"The universe did not come into existence, nor will it cease to exist. And this is not just a matter of empirical fact: there is nothing we could intelligibly describe as the universe's coming into existence or its ceasing to exist. Beginnings and endings join causation in being concepts which, while having innumerable instances within our world, resist extrapolation to the universe itself.
So, if the universe cannot have come into existence and cannot cease to exist, is it temporally infinite in both directions?
[W]hile the notion of beginning to exist is suspect in application to the universe as a whole, we have an allowable approximation to this notion in terms of the universe's having been in existence for only a finite time. This is not: there was a time at which the universe did not exist; the universe will have existed at all times. Nor will there be a future time when it does not exist. But, for all that, there could be a finite limit to its duration to date as well as to its future history.
Similar caveats apply when speaking of an initial event, as the Big Bang may be thought to provide. Consider the kinds of physical process required to make sense of temporal notions, and in particular that of an instant. An instant, a point of time, can be annexed to the end-point of a change, as with starting or ceasing to move. However, if there is no preceding state of rest, there is no starting to move, so no possibility of marking a point of time by reference to such a happening. With respect to an initial physical happening, we cannot invoke the model of an event to which an instant of time might be assigned which is in any sense a temporal boundary, a point of division between the start of the event and an eventless period which preceded. We must always remain temporally within the universe, never somehow break the barrier and find ourselves on the other side, but if there were an initial instant there would be another side—just as, for a body to have a surface requires there to be a contiguous space. None of this is to deny anything involved in a Big Bang cosmology. It is just that any talk about the universe as coming into existence is to be replaced by talk of its finite duration—a matter of a more apposite redescription. Big Bang or no Big Bang, there has never been a time when there was nothing, and our conception of the Big Bang has to be accommodated to that consideration.
The notion of a finite duration has been offered as an approximation to that of a beginning of existence for the universe, but someone who wished to speak of the universe as having begun to exist might reasonably protest that they had not meant anything more than what is captured by the former: to say that the universe came into existence so many years ago is simply to say that it is so many years old. There need be no implication that some event took place at a first moment of time. What is important is that we should avoid conceptions of a temporal boundary that carry over conditions applicable only to happenings within the universe; likewise with the notion of the universe as ceasing to exist. It is not as if, were this to happen, there would then be nothing there. States, more generally, may be handled in this way. If a body has at no time been stationary, then it will have been in motion at every moment of its existence, in which case we cannot say that it began to move, but we could still, it would seem, speak of it as having been in motion for only a finite time. What is important is that a beginning, whether of change or of existence, rather than a merely finite duration, appears to be needed if we are to have an event, a happening in time, which raises a question of causation."
(Rundle, Bede. Why there is Something rather than Nothing. Oxford: Oxford University Press, 2004. pp. 122-4)
"We may philosophize well or ill, but we must philosophize." – Wilfrid Sellars