Where Does Space End? It Must End Somewhere!

[Airbrush]

Having an idea doesn't get you much in terms of accolodates in science, even if you are really sure and the idea turns out to be right. It especially doesn't get you much if it's an idea that everyone is already aware of even if not everyone is onboard with it being correct.

 

snip.....

You don't get celebrated for the apple. You get celebrated for doing all the rest of it.

 

Sure, I'm on board with all that.

 

I'm not here to get celebrated. I'm here to get an expert to explain to me how an infinite big bang happens. And if they can't do that, I'd rather they stop saying that an infinite big bang is just as plausible as a FINITE one. If that is a possible outcome of the big bang, then why do cosmologists avoid even discussing it? They go into great detail about the mechanics of what happened in a FINITE big bang...after it got started, of course.

 

Where Does This Discussion End? It Must End Somewhere.

Edited April 17, 2016 by Airbrush

[Strange]

 

Sure, I'm on board with all that.

 

I'm not here to get celebrated. I'm here to get an expert to explain to me how an infinite big bang happens. And if they can't do that, I'd rather they stop saying that an infinite big bang is just as plausible as a FINITE one. If that is a possible outcome of the big bang, then why do cosmologists avoid even discussing it? They go into great detail about the mechanics of what happened in a finite big bang...after it got started, of course.

 

You (still) seem to be talking about the "big bang" as an event, presumably where the universe appeared from nothing. That is not part of the mainstream big bang model.

 

The big bang theory is a model of the evolution of the universe from an early hot, dense state. It describes the universe now and for the past 13.8 billion years. It does NOT describe the universe at time 0.

[Airbrush]

There again, that is where we differ. Probably my poor choice of words, but you are smart enough to figure out what I mean. I believe I am talking about the "event" and the big bang is certainly an event, or if you prefer an "evolution", that has been unfolding for 13.8 Billion years and continues indefinitely. Not a brief moment. I have said nothing about time 0 or before that, only about what happens AFTER that, and I do not presume anything about the conditions under which the big bang event originated. There could have been a pre-existing universe or not. You consistently evade my questions.

Edited April 17, 2016 by Airbrush

[Strange]

I'm not trying to evade. I thought I had explained how an infinite universe can expand, cool, become less dense.

 

 

#1 When does an infinite universe achieve an infinite size?

 

#2 Was it before the first Planck Time?

 

If the universe is infinite then it has always been infinite.

 

#3 Does infinite in size imply a universe infinite in mass?

 

Yes.

 

#4 An isotropic, homogenious, INFINITE big bang would spread out evenly to infinity in all directions, right?

 

Yes.

BTW, I think Delta's comment about having an idea was directed at the (removed) hijack, not you.

[Delta1212]

^ That is correct.

[Airbrush]

Thanks for answering my questions, carry on.

[RobC673]

We are told that the universe is not a collection of galaxies sitting in space, all moving away from a central point. Instead, a more appropriate analogy is to think of the universe as a giant blob of dough with raisins spread throughout it (the raisins represent galaxies; the dough represents space). When the dough is placed in an oven, it begins to expand, or, more accurately, to stretch, keeping the same proportions as it had before but with all the distances between galaxies getting bigger as time goes on.

 

I have a query, is if this is the case why are certain galaxies colliding?

[Strange]

It would be (slightly¹) more accurate to think of the raisins as clusters of galaxies. Within each cluster, the galaxies are orbiting one another and occasionally collide.

 

1. I think it is a terrible analogy to start with.

Edited April 23, 2016 by Strange

[RobC673]

We are told that space is expanding and that the rate of expansion is increasing so taking these factors into account how do galaxies occasionally collide?

[Strange]

Galaxies in clusters are gravitationally bound to one another and so not affected by the overall expansion of the universe.

[pzkpfw]

We are told that space is expanding and that the rate of expansion is increasing so taking these factors into account how do galaxies occasionally collide?

The expansion of the Universe means that in general everything is getting further away from everything else. Expansion isn't about the Galaxies moving through space away from each other.

 

But - Galaxies are still free to move in space, as caused by gravity. (Pretty much the same reason Earth is still orbiting our Sun.)

 

You raised the dough-and-raisins analogy, so I'll assume you know about the "balloon analogy". That is, where the Universe is represented as the surface of an inflating balloon. Everything on that balloon is getting further away from everything else, and none (or all) are the "centre".

 

Think of the Galaxies on that balloon as ants. If the surface was sticky and they couldn't move, then they'd simply get further away from each other. None would collide. But, those ants can move. Some, being close enough, detect another ant and move close to discuss Wittgenstein.

[RobC673]

It is said that the universe is expanding faster than the speed of light. But even if it is not can the force of gravity override this expansion to cause galaxies to collide?

[ajb]

But even if it is not can the force of gravity override this expansion to cause galaxies to collide?

Locally yes. Gravity overcomes this global expansion within clusters of galaxies. So galaxies that are near each other can collide.

[Strange]

How close Andromeda and Triangulum were in the past?

We might be able to verify this question based on the following Hydrogen bridge between the galaxies:

"Two large neighbors of our own Milky Way galaxy—Andromeda (upper right) and Triangulum (lower left)—experienced a close encounter in the distant past."

http://www.sciencemag.org/news/2012/06/scienceshot-hydrogen-bridge-connects-two-galaxies

However, Andromeda is a supper massive spiral galaxy (with about one billion stars), while Triangulum is like a baby spiral galaxy with only 40 M stars.

So, as close they were - as the gravitational power was stronger.

Hence:

- If they were very close to each other in the past, then how could it be that Andromeda didn't destroy Triangulum galaxy?

- Why now they are moving further away from each other – against the gravitational power?

 

I suggest you look up the word ORBIT in a dictionary.

It is said that the universe is expanding faster than the speed of light. But even if it is not can the force of gravity override this expansion to cause galaxies to collide?

 

Note that expansion can't really be described as a speed. It is a scaling effect so the speed at which two points separate depends on how far apart they are. There are always points which are far enough apart that their speed of separation is greater than the speed of light.

[RobC673]

I have read this reply many times but still do not understand it. Could you give a reply that would be readily comprehensible to a 16yr old school boy?

Thanks RobC673.

[Strange]

I have read this reply many times but still do not understand it. Could you give a reply that would be readily comprehensible to a 16yr old school boy?

Thanks RobC673.

 

Which one?

 

(Edit: which reply, not which schoolboy )

Edited April 25, 2016 by Strange

[swansont]

!

Moderator Note

 

Question about galactic orbital motion has been split

 

http://www.scienceforums.net/topic/94711-galactic-motion-hijack-from-where-does-space-end/

 

[RobC673]

 

Which one?

 

(Edit: which reply, not which schoolboy )

The one about the scaling effect.

[Delta1212]

The one about the scaling effect.

Let's imagine for a moment that you are shrunk down to 1/10th your current size. As you shrink, everything around you looks like it is growing and, by extension, getting farther away from you.

 

A chair that was a foot away from you now appears to be 10 feet away from you from your new, smaller perspective. A wall behind the chair that was 10 feet away now appears to be the equivalent of 100 feet away.

 

Now, as you were shrinking, all of the stuff seemed to grow and move away from you. But the chair seems to have moved from 1 foot away to 10 feet away in the same period that the wall seems to have moved from 10 feet away to 100 feet away. It looks as if the wall moved 90 feet in the time it took the chair to move 9 feet. And tree out the window beyond the wall would have moved farther still. So the farther something is away from you, the faster it would appear to have receded as you shrank, and the farther away it ultimately would have ended. Of course, none of it was actually moving. The apparent distance simply changed.

 

That is, essentially, what happens metric expansion of space, except instead of you shrinking, it is the distance that is growing and all of the "stuff" remains the same size. The effects are otherwise the same. The farther something is from you, the faster it appears to recede because the rate at which the distance grows is directly proportional to how much distance there is between you and it to begin with. And likewise, those things that are receding are not actually moving, as such, it is simply the distance between you that has increased. That is how things that are very, very, very far away can conceivably recede at speeds greater than the speed of light, even though nothing can move that fast through space.

 

It's a bit of a mind-bending concept to really wrap your head around, but that's more or less how it works.

[Strange]

Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

At time 0, they are 1 unit apart:
A.B.C.D.E.F

After some time they are 2 units apart:
A..B..C..D..E..F

After the same time again, they are 3 units apart:
A...B...C...D...E...F

And so on:
A....B....C....D....E....F

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. (But that is OK, because the speed of light limit is a local thing, whereas these objects are in different frames of reference.)

[Delta1212]

I think saying that faster-than-c travel is allowed because they are in different frames of reference is misleading. Everything that is moving is in a different frame of reference.

[Strange]

I think saying that faster-than-c travel is allowed because they are in different frames of reference is misleading. Everything that is moving is in a different frame of reference.

 

Good point!

[Airbrush]

It is incorrect to suggest that galaxy clusters are expanding away from each other. Only SUPERclusters are moving away from each other, not clusters of galaxies. Even clusters are gavitationally bound within their supercluster. It is often overlooked that galaxies are organized in clusters, and then "clusters-of-clusters" which are superclusters.

 

The raisins in the raisin bread, which I agree is a very poor analogy, are superclusters.

Edited April 25, 2016 by Airbrush

[Strange]

Also a good point. But I think it is even more complicated than that. There is the large scale structure of webs and filaments, which is also gravitationally bound...

[fredetuc]

It does and we now have prove. Dark matter is expanding our universe. Thus meaning that the universe as a "wall" to say.