General Relativity and Quantum Cosmology (gr-qc)

Abstract: We consider a type of k-inflation under the Hamilton-Jacobi approach. We calculate various observables such as the scalar power spectrum, the tensor-to-scalar ratio, the scalar spectra index for the case where the Hubble parameter is a power-law function of k-field. The model's parameters are constrained with Planck data and the concrete form of the potential is presented. The results show that the model can be in good agreement with observations.

Abstract: This review introduces Markov Chain Monte Carlo simulations in causal set theory, with a focus on the study of the 2d orders. It will first introduce the Benincasa-Dowker action on causal sets, and cover some musings on the philosophy of computer simulations. And then proceed to review results from the study of the 2d orders, first their general phase transition and scaling behavior and then results on defining a wave function of the universe using these orders and on coupling the 2d orders to an Ising like model. Including matter in this type shows a strong coupling between matter and geometry, that leads to new phase transitions. However, while the matter does induce a new phase transition, it does not change the order of the phase transition of geometry.

Abstract: Recently, it was suggested that the Hawking radiation may originate not at the event horizon but in the quantum region outside of it, known as the quantum atmosphere. The present study attempts to explore this argument further by assessing its role in shaping quantum correlations near a black hole. Herein, these are conveniently captured within the geometric measure of nonlocality, termed as the measurement-induced nonlocality, and found to exhibit signatures of the atmosphere. In particular, a notable loss of correlations is observed well outside the event horizon, coinciding with the peak of particles radiation in the atmosphere region. Still, the correlations are shown to be always finite therein and to continuously scale with not only the radiation temperature but also with the horizon size. Hence, some characteristics of the atmosphere appears to be detectable at the quantum correlations level, providing novel insight and means to help verify the concept of interest.

Abstract: Quantum entanglement harvesting in the relativistic setup attracted a lot of attention in recent times. Acquiring more entanglement within two qubits may be very desirable to establish fruitful communication between them. On the other hand use of reflecting boundaries in a spacetime has close resemblance to the cavity quantum optomechanical systems. Here, in presence of two reflecting boundaries, we study the generation of entanglement between two uniformly accelerated Unruh- DeWitt detectors which are interacting with the background scalar fields. Like no boundary and single boundary situations, entanglement harvesting is possible for their motions in opposite Rindler wedges. We observe that the reflecting boundaries can play double roles. In some parameter space it causes suppression, while in other parameter space we can have enhancement of entanglement compared to no boundary and single boundary cases. Thus increase of boundaries has significant impact in this phenomena and a suitable choices of parameters provides diresable increment of it.

Abstract: Einstein's theory of gravity admits a low energy effective quantum field description from which predictions beyond classical general relativity can be drawn. As gravitational wave detectors improve, one may ask whether non-classical features of such theory can be experimentally verified. Here we argue that nonlinear effects in black hole ringdowns can be sensitive to the graviton number statistics and other quantum properties of gravitational wave states. The prediction of ringdown signals, potentially measurable in the near future, might require the inclusion of quantum effects. This offers a new route to probing the quantum nature of gravity and gravitational wave entanglement.