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Register a new userInflation-Produced Magnetic Fields in R^n F^2 and I F^2 models
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Leonardo Campanelli
Publication date
2008
fields
Physics
and research's language is
English
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We re-analyze the production of seed magnetic fields during Inflation in (R/m^2)^n F_{mu u}F^{mu u} and I F_{mu u}F^{mu u} models, where n is a positive integer, R the Ricci scalar, m a mass parameter, and I propto eta^alpha a power-law function of the conformal time eta, with alpha a positive real number. If m is the electron mass, the produced fields are uninterestingly small for all n. Taking m as a free parameter we find that, for n geq 2, the produced magnetic fields can be sufficiently strong in order to seed dynamo mechanism and then to explain galactic magnetism. For alpha gtrsim 2, there is always a window in the parameters defining Inflation such that the generated magnetic fields are astrophysically interesting. Moreover, if Inflation is (almost) de Sitter and the produced fields almost scale-invariant (alpha simeq 4), their intensity can be strong enough to directly explain the presence of microgauss galactic magnetic fields.
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In the context of f(R)=R + alpha R^2 gravity, we study the existence of neutron and quark stars with no intermediate approximations in the generalised system of Tolman-Oppenheimer-Volkov equations. Analysis shows that for positive alphas the scalar curvature does not drop to zero at the star surface (as in General Relativity) but exponentially decreases with distance. Also the stellar mass bounded by star surface decreases when the value alpha increases. Nonetheless distant observers would observe a gravitational mass due to appearance of a so-called gravitational sphere around the star. The non-zero curvature contribution to the gravitational mass eventually is shown to compensate the stellar mass decrease for growing alphas. We perform our analysis for several equations of state including purely hadronic configurations as well as hyperons and quark stars. In all cases, we assess that the relation between the parameter $alpha$ and the gravitational mass weakly depend upon the chosen equation of state. Another interesting feature is the increase of the star radius in comparison to General Relativity for stars with masses close to maximal, whereas for intermediate masses around 1.4-1.6 solar masses, the radius of star depends upon alpha very weakly. Also the decrease in the mass bounded by star surface may cause the surface redshift to decrease in R^2-gravity when compared to Einsteinian predictions. This effect is shown to hardly depend upon the observed gravitational mass. Finally, for negative values of alpha our analysis shows that outside the star the scalar curvature has damped oscillations but the contribution of the gravitational sphere into the gravitational mass increases indefinitely with radial distance putting into question the very existence of such relativistic stars.
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