early universe [@Notzold:1987ik; @Hannestad:1995rs; @Dolgov:2002ab; @Abazajian:2002qx; @Mangano:2005cc] and in core-collapse supernovae [@Qian:1994wh; @Buras:2002wt; @Duan:2005cp; @Hannestad:2006nj; @Raffelt:2007yz; @Dasgupta:2009mg; @Duan:2010bg; @Friedland:2010sc; @Cherry:2012zw; @Raffelt:2013rqa], the conditions are so extreme that even standard model scale neutrino self-scattering and their self-induced matter mixing potential are important. In the early universe, $\nu$SI could cause neutrinos to annihilate or decay into light particles, modifying the expansion history [@Masso:1994ww; @Beacom:2004yd; @Hannestad:2005ex; @Bell:2005dr; @Ahlgren:2013wba]; change the free-streaming property of neutrinos during photon decoupling [@Chacko:2003dt; @Hannestad:2004qu; @Friedland:2007vv; @Cyr-Racine:2013jua; @Archidiacono:2013dua]; or induce new mixing effects [@Hannestad:2013ana; @Dasgupta:2013zpn]. [ In supernovae, the effect of elastic scattering on neutrino escape time [@Manohar:1987ec] is irrelevant [@Dicus:1988jh],]{} but $\nu$SI could cause a phase transition [@Davoudiasl:2005fd; @Sher:2011mx], change the cooling process [@Choi:1989hi; @Kachelriess:2000qc; @Farzan:2002wx; @Fayet:2006sa; @Zhou:2011rc], or induce non-standard flavor mixing [@Blennow:2008er].

There are several specific limits. In the early universe, if the $\nu$SI mediator is not too massive, it could be in thermal equilibrium, changing the number of relativistic degrees of freedom [@Masso:1994ww; @Ahlgren:2013wba], which can be tested through Big Bang Nucleosynthesis (BBN). We show the “maximally conservative" case from Ref. [@Ahlgren:2013wba], which assumed vector boson mediators. The BBN limits extend down to $g\sim 10^{-8}$. The presence of $\nu$SI can also change the free-streaming property 