Narrow Results

In this paper, we implement the Einsenhart–Duval lift in scalar–tensor gravity as a means to construct integrable cosmological models and analytic cosmological solutions. Specifically, we employ a geometric criterion to constrain the free functions of the scalar–tensor theory such that the field equations can be written in the equivalent form of linear equations. This geometric linearization is achieved by the introduction of an extended minisuperspace description. The results are applied to construct analytic solutions in modified theories of gravity such as the $f(R)$-theory and the hybrid metric-Palatini $f({ℛ})$-gravity.

The timeline of the expansion rate ultimately defines the interplay between high-energy physics, astrophysics and cosmology. The guiding theme of this topical review is provided by the scrutiny of the early history of the space–time curvature through the diffuse backgrounds of gravitational radiation that are sensitive to all the stages of the evolution of the plasma. Due to their broad spectrum (extending from the aHz region to the THz domain) they bridge the macroworld described by general relativity and the microworld of the fundamental constituents of matter. It is argued that during the next score year the analysis of the relic gravitons may infirm or confirm the current paradigm where a radiation plasma is assumed to dominate the whole post-inflationary epoch. The role of high frequency and ultra-high frequency signals between the MHz and the THz is emphasized in the perspective of quantum sensing. The multiparticle final state of the relic gravitons and its macroscopic quantumness is also discussed with particular attention to the interplay between the entanglement entropy and the maximal frequency of the spectrum.

In the summer of 2023, the pulsar timing arrays (PTAs) announced a compelling evidence for the existence of a nanohertz stochastic gravitational wave background (SGWB). Despite this breakthrough, however, several critical questions remain unanswered: What is the source of the signal? How can cosmic variance be accounted for? To what extent can we constrain nanohertz gravity? When will individual supermassive black hole binaries become observable? And how can we achieve a stronger detection? These open questions have spurred significant interests in PTA science, making this an opportune moment to revisit the astronomical and theoretical foundations of the field, as well as the data analysis techniques employed. In this review, we focus on the theoretical aspects of the SGWB as detected by PTAs. We provide a comprehensive derivation of the expected signal and its correlation, presented in a pedagogical manner, while also addressing current constraints. Looking ahead, we explore future milestones in the field, with detailed discussions on emerging theoretical considerations such as cosmic variance, the cumulants of the one- and two-point functions, subluminal gravitational waves, and the anisotropy and polarization of the SGWB.

This paper explores a cosmological reconstruction scheme in the background of $f(Q)$ gravity theory from a Holographic perspective. The basic motivation for this work is that the reconstruction is performed from a holographic origin, which has its roots in the black hole thermodynamics and quantum gravity. Dark energy models inspired by holographic prescription are used to reconstruct the $f(Q)$ gravity models. Two such models, namely, the Granda–Oliveros holographic dark energy model and its generalization, the Chen-Jing model, are considered for the study. Different scale factors are used and a thorough reconstruction scheme is set up using the dark energy models. The observationally constrained values of the free model parameters have been used to form the reconstructed models. Finally, a thorough investigation of the energy conditions has been performed to check the cosmological viability of the reconstructed $f(Q)$ models. As an outcome, we get some very promising and cosmologically viable $f(Q)$ models that present some interesting properties and demand further investigation. Finally, a method is discussed how the constructed $f(Q)$ models can be reconciled with a generalized holographic dark energy.

Using the FLRW cosmological model, this paper explores the dynamics of perfect fluid as a source in the context of modified gravity, where the non-metricity $Q$, which causes the gravitational interaction, is represented by the arbitrary function Lagrangian as the trace of the non-metricity tensor $Q$, say $f(Q)$ gravity. We govern the features of the derived cosmological model in view of the parameterization of Hubble’s parameter of the form, $H(z)=\frac{H_0}{(\gamma +1)}(\gamma +{(1+z)}^{\xi })$. We have spoken about how the energy density, pressure, equation of state parameter, and skewness parameter in our model represent the physical behavior of the cosmos. In addition, we have looked at the kinematic parameters in our model that describe the cosmos, including the jerk, deceleration, and Hubble parameters. The universe’s phase transition from deceleration to acceleration is indicated by our model’s deceleration parameter, $q(z)$. Furthermore, the deceleration parameter’s present value, $q_0$ clearly aligns with the essential $\mathrm{\Lambda }$CDM model. In order to determine the nature of the dark energy model, we also examined geometrical diagnostics such as the Statefinder pairs and $O_m(z)$ diagnostic. Additionally, we used the squared speed of sound test to examine the stability of the cosmos in our model. In the end, at present, the universe in our model is expanding, accelerating, and behaving in a manner consistent with a quintessential dark energy concept while at late the cosmos is dominated by $\mathrm{\Lambda }$CDM.

In this paper, we explore a new version of dark energy called Barrow holographic dark energy within the framework modified gravity called $f(Q,T)$ gravity by adopting the simple homogeneous, isotropic, and spatially flat Friedmann–Robertson–Walker (FRW) model of the universe. Our goal is to understand how the universe evolved over time. To do this, we use parameterizetion of Hubble’s parameter method. We then use a powerful tool called Monte-Carlo Markov Chain to find the best values for the constants in our formula. We do this by comparing our formula to actual data from observations of the universe. Once we have the best values for the constants, we calculate other important parameters that describe the universe’s evolution. These include: Deceleration parameter which measures how quickly the expansion is slowing down. We found $q_0=-0.601_{-0.0131}^{+0.0131}$. Equation of state parameter measures the properties of dark energy. We find ${\omega }_0=-0.7018_{-0.0101}^{+0.0101}$. We also study the stability and energy conditions along with the state-finder and $O_m(z)$-parameter of our model to ensure it is consistent with our understanding of the universe.

In this paper, we explore the possibility of formation of a traversable wormhole in General Relativity supported by particle creation mechanism. The repulsive back-reaction pressure generated through this mechanism can be thought of as a source of sustaining a traversable wormhole. In the first part of this paper, we model a wormhole geometry by assuming an inverse powerlaw variation of particle creation pressure within the wormhole geometry, and the shape function for the wormhole is obtained, assuming a finite redshift function. By stabilizing the wormhole structure, based on the causality of sound-speed we obtained the existence range of the parameter $\beta$ associated with particle creation mechanism. The general shape function obtained is found to adhere to the feasibility conditions of a viable wormhole geometry. Then we studied the 2D and 3D embedding derived for the obtained shape function. In the second part of the paper, we followed the reverse approach of the aforementioned treatment, where we derived the particle creation pressures by assuming three commonly utilized toy shape-functions. The energy conditions are then investigated for all these cases. In essence, particle creation phenomena inside the wormhole may indeed provide for the possibility of sustenance of stable wormhole structure.

The treatment of a quantized field in a curved spacetime requires the study of backreaction of the field on the spacetime via the semiclassical Einstein equation. We consider a free scalar field in spatially flat Robertson–Walker spacetime. We require the state of the field to allow for a renormalized semiclassical stress tensor. We calculate the singularities of the stress tensor restricted to equal times in agreement with the usual renormalization prescription for Hadamard states to perform an explicit renormalization. The dynamical system for the Robertson–Walker scale parameter a(t) coupled to the scalar field is finally derived for the case of conformal and also general coupling.

The article presents a series of numerical simulations of exact solutions of the Einstein equations performed using the Cactus code, a complete three-dimensional machinery for numerical relativity. We describe an application ("thorn") for the Cactus code that can be used for evolving a variety of exact solutions, with and without matter, including solutions used in modern cosmology for modeling the early stages of the universe. Our main purpose has been to test the Cactus code on these well-known examples, focusing mainly on the stability and convergence of the code.

The phase transition associated with the standard electroweak model is very weakly first order. The weakness of the transition means that around the critical temperature the finite-temperature Higgs mass is much less than the critical temperature. This leads to infrared problems in the calculation of the parameters of the potential. Therefore, theories of electroweak baryogenesis, which depend on the details of the transition, must be calculated with care.

Starting from the Klein–Gordon equation, we calculate the entropy of Schwarzschild–de Sitter black hole in non-thermal-equilibrium by using the improved brick-wall method-membrane model. When taking the proper cutoff in the obtained result, we obtain that both black hole's entropy and cosmic entropy are proportional to the areas of event horizon. We avoid the logarithmic term and stripped term in the original brick-wall method. It offers a new way of studying the entropy of the black hole in non-thermal-equilibrium.

We study a varying electric charge brane world cosmology in the RS2 model obtained from a varying-speed-of-light brane world cosmology by redefining the system of units. We elaborate conditions under which the flatness problem and the cosmological constant problem can be resolved by such cosmological model.

The low energy physics as predicted by strings can be expressed in two (conformally related) different variables, usually called frames. The problem is raised as to whether it is physically possible in some situations to differentiate one from the other.

The dynamics of cosmic scalar fields with flat potential is studied. Their contribution to the expansion rate of the universe is analyzed, and their behavior in a simple model of phase transition is discussed.

A natural way to obtain quintessence, i.e. negative pressure contributions in cosmological dynamics and then accelerated behavior of the Hubble fluid, is to take into account a torsion fluid whose effects become relevant at large scale. We investigate a model where a totally antisymmetric torsion field is taken into account and discuss the conditions to obtain quintessence. We obtain exact solutions where dust dominated Friedmann behavior is recovered as soon as torsion effects are not relevant.

We present three different theoretically foreseen, but unusual, astrophysical situations where the gravitational lens equation ends up being the same, thus producing a degeneracy problem. These situations are: (a) the case of gravitational lensing by exotic stresses (matter violating the weak energy condition and thus having a negative mass, particular cases of wormhole solutions can be used as an example), (b) scalar field gravitational lensing (i.e. when considering the appearance of a scalar charge in the lensing scenario), and (c) gravitational lensing in closed universes (with antipodes). The reasons that lead to this degeneracy in the lens equations, the possibility of actually encountering it in the real universe, and eventually the ways to break it, are discussed.

In this article we compute the density of Dirac particles created by a cosmological anisotropic Bianchi I universe in the presence of a constant electric field. We show that the particle distribution becomes thermal when one neglects the electric interaction.

In this paper we propose a superfield description for all Bianchi-type cosmological models. The action is invariant under world-line local n = 4 supersymmetry with SU(2)_local ⊗ SU(2)_global internal symmetry. Due to the invariance of the action we obtain the constraints, which form a closed superalgebra of the n = 4 supersymmetric quantum mechanics. This procedure provides the inclusion of supermatter in a systematic way.

The mixing of the photon with a hypothetical sterile paraphotonic state would have consequences on the cosmological propagation of photons. The absence of distortions in the optical spectrum of distant Type Ia supernovae allows to extend by two orders of magnitude the previous limit on the Lorentz-violating parameter δ associated to the photon–paraphoton transition, extracted from the absence of distortions in the spectrum of the cosmic microwave background. The new limit is consistent with the interpretation of the dimming of distant Type Ia supernovae as a consequence of a nonzero cosmological constant. Observations of gamma-rays from active galactic nuclei allow to further extend the limit on δ.

We present the cosmological solution of the multi-brane/crystal universe model, to leading order in an expansion about the energy densities on branes, by using the generalized Goldberger–Wise mechanism to stabilize the extra dimension, and give general formula of the Friedmann equations. This is very interesting in the study of the cosmological phenomenon in the multi-brane/crystal universe model. We also compute the shift of the extra dimension between the adjacent branes.