The universe has an edge.
In order for every action to have an opposite reaction, the universe must have an edge—it must be finite. For the energy of the action must ricochet off of some boundary, in order for the returning reaction to exist.
What does this mean for the concept of infinity?
In a singular instant, infinity does not exist as a discrete entity. Of course, why would it? Infinity is a function of spacetime, rather than the separation of time and space.
In this singular moment, the universe is not infinite, nor is anything else. The universe is not absolutely infinite.
However, the universe is relatively infinite.
How is that? That can be the case in a universe that is expanding.
You see, when we say a thing is infinite, we cannot grasp its extent concretely. We can keep track of where it goes for only so long. How, then, can we know that it is truly infinite? For all we know, it stops beyond some point we simply have not ventured to. It follows, then, that infinity may well be no more than an abstraction.
Take this as a definition of infinity: the edge. If the universe is ever-expanding, we can never reach its edge. That edge exists, but we do not see it. What we experience, then, is that the universe is infinite. It is like chasing the horizon: no matter how far we walk, we are always an infinite distance from the horizon.
It is infinite expansion that carries us into forever.