docs: def epsilon_crit(self):
          """
          returns the critical projected mass density in units of M_sun/Mpc^2 (physical units)
          """
          const_SI = const.c**2 / (4*np.pi * const.G)  #c^2/(4*pi*G) in units of [kg/m]
          conversion = const.Mpc / const.M_sun  # converts [kg/m] to [M_sun/Mpc]
          pre_const = const_SI*conversion   #c^2/(4*pi*G) in units of [M_sun/Mpc]
          Epsilon_Crit = self.D_s/(self.D_d*self.D_ds) * pre_const #[M_sun/Mpc^2]
          return Epsilon_Crit