Primordial magnetic fields lead to non-Gaussian signals in the cosmic microwave background (CMB) even at the lowest order, as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. In contrast, CMB non-Gaussianity due to inflationary scalar perturbations arises only as a higher order effect. Apart from a compensated scalar mode, stochastic primordial magnetic fields also produce scalar anisotropic stress that remains uncompensated till neutrino decoupling. This gives rise to an adiabatic-like scalar perturbation mode that evolves passively thereafter (called the passive mode). We compute the CMB reduced bispectrum (b l 1 l 2 l 3 ) induced by this passive mode, sourced via the Sachs-Wolfe effect, on large angular scales. For any configuration of bispectrum, taking a partial sum over mode-coupling terms, we find a typical value of l1(l1 +1)l3(l3 +1)b l 1 l 2 l 3 ∼ 6−9×10−16 , for a magnetic field of B0 ∼ 3 nG, assuming a nearly scale-invariant magnetic spectrum . We also evaluate, in full, the bispectrum for the squeezed collinear configuration over all angular mode-coupling terms and find l1(l1 + 1)l3(l3 + 1)b l 1 l 2 l 3 ≈ −1.4 × 10 −16 . These values are more than ∼ 10 6 times larger than the previously calculated magnetic compensated scalar mode CMB bispectrum. Observational limits on the bispectrum from WMAP7 data allow us to set upper limits of B0 ∼ 2 nG on the present value of the cosmic magnetic field of primordial origin. This is over 10 times more stringent than earlier limits on B0 based on the compensated mode bispectrum.

Cosmic magnetic fields are observed to be coherent on large scales and could have a primordial origin. NonGaussian 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.

Primordial magnetic fields will generate non-Gaussian signals in the cosmic microwave background (CMB) as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. We compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales, 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 a few times 10 −29 and 10 −19 , respectively. Observational limits on CMB non-Gaussianity from WMAP data allow us to conservatively set upper limits of a nG, and plausibly sub-nG, on the present value of the primordial cosmic magnetic field. This represents the tightest limit so far on the strength of primordial magnetic fields, on Mpc scales, and is better than limits from the CMB bispectrum and all modes in the CMB power spectrum. Thus, the CMB trispectrum is a new and more sensitive probe of primordial magnetic fields on large scales.Magnetic fields are ubiquitous in the Universe from planets and stars to galaxies and galaxy clusters [1,2], yet the origin and evolution of large-scale magnetic fields remains a puzzle. A popular paradigm is that magnetic fields in collapsed structures could arise from dynamo amplification of seed magnetic fields [2]. The seed field could in turn be generated in astrophysical batteries [3] or due to processes in the early universe [4,5]. Indeed recent γ-ray observations claim to find a lower limit to an all-pervasive intergalactic magnetic field that fills most of the cosmic volume [6], which would perhaps favor a primordial origin. A primordial magnetic field can be generated at inflation [4], or arise out of other phase transitions in the early Universe [5]. As yet there is no compelling mechanism which produces strong coherent primordial fields. Equally, the dynamo paradigm is not without its own challenges in producing sufficiently coherent fields and sufficiently rapidly [2]. Therefore, it is useful to keep open the possibility that primordial magnetic fields originating in the early universe play a crucial role in explaining the observed cosmic magnetism.In this context it is important to investigate every observable signature of the putative primordial magnetic fields. Constraints on large-scale primordial magnetic fields have already been derived using the cosmic microwave background (CMB) power spectrum [7,8] and Faraday rotation [9]. However, the effects of a magnetic field on the CMB are relatively more prominent in its non-Gaussian correlations. This is because magnetic fields induce non-Gaussian signals at lowest order as the magnetic energy density and stress are quadratic in the field. On the other hand, the standard inflationary perturbations, dominated by their linear component, can source non-Gaussian correlations only with higher-order perturbations and thus necessarily produce a small amplitude of CMB non-Gaussianity (cf. [10,11]). Primordial magnetic fields can induce apprec...

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We investigate the possible parametric growth of photon amplitudes in a background of axion-like particle (ALP) dark matter. The observed extragalactic background radiation limits the allowed enhancement effect. We derive the resulting constraints on the axion-photon coupling constant gaγ from Galactic ALP condensates as well as over-densities. If ALP condensates of size R exist in our Galaxy, a scan for extremely narrow unresolved spectral lines with frequency ν can constrain the axion-photon coupling at ALP mass ma = 4πν to gaγ 2 × 10 −14 (10 kpc/R) GeV −1 . Radio to optical background data yield constraints at this level within observed wavebands or ALP mass windows over a broad range 0.08 µeV ma 8 eV. These condensate constraints on gaγ probe down to the QCD axion band for ma 10µ eV.