# Category: Uncategorized

## Gravitational Collapse of Rotating Magnetised Neutron Stars to Black Holes

*Elias Most, ITP, Goethe Universität, Frankfurt*

Fast Radio Bursts are short emissions of unknown extragalactic origin. This talk discusses, how to model them by collapsing neutron stars in resistive GRMHD simulations. Results on gravitational and electromagnetic emission will be presented. Furthermore, the possibility of forming charged black holes as the end states of gravitational collapses will be discussed.

## Cosmic Structures in Brans-Dicke-like theories

The recent discovery of the accelerated cosmic expansion suggests that our Universe may be endowed with a positive cosmological constant Λ. In addition, even though general relativity (GR) is a very successful and well-tested theory, it could be that it is not the final theory of gravity. Brans-Dicke theory is one of the first and simplest modifications of Einstein’s theory. During this talk, I will present the study of cosmic structures in Brans-Dicke-like theories in the presence of a positive cosmological constant. I will discuss the validity of the no-hair theorem in the context of such theories. Moreover, I will show that, the black hole solutions in these theories are no different from those in GR and also that the presence of a stationary cosmological event horizon rules out any regular spherical stationary solution, appropriate for the description of a star.

## Stable hybrid stars within a SU(3) Quark-Meson-Model

The inner regions of the most massive compact stellar objects might be occupied by a phase of quarks. Since the observations of the massive pulsars PSR J1614-2230 and of PSR J0348+0432 with about two solar masses, the equations of state constructing relativistic stellar models have to be constrained respecting these new limits. We discuss stable hybrid stars, i.e. compact objects with an outer layer composed of nuclear matter and with a core consisting of quark matter (QM). For the outer nuclear layer we utilise a density-dependent nuclear equation of state and we use a chiral SU(3) Quark-Meson model with a vacuum energy pressure to describe the object’s core.

The appearance of a disconnected mass-radius branch emerging from the hybrid star branch implies the existence of a third family of compact stars, so called “twin” stars. Twin stars did not emerge as the transition pressure has to be relatively small with a large jump in energy density, which could not be satisfied within our approach. This is, among other reasons, due to the fact that the speed of sound in QM has to be relatively high, which can be accomplished by an increase of the repulsive coupling. This increase on the other hand yields too high transition pressures for twins stars to appear.

## Indian contributions to GR: a centenary review

I would review the contributions to GR from Indian scientists of which the prominent ones are Datt’s homogeneous collapse, Raychaudhuri equation, Vaidya’s and Majumdar’s solutions, and so on.

## On the semi-classicality of cosmological perturbations and the non-Bunch-Davies vacua

Moving from the consideration that matter fields must be treated in terms of their fundamental quantum counterparts, we show straightforward arguments, within the framework of ordinary quantum mechanics and quantum field theory, in order to convince readers that cosmological perturbations must be addressed in term of the semiclassical limit of the expectation value of the quantum fields. We first take into account cosmological perturbations originated by a quantum scalar field, and then extend our treatment in order to account for the expectation values of bilinears of Dirac fermion fields. The latter can indeed transform as scalar quantities under diffeomorphisms, as well as all the other bilinear Dirac fermion elements of the Clifford algebra. Phenomenological consequences follow, including the possibility of generating cross-correlation spectra from fermion perturbations. We then discuss how the macroscopic state for matter we propose can be interpreted as a non-Bunch-Davies vacuum, and then generalise this construction likening it to representation theory. Phenomenological consequences of this identification are then outlined.

## Hydrodynamical study on the conversion of hadronic matter to quark matter

We study transitions of hadronic matter to three-flavour quark matter (3QM) locally both for shock-induced and diffusion-induced conversions. The former is the transition via two-flavour quark matter triggered by a rapid density rise in a shock wave and the latter is induced by diffusions of a seed 3QM. Not only the jump condition on both sides of the conversion front but the structures inside the front are also considered by taking into account what happens during the conversion processes on the time scale of weak interactions. We demonstrated that the combustion will occur in the so-called endothermic regime which has been ignored in the discussion so far. We also find that the deflagration front is unstable in the exothermic regime but stable in the endothermic regime, which is quite contrary to the ordinary combustion. It is also confirmed that strong detonation and weak deflagration are always obtained for shock-induced and diffusion-induced combustion, respectively, regardless of whether in exothermic or endothermic regime.

## General Relativity and Beyond

Extended theories of gravity represent a straightforward generalisation of Einstein theories aimed to cure its shortcomings at ultra-violet and infra-red scales. Here we review some theoretical aspects of metric and metric affine extended theories of gravity in view of astrophysical application. In particular, after a general exposition, we discuss some exact black hole solutions, gravitational waves and stellar structures.