International Journal of Modern Physics D

The X-Ray Imaging and Spectroscopy Mission (XRISM) is a JAXA/NASA X-ray observatory with collaboration from ESA and several institutes and academic institutions worldwide. It is proposed to fulfill the promise of high-resolution X-ray spectroscopy with imaging once realized but unexpectedly terminated by a mishap of ASTRO-H/Hitomi. XRISM carries two sets of X-ray mirror assemblies and is equipped on the focal plane with a $6\times 6$ pixelized X-ray micro-calorimeter array and an aligned X-ray CCD camera. With the combination of high-resolution spectroscopy imaging and the broader field of view, XIRSM is expected to pioneer a new horizon of the Universe in X-ray astrophysics. Aiming to launch the satellite in the Japanese Fiscal Year 2022, we fabricate the instruments and test for the satellite integration starting at the beginning of 2022. This paper reports the development status, reviewing the science objectives and the operation plan.

We study the nonlinear structure formation in cosmology accounting for the quantum nature of the dark matter (DM) particles in the initial conditions at decoupling, as well as in the relaxation and stability of the DM halos. Different from cosmological N-body simulations, we use a thermodynamic approach for collisionless systems of self-gravitating fermions in general relativity, in which the halos reach the steady state by maximizing a coarse-grained entropy. We show the ability of this approach to provide answers to crucial open problems in cosmology, among others: the mass and nature of the DM particle, the formation and nature of supermassive black holes in the early Universe, the nature of the intermediate mass black holes in small halos, and the core-cusp problem.

In this work, we shall study a class of $f(R,\varphi )$ gravity models which during the inflationary era, which is the large curvature regime, result to an effective inflationary Lagrangian that contains a rescaled Einstein–Hilbert term $\alpha R$ in the presence of a canonical minimally coupled scalar field. The dimensionless parameter $\alpha$ is chosen to take values in the range $0<\alpha <1$ and the main motivation for studying these rescaled Einstein–Hilbert $f(R,\varphi )$ gravities, is the fact that the rescaled action may render an otherwise incompatible canonical scalar field theory with the Swampland criteria, to be compatible with the Swampland criteria and specifically the distance conjecture and the de Sitter conjecture, without considering the refined de Sitter conjecture however. As we will show, by studying a large number of inflationary potentials appearing in the 2018 Planck collaboration paper for the constraints on inflation, the simultaneous compatibility with both the Planck constraints and the aforementioned two Swampland criteria is achieved for some models, and the main characteristic of the models for which this is possible is the small value that the parameter $\alpha$ must take.

In this work, we study the cosmological consequences of Rényi holographic dark energy model in the framework of the Finsler–Randers cosmology in which the Hubble horizon is considered as the IR cutoff. In this setup, we derive the evolution equation for the Rényi holographic dark energy density parameter, the equation of state (EoS) parameter and deceleration parameter. Our study shows that this model can describe the current accelerating universe in both noninteracting and interacting scenarios, and also a transition occurs from the deceleration phase to the accelerated phase at the late-time. Moreover, we discuss the statefinder diagnosis of this model, meanwhile, plot the curves of r and s versus redshift z and the evolutionary trajectories of $\{r,s\}$. We find that statefinder can not only break the degeneracy of different coupling parameter values in this model, but also effectively distinguish the difference between the Rényi holographic dark energy model and the $\mathrm{\Lambda }$CDM model. In addition, we find that the statefinder pair $\{r,s\}$ performs better than $\{r,z\}$ and $\{s,z\}$ in this model.

A modified and generalised Chaplygin gas (MCG, $B\ne 0.0$ and GCG, $B=0.0$) has been separately chosen here as a constituent of the universe. Concept of state finder and Om diagnostics are introduced to track the dark energy in the models. Here, observed Hubble data (OHD) and binned Pantheon data of supernovae are used to determine the best-fit equation-of-state (EoS) parameters of these models and these are compared with the $\mathrm{\Lambda }$CDM model. The best-fit value and expected values of cosmological jerk parameter $j$, snap parameter $s$ are determined, which are close to each other. A plot of $j,s$ with red-shift, with themselves, as well as with deceleration parameter $q$, shows the evolution of the universe and its possible future. Variations of $q$ and EoS parameter $\omega$ with red-shift show acceleration–deceleration phase transition in the recent past. Lastly, the state finder pair $(j,s)$ and Om diagnostic have been utilized to discriminate the models.

In this paper, we explore the dynamics of test particles in spacetime around regular Bardeen black holes (BHs) in the novel four-dimensional Einstein–Gauss–Bonnet (4D EGB) theory. First, we analyze properties of the spacetime around the Bardeen BH with respect to the Gauss–Bonnet (GB) coupling parameter $\alpha$, and we show the range of values of the BH charge and the parameter $\alpha$ that horizon exists as $\alpha /M^2\in (-0.15869,1)$ and $g/M\in (0,0.886247)$, respectively. We also investigate the effects of the magnetic charge of the regular BH on innermost stable circular orbits (ISCOs) radius and efficiency of energy release from infalling test particles to the regular Bardeen BH in 4D EGB gravity. Moreover, fundamental frequencies such as Keplerian and harmonic oscillations are also studied with the applications of quasi-periodic oscillations (QPOs) where we proposed a method to test gravity theories through data from astrophysical observations of twin-peak QPOs in relativistic precession (RP) model. Finally, we show the dependence of the distance from the central BH to the orbit where twin-peak QPOs shine from the spin of Kerr BH, magnetic charge of Bardeen and Reissner–Nordström (RN) BHs, and GB coupling parameter.

In this work, Friedmann–Robertson–Walker model is investigated in $f(R,T)=R+\alpha T+\beta T^2$ gravity for a perfect fluid by a numerical analysis. In order to solve the modified field equations, we use a new solution method based on to propose the functional forms of the energy density and the pressure. In this context, in keeping with the recent indications of the observed universe, we consider two different propositions. We show for both propositions that the behaviors of the Hubble and the deceleration parameters are consistent with astrophysical observations and the model depicts de Sitter universe. It is also shown that the model incorporates the initial radiation-dominated era, and it is dominated by a quintessence like fluid at late era for both the propositions.

Recent observational evidence reveals the existence of black holes in the galaxy. Using strong field lensing effect, we find out the expressions of the radius of the photon sphere, the deflection angle, the separation between the first and the other images, the ratio between the flux of the first image as well as the flux coming from all the other images for dilaton–axion black holes and compare the results with Reissner–Nordstrom black holes. We also investigate the variations of primary and secondary image positions as well as their magnifications with respect to the source position for this kind of black holes.

In this paper, we consider a specific model, implementing the existence of a fundamental limit distance $L_0$ between (space or time separated) points in spacetime, which in the recent past has exhibited the intriguing feature of having a minimum-length Ricci scalar $R_{(q)}$ that does not approach the ordinary Ricci scalar $R$ in the limit of vanishing $L_0$. $R_{(q)}$ at a point has been found to depend on the direction along which the existence of minimum distance is implemented.

Here, we point out that the convergence $R_{(q)}\to R$ in the $L_0\to 0$ limit is anyway recovered in a relaxed or generalized sense, which is when we average over directions, this suggesting we might be taking the expectation value of $R_{(q)}$ promoted to be a quantum variable.

It remains as intriguing as before the fact that we cannot identify (meaning this is much more than simply equating in the generalized sense above) $R_{(q)}$ with $R$ in the $L_0\to 0$ limit, namely, when we get ordinary spacetime. Thing is like if, even when $L_0$ (read here the Planck length) is far too small to have any direct detection of it feasible, the intrinsic quantum nature of spacetime might anyway be experimentally at reach, witnessed by the mentioned special feature of Ricci, not fading away with $L_0$ (i.e. persisting when taking the $\hslash \to 0$ limit).

In this paper, we develop the Ostrogradsky–Hamilton formalism for geodetic brane gravity, described by the Regge–Teitelboim geometric model in higher codimension. We treat this gravity theory as a second-order derivative theory, based on the extrinsic geometric structure of the model. As opposed to previous treatments of geodetic brane gravity, our Lagrangian is linearly dependent on second-order time derivatives of the field variables, the embedding functions. The difference resides in a boundary term in the action, usually discarded. Certainly, this suggests applying an appropriate Ostrogradsky–Hamiltonian approach to this type of theories. The price to pay for this choice is the appearance of second-class constraints. We determine the full set of phase space constraints, as well as the gauge transformations they generate in the reduced phase space. Additionally, we compute the algebra of constraints and explain its physical content. In the same spirit, we deduce the counting of the physical degrees of freedom. We comment briefly on the naive formal canonical quantization emerging from our development.

In this paper, we propose a simple generalization of axion-electrodynamics (AED) for the general case in which Dirac fermion fields and scalar/pseudoscalar axion-like fields are present in the local $U$($1$)${}_e$ gauge-invariant Lagrangian of the system. Our primary goal (which is not explored here) is to understand and predict novel phenomena that have no counterpart in standard (pseudoscalar) AED. With this end in view, we discuss on very general grounds, possible processes in which a Dirac field is coupled to axionic fields via the electromagnetic (EM) field.

We come back on the dynamical properties of $k$-essential cosmological models and show how the interesting phenomenological features of those models are related to the existence of boundaries in the phase surface. We focus our attention to the branching curves where the energy density has an extremum and the effective speed of sound diverges. We discuss the behaviour of solutions of a general class of cosmological models exhibiting such curves and give two possible interpretations; the most interesting possibility regards the arrow of time that is reversed in trespassing the branching curve. This study teaches to us something new about general FLRW cosmologies where the fluids driving the cosmic evolution have equations of state that are multivalued functions of the energy density and other thermodynamical quantities.