Maximiliano Ujevic

A mathematical analysis of the evolution of perturbations in a modified Chaplygin gas model

One approach in modern cosmology consists in supposing that dark matter and dark energy are different manifestations of a single “quartessential” fluid. Following such idea, this work presents a study of the evolution of perturbations of density in a flat cosmological model with a modified Chaplygin gas acting as a single component. Our goal is to obtain properties of the model which can be used to distinguish it from another cosmological models which have the same solutions for the general evolution of the scale factor of the universe, without the construction of the power spectrum. Our analytical results, which alone can be used to uniquely characterize the specific model studied in our work, show that the evolution of the density contrast can be seen, at least in one particular case, as composed by a spheroidal wave function. We also present a numerical analysis which clearly indicates as one interesting feature of the model the appearance of peaks in the evolution of the density contrast.

Numerical relativity simulations of neutron star merger remnants using conservative mesh refinement

We study equal- and unequal-mass neutron star mergers by means of new numerical relativity simulations in which the general relativistic hydrodynamics solver employs an algorithm that guarantees mass conservation across the refinement levels of the computational mesh. We consider eight binary configurations with total mass M=2.7M_⊙, mass ratios q=1 and q=1.16, four different equations of state (EOSs) and one configuration with a stiff EOS, M=2.5M_⊙ and q=1.5, which is one of the largest mass ratios simulated in numerical relativity to date. We focus on the postmerger dynamics and study the merger remnant, the dynamical ejecta, and the postmerger gravitational wave spectrum. Although most of the merger remnants are a hypermassive neutron star collapsing to a black hole+disk system on dynamical time scales, stiff EOSs can eventually produce a stable massive neutron star. During the merger process and on very short time scales, about ∼10^(−3) –10^(−2) M_⊙ of material become unbound with kinetic energies ∼10^(50) erg. Ejecta are mostly emitted around the orbital plane and favored by large mass ratios and softer EOS. The postmerger wave spectrum is mainly characterized by the nonaxisymmetric oscillations of the remnant neutron star. The stiff EOS configuration consisting of a 1.5M_⊙ and a 1.0M_⊙ neutron star, simulated here for the first time, shows a rather peculiar dynamics. During merger the companion star is very deformed; about ∼0.03M_⊙ of the rest mass becomes unbound from the tidal tail due to the torque generated by the two-core inner structure. The merger remnant is a stable neutron star surrounded by a massive accretion disk of rest mass ∼0.3M_⊙. This and similar configurations might be particularly interesting for electromagnetic counterparts. Comparing results obtained with and without the conservative mesh refinement algorithm, we find that postmerger simulations can be affected by systematic errors if mass conservation is not enforced in the mesh refinement strategy. However, mass conservation also depends on grid details and on the artificial atmosphere setup; the latter are particularly significant in the computation of the dynamical ejecta.

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the mass ratio

We present new (

Toward Rapid Transient Identification and Characterization of Kilonovae

3+1

General chelating action of copper, zinc and iron in mammalian cells

)-dimensional numerical relativity simulations of the binary neutron star (BNS) mergers that take into account the NS spins. We consider different spin configurations, aligned or antialigned to the orbital angular momentum, for equal- and unequal-mass BNSs and for two equations of state. All the simulations employ quasiequilibrium circular initial data in the constant rotational velocity approach, i.e. they are consistent with the Einstein equations and in hydrodynamical equilibrium. We study the NS rotation effect on the energetics, the gravitational waves (GWs) and on the possible electromagnetic (EM) emission associated to dynamical mass ejecta. For dimensionless spin magnitudes of

Stability of general relativistic static thick disks: The Isotropic Schwarzschild thick disk

\ensuremath{\chi}\ensuremath{\sim}0.1

Numerical self-consistent stellar models of thin disks

we find that both spin-orbit interactions and spin-induced quadrupole deformations affect the late-inspiral merger dynamics. The latter is, however, dominated by finite-size effects. Spin (tidal) effects contribute to GW phase differences up to

Relativistic ring models

\ensuremath{\sim}5

Static charged fluid around a massive magnetic dipole

(20) radians accumulated during the last eight orbits to merger. Similarly, after merger the collapse time of the remnant and the GW spectrogram are affected by the NSs rotation. Spin effects in dynamical ejecta are clearly observed in unequal-mass systems in which mass ejection originates from the tidal tail of the companion. Consequently kilonovae and other EM counterparts are affected by spins. We find that spin aligned to the orbital angular momentum leads to brighter EM counterparts than antialigned spin with luminosities up to a factor of 2 higher.

Stability of general relativistic Miyamoto–Nagai galaxies

With the increasing sensitivity of advanced gravitational wave detectors, the first joint detection of an electromagnetic and gravitational wave signal from a compact binary merger will hopefully happen within this decade. However, current gravitational-wave likelihood sky areas span