Outside the universe

[questionposter]

The universe can be finite and unbounded; analogous to the surface of a sphere which has finite area but no edge.

 

Doesn't have an "edge" but still has a measurable radius, and with the universe there should be a relatively finite radius before before space folds in on itself.

[StringJunky]

Doesn't have an "edge" but still has a measurable radius, and with the universe there should be a relatively finite radius before before space folds in on itself.

 

I understand that the Observable Universe has a radius but the whole universe, not necessarily so...if it has the topology of a Torus , for example, it doesn't have one. Even the example I first gave doesn't have one because only the surface should be considered.

[questionposter]

I understand that the Observable Universe has a radius but the whole universe, not necessarily so...if it has the topology of a Torus , for example, it doesn't have one. Even the example I first gave doesn't have one because only the surface should be considered.

 

But logically there would be a specific point at which three-dimensional space "loops around". I should be able to say "travel x distance from location A at angle theta before seeing location A and negative angle theta".

Or at the very least there should be a specific point at which you can be equidistant from the Earths on either side of the loop, and if I shot a photon and it kept going in a straight line, it should travel finite distance before hitting the back of my head.

Edited May 2, 2012 by questionposter

[robheus]

Theres no proof that the universe is infinitely vast in any direction.

 

If the universe is infinite, as I assume, it can not be proved. And it is no wonder that every measurement we can ever make will yield a finite value.

 

 

If by analogy we would for example take a straight line extending in both directions without end or begin, then any attempt we make to place two points on them, the distance between them yields a finite distance (measured in some unit). However, we are not allowed to induce from that finite result of any possible measurement taken on the line, that the line itself is infinite. The reason of that is that there can not be a finite measure as an upperbound to the measurements we can take. The proof of that is that we can always show, even for the supposed largest distance on the line, that we can place the points further apart and get a larger distance. This reasoning is similar to why there isn't a largest natural number, even when all natural numbers are finite and countable!

 

So, infinity on the line extending to both directions to infinity, only exists in the form of finite measurements on the line!

 

At least though it can be said that there is no boundary.

 

 

Theres no proof that the universe has no outside.

 

No, but this is by definition the case. There neither is a proof that 1 meter is 100 centimer, but that is just how we defined it.

 

[ in case you use universe in a different context, as an element in a collection of universes or the multiverse, then the same applies of course for the multiverse ]

Edited July 1, 2012 by robheus

[robheus]

I think you need to define more clearly what you mean with "parallel" universe and "outside the universe".

 

For example, the theory of inflation accomodates us to infer that:

1. There is more of the same universe as we can currently observe outside of our horizon

2. That there might be an infinite number of such inflating universe "bubbles" which can have distinct physical featueres and are seperated by domain walls.

 

In the interpretation of quantum physics the "many worlds" hypothesis suggests that any time an "observation" is made (collapse of the wave function) a new universe is born, with all the other possible outcomes.

 

In M theory (the 11-th dimensional theory, of which the 5 existing string theories are different aspects) there can be many branes, and each brane can have a different universe.

 

What these theories have in common is that at least in the topological sense they are connected in at least one point, although in M theory in a higher dimension.

 

Apart from that is the abstract possibility of a "parallel universe" in the sense a different universe but which does not have any common point in all of space time with ours. So there are no spacetime relations between any point in the spacetime of that universe and ours.

 

Such an universe should be rejected since there is no way this can make sense. It can be thought of in the philosophical sense as a "possible world". There is no way we can talk about it, since there is even in principle no way of how we can have any knowledge about it. It makes no sense to talk about it.

[StringJunky]

One thing I can't get my head around is: if the universe is isotropic, homogeneous and infinite, its mass must be infinite also...but apparently not. What gives?

[CCWilson]

What's hard to get our minds around is that when scientists say that the universe is bounded (has limits), what they mean is that spacetime is bounded, not that the three dimensional universe is bounded. If you just consider three dimensions, there would always be empty space outside any boundaries specified. Since we cannot really visualize more than our usual three spatial dimensions - we didn't evolve to need more than that - if we want to sort of visualize it, we have to replace one of the spatial dimensions with a time dimension. Then if you think of the surface of a sphere as spacetime, with the two dimensions we see there representing the three dimensions in our world, and nothing allowed outside that surface (just as in our world nothing is allowed our three dimensions), that's what they mean by "bounded". Theoretically I think that means that if we sent out a beam of light and it didn't hit an obstruction, it could eventually find its way back to our position. Whether there would still be anyone here to observe it, doubtful.

 

The concept of multiverses can have several forms. The one which is postulated to explain quantum weirdness is that every time any quantum event occurs, a new universe branches off. That's too weird to even contemplate. The one which sounds plausible is that our universe could have sprung from a mother universe, maybe from a black hole in that mother universe; think of our universe as an expanding sphere as described above, with its own version of time, completely cut off from all other universes - unless it collides with one. Also possible that new universes could spring from black holes in our universe, somehow, on and on, backward and forward. That could explain how our universe began, since it wouldn't have come from nothing, but of course it doesn't explain how the mother universe - the first one - originated. All speculation, and probably no way to confirm or deny, ever.

 

I'm not a physicist, and this stuff requires a lot of very hard thinking to even approach understanding it, so if there's a professional here who can comment, great.