The Arabic word being translated to 'expander' here is musi. The verse in question uses the plural musi'un (expanders) instead of musi, but that's irrelevant.
Anyway.. the word musi comes from the root word awsa'a. This word is used in many forms in the Quran over 30 times and it has a range of meanings, but the verse quoted by OP is the only verse in the whole book where translators have suspiciously decided that it means 'expand'. Other meanings include:
Before Hubble's discovery, the majority opinion by Quranic Scholars was that the verse either means Allah is all-powerful, all-encompassing, or that he made the heavens vast. You will not find a single classical tafsir where the author says it could mean a continuous expansion of the heavens.
Of course, that's not the only issue with this verse. You will notice that I said 'continuous expansion' was not mentioned in the past. I was specific because there are indeed classical tafsirs that say the word could be translated to 'expand'. For example in Tafsir al-Zamakhshari (died 12th century AD), he says that one of the many translations of this word is 'expand'. However, the verse is so vague that not only is the meaning of musi ambiguous, but so is the tense of the sentence!
So even if you could somehow prove that the verse was talking about an expansion, you would additionally have to show that the verse is in present tense. Musi isn't even a verb, it's an active participle noun. Let's look at Sahih's translation again:
"And the heaven We constructed with strength, and indeed, We are [its] expander."
Imagine I said this to you:
"I built this house, and indeed I am the painter"
Do I mean that I built this house and I already painted it? Or does it mean I will paint it soon? Or maybe it means I'm painting it right now? You won't be able to tell unless I give more context.
I tend to lean towards a past tense interpretation. The verse could be saying "we created the heavens and extended them wide". If you look at the next verse, it says: "And the earth We have spread out, how excellent is the spreader!" So in OP's verse, Allah calls himself the expander, and in the next verse he calls himself the spreader. If you want to be consistent, you would have to interpret the next verse as Allah saying that he is still spreading the earth. But no translator claims that. It is a consensus that it refers to a spreading that has already been done.