Small-scale microwave background anisotropies arising from tangled primordial magnetic fields

Abstract

An inhomogeneous cosmological magnetic field creates vortical perturbations that survive Silk damping on much smaller scales than compressional modes. This ensures that there is no sharp cut-off in anisotropy on arcminute scales. As we had pointed out earlier, tangled magnetic fields, if they exist, will then be a potentially important contributor to small-angular-scale cosmic microwave background radiation anisotropies. Several ongoing and new experiments are expected to probe the very small angular scales, corresponding to multipoles with l > 1000. In view of this observational focus, we revisit the predicted signals arising from primordial tangled magnetic fields, for different spectra and different cosmological parameters. We also identify a new regime, where the photon mean-free path exceeds the scale of the perturbation, which dominates the predicted signal at very high l. A scale-invariant spectrum of tangled fields which redshifts to a present value B₀=3 × 10^-9 G produces temperature anisotropies at the 10-μK level between l ∼ 1000 and 3000. Larger signals result if the universe is lambda-dominated, if the baryon density is larger, or if the spectral index of magnetic tangles is steeper, n >-3. The signal will also have non-Gaussian statistics. We predict the distinctive form of the increased power expected in the microwave background at high l in the presence of significant tangled magnetic fields. We may be on the verge of detecting or ruling out the presence of tangled magnetic fields that are strong enough to influence the formation of large-scale structure in the Universe.