def four_pi_G_over_c_squared(dunits='Mpc'):
    """
    4*pi*G/c^2 Dl * Dls/Ds has units of m^2/kg

    we want units of pc^2/kg and Dl in units specified
    by the dunits keyword
    """
    from math import pi as PI
    C = 299792458.0
    GNEWTON = 6.67428e-11
    KG_PER_SUN = 1.98892e+30
    M_PER_PARSEC = 3.08568025e+16
    fourpiGoverc2 = 4.0 * PI * GNEWTON / C ** 2
    if dunits == 'meters':
        return fourpiGoverc2
    else:
        fourpiGoverc2 *= KG_PER_SUN / M_PER_PARSEC
        if dunits == 'kpc':
            return fourpiGoverc2 * 1000.0
        elif dunits == 'Mpc':
            return fourpiGoverc2 * 1000000.0
        elif dunits == 'Gpc':
            return fourpiGoverc2 * 1000000000.0
        else:
            raise ValueError("Don't support dunits='%s'" % dunits)
