International Journal of Modern Physics D

The LIGO-India project to build and operate an advanced LIGO (aLIGO) gravitational wave (GW) detector in India in collaboration with LIGO-USA was initiated as an Indian national mega-science project in 2011. The project relied on the advantage of a head start and cost saving due to the ready availability of most of the aLIGO interferometer components from LIGO-USA, approved by the National Science Foundation. A suitable site was selected and the provisional governmental approval for project was obtained in 2016, immediately following the first direct detection of gravitational waves with the aLIGO detectors. I had discussed the scope and the science case of the project, as well as the plans for its realization by 2022 in a paper [C. S. Unnikrishnan, Int. J. Mod. Phys. D 22 (2013) 1341010, arXiv:1510.06059]. With the KAGRA GW detector in Japan being tuned to be part of the GW detector network, it is now the occasion to assess the progress of the LIGO-India project, and evaluate its relevance and scope for GW science and astronomy. Various key factors in the implementation of the project are reassessed in the backdrop of the projected evolution of the global GW detector sensitivity. In my realistic estimate, it will take more than a decade, beyond 2034, and nearly thrice the original budget to commission and operate the detector with a good fraction of the projected design sensitivity. My decadal assessment points to the eroded scientific relevance of the LIGO-India detector in the GW detector network of the next decade. I argue that the only solution to regain its scientific relevance is to re-plan and relaunch the LIGO-India detector in an upgraded ${\text{A}}^{\#}$ version, similar to the projected post-O5 upgrade of the LIGO-USA detectors beyond 2029.

This research work provides an exhaustive investigation of the viability of different coupled wormhole (WH) geometries with the relativistic matter configurations in the $f(R,G,T)$ extended gravity framework. We consider a specific model in the context of $f(R,G,T)$-gravity for this purpose. Also, we assume a static spherically symmetric spacetime geometry and a unique distribution of matter with a set of shape functions ($\beta (r)$) for analyzing different energy conditions. In addition to this, we examined WH-models in the equilibrium scenario by employing anisotropic fluid. The corresponding results are obtained using numerical methods and then presented using different plots. In this case, $f(R,G,T)$ gravity generates additional curvature quantities, which can be thought of as gravitational objects that maintain irregular WH-situations. Based on our findings, we conclude that in the absence of exotic matter, WH can exist in some specific regions of the parametric space using modified gravity model as $f(R,G,T)=R+\alpha R^2+\beta G^n+\gamma Gln(G)+\lambda T$.

We investigate a crossover quantum chromodynamical (QCD) phase transition that occurred in the early Universe and explore a potential formation scenario for stable strangeon nuggets during this process. To analyze the thermodynamics of the QCD phase involving u, d, s quarks, we employ the Polyakov–Nambu–Jona-Lasinio model, while the relativistic mean-field model is used to describe the hadronic matter. We consider the participation of strangeons (strange quark clusters with net strangeness) in the quark-hadron phase transition process. During this process, strangeon nuggets are formed, and larger nuggets could survive after the phase transition. The crossover phase transition from quarks to hadrons occur at a cosmic temperature of approximately $T\sim 170$MeV, and these two phases (quark phase and strangeon nugget-hadronic phase) are connected in a three-window model. We introduce a distribution function of the nugget baryon number, A, to describe the nugget’s number density (equivalent to the mass spectrum). All strangeon nuggets with $A>A_c$ are considered to be stable, where the critical number, $A_c$, is determined by both weak and strong interactions. To calculate the thermodynamics of stable strangeon nuggets, a nonrelativistic equation of state was applied, resulting in negligible thermodynamic contributions (pressure, entropy, etc.) compared to the hadronic part. Our study shows that the mass fraction of the strangeon nuggets that survived from the early Universe is comparable to dark matter, suggesting a possible explanation for cold dark matter without introducing any exotic particles beyond the Standard Model.

Inspired by Einstein’s Strong Principle of Equivalence, we consider the effects of quantum mechanics to the gravity-like phenomena experienced by an observer in a uniformly accelerating motion in flat spacetime. Among other things, our model of quantum gravity, derived from the first principles, predicts the Unruh effect, and a discrete area spectrum for spacelike two-surfaces.

A new solution to the Einstein–Maxwell field equations for a charged anisotropic compact star is generated. The presence of both a conformal Killing vector and a quadratic equation of state give rise to a new solution with astrophysical significance. The incorporation of the conformal Killing vector leads to an equation relating the gravitational potentials. By combining the quadratic equation of state with the Einstein–Maxwell field equations, an equation for electric field intensity is obtained. Subsequently, one of the gravitational potentials is chosen on physical merits to obtain the second gravitational potential and other matter variables. The generated solution undergoes a thorough analysis to ensure acceptance of physical conditions for realistic compact star models. A deep analysis of the physical aspects reveals the well-behaved nature of the gravitational potentials and matter variables. A smooth matching of the interior and exterior metrics is achieved at the boundary. The gravitational potentials are free from physical and geometric singularities. Also, the energy conditions are satisfied, and the ratio of mass to radius and the surface redshifts remain within reasonable ranges. The equilibrium is attained by the balanced natural interior physical forces. The stability of the model against gravitational collapse is satisfied. Interestingly, the use of quadratic equation of state in conformal stars is missing in treatments performed in the past.

In this work, we study the effects of the Generalized Uncertainty Principle (GUP) in cosmology. We start with the Friedmann–Robertson–Walker (FRW) model endowed with a scalar field. After introducing the GUP modification to the model, we solve for the quantum and classical cases. Finally, we find the GUP-modified Friedmann equations.

In this paper, we study an isotropic flat FLRW-model in scale-covariant theory of gravity $f_{\gamma \delta }(\varphi )$ which is explained in terms of ordinary and covariant differentiation of scalar field $\varphi$. As we know, the deceleration parameter (DP) is time-dependent, so we consider the DP q as the function of t. Using this methodology, we find all the important cosmological factors in terms of a hyperbolic function of the cosmic time t. In turn, we create the model having the behavior of the late-time universe, which is ever accelerated expanding and faces a Big Freeze at the end. The model shows the quintessence dark energy model from early to late times. We compute the constrained values of Hubble parameter $H_0=70.979_{-0.0043}^{+0.021}$ and the model parameter $n=1.24079_{-0.00079}^{+0.00015}$ using joint analysis of the OHD data of 77-points and Pantheon bin data. The model exhibits point-type singularity as it begins with a point of zero volume, infinite energy density and temperature. Furthermore, we obtain the present value of DP $(q_0)\approx -0.54$. Also, we examine the ultimate behavior of our model by the proper analysis of energy conditions (ECs), cosmographical parameters and Statefinder diagnostic. Finally, the proposed model behaves like a quintessence dark energy model.

The observational evidence of $f(R,\mathcal{𝒢},T)$ gravity in the FRW universe is the focus of the study. We provide a novel cosmological theory in this paper that was created within the $f(R,\mathcal{𝒢},T)$ framework. We used the Markov Chain Monte Carlo (MCMC) method to limit the model parameters using the 31 Cosmic Chronometer points and 1048 Pantheon data points. We can efficiently explore the parameter space using this method. In order to compare our proposed $f(R,\mathcal{𝒢},T)$ model to the well-known $\mathrm{\Lambda }$CDM model, we take into account a parameterization of the Hubble parameter for solving the modified field equation. By studying these factors in the context of our new model, we get essential knowledge into the dynamics and evolution of the cosmos. State finder and Om Diagnostic, among other diagnostic tests, have been used to further analyze the behavior and consistency of our $f(R,\mathcal{𝒢},T)$ model. These tests give us a greater understanding of our model’s properties and their applicability to observational data.