arXiv subjects

Masud Chaichian

Publications and source records attributed to Masud Chaichian.

At least 19 recordsLinked to original sources

arXiv2024

On the Axion Electrodynamics in a two-dimensional slab and the Casimir effect

We analyze the Axion Electrodynamics in a two-dimensional slab of finite width $L$ containing a homogeneous and isotropic dielectric medium with constant permittivity and permeability. We start from the known decomposition of modes in the nonaxion case and then solve perturbatively the governing equations for the electromagnetic fields to which the axions are also coupled. This is a natural approach, since the finiteness of $L$ destroys the spatial invariance of the theory in the $z$ direction normal to the plates. In this way we derive the value of the axion-generated rotation angle of the electric and magnetic fields after their passage through the slab, and use the obtained results to calculate the Casimir force between the two conducting plates. Our calculations make use of the same method as previously outlined in \cite{hoye20} for the case of Casimir calculations in chiral media and extend former results on the Casimir force in the Axion Electrodynamics.

hep-th↗

arXiv2023

Mimetic Tensor-Vector-Scalar Cosmology: Incorporating Dark Matter, Dark Energy and Stiff Matter

Phenomenological implications of the Mimetic Tensor-Vector-Scalar theory (MiTeVeS) are studied. The theory is an extension of the vector field model of mimetic dark matter, where a scalar field is also incorporated, and it is known to be free from ghost instability. In the absence of interactions between the scalar field and the vector field, the obtained cosmological solution corresponds to the General theory of Relativity (GR) with a minimally-coupled scalar field. However, including an interaction term between the scalar field and the vector field yields interesting dynamics. There is a shift symmetry for the scalar field with a flat potential, and the conserved Noether current, which is associated with the symmetry, behaves as a dark matter component. Consequently, the solution contains a cosmological constant, dark matter and a stiff matter fluid. Breaking the shift symmetry with a non-flat potential gives a natural interaction between dark energy and dark matter.

gr-qc↗

arXiv2023

Axion electrodynamics: Green's functions, zero-point energy and optical activity

Starting from the theory of Axion Electrodynamics, we work out the axionic modifications to the electromagnetic Casimir energy using the Green's function, both when the axion field is initially assumed purely time-dependent and when the axion field configuration is a static domain wall. For the first case it means that the oscillating axion background is taken to resemble an axion fluid at rest in a conventional Casimir setup with two infinite parallel conducting plates, while in the second case we evaluate the radiation pressure acting on an axion domain wall. We extend previous theories in order to include finite temperatures. Various applications are discussed. 1. We review the theory of Axion Electrodynamics and particularly the energy-momentum conservation in a linear dielectric and magnetic material. We treat this last aspect by extending former results by Brevik and Chaichian (2022) and Patkos (2022). 2. Adopting the model of the oscillating axion background we discuss the axion-induced modifications to the Casimir force between two parallel plates by using a Green's function approach. 3. We calculate the radiation pressure acting on an axion domain wall at finite temperature T. Our results for an oscillating axion field and a domain wall are also useful for condensed matter physics, where "axionic topological insulators" interact with the electromagnetic field with a Chern-Simons interaction, like the one in Axion Electrodynamics, and there are experimental systems analogous to time-dependent axion fields and domain walls as the ones showed by Jiang, Q. D., \& Wilczek, F. (2019) and Fukushima et al. (2019). 4. We compare our results, where we assume time-dependent or space-dependent axion configurations, with the discussion of the optical activity of Axion Electrodynamics by Sikivie (2021) and Carrol et al. (1990).

hep-th↗

arXiv2023

Axionic and nonaxionic electrodynamics in plane and circular geometry

Various aspects of axion electrodynamics in the presence of a homogeneous and isotropic dielectric medium are discussed. 1. We consider first the "antenna-like" property of a planar dielectric surface in axion electrodynamics, elaborating on the treatment given earlier on this topic by Millar {\it et al.} (2017). We calculate the electromagnetic energy transmission coefficient for a dielectric plate, and compare with the conventional expression in ordinary electrodynamics. 2. We consider the situation where the medium exterior to the plate, assumed elastic, is "bent back" and glued together, so that we obtain a circular dielectric string in which the waves can propagate clockwise or counterclockwise. As will be shown, a stationary wave pattern is permitted by the formalism, and we show how the amplitudes for the two counterpropagating waves can be found. 3. As a special case, by omitting axions for a moment, we analyze the Casimir effect for the string, showing its similarity as well as its difference with the Casimir effect of a scalar field for a piecewise uniform string (Brevik and Nielsen 1990). 4. Finally, including axions again we analyze the enhancement of the surface-generated electromagnetic radiation near the center of a cylindrical haloscope, where the interior region is a vacuum and the exterior region a metal. This enhancement is caused by the curvature of the boundary, and is mathematically a consequence of the behavior of the Hankel function of the second kind for small arguments. A simple estimate shows that enhancement may be quite significant, and can therefore be of experimental interest. This proposal is suggested as an alternative to the reflector arrangement in a similar arrangement recently discussed by Liu {\it et al.} (2022).

hep-th↗

arXiv2023

Alternative approach to the Starobinsky model for inflation scenarios

The $R+R^2$ model of gravity with the corresponding shallow potential in the Einstein frame is consistent with the observations. Recently, many efforts have been made to generalize the $R+R^2$ (Starobinsky) model of inflation or use other shallow potentials to construct a model for the early Universe. We revise the question about the shallow potential. We propose a model in which the Starobinsky model can emerge through a dynamical mechanism. We show that the absence of ghost modes results to constraints on the parameters of the Starobinsky model. We obtain the scalar spectral index and the tensor-to-scalar ratio of the extended model and study the three-point correlation function of the curvature perturbation to estimate the primordial non-Gaussianities of the proposed model.

gr-qc↗

arXiv2022

Electric Current and Heat Production by a Neutral Carrier: An Effect of the Axion

A general axion-electrodynamic formalism is presented on the phenomenological level when the environment is dielectric (permittivity and permeability assumed to be constants). Thereafter, a strong and uniform magnetic field is considered in the $z$ direction, the field region having the form of a long material cylinder (which corresponds to the haloscope setup). If the axion amplitude depends on time only, the axions give rise to an oscillating electric current in the $z$ direction. We estimate the magnitudes of the azimuthal magnetic fields and the accompanying Joule heating in the cylinder, taking the cylinder to have ordinary dissipative properties. We evaluate and calculate the electric current and the heat production separately, without using the effective approximation, both when there is a strong magnetic field and when there is a strong electric one, showing that with the magnetic field there is a heat production, while with the electric field there is not. The heat generation that we consider, is a nontrivial effect as it is generated by the electrically neutral axions, and has obvious consequences for axion thermodynamics. The heat production can moreover have an additional advantage, since the effect is accumulative and so grows with time. The boundary conditions (in a classical sense) are explained and the use of them in a quantum mechanical context is discussed. This point is nontrivial, accentuated in particular in connection with the Casimir effect. For comparison purposes, we present finally some results for heat dissipation taken from the theory of viscous cosmology.

hep-ph↗

arXiv2021

Dispersion of Light Traveling through the Interstellar Space, Induced and Intrinsic Lorentz Invariance Violation

Theoretical models and experimental observations suggest that gamma-ray bursts (GRB) and high-energy neutrino bursts travelling through the interstellar space may reach the Earth at different speeds. We propose and study in details the mechanism i), which always exists, where GRB are slowed down due to the dispersion of light in the interstellar medium. In addition to the standard media such as electrons and photons as CMB, we consider the medium with invisible axions. The amount of GRB delays in different media are calculated in details utilizing a novel technique in QFT by using the hitherto known or estimated densities of particles in the space without introducing any arbitrary parameter. Previously, the GRB delays have been interpreted as a sign of Lorentz invariance violation by modifying the dispersion relation of Special Relativity, which relates the energy, the momentum and the mass of a particle, based on different mechanisms ii), such as a stringy spacetime foam, coming from a quantum gravity effect and using an adjustable parameter. Obviously, all the above-mentioned mechanisms i) and ii) are induced (seeming) Lorentz invariance violations but not an intrinsic (genuine) one. The amount of GRB delay due to the two aforementioned interpretations can be distinguished by observing the time of arrival of light with different frequencies. Namely, dispersion of light i) predicts that the higher energy GRB arrive the Earth earlier, while in the other interpretations ii), they arrive later. We notice that the needed amount for delay due to the dispersion of light shall have the potential power to shed additional light on the microstructure of interstellar media with respect to the densities of constituent particles and the origins of their sources. Finally, we indicate the ways to detect the intrinsic Lorentz invariance violation and to interpret them theoretically.

astro-ph.HE↗

arXiv2021

Can the gamma-ray bursts travelling through the interstellar space be explained without invoking the drastic assumption of Lorentz invariance violation?

Experimental observations indicate that gamma-ray bursts (GRB) and high-energy neutrino bursts may travel at different speeds with a typical delay measured at the order of hours or days. We discuss two potential interpretations for the GRB delay: dispersion of light in interstellar medium and violation of Lorentz invariance due to quantum gravitational fluctuations. Among a few other media, we consider dispersion of light in an axion plasma, obtaining the axion plasma frequency and the dispersion relation from quantum field theory for the first time. We find that the density of axions inferred from observations is far too low to produce the observed GRB delay. However, a more precise estimation of the spatial distribution of axions is required for a conclusive result. Other known media are also unable to account for the GRB delay, although there remains uncertainties in the observations of the delays. The interpretation in terms of Lorentz invariance violation and modified dispersion relation suffers from its own problems: since the modification of the dispersion relation should not be dependent on particle type, delays between photons and neutrinos are hard to explain. Thus neither interpretation is sufficient to explain the observations. We conclude that a crucial difference between the two interpretations is the frequency dependence of the propagation speed of radiation: in dispersive plasma the group speed increases with higher frequency, while Lorentz invariance violation implies lower speed at higher frequency. Future experiments shall resolve which one of the two frequency dependencies of GRB is actually the case.

astro-ph.HE↗

arXiv2020

Remarks on the Abraham-Minkowski problem, from the formal and from the experimental side

We analyze the Abraham-Minkowski problem known from classical electrodynamics from two different perspectives. First, we follow a formal approach, implying use of manifolds with curved space sections in accordance with Fermat's principle, emphasizing that the resulting covariant and contravariant components of the photon four-momentum is a property linked to the {\it Minkowski} theory only. There is thus no link to the Abraham theory in that context. Next we turn to the experimental side, giving a brief account of older and newer radiation pressure experiments that clearly show how the Minkowski photon momentum is preferable under optical conditions. Under low-frequency conditions, where experimental detection of the individual oscillations predicted by the Abraham term are possible, the picture is however quite different.

gr-qc↗

arXiv2018

Sakharov's induced gravity and the Poincaré gauge theory

We explore Sakharov's seminal idea that gravitational dynamics is induced by the quantum corrections from the matter sector. This was the starting point of the view that gravity has an emergent origin, which soon gained impetus due to the advent of black hole thermodynamics. In the generalized framework of Riemann--Cartan spacetime with both curvature and torsion, the induced gravitational action is obtained for free nonminimally coupled scalar and Dirac fields. For a realistic matter content, the induced Newton constant is obtained to be of the magnitude of the ultraviolet cutoff, which implies that the cutoff is of the order of the Planck mass. Finally, we conjecture that the action for any gauge theory of gravity at low energies can be induced by Sakharov's mechanism. This is explicitly shown by obtaining the Poincaré gauge theory of gravity.

hep-th↗

arXiv2018

Condition for confinement in non-Abelian gauge theories

We show that a criterion for confinement, based on the BRST invariance, holds in four dimensions, by solving a non-Abelian gauge theory with a set of exact solutions. The confinement condition we consider was obtained by Kugo and Ojima some decades ago. The current understanding of gauge theories permits us to apply the techniques straightforwardly for checking the validity of this criterion. In this way, we are able to show that the non-Abelian gauge theory is confining and that confinement is rooted in the BRST invariance and asymptotic freedom.

hep-th↗

arXiv2015

Non-Projectable Horava-Lifshitz Gravity without Unwanted Scalar Graviton

We consider a way of eliminating the unwanted scalar graviton from Horava-Lifshitz gravity. That is achieved via introduction of certain additional constraints. We perform canonical analysis of both projectable and non-projectable versions of the theory. We obtain the structure of constraints in each case, and analyze its dependence on the values of the coupling constants involved in the additional constraints. In the non-projectable theory, the scalar graviton is absent when the coupling constants have certain values, while for other values the scalar graviton appears. The projectable theory is free from the scalar graviton regardless of the values of the coupling constants, even though the structure of constraints does depend on the value of a coupling constant.

gr-qc↗

arXiv2014

Mimetic dark matter, ghost instability and a mimetic tensor-vector-scalar gravity

Recently modified gravitational theories which mimic the behaviour of dark matter, the so-called "Mimetic Dark Matter", have been proposed. We study the consistency of such theories with respect to the absence of ghost instability and propose a new tensor-vector-scalar theory of gravity, which is a generalization of the previous models of mimetic dark matter with additional desirable features. The original model proposed by Chamseddine and Mukhanov [JHEP 1311 (2013) 135, arXiv:1308.5410] is concluded to describe a regular pressureless dust, presuming that we consider only those configurations where the energy density of the mimetic dust remains positive under time evolution. For certain type of configurations the theory can become unstable. Both alternative modified theories of gravity, which are based on a vector field (tensor-vector theory) or a vector field and a scalar field (tensor-vector-scalar theory), are free of ghost instabilities.

hep-th↗

arXiv2014

Can TeVeS be a viable theory of gravity?

Among modified gravitational theories, the Tensor-Vector-Scalar (TeVeS) occupies a special place -- it is a covariant theory of gravity that produces the modified Newtonian dynamics (MOND) in the nonrelativistic weak field limit and explains the astrophysical data at scales larger than that of the Solar System, without the need of an excessive amount of invisible matter. We show that, in contrast to other modified theories, TeVeS is free from ghosts. These achievements make TeVeS (and its nonrelativistic limit) a viable theory of gravity. A speculative outlook on the emergence of TeVeS from a quantum theory is presented.

hep-th↗

arXiv2013

Arnowitt-Deser-Misner representation and Hamiltonian analysis of covariant renormalizable gravity

We study the recently proposed Covariant Renormalizable Gravity (CRG), which aims to provide a generally covariant ultraviolet completion of general relativity. We obtain a space-time decomposed form --- an Arnowitt-Deser-Misner (ADM) representation --- of the CRG action. The action is found to contain time derivatives of the gravitational fields up to fourth order. Some ways to reduce the order of these time derivatives are considered. The resulting action is analyzed using the Hamiltonian formalism, which was originally adapted for constrained theories by Dirac. It is shown that the theory has a consistent set of constraints. It is, however, found that the theory exhibits four propagating physical degrees of freedom. This is one degree of freedom more than in Hořava-Lifshitz (HL) gravity and two more propagating modes than in general relativity. One extra physical degree of freedom has its origin in the higher order nature of the CRG action. The other extra propagating mode is a consequence of a projectability condition similarly as in HL gravity. Some additional gauge symmetry may need to be introduced in order to get rid of the extra gravitational degrees of freedom.

gr-qc↗

arXiv2013

Higher derivative gravity with spontaneous symmetry breaking: Hamiltonian analysis of new covariant renormalizable gravity

In order to explore some general features of modified theories of gravity which involve higher derivatives and spontaneous Lorentz and/or diffeomorphism symmetry breaking, we study the recently proposed new version of covariant renormalizable gravity (CRG). CRG attains power-counting renormalizability via higher derivatives and introduction of a constrained scalar field and spontaneous symmetry breaking. We obtain an Arnowitt-Deser-Misner representation of the CRG action in four-dimensional spacetime with respect to a foliation of spacetime adapted to the constrained scalar field. The resulting action is analyzed by using Hamiltonian formalism. We discover that CRG contains two extra degrees of freedom. One of them carries negative energy (a ghost) and it will destabilize the theory due to its interactions. This result is in contrast with the original paper [Phys. Lett. B 701, 117 (2011), arXiv:1104.4286 [hep-th]], where it was concluded that the theory is free of ghosts and renormalizable when we analyze fluctuations on the flat background.

gr-qc↗

arXiv2013

Lorentz-invariant CPT violation

A Lorentz-invariant CPT violation, which may be termed as long-distance CPT violation in contrast to the familiar short-distance CPT violation, has been recently proposed. This scheme is based on a non-local interaction vertex and characterized by an infrared divergent form factor. We show that the Lorentz covariant $T^{\star}$-product is consistently defined and the energy-momentum conservation is preserved in perturbation theory if the path integral is suitably defined for this non-local theory, although unitarity is generally lost. It is illustrated that T violation is realized in the decay and formation processes. It is also argued that the equality of masses and decay widths of the particle and anti-particle is preserved if the non-local CPT violation is incorporated either directly or as perturbation by starting with the conventional CPT-even local Lagrangian. However, we also explicitly show that the present non-local scheme can induce the splitting of particle and anti-particle mass eigenvalues if one considers a more general class of Lagrangians.

hep-th↗

arXiv2012

Electromagnetic interaction in theory with Lorentz invariant CPT violation

An attempt is made to incorporate the electromagnetic interaction in a Lorentz invariant but CPT violating non-local model with particle-antiparticle mass-splitting, which is regarded as a modified QED. The gauge invariance is maintained by the Schwinger non-integrable phase factor but the electromagnetic interaction breaks C, CP and CPT symmetries. Implications of the present CPT breaking scheme on the electromagnetic transitions and particle-antiparticle pair creation are discussed. The CPT violation such as the one suggested here may open a new path to the analysis of baryon asymmetry since some of the Sakharov constraints are expected to be modified.

hep-th↗