We take advantage of the wealth of rotation measures data contained in the NRAO VLA Sky Survey catalogue to derive new , statistically robust , upper limits on the strength of extragalactic magnetic fields .
We simulate the extragalactic magnetic field contribution to the rotation measures for a given field strength and correlation length , by assuming that the electron density follows the distribution of Lyman- \alpha clouds .
Based on the observation that rotation measures from distant radio sources do not exhibit any trend with redshift , while the extragalactic contribution instead grows with distance , we constrain fields with Jeans ’ length coherence length to be below 1.7 nG at the 2 \sigma level , and fields coherent across the entire observable Universe below 0.65 nG .
These limits do not depend on the particular origin of these cosmological fields .