International Journal of Modern Physics D

There is much talk of a “mysterious form of energy”, called “dark energy” that forms the bulk of the energy content of the universe. Perhaps the most mysterious aspect of it is why should it be regarded as mysterious in the first place. This question is discussed in the context of the development of relativity and relativistic cosmology. It will be argued that there is no good reason to treat it as other than Einstein’s cosmological constant.

The gravitational lensing caused by a generalized Dymnikova black hole in weak field conditions is examined in this work. The optical geometry of such a black hole is first examined, and then the Gauss–Bonnet theorem is used to calculate the deflection angle in weak field limitations. Moreover, the effect of black hole mass, the parameters $A$ and $n$ on the deflection angle in leading order terms are probed. Otherwise, we explore the implication of the plasma medium on the weak deflection angle behavior.

Swampland conjectures have attracted quite some interest in the cosmological community. They have been shown to have wide ranging implications, like constraints on inflationary models, primordial black holes, dark energy to name a few. Particularly, their implications on single-field inflationary models in general relativity-based cosmology have gathered huge attention. Swampland conjectures in their usual form have been shown to be incompatible with these kinds of single-field models, or have been shown to induce severe fine tuning in these inflationary models for them to be consistent with the conjectures. In this work, we show that a large class of single-field inflationary models can in fact bypass the problems faced by inflationary paradigms in GR-Based cosmology. We use the exact solution approach to inflation for the same purpose and show how string theoretic motivations of the swampland conjectures can be in perfect symphony with various single-field inflationary models in nonstandard cosmological scenarios.

In this paper, we investigate the most general exact solutions of Brans–Dicke cosmology by choosing the scale factor “a” as the new independent variable. It is shown that a set of three field equations can be reduced to a constraint equation and a first-order linear differential equation. Thus this new set of equations is solvable when one supplies one of the following pairs of functions: ($\varphi (a)$, $\rho (a)$), ($\varphi (a)$, $V(a)$) or ($\varphi (a)$, $H(a)$). A universe with a single component energy-matter density, the early universe with constant energy density and radiation and the present universe with constant energy density and matter are studied. It is seen that in all cases initial values of the universe cause ${\varphi }^2$ potential and a constant term in the Hubble function which causes an exponential expansion of the universe. The relation between cosmological constant and initial values of the universe is found.

In this paper, we investigate the gravitational collapse of a Reissner–Nordström–AdS domain wall for both a nonextremal and overcharged collapse. During the collapse, we investigate the quasi-particle production that arises due to the time-dependent metric by coupling a complex scalar field to the background of the collapsing domain wall. The analysis is done by making use of the Ermakov equations which lead to an exact solution for the auxiliary function found in the Milne–Pinney equation. In the nonextremal case, we find that assuming the domain wall is positively charged, where the negatively charged particle production is suppressed compared to the positively charged particles due to the Coulomb attraction and repulsion of the particles with the domain wall itself. The induced quasi-particles approach thermality as time goes to infinity in the IR region of the spectrum, however the UV region of the spectrum remains nonthermal for all time. In the overcharged case, the domain wall oscillates about a radius outside of the naked singularity, abiding by the cosmic censorship conjecture, due to the competition between the Coulomb repulsion and the gravitational attraction. Due to the periodic behavior of the collapsing domain wall, the quasi-particle production acquires a generalized Berry Phase, which is a real and measurable phase. The generalized Berry phase leads to a suppression of the UV region of the spectrum.

In this paper, we collect 54 Gamma-Ray Burst (GRB) afterglows rebrightening in X-ray, optical, and radio bands to build the biggest sample so far, of which 18 type-Ic/b Supernova (SN)-associated GRBs have been deeply investigated. We confirm that the distribution of spectral luminosities of SN-associated GRBs has a wider scope of five orders of magnitude. After excluding short- and low-luminosity SN/GRBs, we obtain the mean light curves of high-luminosity SN/GRBs at different radio frequencies. It is found that the mean radio light curves peak twice on the tens of days and several months in each since the burst trigger. There is a time delay between radio afterglows at different frequencies, i.e. the high-frequency emissions come earlier than those low-frequency ones. We study in detail the multiband rebrightening behaviors of GRB 980425 and GRB 030329 with the most radio observation data at present. Unlike GRB 980425, the rebrightening component contributed from GRB 030329/SN 2003dh is indistinguishable. However, we predict that both SN/GRBs will produce the considerable mJy-level rebrightening emissions from their individual SN remnants on the late-times of 103th years for GRB 980425 and 64th years for GRB 030329, which is observable for the current facilities and would be detected by the forthcoming radio telescopes.

Motivated by the necessity of a high-quality stray light control in the detection of the gravitational waves in space, the spot size of a flat top beam generated by the clipping of the Gaussian beam (GB) is studied. By adopting the mode expansion method (MEM) approach to simulating the beam, a slight variant of the definition of the mean square deviation (MSD) spot size for the MEM beam is proposed. This enables us to quickly estimate the spot size for arbitrary propagation distances. Given that the degree of clipping is dependent on the power ratio within the surface of an optical element, the power ratio within the MSD spot range is used as a measure of spot size. The definition is then validated in the cases of simple astigmatic Gaussian beam and nearly-Gaussian beam profiles. As a representative example, the MSD spot size for a top-hat beam in a science interferometer in the detection of gravitational waves in space is then simulated. As in traditional MSD spot size analysis, the spot size is divergent when diffraction is taken into account. A careful error analysis is carried out on the divergence and in the present context, it is argued that this error will have little effect on our estimation. Using the results of our study allows an optimal design of optical systems with top-hat or other types of non-Gaussian beams. Furthermore, it allows testing the interferometry of space-based gravitational wave detectors for beam clipping in optical simulations. This work will serve as a useful guide in the future system design of the optical bench and the sizes of the optical components.

In this work, we study a new class of attractor models which we shall call generalized $R^p$-attractor models. This class of models is based on a generalization of the Einstein frame potential of $R^p$ $f(R)$ gravity models in the Jordan frame. We present the attractor properties of the corresponding nonminimally coupled Jordan frame theory, and we calculate the observational indices of inflation in the Einstein frame. As we show, there is a large class of nonminimally coupled scalar theories, with an arbitrary nonminimal coupling which satisfies certain conditions, that yield the same Einstein frame potential, this is why these models are characterized attractors. As we demonstrate, the generalized $R^p$-attractor models are viable and well fitted within the Planck constraints. This includes the subclass of the generalized $R^p$-attractor models, namely the Einstein frame potential of $R^p$ inflation in the Jordan frame, a feature also known in the literature. We also highlight an important issue related to the $R^p$ inflation in the Jordan frame, which is known to be nonviable. By conformal invariance, the $R^p$ inflation model should also be viable in the Jordan frame, which is not. We pinpoint the source of the problem using two different approaches in the $f(R)$ gravity Jordan frame, and as we demonstrate, the problem arises in the literature due to some standard simplifications made for the sake of analyticity. We demonstrate the correct way to analyze $R^p$ inflation in the Jordan frame, using solely the slow-roll conditions.