# Confusion with conversion for the Hubble Constant

Hello!

I want to get my data in terms of h70 -1

Sometimes the estimates are in terms of h50 or h70, and I know how to convert those just fine.

However, sometimes they give H0 = 100 h Mpc-1 kms-1 instead.....

My question is, what is this in terms of h?

I know that the relation is

h = H0 / 100 kms-1 Mpc-1

So, would I be right in thinking that it is as straight forward as h=H0 for this specific case or am I being silly

Thankyou!! :)

cepheid
Staff Emeritus
Gold Member
So, would I be right in thinking that it is as straight forward as h=H0 for this specific case Absolutely not, because as you yourself have noted, h is actually equal to:

h = H0 / 100 kms-1 Mpc-1

Hah. I totally figured it out.

I was confused originally because I did use that equation, but got h=h

I thought h=70, rather than H0 = 70

:(

So is 'h' basically just a fraction then? I understand now.

Gah. Thanks

cepheid
Staff Emeritus
Gold Member
Hah. I totally figured it out.

I was confused originally because I did use that equation, but got h=h
Which is true! But not that informative I guess.

I thought h=70, rather than H0 = 70
Right, so that's where your misconception was. That value of H0 corresponds to h = 0.7

So is 'h' basically just a fraction then? I understand now.
Exactly, you got it now! The parameter h is a dimensionless version of the Hubble constant. It is H0 normalized to a reference value of 100 km s-1 Mpc-1. So h is a fraction...it tells you the ratio of H0 to that reference value.