Thanks for so many responses but there has been a misunderstanding. I didn't ask about the creation of the universe (however it is an interesting subject by itself) but only about the everyday creation of energy that is needed according to the Big Bang Theory to compesate for the presumably lossless movement of photons in the universe.

As any object in the universe, the photons are subject to dynamical friction. Tiny in their case but still. In the Big Bang Theory the loss of energy to this friction is zero supposedly because of constant creation of this tiny amount of energy. As mathematicians investigating this problem say (eg. John Baez, possibly also Chris Hillman), this tiny amount of created energy comes from non conservation of momentum which is somehow converted into energy (John Baez says that only 4-momentum is conserved and not energy and momentum separately).

This was my question: How astronomers explain such conversion of momentum into energy that is required by the Big Bang Theory according to matematicians? I may add that matematicians can calculate that if this loss of energy to dynamical friction wouldn't be compensated somehow by the loss of momentum than there wouldn't be any redshift left to account for the expansion of the universe and astronomers insist that the universe is expanding. That's why the BB Theory necessarily needs this creation of energy (presumably at the expense of momentum, since so called "momenrgy" (J. A. Wheeler's term) has to be conserved otherwise the whole relativity collapses.

So my question is how astronomers, who insist that the universe is expanding (and such great observers can't be wrong -- according to J. A. Wheeler), explain the necessary mechanism of creation of energy needed to compensate for the dynamical friction of photons?

Sorry for not explaining the question from the beginning, but I thought that it is a known problem in astronomy that the astronomers deal every day with. So I just asked about the BB and the conservation of energy since I thought it is obvious what I'm asking about.