neutrino-neutrino scattering. Sufficiently high flux or volume densities of neutrinos for any interactions to occur only exist in astrophysical systems. Even there, the difficulty is revealing (using neutrinos!) the signatures of such interactions. So far, only $\nu$SI interactions much stronger (in a sense explained below) than weak interactions have been constrained. Given the difficulty of probing $\nu$SI in the laboratory, it is therefore interesting to consider more model-independent probes, such as those from astrophysics and cosmology.

One direct probe of $\nu$SI uses astrophysical neutrinos as a beam and the Cosmic Neutrino Background (C$\nu$B) as a target. Kolb and Turner (hereafter KT87) [@Kolb:1987qy] utilized the detection of astrophysical neutrinos from SN 1987A. The agreement of the detected signal with the standard expectation of no neutrino scattering en route yields robust constraints on $\nu$SI. KT87 established a phenomenological approach by considering general interactions with mediator masses either much smaller or much larger than the interaction energy. Their constraints could be applied to many possible $\nu$SI models.

The first detection of astrophysical neutrinos since SN 1987A is the 37 events detected by IceCube [@Aartsen:2013bka; @Aartsen:2013jdh; @Aartsen:2014gkd]. One expects a steady stream of more events in the near future, so the precision will improve 