AR_final file_2018-19

Figure 9: Left panel : Shows the space-time contour of the azimuthal component of mean-field result from realistic MHD simulations of turbulent ISM. Right panel : Shows the same but for the azimuthal field components obtained from 1-D dynamo simulations, with same colour code. Note the overall similarity in both contours. can be active in galaxies of this class despite their slow rotation because their discs are relatively thick. Earlier assessments of the possibility of the mean- field dynamo action in low-mass galaxies relied on estimates applicable to thin discs, such as those in massive spiral galaxies. Using both order-of- magnitude estimates and numerical solutions, they show that the strength of differential rotation re- quired to amplify magnetic field reduces as the as- pect ratio of the galactic gas layer increases. Thus, the puzzle of the origin of large-scale magnetic fields appears to be solved. As a result, this class of galaxies provides a new ground for testing the understanding of galactic magnetism. Gravitational wave generation in a vi- able scenario of inflationary magneto- genesis Generation of magnetic fields during inflation is a promising scenario for the origin of the seed mag- netic fields in the universe. A popular mecha- nism suggested by Ratra is one where one takes a coupling between the inflaton and electromag- netic field. For certain choices of this coupling, the fields generated satisfy all observational constraints. However, it turns out that this model suffers from the strong coupling and back-reaction problems. It is also severely constrained by particle production due to the Schwinger mechanism. In the earlier work, Ramkishor Sharma, Kandaswamy Subra- manian , and T. R. Seshdari suggested a model which avoids these problems, while satisfying all observational constraints. This, model, however, requires a low scale for reheating, and generates magnetic fields with blue spectrum, and power con- centrated on the Hubble scale at reheating. The anisotropic stress associated with such fields can source the production of a stochastic background of gravitational waves. For helical fields, the gen- erated gravitational waves can also be circularly polarized. They have calculated this gravitational wave background to compare with the current lim- its set by pulsar timing arrays and those expected from space based gravitational wave detectors like LISA. The detection of (or limts on) the stochas- tic background of gravitational waves in the future will strongly constrain such models of inflationary magnetogenesis. Cosmic Microwave Background Bayesian estimation of SI violation The Bipolar Spherical Harmonic (BipoSH) repre- sentation proposed a decade back has been steadily ( 72 )