Hi,
I am not qualified to answer your questions, but have added a couple of comments below for what they are worth. However, I would also be interested in any additional comments as to current thinking in mainstream cosmology
mrspeedybob said:
Where does this energy come from?
Is it possible that the energy source which gave rise to the big bang is still adding energy to our universe?
I guess this might depend on whether the universe is considered to be a closed or open energy system. If it is a closed system, then I assume you need to consider whether the universe started off with some a fixed amount of energy or whether it had a net zero amount energy, i.e. positive + negative = zero. Alternatively, quantum theory may point to the possibility of a reservoir of vacuum energy that is not normally taken into account, but can be tapped. However, if the universe were part of some larger system, i.e. open, then it would seem possible that our
‘local’ universe might acquire or lose energy to this larger system.
The assumed inflation process is said to have expanded the universe from the Planck scale to ~10-20cm in a fraction of the first second of existence and, as far as I understand it, what drove this process then went through some sort of phase transition. Based on the subsequent LCDM energy-density model, dark energy is assumed to remain constant throughout the subsequent expansion of the universe, but was initially negligible, i.e. ~0%, in comparison to radiation and matter (both normal and dark). However, the constancy of the energy density of dark energy means that it is now assumed to account for ~73% of the total critical energy density. Again, as far as I understand the details, the LCDM model would then imply a 2.7 fold increase in the total energy density of the universe over the last 13.7 billion years.
Apologises if these comments are tangential to the questions you were trying to raise.