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. We propose a novel probe of stochastic primordial magnetic fields that exploits the characteristic CMB non-Gaussianity that they induce. We compute the CMB bispectrum ($b_{l_1{l}_2{l}_3}$) induced by such fields on large angular scales. We find a typical value of $l_1(l_1+1)l_3(l_3+1)b_{l_1{l}_2{l}_3}\sim {10}^{-22}$, for magnetic fields of strength $B_0\sim 3\mathrm{nG}$ and with a nearly scale invariant magnetic spectrum. Observational limits on the bispectrum allow us to set upper limits on $B_0\sim 35\mathrm{nG}$.

• Received 23 February 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.081303