The Arabic word used for expanding is not a verb but an active participle.
So what does this mean? It means it is identifying an individual that either has done an act (perfect verb) or is in the process of performing an act (imperfect verb). Because 51.47 only has the expand/expanse word as an active participle and not a verb we cannot check if it is referring to a perfect verb or an imperfect verb. Is there anywhere else in the Quran that might give us a clue? Yes.
It just so happens that in the very next verse the Quran continues with the description of the creation of the heavens and the Earth.
Pickthall: And the earth have We laid out, how gracious is the Spreader
This is useful because the action "laid out" is a verb and can therefore be identified as a perfect or imperfect verb. It is also followed by an active participle "the spreader". In this case (51.48) the verb "laid out" is a perfect verb, it has been completed. Allah cannot logically be the one who IS doing an action that has finished, but Allah can logically be the one who IS the actor responsible an action that has finished.
I think this rests the case on the claim that the Quran 100% says the universe is in the process of expansion. It could mean either that it is expanding, or that Allah once made it big. When a statement is true no matter what reality is then it cannot be a miracle by any stretch of the imagination.
Conclusion: 51.47 is a verse that could have been written by a mere human. Look up on a clear night, it looks really big, and the Quran says nothing more than that.