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Cosmic magnetic fields are observed to be coherent on large scales and could have a primordial origin. Non-Gaussian signals in the cosmic microwave background (CMB) are generated by primordial magnetic fields as the magnetic stresses and temperature anisotropy they induce depend quadratically on the magnetic field. We compute the CMB scalar trispectrum on large angular scales, for nearly scale-invariant magnetic fields, sourced via the Sachs-Wolfe effect. The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately 10-29 and 10-19, respectively. The scalar anisotropic stress trispectrum is also calculated in the flat-sky approximation and yields a similar result. Observational limits on CMB non-Gaussianity from the Planck mission data allow us to set upper limits of B0 ≰ 0.6 nG on the present value of the primordial cosmic magnetic field. Considering the inflationary magnetic curvature mode in the trispectrum can further tighten the magnetic field upper limit to B0 ≰ 0.05 nG. These sub-nanoGauss constraints from the magnetic trispectrum are the most stringent limits so far on the strength of primordial magnetic fields, on megaparsec scales, significantly better than the limits obtained from the CMB bispectrum and the CMB power spectrum. [ABSTRACT FROM AUTHOR]

Generation of magnetic fields during inflation is a promising mechanism for the origin of the observed large scale magnetic fields in the universe. Among several attempts, a popular model is one where the inflaton and the electromagnetic field are coupled through a coupling function f leading to a term in the Lagrangian density of the form, f²FμνFμν. A number of potential difficulties with such models have been raised in the literature. In our earlier work, we have suggested viable models of inflationary magnetogenesis which avoid these problems and at the same time can lead to either nonhelical or helical magnetic fields of astrophysical interest. Our models require a low energy scale for inflation and reheating (reheating temperature, TR<104 GeV) and generate a blue spectrum of electromagnetic (EM) field which peaks around the horizon scale of reheating. We show here that the anisotropic stress associated with these EM fields naturally source the production of a stochastic background of Gravitational waves (GW) with frequencies in the range of tens of nano Hertz to milli Hertz. These two extremes of the range can be probed respectively by pulsar timing arrays (PTA) experiments and the upcoming Laser Interferometric Space Array (LISA). The peak value of the GW spectrum energy represented by dΩGW/dlnk is 10-6 for the models which lead to nonhelical primordial fields and 2×10-6 for the helical case for TR=100 GeV. In this case the spectrum peaks at a frequency 30 μHz for nonhelical case and at 40 μHz for helical case. These values are obtained when the ratio of EM energy density to the cosmological density at reheating ε~1 and decrease approximately as ε² for smaller values. The amplitude is similar for a lower value of TR, but the frequency at which the GW spectrum peaks decreases as TR. The gravitational waves generated are unpolarized if the EM fields are nonhelical but are circularly polarized for helical primordial fields. If detected in the future these gravitational waves will provide a unique probe of such models of inflationary magnetogenesis. [ABSTRACT FROM AUTHOR]

We study the generation of helical magnetic fields in a model of inflationary magnetogenesis which is free from the strong coupling and backreaction problems. To generate helical magnetic fields, we add an f2˜FμνFμν term to the Lagrangian of the Ratra model. The strong coupling and backreaction problems are avoided if we take a particular behavior of coupling function f, in which f increases during inflation and decreases postinflation to reheating. The generated magnetic field is fully helical and has a blue spectrum, dρB/dlnk∝k4. This spectrum is obtained when coupling function f∝a2 during inflation. The scale of reheating in our model has to be lower than 4000 GeV to avoid backreaction postinflation. The generated magnetic field spectrum satisfies the γ-ray bound for all the possible scales of reheating. The comoving magnetic field strength and its correlation length are ∼4×10-11 G and 70 kpc respectively, if reheating takes place at 100 GeV. For reheating at the QCD scales of 150 MeV, the field strength increases to ∼ nano gauss, with coherence scale of 0.6 Mpc. [ABSTRACT FROM AUTHOR]

We study the inflationary generation of helical cosmological magnetic fields in a higher-dimensional generalization of the electromagnetic theory. For this purpose, we also include a parity breaking piece to the electromagnetic action. The evolution of an extra-dimensional scale factor allows the breaking of conformai invariance of the effective electromagnetic action in 1 + 3 dimensions required for such generation. Analytical solutions for the vector potential can be obtained in terms of Coulomb wavefunctions for some special cases. We also present numerical solutions for the vector potential evolution in more general cases. In the presence of a higher-dimensional cosmological constant there exist solutions for the scale factors in which both normal and extra dimensional space either inflate or deflate simultaneously with the same rate. In such a scenario, with the number of extra dimensions D = 4, a scale invariant spectrum of helical magnetic field is obtained. The net helicity arises, as one helical mode comes to dominate over the other at the superhorizon scales. A magnetic field strength of the order of 10-9 G can be obtained for the inflationary scale H ≃ 10-3 Mpl Weaker fields will be generated for lower scales of inflation. Magnetic fields generated in this model respects the bounds on magnetic fields by Planck and yray observations (i.e., 10-16 G < Bobs < 3.4 × 10-9 G). [ABSTRACT FROM AUTHOR]

Laboratory experiments measuring neutrino oscillations indicate small mass differences between different mass eigenstates of neutrinos. The absolute mass scale is however not determined, with at present the strongest upper limits coming from astronomical observations rather than terrestrial experiments. The presence of massive neutrinos suppresses the growth of perturbations below a characteristic mass scale, thereby leading to a decreased abundance of collapsed dark matter halos. Here we show that this effect can significantly alter the predicted luminosity function (LF) of high redshift galaxies. In particular we demonstrate that a stringent constraint on the neutrino mass can be obtained using the well measured galaxy LF and our semianalytic structure formation models. Combining the constraints from the Wilkinson Microwave Anisotropy Probe 7 yr (WMAP7) data with the LF data at z~4, we get a limit on the sum of the masses of 3 degenerate neutrinos σm?<0.52??eV at the 95% C.L. The additional constraints using the prior on Hubble constant strengthens this limit to σm?<0.29??eV at the 95% C.L. This neutrino mass limit is a factor ~4 improvement compared to the constraint based on the WMAP7 data alone, and as stringent as known limits based on other astronomical observations. As different astronomical measurements may suffer from different set of biases, the method presented here provides a complementary probe of σm?. We suggest that repeating this exercise on well measured luminosity functions over different redshift ranges can provide independent and tighter constraints on σm?. [ABSTRACT FROM AUTHOR]

Generation of primordial magnetic fields during inflation typically requires the breaking of conformai invariance of the electromagnetic action. In this paper this has been achieved naturally in a higher-dimensional cosmological model with a Gauss-Bonnet term in the action. The evolution of the scale factor of the extra dimension (whose dynamics is influenced by the Gauss-Bonnet term) acts as the cause for the breaking of conformai invariance. Different cases have been investigated, each of which is characterized by the number of higher dimensions, the value of the Gauss-Bonnet parameter, and the cosmological constant. Many of the scenarios considered are highly constrained by the requirements that the cosmic evolution is stable, that the normal dimensions expand, and that there is no backreaction due to growing electric fields. However, there do exist scenarios which satisfy the above requirements and are well suited for magnetogenesis. In particular, a scenario where the number of extra dimensions D = 4 and the cosmological constant is nonzero turns out to be best suited for generating primordial magnetic fields. It is shown that for these values of parameters, a scale-invariant magnetic field of the order of 1010-10-9 G can be produced. Even in these most favorable scenarios, the higher-dimensional space expands during inflation at the same rate as the normal dimension. Hence, if a mechanism could freeze the evolution of the higher dimension, this seems to be a viable mechanism to produce acceptable primordial magnetic fields. [ABSTRACT FROM AUTHOR]

Models of inflationary magnetogenesis with a coupling to the electromagnetic action of the form f² FμνFμν, are known to suffer from several problems. These include the strong coupling problem, the backreaction problem and also strong constraints due to the Schwinger effect. We propose a model which resolves all these issues. In our model, the coupling function, f, grows during inflation and transits to a decaying phase post-inflation. This evolutionary behavior is chosen so as to avoid the problem of strong coupling. By assuming a suitable power-law form of the coupling function, we can also neglect backreaction effects during inflation. To avoid backreaction post-inflation, we find that the reheating temperature is restricted to be below ≈ 1.7 × 104 GeV. The magnetic energy spectrum is predicted to be nonhelical and generically blue. The estimated present day magnetic field strength and the corresponding coherence length taking reheating at the QCD epoch (150 MeV) are 1.4 × 10-12 G and 6.1 × 10-4 Mpc, respectively. This is obtained after taking account of nonlinear processing over and above the flux-freezing evolution after reheating. If we consider also the possibility of a nonhelical inverse transfer, as indicated in direct numerical simulations, the coherence length and the magnetic field strength are even larger. In all cases mentioned above, the magnetic fields generated in our models satisfy the γ-ray bound below a certain reheating temperature. [ABSTRACT FROM AUTHOR]