Does the universe necessarily have an end?

Question

While we understand the universe to have had a beginning in the big bang theory, if gravity is quantum in nature yet still has an infinite range then surely once the distance between galaxies becomes great enough, pair production will begin in between those galaxies as a result of the gravitational potential energy being converted to stable particles? This is assuming that the expansion of the universe eventually reaches a steady rate and that dark energy is infinite.

Am I missing something here? If so, what?

I propose such a fate to the universe be named the "steady-state fate" if it is worthy of serious consideration.

Answer 1

While we understand the universe to have had a beginning in the big bang theory

Well, sort of.

1. We observe that on cosmological scales, the universe is expanding.
2. With certain assumptions, this expansion can be modeled and interpreted through the lens of general relativity.
3. If we build a model based on current observational data and extrapolate backward, then we reach a singularity at $\approx 13.8$ billion years ago where the matter and energy density at every point in the universe would be infinite.
4. There are excellent reasons to believe that this isn't the full story. In particular, the large-scale uniformity of the cosmic microwave background cannot be explained without further hypotheses, the most popular of which is the existence of an inflationary epoch which preceded the expansion mentioned above.

The mechanism behind such an inflation period is not yet understood, though there are a number of plausible models. It's important to note that a viable quantum theory of gravity is crucial for understanding the matter and energy dynamics under such extreme conditions, and the lack thereof makes it very difficult to speak concretely about the nature of the universe at such early times.

if gravity is quantum in nature yet still has an infinite range then surely once the distance between galaxies becomes great enough, pair production will begin in between those galaxies as a result of the gravitational potential energy being converted to stable particles?

Though the development of a quantum theory of gravity is one of the great open problems in physics, it's not clear to me why you'd expect pair production (of what?) once galaxies are effectively isolated from one another. In e.g. an extreme electric field, the production of an electron-positron pair is energetically favorable due to the subsequent reduction in electric field strength. In your example you consider the limit of weak fields (the opposite limit). Even in the strong-field limit (and ignoring some important issues surrounding the fact that gravitational energy cannot be localized in a meaningful way), in what sense would pair production be favorable given that - unlike electromagnetic interactions - gravitational interactions are fundamentally attractive?

Why does the universe necessarily have an end?

It doesn't. See the heat death of the universe for the fate which (arguably) appears most compatible with current observations and theoretical predictions, and the ultimate fate of the universe wiki article for alternatives and additional details.

Answer 2

Here are my two cents on this. Firstly, the beginning of the universe is more or less predicted "properly", in that we know what conditions a spacetime must satisfy in order to contain a singularity, following Hawking-Penrose. The question of the fate of the universe is more or less a very broad question, but if you are asking if the universe has an end similar to the notion of singularity at the beginning, this would depend upon the observed average density vs the critical density.

Secondly, I am not sure what the notion of quantum gravity has to do with particle production simply because two galaxies are far apart, or what the notion of dark energy has to do with this at all; the question at that is rather vague and I do not quite understand it.

Lastly, there are some views that have a different notion of an "end", such as Penrose's conformal cyclic cosmologies, which is linked to the notion of the Weyl curvature hypothesis, which at that has several issues. For that matter, I am not aware of a gravitational entropy description using the Weyl invariant that has given insight into the validity of CCC. Similarly, there are other notions such as a cosmology expanding for infinite time; however, I don't think this qualifies as an "end", since I think your question is particularly about the dynamics of matter fields as a cosmology approaches a future singularity. I may be missing something, if I am, do point it out.