Modern Physics Letters A

We review our recent theoretical results about inequivalence between passive gravitational mass and energy for a composite quantum body at a macroscopic level. In particular, we consider macroscopic ensembles of the simplest composite quantum bodies — hydrogen atoms. Our results are as follows. For the most ensembles, the Einstein’s Equivalence Principle is valid. On the other hand, we discuss that for some special quantum ensembles — ensembles of the coherent superpositions of the stationary quantum states in the hydrogen atoms (which we call Gravitational demons) — the Equivalence Principle between passive gravitational mass and energy is broken. We show that, for such superpositions, the expectation values of passive gravitational masses are not related to the expectation values of energies by the famous Einstein’s equation, i.e. $m_g\ne \frac{E}{c^2}$. Possible experiments at the Earth’s laboratories are briefly discussed, in contrast to the numerous attempts and projects to discover the possible breakdown of the Einstein’s Equivalence Principle during the space missions.

A simple tabletop setup based on a superconductor quantum interference device is proposed to test the gravitational interaction. A D-shaped superconducting loop has the straight segment immersed inside a massive sphere while the half-circle segment is wrapped around the sphere. The superconducting condensate within the straight arm of the loop thus bathes inside a gravitational simple harmonic oscillator potential while the condensate in the half-circle arm bathes in the constant gravitational potential around the sphere. The resulting phase difference at the Josephson junctions on both sides of the straight arm induces a sinusoidal electric current that has a frequency determined by the precise gravitational potential due to the massive sphere.

We have investigated the phenomenological implications of texture one-zero neutrino mass matrix under the lamp post of the latest data on neutrino mass and mixings. In particular, we have obtained the predictions of the model for, yet unknown observables like neutrino mass hierarchy, ${𝜃}_{23}$-octant and CP violation. Out of the six texture one-zero neutrino mass models, $T_1$, $T_2$ and $T_3$ are found to be necessarily CP violating. ${𝜃}_{23}$ can be above or below maximality except for the texture $T_4$ (with NH), wherein ${𝜃}_{23}<45^{\circ }$ at $2.5\sigma$. Also, we have proposed a flavor model based on the non-Abelian group $A_4$ within the paradigm of type-I+II seesaw framework, wherein such textures can be realized.

Noncommutative optical squeezed vacuum states are constructed as eigenstates of an appropriate two-photon annihilation operator corresponding to the Biedenharn–Macfarlane $q$-oscillator. We consider in details the role of noncommutativity parameter $q$ on the nonclassical behaviors including quadrature squeezing and sub-Poissonian statistics. Also, we construct the noncommutative photon-added squeezed vacuum states and consider their Hillery-type higher-order squeezing and single-mode noise band.

According to the Lorentz Invariance Violation originated from the quantum gravitational theory and the string theory, the Rarita-Schwinger equation of arbitrary spin fermions are exactly modified in the high energy case. Then we restudy the dynamic equation of fermions with arbitrary spin in charged Kerr-Newman-Kasuya (KNK) black hole space-time. Moreover, the tunneling radiation characteristics of fermions are studied according to the modified dynamic equation. Therefore, some new expressions for physical quantities such as tunneling rate, surface gravitation, Hawking temperature and entropy of the black hole are corrected. As a result, we calculate that the surface gravitation at the event horizon of the KNK black hole is a constant, and find that the Hawking temperature will increase, but the entropy will decrease with the increasing of correction parameter.

Under conformal transformation, $f(\Re )$ theory of gravity in Palatini formalism leads to a Brans–Dicke type of scalar-tensor equivalent theory with a wrong sign in the effective kinetic energy term. This means that the effective scalar acts as the dark energy and so late-time cosmic acceleration in the matter-dominated era is accountable. However, we unveil some aspects of Palatini formalism, which reveals the fact that the formalism is not suitable to explain the cosmological evolution of the early universe with $f(\Re )$ gravity alone. Additionally, it is noticed that some authors, in an attempt to explore Noether symmetry of the theory changed the sign of the kinetic term and hence obtained the wrong answer. Here, we make the correction and unmask a very interesting aspect of symmetry analysis. Mathematical inequivalence between Jordan’s and Einstein’s frame in Palatini $f(\Re )$ theory has also been revealed.

We study the shadow and energy emission rate of a spherically symmetric noncommutative black hole in Rastall gravity. Depending on the model parameters, the noncommutative black hole can reduce to the Schwarzschild black hole. Since the nonvanishing noncommutative parameter affects the formation of event horizon, the visibility of the resulting shadow depends on the noncommutative parameter in Rastall gravity. The obtained sectional shadows respect the unstable circular orbit condition, which is crucial for physical validity of the black hole image model.

In this paper, we employ the Karmarkar condition to model a spherically symmetric radiating star undergoing dissipative gravitational collapse within the framework of classical general relativity. The collapse ensues from an initial static core satisfying the Karmarkar condition in isotropic coordinates and proceeds nonadiabatically by emitting energy in the form of a radial heat flux to the exterior Vaidya spacetime. We show that the dynamical nature of the collapse is sensitive to the initial static configuration that inherently links the embedding to the final remnant. Our model considered several physical tests on how an initially static stellar structure onset to a radiative collapse.

In this paper, we first obtain the energy density by the approach of the new agegraphic dark energy model, and then the $f(T,B)$ gravity model is studied as an alternative to the dark energy in a viscous fluid by flat-FRW background, in which $T$ and $B$ are torsion scalar and boundary term. The Friedmann equations will be obtained in the framework of modified teleparallel gravity by tetrad components. We consider that the universe dominates with components such as matter and dark energy by an interacting model. The Hubble parameter is parameterized by the power-law for the scale factor, and then we fit the corresponding Hubble parameter with observational data constraints. The variation of the equation of state (EoS) for dark energy is plotted as a function of the redshift parameter, and the accelerated expansion of the universe is explored. In what follows, the stability of the model is also studied on the base of the sound speed parameter. Finally, the generalized second law of thermodynamics is investigated by entropies of inside and on the boundary of the apparent horizon in thermodynamics equilibrium.

The purpose of this paper is to explore concircular vector fields (CVFs) of locally rotationally symmetric (LRS) Bianchi type-V spacetimes and to investigate whether these CVFs are Ricci soliton vector fields. We first obtained the concircular equations and then solved them by integrating directly. The existence of concircular symmetry imposes restrictions on the metric functions. It is shown that Bianchi type-V spacetimes admit four-, five-, six-, seven-, eight- or fifteen-dimensional CVFs. Further, we studied the Ricci soliton vector fields for all the cases where Bianchi type-V spacetimes admit CVFs. For this purpose, the obtained CVFs are substituted into Ricci soliton equations. These equations imposed further restrictions on metric functions and it is shown in each case that either all or some CVFs are also Ricci soliton vector fields. The gradient of Ricci soliton vector fields are also obtained. It is shown that the metrics that admit CVFs represent physically plausible perfect fluid models under certain conditions.