Recently atheists such as Nicholas Everitt and Richard Carrier have appealed to the immense size and age of the Universe as features we wouldn't expect if the latter were the creation of the Christian God. There are various ways to flesh out this argument, but Carrier captures the gist of it in his essay Why I Am Not a Christian:
...the Christian hypothesis actually predicts a completely different universe than the one we find ourselves in. For a loving God who wanted to create a universe solely to provide a home for human beings, and to bring his plan of salvation to fruition, would never have invented this universe, but something quite different. But if there is no God, then the universe we actually observe is exactly the sort of universe we would expect to observe. In other words, if there is no God then this universe is the only kind of universe we would ever find ourselves in, the only kind that could ever produce intelligent life without any supernatural cause or plan. Hence naturalist atheism predicts exactly the kind of universe we observe, while the Christian theory predicts almost none of the features of our universe. Indeed, the Christian theory predicts the universe should instead have features that in fact it doesn't, and should lack features that in fact it has.
Carrier goes on:
After all, what need does an intelligent engineer have of billions of years and trillions of galaxies filled with billions of stars each? That tremendous waste is only needed if life had to arise by natural accident. It would have no plausible purpose in the Christian God's plan. You cannot predict from "the Christian God created the world" that "the world" would be trillions of galaxies large and billions of years old before it finally stumbled on one rare occasion of life. But we can predict exactly that from "no God created this world." Therefore, the facts confirm atheism rather than theism. Obviously, a Christian can invent all manner of additional "ad hoc" theories to explain "why" his God would go to all the trouble of designing the universe to look exactly like we would expect it to look if God did not exist. But these "ad hoc" excuses are themselves pure concoctions of the imagination--until the Christian can prove these additional theories are true, from independent evidence, there is no reason to believe them, and hence no reason to believe the Christian theory.
First of all I should note that Carrier's demand for independent evidence of possible reasons why God created as He did is unreasonable. After all, even scientists introduce ad hoc explanations all the time which cannot be independently verified (at least not at first), but which 'save the appearances' when some empirical evidence appears to complicate the theory. All that is required of the Christian is that he produce an explanation of the apparent discrepancy which is strongly indicated by Christian theism or at least consistent with it.
I have highlighted what seems to be the key assumption in bold: Carrier is assuming that God's sole purpose in creating this Universe is to create a home for human beings and bring his plan of salvation to fruition. I think there is plenty of evidence from the Bible that this is not the case. Certainly Earth has a very special place in God's plan, perhaps the most important place. But it does not follow from this that Earth is the sole focus of God's plans and intentions, or even that the Earth's only purpose is to sustain human beings. If that were the case, what was the point of creating those sea monsters, for example, which serve no useful purpose for human beings either as food or as shelter? In fact, in the book of Job God uses those creatures to put Job in his place: there are purposes to creation which human beings cannot fathom, and the Earth does not exist solely to sustain human beings.
But even assuming that the main purpose of the Universe is to produce human beings and that the salvation drama on Earth is central to God's intentions, there may be a very good reason for making the Universe as big and as old as it is. The key lies in remembering that God intends for nonhuman creation to teach humanity important things about Himself. Paul notes that we learn of God's eternal power and divine nature from the things that have been made (Romans 1:20). But just how powerful and how divine is God? If God really is as 'awesome' as Christians say He is, that should be evident somehow in His creation. Now if the Universe were a tiny little dome a few thousand light years in diameter, with a cozy Earth at the center and its boundaries clearly evident, how would that reflect on God's power? A Universe of the scale we'd be comfortable with would imply a rather safe, comprehensible, limited Deity. But what better way for God to teach us that He is fathomless, unthinkable Power than through the awesome Universe which modern science reveals to us?
“Yet this seeming dialogue also is the end and final cause for which He spreads out Time so long and Heaven so deep; lest if we never met the dark, and the road that leads nowhither, and the question to which no answer is imaginable, we should have in our minds no likeness of the Abyss of the Father, into which if a creature drop down his thoughts for ever he shall hear no echo return to him. Blessed, blessed, blessed be He!"