Scalar Model Research Articles

Zero-dimensional models for gravitational and scalar QED decoherence

We investigate the dynamics of two quantum mechanical oscillator system–bath toy models obtained by truncating to zero spatial dimensions linearized gravity coupled to a massive scalar field and scalar quantum electrodynamics (QED). The scalar-gravity toy model maps onto the phase damped oscillator, while the scalar QED toy model approximately maps onto an oscillator system subject to two-photon damping. The toy models provide potentially useful insights into solving for open system quantum dynamics relevant to the full scalar QED and weak gravitational field systems, in particular operational probes of the decoherence for initial scalar field system superposition states.

Tsallis holographic dark energy scenario in viscous f(Q) gravity with tachyon field

This study investigates a bulk viscous fluid anisotropic cosmological model with [Formula: see text] gravity. Here, [Formula: see text] stands for the nonmetricity factor that drives gravitational interaction. We reconstructed the associated parameters with Tsallis holographic dark energy (THDE). We have solved the modified Einstein’s field equations by considering the bulk viscosity factor [Formula: see text]. Under the viscous and nonviscous THDE frameworks, we have obtained the expressions of [Formula: see text] using the power-law form of expansion. We have investigated the nature of various energy conditions for the stability analysis. The positive behavior of DEC and WEC indicates the model’s validation; on the other hand, SEC is violating, indicating the universe’s accelerated expansion. We have also investigated the reconstructed EoS parameter [Formula: see text] for bulk viscosity and obtained the one that lies in both quintessence and phantom regions. We also discussed the correspondence of the tachyon scalar field with THDE energy density in [Formula: see text] gravity. This correspondence permits the reconstruction of potentials and dynamics for scalar field models describing accelerated expansion.

Collective gradient perception with a flying robot swarm

In this paper, we study the problem of collective and emergent sensing with a flying robot swarm in which social interactions among individuals lead to following the gradient of a scalar field in the environment without the need of any gradient sensing capability. We proposed two methods—desired distance modulation and speed modulation—with and without alignment control. In the former, individuals modulate their desired distance to their neighbors and in the latter, they modulate their speed depending on the social interactions with their neighbors and measurements from the environment. Methods are systematically tested using two metrics with different scalar field models, swarm sizes and swarm densities. Experiments are conducted using: (1) a kinematic simulator, (2) a physics-based simulator, and (3) real nano-drone swarm. Results show that using the proposed methods, a swarm—composed of individuals lacking gradient sensing ability—is able to follow the gradient in a scalar field successfully. Results show that when individuals modulate their desired distances, alignment control is not needed but it still increases the performance. However, when individuals modulate their speed, alignment control is needed for collective motion. Real nano-drone experiments reveal that the proposed methods are applicable in real-life scenarios.

Exploring dust growth in the episodic WCd system WR140

ABSTRACT The wind collision region (WCR) in a colliding wind binary (CWB) is a particularly violent place, as such, it is surprising that it is also a region where significant quantities of interstellar dust can form. In extreme cases, approximately 30 per cent of the total mass-loss rate of a system can be converted into dust. These regions are poorly understood, as observation and simulation of these systems are difficult. In our previous paper, we simulated dust growth in CWB systems using an advected scalar model and found our model to be suitable for qualitative study. For this paper, we simulated the periodic dust forming CWB (WCd) system WR140 with our dust model, to determine how dust growth changes over the systems periastron passage. We found that dust production increases significantly at periastron passage, which is consistent with IR emission of the surrounding dusty shell. We also find that the dust production rate of the system decreases rapidly as the stars recede from each other, though the rate of decrease is significantly lower than the rate of increase during periastron passage. This was found to be due to strong cooling and its associated thermal instabilities, resulting in cool, high-density pockets of gas in the WCR where dust forms. The WCR also shows a degree of hysteresis, resulting in a radiative post-shock flow even when the stars are separated enough for the region to behave adiabatically.

D = 5 static, charged black holes, strings and rings with resonant, scalar Q-hair

A mechanism for circumventing the Mayo-Bekenstein no-hair theorem allows endowing four dimensional (D = 4) asymptotically flat, spherical, electro-vacuum black holes with a minimally coupled U (1)-gauged scalar field profile: Q-hair. The scalar field must be massive, self-interacting and obey a resonance condition at the threshold of (charged) superradiance. We establish generality for this mechanism by endowing three different types of static black objects with scalar hair, within a D = 5 Einstein-Maxwell-gauged scalar field model: asymptotically flat black holes and black rings; and black strings which asymptote to a Kaluza-Klein vacuum. These D = 5 Q-hairy black objects share many of the features of their D = 4 counterparts. In particular, the scalar field is subject to a resonance condition and possesses a Q-ball type potential. For the static black ring, the charged scalar hair can balance it, yielding solutions that are singularity free on and outside the horizon.

DHOST gravity in ultra-diffuse galaxies – part I: the case of NGC1052-DF2

The Ultra-Diffuse galaxy NGC1052-DF2 has recently been revealed to be “extremely deficient” in dark matter, if not lacking it at all. This claim has raised many questions regarding the relationship between baryons and dark matter in Ultra-Diffuse galaxies. But there seems to be a quite unanimous belief that, if such very low dark matter content is confirmed and extended to other similar galactic objects, it might be a deathblow to theories which modify and extend General Relativity. Deficient dark matter galaxies thus represent a fertile ground to test both standard dark matter and modified gravity theories. In this work, we consider a specific Degenerate Higher-Order Scalar Tensor model to study the internal kinematics of NGC1052-DF2. Due to the partial breaking of the corresponding screening mechanism, this model can possibly have large cosmological scale effects influencing the dynamics of smaller structures like galaxies. We consider two scenarios: one in which the model only describes dark energy; and one in which it additionally entirely substitutes dark matter. We find that the best model to explain data is General Relativity with only stellar contribution. But while in the former scenario General Relativity is still statistically favoured, in the latter one the alternative model is as much successful and effective as General Relativity in matching observations. Thus, we can conclude that even objects like NGC1052-DF2 are not in contrast, and are not obstacles, to the study and the definition of a reliable alternative to General Relativity.

A double scalar elastic damage constitutive model of ice materials

The constitutive model of material is the basis of structural analysis and is extremely important. In order to improve the analysis accuracy of the novel ice structure, a double scalar elastic damage constitutive model of ice materials including plain ice and fiber-reinforced ice (FRI) based on continuum damage theory and fracture mechanics is established in this study. The modified Ottosen equivalent strain criterion which can consider the comprehensive mechanical properties of compression, tension and shear is used as the damage initiation criterion and the internal variable of damage evolution. Based on the uniaxial compressive and tensile experiments, the two damage evolution laws of tensile and compressive damage with the modified Ottosen equivalent strain criterion as the internal variables are established. A unified weighted damage evolution function is constructed by the strain decomposition in the principal strain space. Based on the strain equivalent assumption, a complete damage constitutive model is established, which can consider the properties of ice materials at different temperatures and fiber contents. The energy regularization of crack bond model in fracture mechanics is adopted and the softening behavior of materials is described through fracture energy and stress-crack width relationship instead of the relationship between stress and strain, which solves the problem of mesh dependence in finite element analysis. The damage constitutive model is integrated into the large-scale finite element analysis software ABAQUS through the user-defined material subroutine (UMAT), and the establishment of the numerical model is realized, which can be conveniently used for numerical simulation of ice structures. Finally, the proposed damage constitutive model is evaluated according to the comparison between the results of finite element analysis and the results of uniaxial compression, uniaxial tension and four-point bending beam experiments. The results show that the damage constitutive model can well describe the stress-strain nonlinear behavior and capture the basic failure mode of ice materials. This study will lay a foundation for the refined analysis of ice structures.

A minimal self-tuning model to solve the cosmological constant problem

The expansion of the Universe is observed to be accelerating, with the simplest solution being a classical cosmological constant. However, this receives contributions from the quantum vacuum, which are predicted to be many orders of magnitude larger than observations, and suffers from radiative instabilities requiring repeated fine-tuning. In this paper we present a minimal, self-tuning scalar field model that can dynamically cancel a large quantum vacuum energy, avoiding Weinberg's No-Go Theorem, and produce accelerated de Sitter expansion at a lower energy scale as a solution to the problem. Our minimal model, which contains a non-canonical kinetic energy and a linear potential, belongs to the Kinetic Gravity Braiding sub-class of Horndeski theory which is not observationally excluded, and lies outside of the known Fab-Four or Well-Tempered models. We find analytic solutions in the limits of slow-roll and fast-roll, and numerically solve the equations of motion to illustrate our model. We show that the model allows for a matter dominated era, and that the attractor solution is stable under a phase transition in the vacuum energy density. We also consider the energy-scales required to match observations. Our model shows the existence of a wider class of successful self-tuning models than previously assumed.

Constructing CFTs from AdS flows

We study the renormalization group flow equations for correlation functions of weakly coupled quantum field theories in AdS. Taking the limit where the external points approach the conformal boundary, we obtain a flow of conformally invariant correlation functions. We solve the flow for one- and two-point functions and show that the corrections to the conformal dimensions can be obtained as an integral over the Mellin amplitude of the four-point function. We also derive the flow of the Mellin amplitude for higher n-point functions. We then consider the flows at tree level and one loop (in AdS), and show that one obtains exactly the recursion relations for the corresponding Mellin amplitudes derived earlier by Fitzpatrick et al. [1] at tree level and Yuan [2, 3] at one loop. As an application, we furthermore compute one-loop corrections to the conformal dimensions for some operators in the CFT dual to an O(N) scalar model in AdS.

An exploration of dust grain growth within WCd systems using an advected scalar dust model

ABSTRACT Dust production is one of the more curious phenomena observed in massive binary systems with interacting winds. The high temperatures, UV photon flux, and violent shocks should destroy any dust grains that condense. However, in some extreme cases, dust production yields of approximately 30 per cent of the total mass-loss rate of the stellar winds have been observed. In order to better understand this phenomenon, a parameter space exploration was performed using a series of numerical models of dust producing carbon phase Wolf–Rayet (WCd) systems. These models incorporated a passive scalar dust model simulating dust growth, destruction, and radiative cooling. We find that reasonable dust yields were produced by these simulations. Significant changes in the dust yield were caused by changing the mass-loss rates of the stars, with a greater mass-loss rate contributing to increased dust yields. Similarly, a close orbit between the stars also resulted in higher dust yields. Finally, a high-velocity wind shear, which induces Kelvin–Helmholtz (KH) instabilities and wind mixing, drastically increases the dust yields.

Coupling between DBI dark energy model and f(Q) gravity and its effect on condensed body mass accretion

In this paper, we have studied the reconstruction formalism of the Dirac–Born–Infeld (DBI)-essence scalar field model in the background of non-metric gravity or [Formula: see text] gravity which is controlled by the deformation or non-metricity scale [Formula: see text]. The deformation is caused by the fifth dimension of wrapped compact spaces in brane cosmology. The fifth dimension of that wrapped space is controlled by the D-Brane tension [Formula: see text]. Hence, we have reconstructed the formalism of DBI scalar field energy densities and pressures using the coupling of deformation geometry and brane scalar fields, i.e. [Formula: see text] and DBI-Scalar fields. The accretion of dark energy onto black holes and wormholes and its critical analysis had been studied with this reconstructed model. The validity of energy conditions has been studied. We have assumed four types of singularity resolutions of the scale factor to investigate the nature of the black hole and wormhole masses due to accretion. Graphically, the physical quantities like mass, kinetic and potential energies have been analyzed for DBI-essence model in the background of [Formula: see text] gravity.

Momentum dependent gap in holographic superconductors revisited

We reconsider the angular dependence in gap structure of holographic superconductors, which has not been treated carefully so far. For the vector field model, we show that the normalizable ground state is in the p-wave state because s-wave state is not normalizable. On the other hand, in the scalar order model, the ground state is in the s-wave. The angle dependent gap function is explicitly constructed in these models. We also suggest the modified ansatz of the vector order which enables to discuss the order px ± ipy gap. We have also analytically investigated the critical temperature and the behavior of the gap near there. Interestingly, for the fixed conformal dimension of the Cooper pair operator, the critical temperature in vector model is higher than that of the scalar model.

T-channel singularities in cosmology and particle physics

The t-channel singularity of a cross section for a binary 2→2 scattering occurs when the particle exchanged in the t-channel is kinematically allowed to be on its mass shell, that is when the process can be viewed as a sequence of two physical subprocesses: a two-body decay 1→2 and an inverse decay 2→1. We derive conditions for the singularity to be present in a binary process. A class of divergent cross sections for Standard Model processes has been determined and illustrated by the weak analog of the Compton scattering Ze−→Ze−. After critically reviewing regularization methods proposed in literature, we discuss singular processes that occur in a medium composed of particles in thermal equilibrium. The medium is shown to regulate the singularities naturally as particles acquire a non-zero width. We demonstrate a possible cosmological application by calculating thermally averaged cross section for an elastic scattering within a simple scalar model. The transition probability, which is divergent in vacuum, becomes finite when the process occurs in a thermal bath due to the imaginary part of the t-channel mediator's self-energy computed within the Keldysh-Schwinger formalism.

Probing Quintessence using BAO imprint on the cross-correlation of weak lensing and post-reionization H i 21 cm signal

ABSTRACT In this work, we investigate the possibility of constraining a thawing Quintessence scalar field model for dark energy. We propose using the imprint of baryon acoustic oscillation on the cross-correlation of post-reionization 21-cm signal and galaxy weak lensing convergence field to tomographically measure the angular diameter distance DA(z) and the Hubble parameter H(z). The projected errors in these quantities are then used to constrain the Quintessence model parameters. We find that independent 600 h radio interferometric observation at four observing frequencies 916, 650, 520, and 430 MHz with an SKA-1-Mid like radio telescope in cross-correlation with a deep weak lensing survey covering half the sky may measure the binned DA and H at a few per cent level of sensitivity. The Monte Carlo analysis for a power-law thawing Quientessence model gives the 1 − σ marginalized bounds on the initial slope λi, dark energy density parameter Ωϕ0 and the shape of the potential Γ at 8.63, 10.08, and $9.75{{\ \rm per\ cent}}$, respectively. The constraints improve to 7.66, 4.39, and $5.86{{\ \rm per\ cent}}$, respectively, when a joint analysis with supernovae and other probes is performed.

Key characteristics of generic bond graphs for planetary gears

This paper deals with generalized power flow models of compound planetary gear trains based on standard planetary gears. In this context, the Bond Graph approach provides an optimal tool for a large variety of structures. Starting with compound gears based on two standard planetary gears, two kinds of parameter definitions are discussed and two specifications for power ratio and efficiency are compared. Next, the influence of elastic shaft connections is clarified. Essential methods for these findings are the use of symmetry and the definition of sub-systems and, if applicable, of scalar or vectorial models. The second part deals with the effect of the number of constraints on constructions based on three standard planetary gears. Typical consequences, such as the number of free running elements, of complete Willis equations or of self-locking impacts, are systemized and illustrated by means of reference instances. It is shown by example that constructions featuring five external shafts would avoid such effects. Corresponding outcomes for the equivalent dynamic system equations, the parameter definitions and the Bond Graph modelling are given.

Motion induced excitation and electromagnetic radiation from an atom facing a thin mirror

We evaluate the probability of (de-)excitation and photon emission from a neutral, moving, non-relativistic atom, coupled to the quantum electromagnetic field and in the presence of a thin, perfectly conducting plane ("mirror"). These results extend, to a more realistic model, the ones we had presented for a scalar model, where the would-be electron was described by a scalar variable, coupled to an (also scalar) vacuum field. The latter was subjected to either Dirichlet or Neumann conditions on a plane. In our evaluation of the spontaneous emission rate produced when the accelerated atom is initially in an excited state, we pay attention to its comparison with the somewhat opposite situation, namely, an atom at rest facing a moving mirror.

On finite time singularities in scalar field dark energy models based in the RS-II Braneworld

The quest of deciphering the true nature of dark energy has proven to be one of the most exciting in recent times in cosmology. Various ideas have been put forward in this regard besides the usual cosmological constant approach, ranging from scalar field based models like Quintessence and Phantom dark energy to various modified gravity approaches as well. A very interesting idea then is to consider scalar field dark energy models in quantum gravitationally corrected cosmologies with the RS-II Braneworld being one of the most well known in this regard. So in this work, we consider RS-II Braneworld based scalar field dark energy models and try to look out for the existence of finite time singularities in these regimes both through a dynamical system perspective, for which we employ the Goriely–Hyde singularity analysis method, and a physical perspective. Our approach is general in the sense that it is not limited to any particular class of potentials or for any constrained parameter region for the brane tension and is valid for both Quintessence and phantom dark energy regimes. We firstly show through Goriely–Hyde procedure that finite time singularities can exist in these models for a limited set of initial conditions and that this result would hold irrespective of any consideration given to the swampland dS conjecture. We then discuss the physical nature of the singularities that can occur in this regime, where we use a well motivated ansatz for the Hubble parameter and show that these models of dark energy can allow for weak singularities like those of Type III and Type IV and can also allow for strong singularities like the Big Rip (Type I).

Four‐mode Squeezed States in de Sitter Space: A Study With Two Field Interacting Quantum System

AbstractIn this paper we study the application of four‐mode squeezed states in the cosmological context, studying two weakly coupled scalar fields in the planar patch of the de Sitter space. We construct the four‐mode squeezed state formalism and connect this concept with the Hamiltonian of the two coupled inverted harmonic oscillators having a time‐dependent effective frequency in the planar patch of the de Sitter space. Further, the corresponding evolution operator for the quantum Euclidean vacuum state has been constructed, which captures its dynamics. Using the Heisenberg picture coupled differential equations describing the time evolution for all squeezing parameters (amplitude, phase and angle) have been obtained, for the weakly coupled two scalar field model. With the help of these evolutions for the coupled system, we simulate the dynamics of the squeezing parameters in terms of conformal time. From our analysis, we observe interesting dynamics, which helps us to explore various underlying physical implications of the weakly coupled two scalar field system in the planar patch of the de Sitter cosmological background.

Towards hybrid two‐phase modelling using linear domain decomposition

AbstractThe viscous flow of two immiscible fluids in a porous medium on the Darcy scale is governed by a system of nonlinear parabolic equations. If infinite mobility of one phase can be assumed (e.g., in soil layers in contact with the atmosphere) the system can be substituted by the scalar Richards model. Thus, the porous medium domain may be partitioned into disjoint subdomains where either the full two‐phase or the simplified Richards model dynamics are valid. Extending the previously considered one‐model situations we suggest coupling conditions for this hybrid model approach. Based on an Euler implicit discretization, a linear iterative (L‐type) domain decomposition scheme is proposed, and proved to be convergent. The theoretical findings are verified by a comparative numerical study that in particular confirms the efficiency of the hybrid ansatz as compared to full two‐phase model computations.

Quantifying the breakdown of the rotating-wave approximation in single-photon superradiance

We study quantitatively the breakdown of the rotating-wave approximation (RWA) when calculating collective light emission by quantum emitters, in particular in the weak-excitation limit. Our starting point is a known multiple-scattering formalism where the full light–matter interaction leads to induced inter-emitter interactions described by the classical Green function of inhomogeneous dielectric media. When making the RWA in the light–matter interaction, however, these induced interactions differ from the classical Green function, and for free space we find a reduction of the interatomic interaction strength by up to a factor of two. By contrast, for the corresponding scalar model the relative RWA error for the inter-emitter interaction even diverges in the near field. For two identical emitters, the errors due to the RWA in collective light emission will show up in the emission spectrum, but not in the sub- and superradiant decay rates. In case of two non-identical emitters, also the collective emission rates will differ by making the RWA. For three or more identical emitters, the RWA errors in the interatomic interaction in general affect both the collective emission spectra and the collective decay rates. Ring configurations with discrete rotational symmetry are an interesting exception. Interestingly, the maximal errors in the collective decay rates due to making the RWA occur for finite emitter separations.