# B Infinite amount of matter in the universe?

1. Dec 15, 2016

### lifeonmercury

If the universe is indeed flat and the cosmological principle holds true, does this mean that there is an infinite amount of space in the universe as well as an infinite amount of matter?

2. Dec 15, 2016

### Staff: Mentor

Yes.

Edit: Merged another thread into this one.

Last edited: Dec 15, 2016
3. Dec 15, 2016

### Demystifier

Yes it does.

4. Dec 15, 2016

### lifeonmercury

It appears that most scientists subscribe to the flat universe theory and believe in the cosmological principle. This means that our most up-to-date scientific findings suggest that it is more likely than not that space and matter just keep going on and on forever in all directions. That is quite a difficult one to comprehend.

5. Dec 15, 2016

### Staff: Mentor

There is no "flat universe theory". Our cosmological models allow a curvature, but measurements so far have been in agreement with a flat universe, with increasing precision over time. It does not have to be exactly flat, but at least very close to it. Experimentally that difference doesn't matter, so you can often just assume that the universe is flat. It does not mean that it has to be exactly flat.
It is expected, based on existing measurements.

6. Dec 15, 2016

### Demystifier

And a lot of extrapolation. In Ancient times, by similar reasoning people might think that the Earth is flat and infinite.

7. Dec 15, 2016

### lifeonmercury

Are there any scientific models that predict a finite, bounded universe (such as one with a center and edges/borders)? I recall reading about a theory claiming that the universe is a sphere surrounded by a black hole state at the edges.

8. Dec 15, 2016

### phinds

no
this is nonsense

9. Dec 15, 2016

### Bandersnatch

No - in this analogy, application of 'cosmological principle' to the surface of the Earth would prompt the ancient people to conclude that the Earth does not have an edge, and instead that the surface is a continuous plane. Either infinite or curved.
Providing they applied the same measurement analysis we do now, they could also measure the curvature to within some error margin.

The main difference between us and the analogy, though, is in the fact that the ancients could expand the base of their measurements to cover the whole surface. We, on the other hand, are restricted in how big a space can we use as a base for our measurements, since in our universe there exists an event horizon (mere <50% farther than the farthest objects we see now). Which is to say, while given enough effort the ancients could verify whether the application of the principle to Earth was valid globally, we won't be able to do the same with respect to the universe. Our choice to apply the principle to areas outside our potentially observable universe will be always based on mere parsimony, i.e. aesthetics.

This might help:
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html
It ties a bit to the argument about applicability of the cosmological principle mentioned above.

10. Dec 15, 2016

### Chronos

The ancients had good reason to believe earth was of finite size. The moon was one obvious clue and lunar eclipses sealed the deal by around 500 BC. The shadow of the earth on the moon was clearly curved suggesting it too is spherical. Eratosthenes is credited with deducing a fairly precise figure for earth's circumference [40,000 Km] around 240 BC. Modern man has also arrived at a fairly precise figure for the size of the observable universe. There are some obvious practical difficulties in measuring the size of the unobservable universe. We have already tried the ancient approach by measuring the curvature of space. All we have managed to figure out is it must be really big - which, of course, assumes the classical rules of geometry apply to the whole of the universe. For a more comprehensive discussion on ancient measurements of the size of earth, see http://www.metrum.org/measures/measurements.htm

11. Dec 15, 2016

### stoomart

Infinite matter makes as much sense as a singularity of infinite energy. I understand this is what the math predicts, but it is non-sensical to me; LQC seems to offer a reasonable explanation for the source and destination of matter/energy.

12. Dec 15, 2016

### phinds

Then you must of necessity believe that the universe cannot be infinite. You MIGHT be right but you can't prove it so that's just a personal opinion, not science.

EDIT: I assume you know by now that when it comes to cosmology (the very large) and Quantum Mechanics (the very small) human "intuition", "common sense", and so forth are less than worthless, they can actually get in the way of your understanding of reality.

13. Dec 15, 2016

### stoomart

Agreed, the same is true for both sides of the argument. I think threads discussing infinite time/space/energy are too philosophical in general. It's one thing to say "this is what the math predicts", and another to say "this is what must be": my understanding is the latter is a scarce luxury.
I approach all topics from the perspective that common sense is attainable, even in the absence of observable evidence. In this case, scientific observation predicts the universe had a beginning and common sense tells me it's finite.

14. Dec 15, 2016

### Chronos

We are unavoidably prisoners of human intuition. It is the bedrock of all intellectual pursuits. Intuition certainly leads us down dead end roads, where the error of our ways are exposed by paradoxes and infinities, but, that is the price that of admission to the frontiers of knowledge. That same intuition, however, warns us of the peril of forcing nature to comply with our personal preferences. Only by confronting these issues is progress achieved. The answers are not necessarily 'pretty', 'natural' or reflect any deeper, hidden order that satisfies any such expectations. As Freud might say, sometimes a cigar is just a cigar. You need not comprehend the nature of things to acknowledge their reality.

15. Dec 15, 2016

### phinds

I'm not aware of any such. Citations? Our currently accepted model of the universe, the Big Bang Theory, is agnostic on the subject of a creation event.

16. Dec 15, 2016

### stoomart

I'm not talking about a creation event, I'm referring to the big bang.

17. Dec 15, 2016

### Chalnoth

Sort of, but only if it is exactly flat and the cosmological principle is exactly true.

The former we cannot ever be sure of, because our measurement accuracy will always have some non-zero error.
The latter we definitely know is false locally, but works pretty well for large scales within the observable universe. The cosmological principle may break down on scales much larger than our cosmological horizon, but we have no way to know at present.

Basically, as others have stated, this means that the precise answer to your question is yes. The problem is that this requires making two unfounded assumptions that we have no reason to believe are true, so this is a trivial statement: yes, if you assume the universe is infinite, then it's infinite. It's true but content-free.

18. Dec 15, 2016

### nikkkom

Big Bang theory does not say that Universe definitely had a beginning. It merely says that there was a much denser, and much faster expanding state in the past. It is generally understood that before that, "something else" was happening. This "something else" can possibly extend infinitely far into the past. One class of such theories are eternal inflation theories.

19. Dec 15, 2016

### phinds

Which is exactly what I am talking about. See, the clue was when I said "Our currently accepted model of the universe, the Big Bang Theory"

20. Dec 15, 2016

### stoomart

Thanks nikkkom, this is similar to the idea I subscribe to: something along the lines of sequential universes or a multiverse, just not an infinite universe.