Although few atheistic philosophers take such a view, I do occasionally hear opponents of the Leibnizian cosmological argument (LCA) suggest that the universe may exist necessarily. I want to argue against this in a somewhat unconventional way. First of all, to repeat the standard version of the LCA:
1. Every existing thing has an explanation of its existence, either in the necessity of its own nature or in an external cause. (Premise, PSR)
2. If the universe has an explanation of its existence, that explanation is God. (Premise)
3. The universe exists. (Premise)
4. Hence, the universe has an explanation of its existence. (From 1 and 3)
5. Therefore, the explanation of the universe is God. (From 2 and 4)
Since the atheist in this instance rejects (2) by opting to conclude that the universe exists by a necessity of its own nature, we may argue the following:
6. Whatever exists by a necessity of its own nature is either omnipotent or not-omnipotent. (Premise, law of excluded middle)
7. Every not-omnipotent thing possibly has an external cause. (Premise)
8. Whatever is necessary cannot have an external cause. (Premise)
9. Therefore, whatever exists by a necessity of its own nature is omnipotent. (From 6 - 9)
10. The universe is not-omnipotent. (Premise)
11. Therefore, the universe does not exist by a necessity of its own nature. (From 9 and 10)
(6) is obviously true, so in confirmation of (9) we are left with (7) and (8). (7) is fairly benign, since even supposing that something not-omnipotent is uncaused, there is still a possible world in which it does have an external cause. (8), I think, is also indubitably true. If a necessary entity n1 were caused by another necessary entity n2, then n1 and n2 would have to be distinct entities. Yet, whatever is necessary must have an essence identical to its existence, for to exist non-essentially is to exist contingently. Since nothing can cause itself to exist, it follows that whatever is necessary is also self-existent and therefore cannot have an external cause.
(10) is true upon observation. There are many limitations inherent throughout the universe, so the universe cannot be omnipotent. Even if it were omnipotent, that wouldn't bode well for the atheist, since she presumably wants to deny that any existing thing is omnipotent. Of course, (11) follows logically from (9) and (10).