If ultra-high-energy cosmic rays originate from extragalactic sources , the offsets of their arrival directions from these sources imply an upper limit on the strength of the extragalactic magnetic field .
The Pierre Auger Collaboration has recently reported that anisotropy in the arrival directions of cosmic rays is correlated with several types of extragalactic objects .
If these cosmic rays originate from these objects , they imply a limit on the extragalactic magnetic field strength of B < 0.7 – 2.2 \times 10 ^ { -9 } \left ( \lambda _ { B } / { 1 ~ { } Mpc } \right ) ^ { -1 / 2 } G for coherence lengths \lambda _ { B } < 100 Mpc and B < 0.7 – 2.2 \times 10 ^ { -10 } G at larger scales .
This is comparable to existing upper limits at \lambda _ { B } = 1 Mpc , and improves on them by a factor 4–12 at larger scales .
The principal source of uncertainty in our results is the unknown cosmic-ray composition .