Search results for "General Relativity"
showing 10 items of 1057 documents
Running couplings from adiabatic regularization
2019
We extend the adiabatic regularization method by introducing an arbitrary mass scale $\mu$ in the construction of the subtraction terms. This allows us to obtain, in a very robust way, the running of the coupling constants by demanding $\mu$-invariance of the effective semiclassical (Maxwell-Einstein) equations. In particular, we get the running of the electric charge of perturbative quantum electrodynamics. Furthermore, the method brings about a renormalization of the cosmological constant and the Newtonian gravitational constant. The running obtained for these dimensionful coupling constants has new relevant (non-logarithmic) contributions, not predicted by dimensional regularization.
Bounds on Neutrino-Scalar Yukawa Coupling
2015
General neutrino-scalar couplings appear in many extensions of Standard Model. We can probe these neutrino-scalar couplings by leptonic decay of mesons and from heavy neutrino search. Our analysis improves the present limits to $|g_e|^2<1.9\times 10^{-6}$ and $|g_\mu|^2<1.9\times 10^{-7}$ at 90\% C.L. for massless scalars. For massive scalars we found for the first time the constraints for $g^2_{\alpha}$ couplings to be $10^{-6}-10^{-1}$ respectively for scalar masses between below 1 MeV and for 300 MeV.
Fully Covariant and Conformal Formulation of the Z4 System Compared to the BSSN Formulation in Spherical Symmetry
2014
We have generalized a covariant and conformal version of the Z4 system of the Einstein equations by adopting a reference metric approach, that we denote as fCCZ4, well suited for curvilinear as well as Cartesian coordinates. We implement this formalism in spherical polar coordinates under the assumption of spherical symmetry using a partially-implicit Runge-Kutta (PIRK) method, without using any regularization scheme, and show that our code can evolve both vacuum and non-vacuum spacetimes without encountering instabilities. We have performed several tests and compared the Hamiltonian constraint violations of the fCCZ4 system, for different choices of certain free parameters, with these of B…
Stationary models of magnetized viscous tori around a Schwarzschild black hole
2020
We present stationary solutions of magnetized, viscous thick accretion disks around a Schwarzschild black hole. We assume that the tori are not self-gravitating, are endowed with a toroidal magnetic field, and obey a constant angular momentum law. Our study focuses on the role of the black hole curvature in the shear viscosity tensor and in their potential combined effect on the stationary solutions. Those are built in the framework of a causality-preserving, second-order gradient expansion scheme of relativistic hydrodynamics in the Eckart frame description which gives rise to hyperbolic equations of motion. The stationary models are constructed by numerically solving the general relativis…
Misbeliefs and misunderstandings about the non-Markovian dynamics of a damped harmonic oscillator
2003
We use the exact solution for the damped harmonic oscillator to discuss some relevant aspects of its open dynamics often mislead or misunderstood. We compare two different approximations both referred to as Rotating Wave Approximation. Using a specific example, we clarify some issues related to non--Markovian dynamics, non--Lindblad type dynamics, and positivity of the density matrix.
On numerical relativistic hydrodynamics and barotropic equations of state
2012
The characteristic formulation of the relativistic hydrodynamic equations (Donat et al 1998 J. Comput. Phys. 146 58), which has been implemented in many relativistic hydro-codes that make use of Godunov-type methods, has to be slightly modified in the case of evolving barotropic flows. For a barotropic equation of state, a removable singularity appears in one of the eigenvectors. The singularity can be avoided by means of a simple renormalization which makes the system of eigenvectors well defined and complete. An alternative strategy for the particular case of barotropic flows is discussed.
Magnetized discs and photon rings around Yukawa-like black holes
2021
We present stationary solutions of geometrically thick discs (or tori) endowed with a self-consistent toroidal magnetic field distribution surrounding a non-rotating black hole in an analytical, static, spherically-symmetric $f(R)$-gravity background. These $f(R)$-gravity models introduce a Yukawa-like modification to the Newtonian potential, encoded in a single parameter $\delta$ which controls the strength of the modified potential and whose specific values affect the disc configurations when compared to the general relativistic case. Our models span different magnetic field strengths, from purely hydrodynamical discs to highly magnetized tori. The characteristics of the solutions are ide…
A "horizon adapted" approach to the study of relativistic accretion flows onto rotating black holes
1998
We present a new geometrical approach to the study of accretion flows onto rotating (Kerr) black holes. Instead of Boyer-Lindquist coordinates, the standard choice in all existing numerical simulations in the literature, we employ the simplest example of a horizon adapted coordinate system, the Kerr-Schild coordinates. This choice eliminates boundary ambiguities and unphysical divergent behavior at the event horizon. Computations of Bondi-Hoyle accretion onto extreme Kerr black holes, performed here for the first time, demonstrate the key advantages of this procedure. We argue it offers the best approach to the numerical study of the, observationally, increasingly more accesible relativisti…
Dynamical behaviour of an XX central spin model through Bethe ansatz techniques
2009
Following the Bethe ansazt procedure the exact dynamics of an XX central spin model is revealed. Particular initial conditions are analyzed and the consequent time evolution is compared with the exact solution obtained by solving the time-dependent Schrudinger equation. The interest towards spin systems and in particular central spin systems, is motivated by the recent developments in more applicative contexts.
Pair and triple correlations in theA+B→Bdiffusion-controlled reaction
1994
An exact solution for the one-dimensional kinetics of the diffusion-controlled reaction A+B\ensuremath{\rightarrow}B is obtained by means of the three-particle correlation functions. Because of a lattice discreteness each site could be occupied by a single particle only which leads to the so-called ``bus effect'': Recombination of any particle A is defined by a spatial configuration of two nearest particles B only surrounding A from its left and right. This results in the unusual algebraic decay law, n(t)\ensuremath{\propto}${\mathit{t}}^{\mathrm{\ensuremath{-}}1}$, which asymptotically (as t\ensuremath{\rightarrow}\ensuremath{\infty}) does not depend on the trap B concentration.