International Journal of Modern Physics D

Here, we concisely review the nonminimal coupling dynamics of a single scalar field in the context of purely affine gravity and extend the study to multifield dynamics. The coupling is performed via an affine connection and its associated curvature without referring to any metric tensor. The latter arises a posteriori and it may gain an emergent character like the scale of gravity. What is remarkable in affine gravity is the transition from nonminimal to minimal couplings which is realized by only field redefinition of the scalar fields. Consequently, the inflationary models gain a unique description in this context where the observed parameters, like the scalar tilt and the tensor-to-scalar ratio, are invariant under field reparametrization. Overall, gravity in its affine approach is expected to reveal interesting and rich phenomenology in cosmology and astroparticle physics.

In this paper, we study cosmic evolution and current expansion via generalized ghost pilgrim dark energy (PDE) model for FRW universe in $f(R)$ gravity. For this purpose, we consider both interacting and non-interacting forms of dark energy (DE) model and reconstruct $f(R)$ model using red-shift parameter. In order to analyze the effects of the DE model, we formulate some standard cosmological parameters such as Hubble, effective equation of state (EoS) and statefinder parameters as well as ${\omega }_c$–${\omega }_c^{\prime }$ plane. We also investigate the stability criteria for both interacting and noninteracting models through the squared speed of sound. It is concluded that the best-fit results are found for the positive curvature parameter in the presence of the noninteracting model. For the interacting model, the cosmic evolution is well explained by closed and open universe models while the flat geometry effectively describes the current cosmos.

In this paper, we study the contribution of the tidal charge in the electromagnetic field in the vicinity of the black hole in the braneworld, and present an analytical solution of Maxwell equations for the dipolar configuration of the vector potential of the electromagnetic field of the stationary current loop generated by the charged matter surrounding the black hole. In order to find the location of the current loop we find the radius of the stable circular orbit of the test particle, by considering the particle motion around the tidal charged black hole. Furthermore, we study the dipolar magnetic field in both interior and exterior regions of the current loop, respectively. Although all results are compared with those previously obtained in Schwarzschild space.

We study for the first time how a new class of stars could impact an ensemble of pulsars with known masses and spin-periods. These new compact objects are strange stars admixed with condensed dark matter (DM). In this exploratory theoretical work, our goal is to determine how the basic parameters of pulsars are modified for such a new class of compact objects. In particular, we consider three different scenarios that correspond to a DM mass fraction of $5\%$, $11\%$ and $25\%$. Within each scenario with fixed parameters, we predict theoretically other properties of the pulsars, such as the radius, the compactness, the moment of inertia, as well as the angular momentum. Our numerical results are summarized in tables and also shown graphically for better visualization, where a comparison between the different scenarios can be made.

The recently found alignment of the polarization axes (PA) of quasars in large quasar groups (LQGs) on Mpc scales can be explained by general relativistic cosmic string networks. By considering the cosmic string as a result of spontaneous symmetry breaking of the gauged U(1) abelian Higgs model with topological charge $n$, many stability features of $n$-vortex solutions of superconductivity can be taken over. Decay of the high multiplicity ($n$) super-conducting vortex into a lattice of $n$ vortices of unit magnetic flux is energetically favorable. The temporarily broken axial symmetry will leave an imprint of a preferred azimuthal-angle on the lattice. The stability of the lattice depends critically on the parameters of the model, especially when gravity comes into play.

In order to handle the strong nonlinear behavior of the time-dependent coupled field equations of gravity and the scalar-gauge field, we will use a high-frequency approximation scheme to second order on a warped 5D axially symmetric spacetime with the scalar-gauge field residing on the brane. We consider different winding numbers for the subsequent orders of perturbations of the scalar field. A profound contribution to the energy–momentum tensor comes from the bulk spacetime and can be understood as “dark”-energy. The cosmic string becomes super-massive by the contribution of the 5D Weyl tensor on the brane and the stored azimuthal preferences will not fade away. During the recovery to axial symmetry, gravitational and electro-magnetic radiation will be released.

The perturbative appearance of a nonzero energy–momentum component $T_{t\phi }$ can be compared with the phenomenon of bifurcation along the Maclaurin–Jacobi sequence of equilibrium ellipsoids of self-gravitating compact objects, signaling the onset of secular instabilities. There is a kind of similarity with the Goldstone-boson modes of spontaneously broken symmetries of continuous groups. The recovery of the SO(2) symmetry from the equatorial eccentricity takes place on a time-scale comparable with the emission of gravitational waves.

The emergent azimuthal-angle dependency in our model can be used to explain the aligned PA in LQGs on Mpc scales. Spin axis direction perpendicular to the major axes of LQGs when the richness decreases can be explained as a second-order effect in our approximation scheme by the higher multiplicity terms. The preferred directions are modulo $\frac{180^o}{i}$, with $i$ being an integer dependent on the $i$th order of approximation.

When more data of quasars of high redshift becomes available, one could prove that the alignment emerged after the symmetry breaking scale and must have a cosmological origin. The effect of the warp factor on the second-order perturbations could also be an indication of the existence of extra large dimensions.

We test the hypothesis that in models with an explicit breaking of Lorentz invariance violation (LIV), the problem with the cosmological constant may, if not already be solved, then at least be partially mitigated by improving its UV behavior. Certain class of noncommutative models that have a well-defined dual momentum space geometry and in addition, setting up a suitable frame for developing the appropriate doubly (or deformed) special relativity (DSR) models are also subject to the same test.

We show that the parametric resonance of gravitational waves (GWs) occurs due to the axion coherent oscillation and the circular polarization of GWs is induced by the Chern–Simons coupling. However, we have never observed these signatures in the data of GWs. Using this fact, we give stringent constraints on the Chern–Simons coupling constant $\ell$ and the abundance of the light string axion. In particular, we improved the current bound $\ell \le 10^8\mathrm{\text{km}}$ by many orders of magnitude.

We review the Schrödinger picture of field theory in curved spacetime and using this formalism, the power spectrum of massive noninteracting, minimally coupled scalars in a fixed de Sitter background is obtained. To calculate the $N$-point function in Schrödinger field theory, the in-in formalism is extended in the Friedmann–Lemaître–Robertson–Walker (FLRW) universe. We compute the three-point function for primordial scalar field fluctuation in the single field inflation by this in-in formalism. The results are the same as the three-point function in the Heisenberg picture.

We study the influence of a periodic perturbation of the effective masses of fermions, due to the assumed semiclassical ultra-light dark matter (ULDM) background, on the motion of neutrons in a gravitational quantum well (GQW). Our focus is on the transition probability between the lowest two energy states, with the Rabi frequency in the kHz region corresponding to the series of “sweet spot” DM masses in the $10^{-11}$eV ballpark. The relevant probability is written in terms of the specific mass and of the effective coupling to the ordinary matter. These parameters can be constrained by the nonobservation of any significant deviations of the measured transition probabilities from the DM-free picture.

Since Dirac stated his Large Number Hypothesis the space-time variation of fundamental constants has been an active subject of research. Here we analyze the possible spatial variation of two fundamental constants: the fine structure constant $\alpha$ and the speed of light $c$. We study the effects of such variations on the luminosity distance and on the peak luminosity of Type Ia supernovae (SNe Ia). For this, we consider the change of each fundamental constant separately and discuss a dipole model for its variation. Elaborating upon our previous work, we take into account the variation of the peak luminosity of Type Ia supernovae resulting from the variation of each of these fundamental constants. Furthermore, we also include the change of the energy release during the explosion, which was not studied before in the literature. We perform a statistical analysis to compare the predictions of the dipole model for $\alpha$ and $c$ variation with the Union 2.1 and JLA compilations of SNe Ia. Allowing the nuisance parameters of the distance estimator ${\mu }_0$ and the cosmological density matter ${\mathrm{\Omega }}_{\mathrm{\text{m}}}$ to vary. As a result of our analysis, we obtain a first estimate of the possible spatial variation of the speed of light $c$. On the other hand, we find that there is no significant difference between the several phenomenological models studied here and the standard cosmological model, in which fundamental constants do not vary at all. Thus, we conclude that the actual set of data of Type Ia supernovae does not allow to verify the hypothetical spatial variation of fundamental constants.

We consider the hypothesis that dark matter (DM) and dark energy (DE) consist of ultra-light self-interacting scalar particles. It is found that the Klein–Gordon equation with only two free parameters (mass and self-coupling) on a Schwarzschild background, at the galactic length-scales has the solution which corresponds to Bose–Einstein Condensate (BEC), behaving as DM, while the constant solution at supra-galactic scales can explain DE.

A self-sustained traversable wormhole is obtained as a vacuum solution of a scale-dependent gravitational theory. Comparisons with other approaches towards wormhole self-sustainability are presented with emphasis on the running of the gravitational coupling and on a possible effective description of gravity near the Planck scale.

The recent analysis of low-redshift supernovae (SN) has increased the apparent tension between the value of $H_0$ estimated from low and high redshift observations such as the cosmic microwave background (CMB) radiation. At the same time other observations have provided evidence of the existence of local radial inhomogeneities extending in different directions up to a redshift of about $0.07$. About $40\%$ of the Cepheids used for SN calibration are directly affected because they are located along the directions of these inhomogeneities. We compute with different methods the effects of these inhomogeneities on the low-redshift luminosity and angular diameter distance using an exact solution of the Einstein’s equations, linear perturbation theory and a low-redshift expansion. We confirm that at low redshift the dominant effect is the nonrelativistic Doppler redshift correction, which is proportional to the volume averaged density contrast and to the comoving distance from the center. We derive a new simple formula relating directly the luminosity distance to the monopole of the density contrast, which does not involve any metric perturbation. We then use it to develop a new inversion method to reconstruct the monopole of the density field from the deviations of the redshift uncorrected observed luminosity distance respect to the $\mathrm{\Lambda }$CDM prediction based on cosmological parameters obtained from large scale observations. The inversion method confirms the existence of inhomogeneities whose effects were not previously taken into account because the $2M++$ [G. Lavaux and M. J. Hudson, Mon. Not. R. Astron. Soc. 416 (2011) 2840] density field maps used to obtain the peculiar velocity [J. Carrick et al., Mon. Not. R. Astron. Soc. 450 (2015) 317] for redshift correction were for $z\le 0.06$, which is not a sufficiently large scale to detect the presence of inhomogeneities extending up to $z=0.07$. The inhomogeneity does not affect the high redshift luminosity distance because the volume averaged density contrast tends to zero asymptotically, making the value of $H_0^{\mathrm{\text{CMB}}}$ obtained from CMB observations insensitive to any local structure. The inversion method can provide a unique tool to reconstruct the density field at high redshift where only SN data is available, and in particular to normalize correctly the density field respect to the average large scale density of the Universe.

In this paper, we will propose a universal relation between the holographic complexity (dual to a volume in AdS) and the holographic entanglement entropy (dual to an area in anti-de Sitter (AdS)). We will explicitly demonstrate that our conjuncture holds for all metrics asymptotic to ${\mathrm{\text{AdS}}}_3$, and then argue that such a relation should hold in general due to the AdS version of the Cavalieri principle. We will demonstrate that it holds for Janus solution, which have been recently been obtained in type IIB string theory. We will also show that this conjecture holds for a circular disk. This conjecture will be used to show that the proposal that the complexity equals action and the proposal that the complexity equals volume can represent the same physics. Thus, using this conjecture, we will show that the black holes are fastest computers, using the proposal that complexity equals volume.

The external field approximation (EFA) provides a mechanism whereby classical gravity and inertia emerge from a quantum background. The two-point vector, that in EFA represents gravity, is the byproduct of symmetry breaking and is accompanied by classical, vortical structures. Its interaction range is, in general, that of the metric tensor, but, in the context of a simple symmetry-breaking model, the range can be made finite by the presence of massive scalar particles. Vortices that conceal matter can then be produced, making it effectively “dark”. In EFA, fermion relativistic vortices can be induced, in particular, by rotation.