Consequences of f(R)-theories of gravity on gravitational leptogenesis

f(R)-theories of gravity are reviewed in the framework of the matter-antimatter asymmetry in the Universe. The asymmetry is generated by the gravitational coupling of heavy (Majorana) neutrinos with the Ricci scalar curvature. In order that the mechanism works, a time varying non-zero Ricci curvature is necessary. The latter is provided by f(R) cosmology, whose Lagrangian density is of the form {\cal L}(R)\sim f(R). In particular we study the cases f(R)\sim R+\alpha R^n and f(R)\sim R^{1+\epsilon}.Comment: 14 page

Neutrino coupling to cosmological background: A review on gravitational Baryo/Leptogenesis

In this work we review the theories of origin of matter-antimatter asymmetry in the Universe. The general conditions for achieving baryogenesis and leptogenesis in a CPT conserving field theory have been laid down by Sakharov. In this review we discuss scenarios where a background scalar or gravitational field spontaneously breaks the CPT symmetry and splits the energy levels between particles and anti-particles. Baryon or Lepton number violating processes in proceeding at thermal equilibrium in such backgrounds gives rise to Baryon or Lepton number asymmetry.Comment: 62 pages, no figures. Invited review to appear in IJMP

Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii

We present mass and radius derivations for a sample of very young, mid- to late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a previous paper, we determined effective temperatures and surface gravities for these targets, from an analysis of their high-resolution optical spectra and comparisons to the latest synthetic spectra. We now derive extinctions, radii, masses and luminosities by combining our previous results with observed photometry, surface fluxes from the synthetic spectra and the known cluster distances. These are the first mass and radius estimates for young, very low mass bodies that are independent of theoretical evolutionary models (though our estimates do depend on spectral modeling). We find that for most of our sample, our derived mass-radius and mass-luminosity relationships are in very good agreement with the theoretical predictions. However, our results diverge from the evolutionary model values for the coolest, lowest-mass targets: our inferred radii and luminosities are significantly larger than predicted for these objects at the likely cluster ages, causing them to appear much younger than expected. We suggest that uncertainties in the evolutionary models - e.g., in the choice of initial conditions and/or treatment of interior convection - may be responsible for this discrepancy. Finally, two of our late-M objects (USco 128 and 130) appear to have masses close to the deuterium-fusion boundary (9--14 Jupiters, within a factor of 2). This conclusion is primarily a consequence of their considerable faintness compared to other targets with similar extinction, spectral type and temperature (difference of 1 mag). Our result suggests that the faintest young late-M or cooler objects may be significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap

Comment on "Cherenkov Radiation by Neutrinos in a Supernova Core"

Mohanty and Samal have shown that the magnetic-moment interaction with nucleons contributes significantly to the photon dispersion relation in a supernova core, and with an opposite sign relative to the usual plasma effect. Because of a numerical error they overestimated the magnetic-moment term by two orders of magnitude, but it is still of the same order as the plasma effect. It appears that the Cherenkov processes gamma+nu -> nu and nu -> nu+gamma remain forbidden, but a final verdict depends on a more detailed investigation of the dynamical magnetic susceptibility of a hot nuclear medium.Comment: 2 pages, REVTEX. Submitted as a Comment to PR

Hydrogen spin oscillations in a background of axions and the 21-cm brightness temperature

The 21-cm line signal arising from the hyperfine interaction in hydrogen has an important role in cosmology and provides a unique method for probing of the universe prior to the star formation era. We propose that the spin flip of Hydrogen by the coherent emission/absorption of axions causes a lowering of their spin temperature and can explain the stronger than expected absorption of 21-cm light reported by the EDGES collaboration. We find the analogy of axion interaction with the two level HI with the Jaynes-Cummings model of a two level atom in a cavity and we derive the spin flip frequency in this formalism and show that the coherent oscillations frequency $\Omega \propto 1/f_a$ in contrast with the incoherent transitions between the HI hyperfine levels where the transition rates $\propto 1/f_a^2$. The axion emission and absorption rates are equal but the spin temperature is still lowered due to different selection rules for the spin flip transitions compared to the photon process. We show that the axion process goes in the right direction for explaining the EDGES observation. For this mechanism to work we require a coherent field of relativistic axions with energy $E_\nu$ peaked at the 21-cm spin-flip energy. Such a coherent background of relativistic axions can arise from the decay of cosmic strings if the decay takes place in the electroweak era.Comment: 6 pages - Accepted for publication in MNRA

Angular momentum evolution of young low-mass stars and brown dwarfs: observations and theory

This chapter aims at providing the most complete review of both the emerging concepts and the latest observational results regarding the angular momentum evolution of young low-mass stars and brown dwarfs. In the time since Protostars & Planets V, there have been major developments in the availability of rotation period measurements at multiple ages and in different star-forming environments that are essential for testing theory. In parallel, substantial theoretical developments have been carried out in the last few years, including the physics of the star-disk interaction, numerical simulations of stellar winds, and the investigation of angular momentum transport processes in stellar interiors. This chapter reviews both the recent observational and theoretical advances that prompted the development of renewed angular momentum evolution models for cool stars and brown dwarfs. While the main observational trends of the rotational history of low mass objects seem to be accounted for by these new models, a number of critical open issues remain that are outlined in this review.Comment: 22 pages, 8 figures, accepted for publication in Protostars & Planets VI, 2014, University of Arizona Press, eds. H. Beuther, R. Klessen, K. Dullemond, Th. Hennin

Bianchijev kozmološki model tipa V u Lyrinoj mnogostrukosti

Bianchi type-V space-time is considered in the presence of a perfect fluid source in the framework of Lyra manifold with pressure equal to energy density (p=ρ). Some physical and geometrical properties of the model are discussed.Razmatramo Bianchijev prostor-vrijeme tipa V uz prisutnost perfektne tekućine u okviru Lyrine mnogostrukosti i jednak tlak i gustoću energije (p = ρ). Raspravljaju se neke fizičke i geometrijske odlike modela

Bianchijev kozmološki model tipa V u Lyrinoj mnogostrukosti

Bianchi type-V space-time is considered in the presence of a perfect fluid source in the framework of Lyra manifold with pressure equal to energy density (p=ρ). Some physical and geometrical properties of the model are discussed.Razmatramo Bianchijev prostor-vrijeme tipa V uz prisutnost perfektne tekućine u okviru Lyrine mnogostrukosti i jednak tlak i gustoću energije (p = ρ). Raspravljaju se neke fizičke i geometrijske odlike modela