I'm not sure if this is heresy or not but an eternally existing universe doesn't seem to contradict orthodox doctrine. It is my understanding that Aquinas thought that an eternally existing creation is possible, but that at least there was a temporal beginning since this is a revealed doctrine.
Creation from nothing isn't a change in some primordial substance of "nothingness", there is no material cause for the origin of the cosmos, rather it is the imparting of existence to things which cannot exist by their own power, thus being created and already having been created are simultaneous and not actually distinct. What matters is not temporal beginning, but ontological dependence and participation. Things are simply brought from non-existence to existence, and this is a continuous act on the part of God.
Perhaps the big bang vindicates this, as it is perfectly reasonable to interpret this as giving us a temporal and spatial beginning to the universe, but that perhaps "before" this point there was something existing quite distinct from the universe as we know it, quantum in nature, maybe having come from a previous universe, and which was neither temporal nor spatial as we now know it to be.
I don't know though maybe this is a heretical take on Aquinas though. Apologies if it is. It's just my musings. I want to see what you guys think. However if this is a correct way of seeing Aquinas's doctrine then it is weirdly vindicated by modern science.