Inconsistent units for luminosity_distance

[jeremykubica]

The documentation says that the units for luminosity_distance are Mpc (and the values are consistent with Mpc), but the unit annotation is in km / s.

[ColmTalbot]

Hi @jeremykubica thanks for reporting this, I've seen some issues with the units. I suspect I know how this could happen, but please can you provide a reproducible example so that I can track down the issue.

[jeremykubica]

Of course. Sorry for not including that before.

Here is the code I was using in a notebook:

from wcosmo import wcosmo

# Use the Planck 2018 cosmological parameters
cosmo_H0=67.66
cosmo_Om0=0.30966

wcosmo.luminosity_distance(0.1, cosmo_H0, cosmo_Om0)

The displayed output is 475.87628 km / s. I think it should be 475.87628 Mpc.

[ColmTalbot]

The issue here is that the value of H0 you are passing has implicit units of km/s/Mpc, this means that your answer is off by that factor. I always assume that values passed without units are intended to be dimensionless. The only alternatives I can think of are:

• provide default units. I'm not a fan of this as it opens up the possibility of getting stealth errors where people think they are working with different units.
• raise an error if it something I think should have non-trivial units doesn't have units. This is probably a better option, but I'd have to think about the best way to implement it.

[jeremykubica]

Do I need to scale the value to get Mpc? Or is it just the units? The answer itself seems to be correct (when compared with the same computation in astropy) if interpreted as Mpc. I can just use .value in that case.