The Friedmann derivation is indeed introductory physics today. I am happy to see if someone have derived it also with relativistic mass as I have done here, second derivation. Einstein, Wheeler, Adler etc. where or are all allergic to relativistic mass, this is well documented in literature. Lorentz got the relativistic mass correct already in 1904. Einstein could just have divided his relativistic energy by c^2, but instead derived two relativistic mass equations in end of his paper, these solutions are today known to be wrong, but hardly talked about as Einstein simply abandoned relativistic mass concept.
They can not get their theory to match all the Planck units same time (for what they interpret as micro black holes), and they get strange interpretations at Hubble scale...
My just updated draft here
The Collision Time of the Observable Universe is 13.8 Billion Years per Planck time: A New Understanding of the Cosmos based on Collision Space-Time
basic math, but a small difference in foundation can give massive differences in interpretations...
the paper still need some re-writing to clarify thoughts, but it is basically there already. My theory for example matches all Planck units at the same time when linked to escape velocity c, unlike GR.
By re-introducing relativistic mass we also get perfect supernova fit without need for any dark energy, yes it is that simple. This naturally support my universe equation where no cosmological constants needed except the observed Hubble constant, which is nothing more than one divided by the collision-time inside a sphere with escape velocity c.
In one sweep (re-introducing Lorentz 1904 relativistic mass) one get rid of curvature of space, and no need for dark energy. Tested against data already!
No need for dark energy when re-intorducing relativistic mass
Einstein was right, his cosmological constant was a blunder, he should instead have picked up on Lorentz relativistic mass, and simply divided his relativistic energy by c^2 and said it is fully compatible. He followed on a different path...that has lead us to dark energy that never have been detected, but makes up most of today's hypotetical universe. I am not buying into it, even if it makes me unpopular.