Is the Universe infinite?

[Strange]

5 hours ago, Airbrush said:

How can two things occupy the same space at the same time?

By being bosons. 

[Airbrush]

Does that mean a universe composed of bosons can overlap our universe?

I'm ok with the prospect that an infinite number of universes could overlap in the same space, just on different dimensions.

[Meir Gilady]

time and space are infinite

 

[Strange]

2 minutes ago, Meir Gilady said:

time and space are infinite

 

And your evidence for that is, what exactly?

[Meir Gilady]

What evidence you have that it is not?

if you fire a beam of light in 1 direction in the universe,will it stop moving after 14 billion years or so?or will it move forever(exclude the energy loss)if you fire it same time to opposite direction it also move as far is we observe at least 14 biilion years/ now add this 2 and already you get twice the age of the universe as scientist claim (unless you belive that big bang started  on Earth) time couldn't start and will never end at it its core definition suggest.If the universe was not infilite matter should collapse over time into single dot, 

[Strange]

23 minutes ago, Meir Gilady said:

What evidence you have that it is not?

I am not claiming it is not (because there is no evidence either way). 

24 minutes ago, Meir Gilady said:

if you fire a beam of light in 1 direction in the universe,will it stop moving after 14 billion years or so?

Why would it stop after 14 billion years? The radius of the observable universe currently about 45 billion light years. The universe is almost certainly many times larger than that. 

27 minutes ago, Meir Gilady said:

you get twice the age of the universe as scientist claim

Nonsense

28 minutes ago, Meir Gilady said:

If the universe was not infilite matter should collapse over time into single dot, 

That was certainly Newton’s argument for the universe being infinite. 

But the universe is expanding (and the rate of expansion is increasing) so there is no reason to think it will collapse. 

[Itoero]

On ‎1‎/‎08‎/‎2017 at 3:32 PM, dordle-loddle said:

Is the Universe infinite or just really, really big?

What is meant with 'infinity'?

A hamster can walk infinitely on its running wheel.

I'm sorry if this has been asked before.

[taeto]

3 minutes ago, Itoero said:

What is meant with 'infinity'?

The original question does not contain the noun 'infinity', it contains the adjective 'infinite'.

The noun 'infinity' is used in mathematics mostly as the name of the symbol \(\infty\), which figures in expressions that involve a limit operation. Additionally 'infinity' is a name given to some element that is added 'extra' to a structure in order to extend it to a larger structure that has some desirable property, e.g. to construct the extended real numbers, or to make a topological space compact, or to make an elliptic curve closed under the operation of the group. In physics I believe you can find both meanings as well.

The adjective 'infinite', in this context, seems to refer to the size, or possibly the mass, or the number of particles, or the amount of time of existence. It is hard to answer the question if you are not specific. If the spacial extent of the universe is infinite, then it becomes very hard to imagine that all those other quantities are not infinite as well. If it is finite, then it becomes hard to imagine that the mass is infinite, whereas the number of particles might be in question, and the time extent could be either finite or infinite. At least that is my naive take on it. I am also interested in the implications either way.   

[Strange]

18 hours ago, Itoero said:

What is meant with 'infinity'?

A hamster can walk infinitely on its running wheel.

I'm sorry if this has been asked before.

The hamster isn't going anywhere. The concept of infinite size means (by analogy with the integers, for example) that however far you walk, you can always take one more step (with the caveat that, because the universe could be finite but unbounded, you never get back to your starting point).

[Carrock]

4 hours ago, Strange said:

The hamster isn't going anywhere. The concept of infinite size means (by analogy with the integers, for example) that however far you walk, you can always take one more step (with the caveat that, because the universe could be finite but unbounded, you never get back to your starting point).

Sorry, not very rigorous like many in this thread but this one's easy to respond to.

e.g. The concept of unlimited size means (by analogy with an unlimited but finite set of integers, for example) that however far you walk, you can always take one more step.

i.o.w. the concept of a finite or countably infinite universe expanding for unlimited (but not infinite) time is perfectly OK.

A universe which has expanded exponentially for infinite time is inconsistent with any current theory as its volume would be uncountably infinite. The 'I can't get to the infinite future from here' argument doesn't work very well since photons, which have infinite range, can get there from here.

 

Models which predict unending expansion are therefor problematic.

 

 

[Strange]

18 hours ago, Carrock said:

A universe which has expanded exponentially for infinite time is inconsistent with any current theory as its volume would be uncountably infinite.

My analogy was only countably infinite because it used steps. 

In reality, surely the universe would be uncountably infinite (if it were infinite) because space is (in current models) smooth and differentiable, which means it is continuous.

But I think the concept of countable or uncountably infinite only applies to how you measure it, not how large the universe is. So it is, to some extent, model dependent.

(But I may well be wrong about all the above!)