Alpher Bethe Gamow: Factor characterizing the decrease of density with time

[pianoplayer98]

Alpher Bethe Gamow: "Factor characterizing the decrease of density with time"

Directly from the article:

http://cache.artofproblemsolving.com/texer/images/c957650eca2c34559b5fd5931bd74440410d6ee3.png

where f(t) is a factor characterizing the decrease of the density with time.

What specifically is this f(t)? Is it just the density of the universe at time t? Where would this be available?

It seems to me that the Friedmann equation is a good place to derive this information, having both a term for density and time. However, I'm very new to cosmology.

 

[pianoplayer98]

By the way, I know that αβγ is not a valid model for non-H/He elements, but I'm just carrying out the calculations myself.