Friedmann
Assuming a Universe that is homogeneous and isotropic, Alexandr Friedmann found exact solutions to the equations of general relativity. His solutions start at a point singularity and explosively expand, a process now known as the Big Bang. The closed solution expands to a maximum size and then contracts, accelerating back to a singularity. The open solutions continue to expand forever but at decreasing rates.One characteristic of general relativity and specifically Friedmann geometry is that energy is not always conserved, a result of replacing gravity with curved geometry. This has the interesting result that in Friedmann universes energies of both particles and photons change with time. When universes expand, energies decrease. Energies increase when universes contract.
The Big Bang and the dynamic universes of Friedmann solutions have found wide acceptance and explain many observations. Friedmann provides immediate explanations for the observed low temperature cosmic microwave background and for Hubble redshift, the changes in the characteristic colors of light from elements in distant regions of space.
Closed Friedmann geometry was used by Schrödinger with relativistic quantum mechanics to show that quantum wave functions change in time exactly as particle and photon momenta do, a result critical to the correct interpretation of Hubble redshift.
Alexandr Friedmann