def conformal_factor(r)
      tmp = sqrt(@k)
      if @Omega > 1.0
        sin(tmp*r)/tmp
      elsif @Omega < 1.0
        sinh(tmp*r)/tmp
      else
        r
      end
    end
def conformal_factor(r)
      tmp = sqrt(@k)
      if @Omega > 1.0
        sin(tmp*r)/tmp
      elsif @Omega < 1.0
        sinh(tmp*r)/tmp
      else
        r
      end
    end
r: comoving distance