Infinite Universe = Impossible

[Darren9]

It's not expanding into anything.

How do you know what is or isn't outside of the universe to make what seems like a definite statement?

[CjStaal]

ITT people who took a philosophy class vs STEM majors.

[Technical Ben]

It's not expanding into anything.

Is the expansion infinite though? If so, it's a matter of relative perspective.

(relative comic, though we tend to be less linear and more circular as people...)

http://xkcd.com/435/

The robot would detect it if the extra dimension influences the laws of its 2D world.

Let's assume the robot stands in a valley before a mountain. We also assume the valley is flat. At this moment height is still an unknown dimension for the robot. Its vision is limited to length and width so it would only detect a more or less irregular shaped plane it is in.

Now it moves to the edge of the plane (= moves to the border of mountain and valley). At the exact edge the robot would rotate in an additional space dimension as it is now trying to climb the mountain.

-> The robot now 'sees' that the irregular shaped plane changes.

-> The robot also experiences a kind of drag it never felt like that before. (In the 3D world gravity tries to pull the robot down the mountain.)

-> Other stuff influences the movement and perception, example falling rocks.

If the AI in the robot is smart enough it can now conclude there's something more to the world it knows about. And if it does an Einstein it'll discover that there's a height dimension.

That is a 2d robot existing in a 3d universe. What if the robot is 2d in a 2d universe. The possibility of 3 dimensions still exists (we know it does, not just as a possibility but an actuality) but can it physically detect the 3rd dimension? It could mathematically theorise the existence of one. I'm not sure it could "detect" it from within, it requires the 3rd dimension to intersect or interact with it. As in your example of a mountain, right?

IIRC a 2d universe can exist (but there are some limitations, I fail to recall and need time to find the wiki page on it...), thus so can our 2d robot. If our robot is in a 3d universe (say computer simulated, or mechanically simulated with lego ) then can it say if a third dimension exists? Can it ever detect something "outside" it's universe? It's universe would exist entirely consistent and internally valid. However other universes could exist outside.

A 3d robot/mechanism could interact with the 2d one, but I do not see a necessity of the 2d to be able to interact with the 3rd dimension.

Interestingly, the 2d universe can be infinite, and then the 3rd dimension can be infinite also. There are of cause some infinities greater than others.

Edited February 2, 2015 by Technical Ben

[K^2]

I understand that there's nothing there, but it's still expanding into...empty "space".

No, it does not. It does not expand like the balloon that you inflate. It expands like a surface of the balloon that you inflate. Except, it's not an embedded object. All that exists is that surface whose area is getting larger.

[Poynting]

The universe as far as we can tell is infinite in spacial dimensions. It had a start at the big bang. Measurements from standard brightnest supernovae in other galaxies can be used to measure the rate at which the universe is expanding. The measured rate of expansion determines that it is expanding faster that the gravitation of everything in the universe can slow it down (Its travelling faster than escape velocity, like a rocket leaving kerbin's SOI in KSP). Therefore there is no end to the universe it will last forever (or if you understand general relitivity it is not a bound universe, it has a saddle geometry). However saying it will last forever, you have to realise as the universe expands everything flys apart untill the universe pretty much appears empty (You couldn't reach the next object closest to you in your lifetime, even traveling at the speed of light). So in any useful sense it does end for us.

Now for the expanding problem...

First you have to comprehend what we mean by infinity and expansion. Its not easy by any means as this kind of expansion is like nothing you have witnessed.

First of all lets imagine the universe is an infinite sheet of rubber (2D for simplicity). And by infinite we really mean literally goes on forever.

Now imagien stretching a small part of this rubber sheet, pulling it inwards towards you. Its possible right? the rubber compresses near you and stretches further out? Gradually that stretching reduces to basically nothing far out from you. Now imagine the opposite, pushing out and stretching the rubber near you. It compresses further out, until eventually really far out you can't tell that the rubber is compressed at all?

But we've measured (with those supernovae) that the entire universe is expanding. It isn't getting compressed somewhere else. So how does that work? where is the compressed bit? Well thats the trick, with an infinite universe you don't need a compressed bit. You just need the furest parts of the universe from you to constantly move outwards. If the universe were finite that would mean the outer edge moves into a new area. But the universe is infinite so all you need is for the infinitely far away section of the universe to be traveling away from you infinitely quickly. It never moves into a new area beacause we aren't taking about a point in space when we say infintely far away from us. We are just talking about the fact that as we get furthar away from our observation point, the point we moved to is flying away from us faster and faster. Even exceedign the speed of light at a large enough distance (This doesn't break relativity, nothing is actually travelling faster than the speed of light locally, it just appears that way from our point of view).

In essance you can do want you want with an infinite space because its very definition means you don't have to worry about the edge of it doing anything naughty, because it doesn't have one!

[YNM]

The key is that our universe must be borderless. Regarding boundedness, that's an open answer anyone can do ; no need to butt head around (unless your life is as long as a small red dwarf star...)

Borderless : as with what we know as borders, ie. edges. Edges means a center, and center is a fairly preferred direction (so anisotropy).

Boundedness : infinite or not. Think about the surface of a bagel or a sphere - bounded but borderless. Or an infinite sheet do the same as well. Of course there's an effect because of that for the things on it...

Regarding SN observations : they show that the expansion rate is accelerated. The expansion itself is known since Hubble plotted velocity vs. distance. And predicted since Friedmann fickled with EFE.

[PB666]

No.

Damn too short =)

The univers is not infinit but closed in it self, so there is no visible border for us, like there is no vissible border for some 2 dimensional lifeform living on the surface of a sphere.

Greetings

Ben

There are two universes. The first is the known universe. It has no center, no edge, and is flat euclidian in all directions and has a constant cmb radiation. This is the defined universe.

The other universe is the hypothetical universe that, in our space timt, began thirteen biliion years ago, its beginnig is shrouded in opaqueness of annihilation may have inflated from a quantum singularity may have a central reference space time which is much older than ours and there are very young stuff looking back on all us dying old geezer galaxies hurling outward toward the unkwon expanse.