Weak Field Regime Research Articles

Dynamics of magnetization growth and relaxation in ferrofluids.

The dynamics of the growth and relaxation of the magnetization in ferrofluids are determined using theory based on the Fokker-Planck-Brown equation, and Brownian-dynamics simulations. Magnetization growth starting from an equilibrium nonmagnetized state in zero field, and following an instantaneous application of a uniform field of arbitrary strength, is studied with and without interparticle interactions. Similarly, magnetization relaxation is studied starting from an equilibrium magnetized state in a field of arbitrary strength, and following instantaneous removal of the field. In all cases, the dynamics are studied in terms of the time-dependent magnetization m(t). The field strength is described by the Langevin parameter α, the strength of the interparticle interactions is described by the Langevin susceptibility χ_{L}, and the individual particles undergo Brownian rotation with time τ_{B}. For noninteracting particles, the average growth time decreases with increasing α due to the torque exerted by the field, while the average relaxation time stays constant at τ_{B}; with vanishingly weak fields, the timescales coincide. The same basic picture emerges for interacting particles, but the weak-field timescales are larger due to collective particle motions, and the average relaxation time exhibits a weak, nonmonotonic field dependence. A comparison between theoretical and simulation results is excellent for noninteracting particles. For interacting particles with χ_{L}=1 and 2, theory and simulations are in qualitative agreement, but there are quantitative deviations, particularly in the weak-field regime, for reasons that are connected with the description of interactions using effective fields.

Electric Penrose, circular orbits and collisions of charged particles near charged black holes in Kalb–Ramond gravity

General relativity (GR) is a well-tested theory of gravity in strong and weak field regimes. Many modifications to this theory were obtained, including different scalar, vector, and tensor fields to the GR with non-minimal coupling to gravity. Kalb–Ramond (KR) gravity is also a modified theory formulated in the presence of a bosonic field. One astrophysical way to test gravity is by studying the motion of test particles in the spacetime of black holes (BH). In this work, we study the circular motion of charged particles and explore energetic processes around charged BHs in KR theory. First, we investigated the event horizon radius and analyzed horizon-no horizon regions in the BH charge and KR parameter space. Considering the Coulomb interaction, we derive and analyze the effective potential for charged particles around a charged KR BH. We investigate charged particles’ angular momentum and energy corresponding to circular orbits. We also investigate how the KR non-minimal coupling parameter affects the radius of the innermost stable circular orbits, the corresponding energy, and the angular momentum. We also investigated the electric Penrose process and charged-particle collisions near the KR BH. The presence of the nonzero KR parameter results in a decrease in the energy efficiency of the Penrose process. Also obtained is that the KR parameter’s positive (negative) values cause a decrease (increase) in the center of mass energy of colliding particles near the BH horizon.

Analytical and numerical study of light-induced optical microcavity generation by half-cycle light pulses in the resonant medium

An optical Bragg-like microcavity can be formed when extremely short light pulses interacting coherently with the medium collide in the resonant medium. Here we present an analytical approach, based on the approximate solution of the time-dependent Schrödinger equation, that reveals both the cavity formation and its control by the half-cycle pulses colliding in the medium. This approach allows the calculation of cavity properties in the weak-field regime without the need for complex numerical simulations. Beyond the approximate analytical solutions, our analytical results are confirmed by numerical simulations of the Maxwell-Bloch equations. Grating formation is also studied in the strong-field regime, where collisions of self-induced transparency half-cycle pulses occur. The results presented here reveal, to our knowledge, a new type of spatio-temporal photonic crystal in the form of micro-cavities in a simple atomic medium. The results obtained demonstrate the feasibility of attosecond optical switching in a simple atomic medium with half-cycle pulses.

Nonlinear Current Injection in Hexagonal Boron Nitride using Linearly Polarized Light in a Deeply Off‐Resonant Regime

AbstractLight‐induced electron dynamics in monolayer hexagonal boron nitride is theoretically investigated under the influence of two‐color linearly‐polarized laser fields at frequencies ω and 2ω, by solving the time‐dependent Schrödinger equation with a tight‐binding model. In the weak field regime, it is confirm that the injection of ballistic current arises from the breakdown of time‐reversal symmetry. This phenomenon is attributed to quantum interference between two distinct excitation paths: a one‐photon (2ℏω) absorption path and a two‐photon (ℏω) absorption path. In a strong field regime, the analysis reveals that the two‐color laser fields may generate a substantial population imbalance within momentum space, consequently facilitating the injection of ballistic current even in a deeply off‐resonant regime. The findings demonstrate that a pronounced population imbalance exceeding 30% of excited electrons can be realized without relying on the ellipticity of the fields. This highlights the potential of linearly polarized light for efficient photovoltaic effects and valley population control in 2D systems and heterostructures.

Tachyonic instability and spontaneous scalarization in parameterized Schwarzschild-like black holes

We study the phenomenon of spontaneous scalarization in parameterized Schwarzschild-like black holes. Two metrics are considered, the Konoplya–Zhidenko metric and the Johannsen–Psaltis metric. While these metrics can mimic the Schwarzschild black hole well in the weak-field regime, they have deformed geometries in the near-horizon strong-field region. Such deformations notably influence the emergence of tachyonic instability and subsequent spontaneous scalarization, enabling a clear distinction between these parameterized metrics and the standard Schwarzschild metric. These results suggest a possible way to test the parameterized black holes and thus the Kerr hypothesis by observing the phenomenon of spontaneous scalarization.

Aether Scalar Tensor (AeST) theory: quasistatic spherical solutions and their phenomenology

ABSTRACT There have been many efforts in the last three decades to embed the empirical Modified Newtonian Dynamics (MOND) programme into a robust theoretical framework. While many such theories can explain the profile of galactic rotation curves, they usually cannot explain the evolution of the primordial fluctuations and the formation of large-scale structures in the Universe. The Aether Scalar Tensor theory seems to have overcome this difficulty, thereby providing the first compelling example of an extension of general relativity able to successfully challenge the particle dark matter hypothesis. Here, we study the phenomenology of this theory in the quasistatic weak-field regime and specifically for the idealized case of spherical isolated sources. We find the existence of three distinct gravitational regimes, that is, Newtonian, MOND, and a third regime characterized by the presence of oscillations in the gravitational potential which do not exist in the traditional MOND paradigm. We identify the transition scales between these three regimes and discuss their dependence on the boundary conditions and other parameters in the theory. Aided by analytical and numerical solutions, we explore the dependence of these solutions on the theory parameters. Our results could help in searching for interesting observable phenomena at low redshift pertaining to galaxy dynamics as well as lensing observations, however, this may warrant proper N-body simulations that go beyond the idealized case of spherical isolated sources.

Weak-field regime of scalar-tensor theories with an instantaneous mode

Weak-field regime of scalar-tensor theories with an instantaneous mode

A thermo-magnetic bag model for the quark-gluon plasma

In this work we study the pressure of the quark-gluon plasma (QGP) in the presence of a weak magnetic field, using a minimally enhanced model of a weakly interacting gas of quasi-particles in a thermal bath. We include the magnetic field effects through the quark mass that has been modified using a recently proposed thermo-magnetic coupling. This thermo-magnetic coupling emerges form the quark-gluon vertex in the HTL approximation [1]. We use Lattice QCD [2] data, to constrain the thermo-magnetic bag function of the quasi-particle model and provide an estimate of the thermo-magnetic vacuum energy density. We then compute the transverse pressure of the system and compare with similar results from the literature. We find that the inverse magnetic catalysis already built within this thermo-magnetic coupling allows a robust description of this Lattice QCD data for the pressure of the QGP in the presence of a weak magnetic field. The extension to the thermal quasi-particle model we have introduced here, makes it easier to pursue further phenomenological studies that require simulations with an EoS that has integrable quasi-particle thermodynamic variables which have the general features of lattice data in the weak magnetic field regime.

Solar Tachocline Confinement by the Nonaxisymmetric Modes of a Dynamo Magnetic Field

We recently presented the first 3D numerical simulation of the solar interior for which tachocline confinement was achieved by a dynamo-generated magnetic field. In this follow-up study, we analyze the degree of confinement as the magnetic field strength changes (controlled by varying the magnetic Prandtl number) in a coupled radiative zone (RZ) and convection zone (CZ) system. We broadly find three solution regimes, corresponding to weak, medium, and strong dynamo magnetic field strengths. In the weak-field regime, the large-scale magnetic field is mostly axisymmetric with regular, periodic polarity reversals (reminiscent of the observed solar cycle) but fails to create a confined tachocline. In the strong-field regime, the large-scale field is mostly nonaxisymmetric with irregular, quasi-periodic polarity reversals and creates a confined tachocline. In the medium-field regime, the large-scale field resembles a strong-field dynamo for extended intervals but intermittently weakens to allow temporary epochs of strong differential rotation. In all regimes, the amplitude of poloidal field strength in the RZ is very well explained by skin-depth arguments, wherein the oscillating field that gives rise to the skin depth (in the medium- and strong-field cases) is a nonaxisymmetric field structure at the base of the CZ that rotates with respect to the RZ. These simulations suggest a new picture of solar tachocline confinement by the dynamo, in which nonaxisymmetric, very long-lived (effectively permanent) field structures rotating with respect to the RZ play the primary role, instead of the regularly reversing axisymmetric field associated with the 22 yr cycle.

Confinement and nonlinear electrodynamics: Asymptotic Schwarzschild charged black hole

While most nonlinear electrodynamics (NED) models tend to Maxwell’s linear theory in the weak field regime, very recently a model of NED has been proposed in the context of heavy quark–antiquark confinement that gives the same limit in the strong field regime. This form of correction to Maxwell’s linear theory is shown to be applicable in such a confined system. In this model, the radial electric field of a point charge is described by E=qr2+ζqr which consists of a Coulomb term plus a term proportional to the NED parameter ζ. Here in this paper, we extend the previous study on the model and accordingly, first, present more on the properties of the model and then introduce the magnetic black hole solution obtained in the gravity coupled minimally to this NED. The solution due to the unique property of the NED is asymptotically Schwarzschild black hole. In addition to the thermodynamics of the black hole, we investigate the circular speed–radius curve of the hypothetical star orbiting the mentioned black hole. It is shown that again due to the correction of Maxwell’s theory in the weak field limit these curves are equivalent to the corresponding curves in the Schwarzschild black hole.

Unveiling Coherent Control of Halomethane Dissociation Induced by a Single Strong Ultraviolet Pulse.

We present a theoretical investigation into the coherent control of photodissociation reactions in halomethanes, specifically focusing on CH2BrCl by manipulating the spectral phase of a single femtosecond laser pulse. We examine the photodissociation of CH2BrCl under an ultrashort pulse with a quadratic spectral phase and reveal the sensitivity of both the total dissociation probability and the resulting radical products (Br+CH2Cl and Cl+CH2Br) to chirp rates. To gain insights into the underlying mechanism, we calculate the population distributions of excited vibrational states in the ground electronic state, demonstrating the occurrence of resonance Raman scattering (RRS) in the strong-field limit regime. By utilizing chirped pulses, we show that this RRS phenomenon can be suppressed and even eliminated through quantum destructive interference. This highlights the high sensitivity of photodissociation into Cl+CH2Br to the spectral phase, showcasing a phenomenon that goes beyond the traditional one-photon photodissociation of isolated molecules in the weak-field limit regime. These findings emphasize the importance of coherent control in the exploration and utilization of photodissociation in polyatomic molecules, paving the way for new advancements in chemical physics and femtochemistry.

General formulae for the periapsis shift of a quasi-circular orbit in static spherically symmetric spacetimes and the active gravitational mass density

We study the periapsis shift of a quasi-circular orbit in general static spherically symmetric spacetimes. We derive two formulae in full order with respect to the gravitational field, one in terms of the gravitational mass [Formula: see text] and the Einstein tensor and the other in terms of the orbital angular velocity and the Einstein tensor. These formulae reproduce the well-known ones for the forward shift in the Schwarzschild spacetime. In a general case, the shift deviates from that in the vacuum spacetime due to a particular combination of the components of the Einstein tensor at the radius [Formula: see text] of the orbit. The formulae give a backward shift due to the extended-mass effect in Newtonian gravity. In general relativity, in the weak-field and diffuse regime, the active gravitational mass density, [Formula: see text], plays an important role, where [Formula: see text], [Formula: see text] and [Formula: see text] are the energy density, the radial stress and the tangential stress of the matter field, respectively. We show that the shift is backward if [Formula: see text] is beyond a critical value [Formula: see text][Formula: see text]g/cm3[Formula: see text], while a forward shift greater than that in the vacuum spacetime instead implies [Formula: see text], i.e. the violation of the strong energy condition, and thereby provides evidence for dark energy. We obtain new observational constraints on [Formula: see text] in the Solar System and the Galactic Centre.

Phonon motion around (2+1)-dimensional acoustic black hole

We study motion of a phonon, a particle representing the quanta of the sound wave in the (2+1) spacetime of the acoustic analogous axially symmetric black hole, so-called acoustic (sonic) black hole. Similar to the real objects known as black holes in relativity theories, the phenomenon called acoustic black hole possesses the ergoregion whose area is increasing with increasing rotation of the black hole, leading to more phonons being affected by the supersonic flow. It is found that phonons in the ergoregion of an acoustic black hole behave differently than those outside of it. Specifically, we found that the phonons in the ergoregion are affected by the supersonic flow of the fluid, causing them to move in different directions than those outside the ergoregion. Moreover, we presented calculations of the deflection angle and time delay of the phonon in the field of the acoustic black hole in the weak field regime that can be useful to test the geometry of the acoustic black hole in the laboratory.

Constraints on charged symmergent black hole from shadow and lensing

In this paper, we report on exact charged black hole solutions in symmergent gravity with Maxwell field. Symmergent gravity induces the gravitational constant G, quadratic curvature coefficient , and the vacuum energy from the flat spacetime matter loops. In the limit in which all fields are degenerate in mass, the vacuum energy can be expressed in terms of G and . We parametrize deviation from this limit by a parameter such that the black hole spacetime is de Sitter (dS) for and anti-de Sitter (AdS) for . In our analysis, we study horizon formation, shadow cast and gravitational lensing as functions of the black hole charge, and find that there is an upper bound on the charge. At relatively low values of charge, applicable to astronomical black holes, we determine constraints on and using the Event Horizon Telescope (EHT) data from Sgr. A* and M87*. We apply these constraints to reveal how the shadow radius behaves as the observer distance varies. It is revealed that black hole charge directly influences the shadow silhouette, but the symmergent parameters have a tenuous effect. We also explored the weak field regime by using the Gauss–Bonnet theorem to study the weak deflection angle caused by the M87* black hole. We have found that impact parameters comparable to the actual distance D = 16.8 Mpc show the potential detectability of such an angle through advanced astronomical telescopes. Overall, our results provide new insights into the behavior of charged black holes in the context of symmergent gravity and offer a new way to test these theories against observational data.

Cosmological perturbations engendered by discrete relativistic species

Within the extension of the varLambda CDM model, allowing for the presence of neutrinos or warm dark matter, we develop the analytical cosmological perturbation theory. It covers all spatial scales where the weak gravitational field regime represents a valid approximation. Discrete particles – the sources of the inhomogeneous gravitational field – may be relativistic. Similarly to the previously investigated case of nonrelativistic matter, the Yukawa interaction range is naturally incorporated into the first-order scalar metric corrections.

Stark Localization as a Resource for Weak-Field Sensing with Super-Heisenberg Precision.

Gradient fields can effectively suppress particle tunneling in a lattice and localize the wave function at all energy scales, a phenomenon known as Stark localization. Here, we show that Stark systems can be used as a probe for the precise measurement of gradient fields, particularly in the weak-field regime where most sensors do not operate optimally. In the extended phase, Stark probes achieve super-Heisenberg precision, which is well beyond most of the known quantum sensing schemes. In the localized phase, the precision drops in a universal way showing fast convergence to the thermodynamic limit. For single-particle probes, we show that quantum-enhanced sensitivity, with super-Heisenberg precision, can be achieved through a simple position measurement for all the eigenstates across the entire spectrum. For such probes, we have identified several critical exponents of the Stark localization transition and established their relationship. Thermal fluctuations, whose universal behavior is identified, reduce the precision from super-Heisenberg to Heisenberg, still outperforming classical sensors. Multiparticle interacting probes also achieve super-Heisenberg scaling in their extended phase, which shows even further enhancement near the transition point. Quantum-enhanced sensitivity is still achievable even when state preparation time is included in resource analysis.

All Order Gravitational Waveforms from Scattering Amplitudes.

Waveforms are classical observables associated with any radiative physical process. Using scattering amplitudes, these are usually computed in a weak-field regime to some finite order in the post-Newtonian or post-Minkowskian approximation. Here, we use strong-field amplitudes to compute the waveform produced in scattering of massive particles on gravitational plane waves, treated as exact nonlinear solutions of the vacuum Einstein equations. Notably, the waveform contains an infinite number of post-Minkowskian contributions, as well as tail effects. We also provide, and contrast with, analogous results in electromagnetism.

Analytic Post-Newtonian Astrometric and Spectroscopic Models of Orbits around Black Holes

Observations of the S stars, the cluster of young stars in the inner 0.1 pc of the Galactic center, have been crucial in providing conclusive evidence for a supermassive black hole at the center of our galaxy. Since some of the stars have orbits less than that of a typical human lifetime, it is possible to observe multiple orbits and test the weak-field regime of general relativity. Current calculations of orbits require relatively slow and expensive computations in order to perform numerical integrations for the position and momentum of each star at each observing time. In this paper, we present a computationally efficient, first-order post-Newtonian model for the astrometric and spectroscopic data gathered for the S stars. We find that future, 30 m class telescopes—and potentially even current large telescopes with very high spectroscopic resolution—may be able to detect the Shapiro effect for an S star in the next decade or so.

X-ray Tests of General Relativity with Black Holes

General relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak-field regime with experiments in the solar system and radio observations of binary pulsars. Until 2015, the strong-field regime was almost completely unexplored. Thanks to new observational facilities, the situation has dramatically changed in the last few years. Today, we have gravitational wave data of the coalesce of stellar-mass compact objects from the LIGO-Virgo-KAGRA collaboration, images at mm wavelengths of the supermassive black holes in M87* and Sgr A* from the Event Horizon Telescope collaboration, and X-ray data of accreting compact objects from a number of X-ray missions. Gravitational wave tests and black hole imaging tests are certainly more popular and are discussed in other articles of this Special Issue. The aim of the present manuscript is to provide a pedagogical review on X-ray tests of general relativity with black holes and to compare these kinds of tests with those possible with gravitational wave data and black hole imaging.

Gravitational and electromagnetic radiation from binary black holes with electric and magnetic charges: hyperbolic orbits on a cone

We derive the hyperbolic orbit of binary black holes with electric and magnetic charges. In the low-velocity and weak-field regime, by using the Newtonian method, we calculate the total emission rate of energy due to gravitational and electromagnetic radiation from binary black holes with electric and magnetic charges in hyperbolic orbits. Moreover, we develop a formalism to derive the merger rate of binary black holes with electric and magnetic charges from the two-body dynamical capture. We apply the formalism to investigate the effects of the charges on the merger rate for the near-extremal case and find that the effects cannot be ignored.