Modern Physics Letters A

Understanding the role of higher derivatives is probably one of the most relevant questions in quantum gravity theory. Already at the semiclassical level, when gravity is a classical background for quantum matter fields, the action of gravity should include fourth derivative terms to provide renormalizability in the vacuum sector. The same situation holds in the quantum theory of metric. At the same time, including the fourth derivative terms means the presence of massive ghosts, which are gauge-independent massive states with negative kinetic energy. At both classical and quantum level such ghosts violate stability and hence the theory becomes inconsistent. Several approaches to solve this contradiction were invented and we are proposing one more, which looks simpler than those what were considered before. We explore the dynamics of the gravitational waves on the background of classical solutions and give certain arguments that massive ghosts produce instability only when they are present as physical particles. At least on the cosmological background one can observe that if the initial frequency of the metric perturbations is much smaller than the mass of the ghost, no instabilities are present.

The effective dimension-5 operators can be induced by quantum gravity or inspired by string and M theories. They have important impacts on grand unified theories. We investigate their effects for the well known E(6) model. Considering the breaking chains E₆↦H = SO(10)×U_V′(1)↦SU(5)×U_V(1)×U_V′(1)↦SU(3)×SU(2)×U_Z(1)×U_V(1)×U_V′(1) and E₆↦H = SO(10)×U_V′(1)↦SU(4)×SU_L(2)×SU_R(2)×U_V′(1)↦SU(3)×SU_L(2)×SU_R(2)×U_S(1)×U_V′(1), we derive and give all of the Clebsch–Gordan coefficients associated with E₆ breaking to the Standard Model (SM). Physical effects of nonzero vacuum expectations of SM singlets Higgs in E₆ grand unified theories are discussed.

Recently, a new class of inflationary models, so-called gauge-flation or non-Abelian gauge field inflation has been introduced where the slow-roll inflation is driven by a non-Abelian gauge field A with the field strength F. This class of models are based on a gauge field theory having F² and F⁴ terms with a non-Abelian gauge group minimally coupled to gravity. Here, we present a new class of such inflationary models based on a gauge field theory having only F² term with non-Abelian gauge fields non-minimally coupled to gravity. The non-minimal coupling is set up by introducing the Einstein tensor besides the metric tensor within the F² term, which is called kinetic coupled gravity. A perturbation analysis is performed to confront the inflation under consideration with Planck and BICEP2 results

At sufficiently large separation between a quark and an antiquark the quantum chromodynamics (QCD) string breaks into parts due to creation of light quark–antiquark pairs. We show that a background magnetic field affects the breaking of the QCD string oriented in the transverse directions with respect to the axis of the magnetic field. Using semiclassical approach we argue that above certain, the background magnetic field eB ≈0.3 GeV² breaking of the transverse string should become energetically unfavorable.

We discuss the implications of a purely classical (instead of quantum) theory of gravity for the primordial gravitational wave spectrum generated during inflation. We argue that for a scalar field driven inflation in a classical gravity the amplitude of the gravitational wave will be too small, irrespective of its primordial seed, to be detected in any forthcoming experiments. Therefore, a positive detection of the B-mode polarizations in the Cosmic Microwave Background (CMB) spectrum will naturally confirm the quantum nature of gravity itself. Furthermore there will be no upper limit on the scale of inflation in the case of classical gravity, and a high-scale model of inflation can easily bypass the observational constraints.

We discuss the Noether Symmetry Approach in the framework of Gauss–Bonnet cosmology showing that the functional form of the F(R, 𝒢) function, where R is the Ricci scalar and 𝒢 is the Gauss–Bonnet topological invariant, can be determined by the presence of symmetries. Besides, the method allows to find out exact solutions due to the reduction of cosmological dynamical system and the presence of conserved quantities. Some specific cosmological models are worked out.

This paper investigates the dynamics of anisotropic viscous spherical star under the effects of electromagnetic field for a radially moving observer relative to the matter distribution, i.e. a tilted observer. We formulate relationship between tilted and non-tilted quantities using the Einstein–Maxwell field equations. The dynamical equations and equations for the Weyl tensor are constructed to examine the inhomogeneities in the fluid configuration. It is found that different factors like heat radiation, shear viscosity, electric charge and in particular congruence of the tilted observer, affect the energy density inhomogeneity of the spherical star. Finally, we study stability of the system with non-tilted frame in the presence of charge.