def SFR(z):
   """ returns the cosmic star formation rate in M_\odot/yr/Mpc^3 following Strolger+15"""
   A = 0.015 #M_\odot/yr/Mpc^3
   B = 1.5
   C = 5.0
   D = 6.1
   return A*(1+z)**C/( ((1+z)/B)**D + 1 )