Question about "expansion of the universe"

[DyslexicHobo]

First off, I want to apologize is this question is too philosophical rather than scientific. I'm not sure if there's any scientific evidence on the topic (hence the reason I'm posting ).

Now, I believe it was in one of Hawking's (maybe Greene's?) books that I read something about how when the big bang occurred, it did not happen at a point in our spacetime--all of spacetime was contained within the big bang (so his point makes sense to me). He also states that the universe has no center, and all of the bodies are expanding away from every other body. He uses the analogy of having a deflated balloon with marker dots all over; when the balloon inflates, this is simulating how our galaxies are behaving--every dot is moving away from every other dot.

First off, am I understanding this first part correctly? If so, please read on.

If there is no central point to the galaxy, I believe I am correct in saying that it cannot have any edges (or a central point could be determined). If the universe does not have edges, then where is the expansion of the universe happening? Is empty space just popping up between nothingness?


Edit: Also, why can't they find the "center of the universe"? If our universe has a finite amount of mass, wouldn't it be possible to calculate where the center of mass was located (obviously not experimentally possible, but theoretically). Does dark matter (perhaps is properties, or our lack of knowledge about it) play a role in stopping us from finding this piece of information?

Blah there's so much I don't know.

 

[PhatoseAlpha]

Hmm. Trying to remember, Hawking isn't easy to understand.

Empty space popping up between nothingness is something akin to what's theorized, though not precisely. Rather then nothingness appearing, the space itself is getting bigger. What's already there is being spread out, but nothing new is appearing - like on the balloon, there is no new balloon surface appearing as it's inflated - the balloon surface is just getting bigger.

As for calculating a center, I would think like this: you've got a situation where space and time are the same thing. Therefore, you can't just take a 'snapshot' of the universe in time and calculate the spatial center, but you need a temporal/spatial center. This means path's of objects are going to be relevant in addition to their mass, since you need to know when as well as where, since time and space are one big thing. But with Heisenberg uncertainty, you just can't know an objects position and it's velocity at the same time, so you can't really know it's position over time either. You have nowhere to start your calculations.

Though, I imagine some nuclear physicist is gonna pop in and tell me I'm wrong in ways I can't comprehend, so take this all with a grain of salt.

 

[f95toli]

I think it is a bit simpler than that. The reason why there is no centre of gravity is because the universer does not have an "edge" (which does NOT mean that it is infinite).
Also, interactions are limited by the speed of light meaning if the stars are too far apart they can't interact (since the universe is expanding the distance BETWEEN two stars can grow FASTER than the speed of light), hence there can't be a centre of gravity.

 

[Biftheunderstudy]

We can't actually tell how big our universe is, we can only tell its as big as our so called particle horizon. This is the distance it takes light to travel from the big bang, currently this is around 13.7 billion years. So even though we have said that the universe is isotropic and homogeneous which means we can't say our position in the universe is special, we can say we are at the center of our universe, meaning we are at the center of our particle horizon. The consequences of this are that the universe could be much larger than we can see and we would never know or be able to tell.

The fact that the universe has no edge is the same as saying the earth has no edge in 2 dimensions, the universe has no edge in 3(time as well).

 

[DyslexicHobo]

Originally posted by: PhatoseAlpha
Hmm. Trying to remember, Hawking isn't easy to understand.

Empty space popping up between nothingness is something akin to what's theorized, though not precisely. Rather then nothingness appearing, the space itself is getting bigger. What's already there is being spread out, but nothing new is appearing - like on the balloon, there is no new balloon surface appearing as it's inflated - the balloon surface is just getting bigger.

As for calculating a center, I would think like this: you've got a situation where space and time are the same thing. Therefore, you can't just take a 'snapshot' of the universe in time and calculate the spatial center, but you need a temporal/spatial center. This means path's of objects are going to be relevant in addition to their mass, since you need to know when as well as where, since time and space are one big thing. But with Heisenberg uncertainty, you just can't know an objects position and it's velocity at the same time, so you can't really know it's position over time either. You have nowhere to start your calculations.

Though, I imagine some nuclear physicist is gonna pop in and tell me I'm wrong in ways I can't comprehend, so take this all with a grain of salt.

I think I get what you're trying to say. I've actually thought that maybe the fabric of space was just stretching--makes more sense than nothingness randomly popping up. I don't, however, understand why we can't find the center of mass of the universe (I understand why we aren't going to... because it's physically impossible right now, but does this imply that there is no center of mass?)

So the universe is constantly expanding. Lets use Earth for my example: We send signals out (traveling at c) in all directions, and figure out a way to find the planetary body that is the furthest away from us in every single direction. We can then plot the points, and by determining which sides are furthest away from us, we can determine that the center of mass is in that direction.

I know this reasoning is probably not correct, but can someone explain why?

Edit: I posted this before I saw the guy above me.

I like the analogy of why space has no physical boundaries.

But I still don't understand why it's impossible to calculate center of MASS. Because all of the mass in the universe is traveling at speeds slower than c (right?). So we could send signals traveling at the speed of c and determine the instantaneous center of mass. No?

 

[Biftheunderstudy]

For ease lets say the the experiment was perfect.
We would find that the center of mass of the universe is right here, but if we moved to a different galaxy and redid the experiment we would find it to be there as well. The reason for this is that the universe is hypothesized to be homogeneous and isotropic. Meaning that it is the same in any direction we look or anywhere we go. Take same to mean 'roughly' the same as there are always local features but on the largest scales the universe is isotropic and homogeneous.

 

[f95toli]

Originally posted by: DyslexicHobo
But I still don't understand why it's impossible to calculate center of MASS. Because all of the mass in the universe is traveling at speeds slower than c (right?). So we could send signals traveling at the speed of c and determine the instantaneous center of mass. No?

No, becasue as I pointed out above the DISTANCE between stars can increase at a rate larger than c; simply because space itself between the stars is "expanding". Nothing with mass can travel at speeds >c and the same is true for all forms of interaction, but space-time is not limited in the same way. Note that this also implies that there are regions in the universe that we can neither see nor interact with, since they are so far away they are moving away from us at a speed larger than c.

 

[DyslexicHobo]

Biftheunderstudy and f95toli, thank you very much for your explanations. I believe I understand why it is this way now.