Físicahttp://hdl.handle.net/10669/2752020-01-21T10:29:02Z2020-01-21T10:29:02ZExact solution of an ekpyrotic fluid and a primordial magnetic field in an anisotropic cosmological space-time of Petrov Dhttp://hdl.handle.net/10669/793542019-10-14T16:15:02Z2017-01-01T00:00:00ZExact solution of an ekpyrotic fluid and a primordial magnetic field in an anisotropic cosmological space-time of Petrov D
This document obtained and analyzed an exact solution of an ekpyrotic fluid (P = 5/3 µ) and a primordial magnetic field, so that the latter does not induce currents or electric fields. It is determined that the main role, in relation to the anisotropy in the solution, is played by the magnetic field. The solution is analyzed in points where, in appearance, some extent of singularity could exist, using the Kretschmann invariant and analyzing the metric’s functions, and it is established that this is singular at t = 0. The value of the Kretschmann invariant, in these proximities, tends to be the same as the one obtained in the case of a free ekpyrotic fluid of the magnetic field; therefore, the magnetic field is left at the background when t → 0. It is determined that the solution presents this behavior in small time values, and tends to be isotropic and equivalent to the solution of the ekpyrotic fluid obtained for the flat model of Friedmann, Robertson, Walker and Lemaitre (FRWL); for big time values, it tends to behave as the Kasner LRS solution.
2017-01-01T00:00:00ZExact Solutions of a Chaplygin Gas in an Anisotropic Space-Time of Petrov Dhttp://hdl.handle.net/10669/793432019-10-10T15:01:55Z2017-01-01T00:00:00ZExact Solutions of a Chaplygin Gas in an Anisotropic Space-Time of Petrov D
This document obtains two exact solutions to the anisotropic spacetime of Petrov D by using the model of a perfect fluid. These solutions represent a scenario of a universe in which the pressure P and the energetic density µ of the fluid are inversely proportional (Chaplygin’s type P = −Q2/µ), where Q is a constant of proportionality. It is established that the symmetry of those models, in the proximities when t → 0, is equivalent to the analogues for the dust model, and might tend to behave as the solutions of the flat or vacuum LRS Kasner solution (Local Rotational Symmetry). Although the solutions are not flat or vacuum in any of the cases, in those proximities, the density tends to infinite and with no pressure. When t → ∞, the models tend to behave as the isotropic flat model of the type FRWL. In the analysis of the Hubble and the deceleration parameters obtained that in those solutions, the Hubble’s constant and the deceleration parameter, depend on time and manner in which their values, or tendency, significantly evolve. The deceleration parameter q changes its sign as times passes, so that it represents an initial deceleration process that, in continuity, constantly changes to a process of acceleration.
2017-01-01T00:00:00ZA study of a coronal hole associated with a large filament eruptionhttp://hdl.handle.net/10669/793022019-10-03T15:21:31Z2017-01-01T00:00:00ZA study of a coronal hole associated with a large filament eruption
We report the results of a detailed study of an equatorial coronal hole and a dimming region related to the eruptions of a nearby large filament and subsequent coronal mass ejections (CMEs). The dynamic eruptions of the filament and the associated CMEs are probably related to the magnetic reconnection involving the magnetic field lines at the filament footpoints. During the starting processes of the filament eruption, we observed several newly emerged small magnetic flux concentrations close to the filament footpoints. Disturbance increase in the prominence body was observed during the pre-eruption processes. After the filament eruption, we observed evacuated filament material from the filament channel towards the coronal hole. Thus, all the region is perturbed and EUV loops and bright points are observed before and after the eruptions. Additionally, after the CME, we observed the disappearance of the dimming region and the coronal hole, followed by photospheric magnetic diffusion. We discussed a possible magnetic reconnection scenario and MHD waves involved during these processes.
2017-01-01T00:00:00ZDo Magnetic Fields Destroy Black Hole Accretion Disk g-Modes?http://hdl.handle.net/10669/774212019-06-28T19:36:57Z2015-08-04T00:00:00ZDo Magnetic Fields Destroy Black Hole Accretion Disk g-Modes?
Diskoseismology, the theoretical study of normal-mode oscillations in geometrically thin, optically thick accretion disks, is a strong candidate for explaining some quasi-periodic oscillations in the power spectra of many black hole X-ray binary systems. The existence of g-modes, presumably the most robust and visible of the modes, depends on general relativistic gravitational trapping in the hottest part of the disk. As the existence of the required cavity in the presence of magnetic fields has been put into doubt by theoretical calculations, we will explore in greater generality what effect the inclusion of magnetic fields has on the existence of g-modes. We use an analytical perturbative approach on the equations of MHD to assess the impact of such effects. Our main conclusion is that there appears to be no compelling reason to discard g-modes. In particular, the inclusion of a non-zero radial component of the magnetic field enables a broader scenario for cavity non-destruction, especially taking into account recent simulations' saturation values for the magnetic field.
2015-08-04T00:00:00Z