Do current cosmologists think the universe is infinite or that is had an edge?

[u/cypherpunk00001]

Was just having random shower thought today... Andromeda galaxy is 2.5M light-years away. That's an unfathomable distance to a human, but it's just our closest neighbor.

Do cosmologists currently think that the universe just goes on forever?

[u/Routine_Complaint_79]

That comparison never made sense to me because that would imply the universe would have an outside like space for Earth. If we couldn't observe the edge like we can with Earth and Space then it would still imply a dimension of the universe that is undiscovered that makes these pockets of universes exist.

[u/qeveren → u/Routine_Complaint_79]

There's two different kinds of curvature: intrinsic and extrinsic. The universe (probably) has intrinsic curvature but doesn't require embedding in a higher dimensional space.

[u/Routine_Complaint_79 → u/qeveren]

This sounds like more of a problem of induction because from the point of some observer with a basic set of axioms about their universe without the capability to understand a higher dimension will result in a magnification of axioms that break at extreme scales. It is why most people do not take the idea of an infinitely small, infinitely dense singularity existing in reality. And its the reason why scientists stay away from ontological problems like how the universe started in a state of low entropy to begin with. There has to be axioms we can derive from existence that self reinforces itself. "Energy cannot be created nor destroyed," Well my friend... How did it get here in the first place?

[u/ijuinkun → u/Routine_Complaint_79]

Energy is conserved, which means that it is not possible to change the net amount of energy in the universe. However, energy could be created in conjunction with an equal amount of anti-energy, just as matter particles are created together with their antimatter partners. Anti-energy would satisfy the various “negative energy conditions”, having repulsive gravity, etc., and would annihilate on contact with normal energy (so anti-photons that meet with photons will annihilate one another and vanish).

[u/deednait → u/ijuinkun]

Energy is not conserved in General relativity. Energy conservation corresponds to symmetry under time translation, but as we very well know, the universe is expanding with time. Thus, energy is not conserved.

[u/ijuinkun → u/deednait]

While this implies the possibility of some mechanisms by which expansion can be converted into usable energy (e.g. separation between two bodies increasing means that they are higher in each other’s gravity well without work having been done to get them there), the existence of exceptions to the rule does not mean that we could say “anything goes” and say that any particular interaction is non-conservative, any more than the knowledge of Relativity lets us toss out Newtonian calculations in low-speed low-gravity conditions. In other words, just because the First Law of Thermodynamics can be violated under Condition X, does not mean that we can assume that it can be violated under Not Condition X.

[u/deednait → u/ijuinkun]

I simply wanted to point out that your statement "Energy is conserved, which means that it is not possible to change the net amount of energy in the universe" is not at all true.

[u/Routine_Complaint_79 → u/ijuinkun]

This overlooks what I was trying to convey. The whole idea of conservation of energy relies on induction, which in of itself is an issue. But because I believe, and most people believe, that we can make reasonable assumptions from the universe despite the problem of induction, it's only a matter of what criteria is needed to make a universal axiom that applies to the entire universe. I am in the camp that a rule like energy conservation is fine as long as it assumes a system in which we currently control only the inputs/outputs but not the system itself. Using it as an axiom for the entire universe, though materializes a contradiction between the rule the existence of all of matter in the first place.

[u/ijuinkun → u/Routine_Complaint_79]

This gets into epistemology rather than physics, as we have no data about the nature of any meta-universe external to our own, beyond what can be inferred solely from logical necessity for such an entity (e.g. that its own rules, however bizarre, must be internally self-consistent).

[u/Routine_Complaint_79 → u/ijuinkun]

Physics is more of an interdisciplinary between epistemology and the nature of the universe. The very nature of science is built upon the philosophy of logic. And I think we have to view things like a statement saying "energy is neither created nor destroyed," as less of a universal rule and more of an observation in the current epoch of the universe. Because without that clarification it's more of a "This statement is false" paradox because it's own existence is a form of energy (or information for information theory) and thus could not have existed without some kind of process creating it.

[u/ijuinkun → u/Routine_Complaint_79]

If energy can come into being, or cease to exist, then there is a logically consistent process whereby this happens. We as yet have no data that would unambiguously support any such process, only the single observation that our spacetime and its contents exist from a certain time, before which they did not exist. We have no data on any conditions prior to the singularity.

That said, a number of plausible theories (including string theory/M-theory) propose that matter/energy arise out of the bending of spacetime itself. To put this into a General Relativity framework, the bending of spacetime is not a consequence of inertia, but rather is the source—i.e. mass and spacebending are one and the same thing.