source of counter-propagating waves necessary to drive solar wind turbulence at low frequencies.

physics.plasm-ph↗

arXiv2026

Finite Orbital Angular momentum Bessel beams propagating along light-cone coordinates

New solutions for Bessel electromagnetic beams, propagating along the light cones, are investigated. Of the variety of structures possible in the light cone variables, the one involving a product of Airy functions is discussed in detail. This class of solutions, representing an asymmetry on the light-cone coordinates dependence, is a non-trivial extension to the usual plane wave solutions. We also explore the conditions under which these solutions will carry finite orbital angular momentum density.

physics.gen-ph↗

arXiv2026

Electromagnetic plasma wave modes propagating along light-cone coordinates

We present new electromagnetic plasma wavepacket solutions that propagates in one time and one space coordinates. Differently to the usual plane wave solution, which is written in terms of separation of variables, all our solutions are along the light-cone coordinates. This allow us to find several new solutions whose functionality properties rely on the conditions imposed on the choice for their light-cone coordinates dependence. The presented wavepacket solutions are constructed in terms of multiplications of Airy functions, Parabolic cylinder functions, Mathieu functions, or Bessel functions. We thoroughly analyze the case of a double Airy solution, which have new electromagnetic properties, as a defined wavefront, and velocity faster than the electromagnetic plane wave counterpart solution. It is also mentioned how more general structured wavepackets can be constructed from these new solutions.

physics.plasm-ph↗

arXiv2025

Laser-Driven Annular Shock Waves as Laboratory Analogues of $w$CDM Cosmologies and Cosmological Gravitational Waves

We demonstrate that the experimental evolution of an annular, laser-driven plasma shock wave, expanding over time and undergoing self-interaction gives rise to multiple shock structures that evolve analogously to a multicomponent cosmological universe. Different propagation trajectories along the shock surface correspond to various forms of $w$CDM cosmologies, enabling the study of scenarios ranging from simple radiation- or matter-dominated universes to those including dark energy. We further show that the dynamics of the Mach stems approximately follows a Hubble-like law. Additionally, perturbations in the shock fronts serve as experimental analogues of cosmological gravitational perturbations in a matter-dominated universe. This work opens a new experimental pathway for classically simulating complex cosmological models and gravitational waves at macroscopic scales in the laboratory.

physics.plasm-ph↗

arXiv2025

The effect of plasma expansion on the dispersion properties of MHD waves

In this work, we employ the set of ideal expanding magnetohydrodynamic (MHD) equations within the Expanding Box Model (EBM) framework to theoretically characterize the effects of radial solar wind expansion on its characteristic linear MHD waves. Through the analytical derivation of dispersion relations by a first-order expansion of the MHD-EBM equations, we explore the changes in wave propagation across a range of heliocentric distances on the linear magnetohydrodynamic modes: the Alfvén mode and the fast and slow magnetosonic modes, as obtained from the ideal MHD-EBM equations. Our findings reveal a spatial dependence in the derived dispersion relations that aligns with both the literature and the traditional ideal MHD case in the non-expanding limit, thereby helping to bridge the gap between theory and observation in solar wind dynamics. We observe a general decrease in wave frequencies as the plasma expands farther from the Sun. This decrease is reflected in the dispersion relations through the radial decrease of both the Alfvén and sound speeds, which decrease proportionally to $1/R$ and $1/R^{γ- 1}$, respectively, where $γ$ is the plasma polytropic index. The fast magnetosonic mode frequency and phase speed are significantly affected by the polytropic index value. We consider three models for the polytropic index evolution in the expanding solar wind: a constant (quasi-adiabatic) case, a radially decreasing profile in the outer heliosphere, and a model incorporating thermodynamic heating effects. Notably, we find that in the case of a decreasing polytropic index, the fast magnetosonic mode experiences an acceleration in the distant heliosphere, highlighting the significant influence of expansion on solar wind dynamics.

physics.plasm-ph↗

arXiv2025

Dark energy as a battery for magnetic field generation in plasmas

It is shown that in the spacetime dominated by a cosmological constant, in the far region of a Schwarzschild-de Sitter black hole, a seed magnetic field can be generated in an ambient plasma (in a state of no magnetic field) by a general-relativistic battery, which depends on the interaction of spacetime curvature with inhomogeneous plasma thermodynamics. Thus, at large distances, dark energy becomes the only gravitational source for magnetic field generation. This allows a mechanism that make dark energy manifest through its conversion to cosmic magnetic fields.

physics.plasm-ph↗

arXiv2025

Electromagnetic plasma waves in Big Bounce and Big Crunch cosmologies

We study the exact dynamics of electromagnetic waves in cold electron-positron plasma in the simplest background of Big Bounce and Big Crunch cosmologies. We show that these waves are described by a Mathieu equation in a Big Bounce cosmology, which opens the possibility of electromagnetic wave amplification due to the cosmological constant in this Universe. On the contrary, in a Big Crunch cosmology, electromagnetic plasma waves are described by a modified Mathieu equation. In both cases, the features (grow/collapse) of the Universes imprint similar characteristics in the temporal evolution of these waves.

physics.plasm-ph↗

arXiv2025

Ultrarelativistic outflows in asymmetric magnetic reconnection

We present a one-fluid pair plasma magnetohydrodynamical model for asymmetric relativistic magnetic reconnection that incorporates the thermal-inertial effects of the plasma. We find the general scaling relation for the reconnection rate in a Sweet-Parker-type configuration. However, we show that under a specific highly asymmetric scenario, this magnetic reconnection process can produce ultrarelativistic plasma outflows, with velocities surpassing those of the inflow particles, and also, those found in symmetric cases. We highlight the significance of the asymmetry in enhancing particle acceleration and energy release.

physics.plasm-ph↗

arXiv2024

Magnetic seed generation by plasma heat flux in accretion disks

Context. Magnetic batteries are potential sources that may drive the generation of a seed magnetic field, even if this field is initially zero. These batteries can be the result of non-aligned thermodynamic gradients in a plasma, as well as of special and general relativistic effects. So far, magnetic batteries have only been studied in ideal magnetized fluids. Aims. We study the non-ideal fluid effects introduced by the energy flux in the vortical dynamics of a magnetized plasma in curved spacetime. We propose a novel mechanism for generating a heat flux-driven magnetic seed within a simple accretion disk model around a Schwarzschild black hole. Methods. We use the 3+1 formalism for the splitting of the space-time metric into space-like and time-like components. We study the vortical dynamics of a magnetized fluid with a heat flux in the Schwarzschild geometry in which thermodynamic and hydrodynamic quantities are only dependent on the radial coordinate. Assuming that the magnetic field is initially zero, we estimate linear time evolution of the magnetic field due to the inclusion of non-ideal fluid effects. Results. When the thermodynamic and hydrodynamic quantities vary only radially, the effect of the coupling between the heat flux, spacetime curvature and fluid velocity acts as the primary driver for an initial linearly time growing magnetic field. The plasma heat flux completely dominates the magnetic field generation at an specific distance from the black hole, where the fluid vorticity vanishes. This distance depends on the thermodynamical properties of the Keplerian plasma accretion disk. These properties control the strength of the non-ideal effects in the generation of seed magnetic fields.

astro-ph.HE↗

arXiv2024

Accelerating solutions of the Korteweg-de Vries equation

The Korteweg-de Vries equation is a fundamental nonlinear equation that describes solitons with constant velocity. On the contrary, here we show that this equation also presents accelerated wavepacket solutions. This behavior is achieved by putting the Korteweg-de Vries equation in terms of the Painlevé I equation. The accelerated waveform solutions are explored numerically showing their accelerated behavior explicitly.

nlin.SI↗

arXiv2024

Upshifted frequency of electromagnetic plasma waves due to reflecting gravitational waves acting as almost-luminal mirrors

We show that dispersive gravitational waves, as a background spacetime, can reflect electromagnetic waves in a plasma. This reflection upshifts the frequency of the reflected wave, being larger for low-frequency incident waves. This effect takes place when the gravitational wave background propagates almost at the speed of light, allowing it to behave similar to a luminal mirror to electromagnetic plasma waves.

astro-ph.HE↗

arXiv2024

Different kinds of accelerated propagation of relativistic electromagnetic plasma wavepackets

Relativistic electromagnetic plasma waves are described by a dynamical equation that can be solved not only in terms of plane waves, but for several different accelerating wavepacket solutions. Depending on the spatial and temporal dependence of the plasma frequency, different kinds of accelerating solution can be obtained, for example, in terms of Airy or Weber functions. Also, we show that an arbitrary accelerated wavepacket solution is possible, for example, for a system with a luminal plasma slab.

physics.plasm-ph↗

arXiv2024

Exact solutions to the Telegraph equation in terms of Airy functions

Two exact different solutions to the Telegraph equation in three-dimensional space are obtained in terms of Airy functions. As a result, these solutions unveil a distinctive propagation pattern along a coordinate that resembles a speed-cone-like coordinate of the system. This unique characteristic leads to effective Schrödinger-like equations, amenable to exact solutions through Airy functions.

physics.gen-ph↗

arXiv2024

Cosmological electromagnetic hopfions

It is shown that any mathematical solution for null electromagnetic field knots in flat spacetime is also a null field knotted solution for cosmological electromagnetic fields. This is obtained by replacing the time $t\rightarrow τ=\int dt/a$, where $a=a(t)$ is the scale factor of the Universe described by the Friedman-Lemaître-Robertson-Walker (FLRW) cosmology, and by adequately rewriting the (empty flat spacetimes) electromagnetic fields solutions in a medium defined by the FLRW metric. We found that the dispersion (evolution) of electromagnetic Hopfions is faster on cosmological scenarios. We discuss the implications of these results for different cosmological models.

gr-qc↗