# Thread: Properties of void outside the universe*?

1. ## Properties of void outside the universe*?

*By "outside the universe" I mean... naively suppose you can pinpoint where in 4-space the big bang occurred. Time passes, universe expands, etc.

Void inside the (observable) universe has properties, like permittivity and permeability of free space. Question is, would the same hold for outside? Would there be a way to know?

2. ## Re: Properties of void outside the universe*?

Usual answer I hear for this kind of questions on science-show is that it's hard to talk about what is outside of the universe as there is nothing before big bang or outside the universe. But there is inflation model of big bang which states that there are bubble universes stretching over infinite space time and, each of those universes can have it's own rules and constants.

At the moment there is no known way to verify this but who knows whether in 100 year some idea come up.

3. ## Re: Properties of void outside the universe*?

Originally Posted by danzibr
*By "outside the universe" I mean... naively suppose you can pinpoint where in 4-space the big bang occurred. Time passes, universe expands, etc.

Void inside the (observable) universe has properties, like permittivity and permeability of free space. Question is, would the same hold for outside? Would there be a way to know?
Actually you can pinpoint the location in 4-space where the big bang occurred (assuming that the 4th dimension is time). The time co-ordinate is <insert current best calculated age of universe>.

The space co-ordinates are "everywhere".

This is what is meant by the expansion of the universe - there is no "outside" to measure it against (or if there is, none of the spacial co-ordinate systems we can use inside have any meaning there). Every point in the universe came from the big bang.

This means that your question isn't meaningful - there isn't an outside (as we currently understand it) to observe. You also cannot go back before the beginning because although you can calculate a theoretical time postion, there's no reference to observe it passing - time itself is also meaningless before the big bang.

If we suppose that there was something before the big bang, and than time and spacial coordinates have meaning, they still don't relate to those we use inside the universe (or at least we cannot relate them) so the best answer becomes "possibly". An equivalent question is "if we postulate the ability to see gravity in the same way we see light, would the gravity of the earth look blue?" It's a meaningless because we don't have the referents to answer it.

4. ## Re: Properties of void outside the universe*?

Originally Posted by danzibr
*By "outside the universe" I mean... naively suppose you can pinpoint where in 4-space the big bang occurred. Time passes, universe expands, etc.

Void inside the (observable) universe has properties, like permittivity and permeability of free space. Question is, would the same hold for outside? Would there be a way to know?
Nice questions, even though the answers seem to be "no".

5. ## Re: Properties of void outside the universe*?

Originally Posted by Khedrac
Actually you can pinpoint the location in 4-space where the big bang occurred (assuming that the 4th dimension is time). The time co-ordinate is <insert current best calculated age of universe>.

The space co-ordinates are "everywhere".

This is what is meant by the expansion of the universe - there is no "outside" to measure it against (or if there is, none of the spacial co-ordinate systems we can use inside have any meaning there). Every point in the universe came from the big bang.

This means that your question isn't meaningful - there isn't an outside (as we currently understand it) to observe. You also cannot go back before the beginning because although you can calculate a theoretical time postion, there's no reference to observe it passing - time itself is also meaningless before the big bang.

If we suppose that there was something before the big bang, and than time and spacial coordinates have meaning, they still don't relate to those we use inside the universe (or at least we cannot relate them) so the best answer becomes "possibly". An equivalent question is "if we postulate the ability to see gravity in the same way we see light, would the gravity of the earth look blue?" It's a meaningless because we don't have the referents to answer it.
I figured someone would say something like this.

Alternatively, right now the observable universe is something like 90 billion ly wide. Suppose you could teleport 90 billion ly in a given direction. You'd either end up in another bubble, or outside of any bubbles, or maybe still in our bubble. As asda fasda mentioned, in the first case void might have different properties. In the second case... I dunno.

6. ## Re: Properties of void outside the universe*?

Originally Posted by danzibr
Alternatively, right now the observable universe is something like 90 billion ly wide. Suppose you could teleport 90 billion ly in a given direction. You'd either end up in another bubble, or outside of any bubbles, or maybe still in our bubble. As asda fasda mentioned, in the first case void might have different properties. In the second case... I dunno.
Oddly enough, I think the current theory is that you would end up in an overlapping bubble with the same properties (and same big bang)!
The logic goes that when two points are sufficiently far enough apart that the expansion of the universe means they are effectively moving apart faster than the speed of light, then they are no longer in the same universe. However a point half-way between (in so much as that is defined once things are in separate universes) is still in the same universe as both of them.
No - I don't like that theory either (so I have probably badly misunderstood it).

Anyway, if the universe is infinite (as I am told the current theory is) then you could travel 180 B ly and remain in the same universe, but outside the observable bubble. In fact, with distance defined by the universe then no matter how far you travel you cannot leave it by travelling a distance - leaving it would require something else.
Back when I was growing up scientists were rather less sure about the size and "shape" of the universe (one through was that it could be 'closed' - travel far enough in one direction and you get back to the starting point coming from the opposite direction).

As for constant properties, there remain alternative theories for the current universe, particularly for gravity, that mean if you travel far enough in this universe the properties can change! Note, they are minority theories, but although the standard model is getting better and better dark matter and dark energy remain big enough problems that respectable physicists accept that one of the other theories could be more correct (though it is thought to be very unlikely)

7. ## Re: Properties of void outside the universe*?

Originally Posted by danzibr
Void inside the (observable) universe has properties, like permittivity and permeability of free space. Question is, would the same hold for outside? Would there be a way to know?
Well, that's kind of the definition of outside the observable universe: the observable universe ends where you stop being able to know those things.

The prevailing philosophy (Copernican principle) is that we should assume what we can see typical. This is not to say identical, for example there was a famous statistic that the typical family had 2.4 children, but obvious no actual family has 2.4 children; the number varies and you should expect it to find variable numbers of children as you see new, normal families.

So going back to permittivity and permeability of free space. Do these numbers vary ever, even in the slightest? Our understanding is no, so we expect the unobservable universe to have these same values.

8. ## Re: Properties of void outside the universe*?

Problem with answering this kind of question is that we don't know where the big bang came from, there are hypothesis's like eternal inflation, string theory.

9. ## Re: Properties of void outside the universe*?

Well, that's kind of the definition of outside the observable universe: the observable universe ends where you stop being able to know those things.

The prevailing philosophy (Copernican principle) is that we should assume what we can see typical. This is not to say identical, for example there was a famous statistic that the typical family had 2.4 children, but obvious no actual family has 2.4 children; the number varies and you should expect it to find variable numbers of children as you see new, normal families.

So going back to permittivity and permeability of free space. Do these numbers vary ever, even in the slightest? Our understanding is no, so we expect the unobservable universe to have these same values.
Yes the current (common) theory is that things are fundamentally the same in every direction, infinitely. We just can't "see that far" because light from there has not reached us yet.

You can imagine that we are on the surface of a baloon that gets inflated since the big bang. The question "where on the surface did the big bang/inflation begin/happen?" is for the baloon: Everywhere on the surface at the same time.

10. ## Re: Properties of void outside the universe*?

This is the sort of thing that can only be asked within the context of a particular mathematical theory or framework and not about reality. So you could ask what a given way of describing the universe would suggest, but by definition this is a question about things you can't obtain evidence about from anything you observe within the universe.

One mathematical formulation which makes this a non-question would be if you take a group theoretical view of things like space and say that space, as a Lie group, is defined by the operators which let you move through it. So only those points which you can reach by a combination of operators exist, and there's nothing 'outside' of them since there are no operators which connect them. But that's a convenient way of formulating a local theory of physics (because you don't need to know what's 'over there', you only need to know what you see where you are after executing a given motion pattern), it doesn't necessarily mean that reality is structured as stuff embedded in a manifold defined by a Lie group...

11. ## Re: Properties of void outside the universe*?

Originally Posted by Rydiro
Yes the current (common) theory is that things are fundamentally the same in every direction, infinitely. We just can't "see that far" because light from there has not reached us yet.
That implies that we eventually will, which is not what is currently thought, it is believed as I understand it that the observable universe is shrinking, because the rate of separation is rising, so the limit at which the speed of light is lower than the movement away of the expanding bits of universe is moving towards us.

I sort of expect that's all wrong, but I don't have the maths to work out how.

12. ## Re: Properties of void outside the universe*?

Originally Posted by halfeye
That implies that we eventually will, which is not what is currently thought, it is believed as I understand it that the observable universe is shrinking, because the rate of separation is rising, so the limit at which the speed of light is lower than the movement away of the expanding bits of universe is moving towards us.

I sort of expect that's all wrong, but I don't have the maths to work out how.
That's two different things. First our universe is bigger then our observable universe, as some parts of our universe has escaped due to higher speed of expansion then the speed of light. This means that outside of our observable universe there is still "our" universe, we can't just communicate with it, and thus should have the same properties.

The other issue is whether is there anything outside of our universe and whether it has to have same properties as our universe. This question is about "where" happened the big bang and what has cause it and more why the whole universe has exact properties that allowed for us to be here. This is more philosophical question and as it was said before has many issues, but physics has made some theories that tackles it and some of more promising models (i.e. inflation models as I pointed before) suggest that there might be thousand of universes, each with each own big bang that started it and different properties which "collapse" to their values during big bang (probably on random)

13. ## Re: Properties of void outside the universe*?

Originally Posted by asda fasda
That's two different things. First our universe is bigger then our observable universe, as some parts of our universe has escaped due to higher speed of expansion then the speed of light. This means that outside of our observable universe there is still "our" universe, we can't just communicate with it, and thus should have the same properties.

The other issue is whether is there anything outside of our universe and whether it has to have same properties as our universe. This question is about "where" happened the big bang and what has cause it and more why the whole universe has exact properties that allowed for us to be here. This is more philosophical question and as it was said before has many issues, but physics has made some theories that tackles it and some of more promising models (i.e. inflation models as I pointed before) suggest that there might be thousand of universes, each with each own big bang that started it and different properties which "collapse" to their values during big bang (probably on random)
As far as I can tell, this rant has no connection to what I was talking about at all.

What I was talking about may or may not have been off topic, but the above seems to have no connection to anything.

14. ## Re: Properties of void outside the universe*?

Originally Posted by halfeye
That implies that we eventually will, which is not what is currently thought, it is believed as I understand it that the observable universe is shrinking, because the rate of separation is rising, so the limit at which the speed of light is lower than the movement away of the expanding bits of universe is moving towards us.

I sort of expect that's all wrong, but I don't have the maths to work out how.
No, you are correct. There are parts of the universe we will never see, because space is expanding, and the rate of expansion (as near as we can tell) is accelerating. That means that in however-many-billion (trillion?) years, all we will be able to see will be our local galacy group (which will probably eventually merge into a single elliptical galaxy by that time).

15. ## Re: Properties of void outside the universe*?

Originally Posted by halfeye
That implies that we eventually will, which is not what is currently thought, it is believed as I understand it that the observable universe is shrinking, because the rate of separation is rising, so the limit at which the speed of light is lower than the movement away of the expanding bits of universe is moving towards us.

I sort of expect that's all wrong, but I don't have the maths to work out how.
As far as we can tell the rate of expansion is constant. So the absolute size of the visible universe - the distance from which a photon that originated at a given point would have had the time to reach us - will only ever increase. For that to change the rate of acceleration would have to increase, we'd wind up facing down a big rip scenario, and there's no evidence that that's the likely ultimate fate of our universe.

Of course the extra space created by the expansion of space is empty, boring space. And the increased distances push away matter that's far more interesting. So in the distant future when all the local galaxies combine into one super galaxy and all the other galaxies have been pushed out past the cosmic event horizon, it doesn't matter how far out a hypothetical photon could have started out in order to reach earth by that point. What would matter is that there wouldn't be any matter or other interesting stuff that could produce such photons, so the point would be moot.

16. ## Re: Properties of void outside the universe*?

Originally Posted by Lord Torath
(which will probably eventually merge into a single elliptical galaxy by that time).
Which has been very creatively dubbed "Milkomeda", I wish I were joking.

17. ## Re: Properties of void outside the universe*?

Originally Posted by halfeye
That implies that we eventually will, which is not what is currently thought, it is believed as I understand it that the observable universe is shrinking, because the rate of separation is rising, so the limit at which the speed of light is lower than the movement away of the expanding bits of universe is moving towards us.

I sort of expect that's all wrong, but I don't have the maths to work out how.
The size of the observable universe isn't shrinking, but to the best of our current understanding there is a "future visibility limit" (if you search on that phrase you'll turn up info on it, I can't post any links yet) a little over 60 billion light years in radius.

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