Expanding Universe Question

[Tommohawk]

I'm currently grappling with the idea of an accelerating expansion - I get the principles more or less, ie increased red-shifts and supernovae used as "standard candle" to assess actual distance.

But a much more fundamental question is: why is some stuff further away than other stuff at all? If there was a singularity as the origin of all matter, and it was all accelerated at the same rate, all matter would occupy a single expanding shell.

The fact that this isn't so suggests one of three things:  

1. all matter didnt originate from the same point, or

2. that it didnt originate at the same time, or 

3. matter had varying rates of acceleration.

...or of course some combination of the above.

Can anyone explain this please?

[Paul73]

It helps to think of it as space itself expanding, rather than every significant object speeding away from the others.

This sort of thing can really make your head hurt!

Paul

[vlaiv]

Here is something that might help:

Don't think of Big Bang as regular explosion where particles are exploded in classical sense - like bomb going off or something like that.

There is no single center of explosion from which everything is getting further and further away. Think of it more in terms of space between stuff expanding. Maybe the easiest way to visually present yourself with concept of expanding space would be the following:

draw couple of concentric circles and same number of dots on each circle. Dots will be the "stuff", circles themselves will be the the space. Smaller circles are further in time. So examine the first, smallest circle. Everything is close together, "total volume", or in this case total length of circle, or amount of space is also small. Now look at ever increasing size of circles. You may notice couple of things: First - amount of space increases (length along the circle), next - there is no increase in stuff content - you have the same number of dots, and last - distance (along the circle) of to adjacent dots is increasing - they are moving further apart, but also note that distance to further dots is increasing more and more as you go along the circle, so while neighbouring dots may moved a bit apart from one circle to next, dots on opposite size moved quite a bit (order of difference in radius). If you go all the way back in time, shrinking the circle you will end up with a dot - singularity of big bang.

There is also explanation with balloon - as balloon gets inflated, dots drawn on balloon get further and further among each other - same explanation as above but in two dimensions.

So we have model in 1d - circle, it takes two dimensions to "visualize" it - on a piece of paper, but it is 1d case - you can only move forward and backward on a circle. We have 2d model - balloon - it is 3d object used to represent expanding universe in 2d - it is 2d because you can only move in 2d on surface of balloon.

Space is 3d stuff so we would need +1 dimension to "visualize" it - something we can't naturally do, but you can extrapolate from 1d and 2d case to figure out what is going on.

(sphere, and hypersphere is not really what the universe is because it has negative curvature, something we still did not measure to be so with certainty, but there are other geometrical shapes that have same properties and might be good model for our universe).

HTH

[Knight of Clear Skies]

It's thought that small-scale quantum fluctuations were the seed of the structure (voids and dense regions) that later evolved.

[vlaiv]

24 minutes ago, Knight of Clear Skies said:

It's thought that small-scale quantum fluctuations were the seed of the structure (voids and dense regions) that later evolved.

That combined with inflation - regular expansion would give enough time for things to smooth over, so there had to be period of really rapid expansion in order to separate areas that fluctuated into different state from each other as to prevent further interaction and smoothing of things.

[Tommohawk]

Hi and thanks for those points... but I'm not sure it answers the question.

I'm familiar with the expanding balloon idea - and in a way this makes my point quite graphically. If the singularity was a single instantaneous event, all matter would occupy a single balloon shell - it would expand, and so there would be separation along the surface of the balloon, but all matter would be equidistant from the origin. Imagine standing on the surface of that shell - we could see matter all around us, and below us, because there is no solid surface, but not above us. That clearly isnt the case.

Of course we can say its the space expanding, not the matter itself, but that doesnt make any difference. Looking at this another way, if any matter has travelled further from the origin than any other matter, it must have either had more acceleration, or not started in the same place.

If we try and account for this by saying a singularity isnt actually a point source, but rather the original event was an expansion from all points simultaneously, this could explain the fact that matter extends throughout space - but it would impact on any assumptions made regarding different degrees of redshift when considering changes in the acceleration of the universe.

[Ruud]

2 hours ago, Tommohawk said:

1. all matter didnt originate from the same point ...

That would be it, I guess.

Although we see only a portion of the universe, its present size may be infinite. If this is the case, and if it has a finite age, the universe was always infinite, even at birth.

'The singularity' is not a thing, I suppose, but the start of everything.

 

[SilverAstro]

2 hours ago, Tommohawk said:

 If there was a singularity as the origin of all matter, and it was all accelerated at the same rate, all matter would occupy a single expanding shell.

A single expanding shell suggests that you think everything started out in the same place ? No, it (whatever it was) must have had dimension(s), incredibly small dimensions granted, nevertheless dimensions that expanded.

Incredibly smaller dimensions than we currently know how to characterize in physics. Even smaller than we can treat mathematically, so we call it a 'singularity' for convenient short-hand

A mathematical singularity tends to infinity ( in this case infinitely small) it doesnt mean that it is actually physically infinite or infinitely small. If it really were infinitely small it would take more than an infinite time at an infinitely high acceleration to become , , well it wouldnt become anything because it would still be infinitly small !

A singularity is a mathematical way of saying "we dont know", that is not trivial, it means that our equations just go off ! No longer make sense !

Being that we dont know, we can not talk of shells. We do know that there is a distributed universe, therefor it could not have really been infinitely small. 

EDIT Ah, while I was typing Rudd said it more briefly !!

[vlaiv]

3 minutes ago, Tommohawk said:

Hi and thanks for those points... but I'm not sure it answers the question.

I'm familiar with the expanding balloon idea - and in a way this makes my point quite graphically. If the singularity was a single instantaneous event, all matter would occupy a single balloon shell - it would expand, and so there would be separation along the surface of the balloon, but all matter would be equidistant from the origin. Imagine standing on the surface of that shell - we could see matter all around us, and below us, because there is no solid surface, but not above us. That clearly isnt the case.

Of course we can say its the space expanding, not the matter itself, but that doesnt make any difference. Looking at this another way, if any matter has travelled further from the origin than any other matter, it must have either had more acceleration, or not started in the same place.

If we try and account for this by saying a singularity isnt actually a point source, but rather the original event was an expansion from all points simultaneously, this could explain the fact that matter extends throughout space - but it would impact on any assumptions made regarding different degrees of redshift when considering changes in the acceleration of the universe.

Hm, no, with balloon you have to think in this way:

There is only left and right, aft and forward. No up and down directions! All the matter is spread over the surface of the balloon and the surface of the balloon is the only thing that exists. Third dimension and center of the balloon are just mathematical concepts not reality for beings living on the surface of the balloon. So balloon is not solid sphere with stuff inside. It is only shell. Similar to the circle - there are no things that are inside circle or outside of the circle - only on the circle itself.

We can argue that there is fourth spatial dimension - but there is no evidence of that, and if we use mathematics to model the universe in this way, then yes, for this particular case, all the matter moved from "origin" the same amount in fourth spatial dimension - same as with the circle (center of it) and balloon (center of it), but those centers are not inside the actual universe (center of circle does not lie on the circle itself, similar to the center of the balloon).

Key here is to try to stop thinking in 3d (that is a bit hard thing to do since our brain evolved to think in 3d), and use basic principles to describe what one might "feel" or "see" in lower dimensions. For example we are a dot on a circle - we can only see one point on each side of us. There is no left or right, up or down, just in front and behind. On surface of balloon / sphere - there are 2 (perpendicular) directions we can observe - left/right and in front / behind (this is why surface of balloon is 2d and circle it self is 1d). If we extend this analogy to 3d world that we live in - we have 3 dimensions - left/right, up/down, front/back, and "center" of Big Bang would be along fourth axes perpendicular to those 3 directions we observe - not something that we can imagine or do unless we mathematically describe our world as 3d hypersphere embedded in 4 dimensions.

These are all spatial dimensions - space dimensions, or mathematical dimensions, don't confuse it with space-time (also 4d but 3 spatial + 1 temporal dimension).

[Paul73]

1 hour ago, vlaiv said:

That combined with inflation - regular expansion would give enough time for things to smooth over, so there had to be period of really rapid expansion in order to separate areas that fluctuated into different state from each other as to prevent further interaction and smoothing of things.

That is the theory.....There was an exceptionally rapid expansion in the first fraction of a second after the Big Bang. There followed Quarks & Leptrons, to nucleons (for about 20 mins ish). Then is all calms down a bit as by now everything is pretty spread out.

Tiny fluctuations at the early sub atomic massively quick stages, cause what we see today. So post the mega quick expansion, we have something that behaves like raw cake mixture containing sultanas. As if cooks the cake mixture (space) expands. This carries the sultanas (big chunks of matter) apart. We need to be wary of pushing the analogy too far, but I find the cake model easier to get my head round.

Paul

 

[Tommohawk]

OK thanks again for further thoughts. Hopefully you would agree that matter now doesn't occupy a single point - is it spread throughout  the "universe".

... aah just got v busy at work .. willpick up later .. please stay with me!

 

[laudropb]

Anyone interested in furthering their knowledge and hopefully understanding of this topic should consider doing the MOOC course on gravity offered  by FutureLearn. It is free and among the tutors there is no less an authority than the Noble prize winner Gearge Smoot. It's free and is currently running.

[cloudsweeper]

The MOOC on Gravity is good - I did it a short while ago.

One point that was raised in this thread - why is matter at various locations if it all started from the same singularity?  Is that not because gas, "dust", and other matter coalesced into galaxies and stars at different times, and indeed are still doing so now (in nebulae for example).  Just a thought.....

Doug.

[SilverAstro]

1 hour ago, Paul73 said:

That is the theory.....There was an exceptionally rapid expansion in the first fraction of a second after the Big Bang.

This carries the sultanas (big chunks of matter) apart. We need to be wary of pushing the analogy too far, but I find the cake model easier to get my head round.

The "exceptionally rapid expansion" inflationary period is why the universe looks the same all over, homogeneous, even those parts that are sufficiently far apart that they cannot have been in communication (light speed ) ?

The cake model is disturbing ! I am reasonably happy with one idea that we (it) is all just a humongous simulation on a megga alien computer and that we might get switched off at any moment, but to be in an alien kitchen about to be eaten would be ignominious !

[saac]

Could be worse, the alien cook may decide that the cake is a dud, throw it in the bin and start over again  

Jim

[SilverAstro]

1 hour ago, saac said:

cake is a dud, throw it in the bin and start over again

Eeeek ! So not to end in a Big Crunch, more like decay or a cold dark Heat Death.

EDIT : Or he misses the bin and we end in a Big Bounce

 

[George Jones]

As some others already have said, the Big Bang singularity is not a point. A small, good, non-technical (but still difficult) book on singular spacetimes is "The Edge of Infinity" by Paul Davies.

Speaking very loosely

On 2013-12-19 at 13:43, George Jones said:

I am not sure that there is a completely accepted technical definition of "spacetime singularity". There is, however, a reasonably generic definition of "singular spacetime". Singular spacetimes have "edges".

Roughly, a spacetime is singular if there is a path that a rocket with a finite amount of fuel could traverse that is either inextandable  at some finite (wristwatch) time in the or at some finite (wristwatch) time in the past.

Inextendable in the future. because the rocket falls "over the edge and off of spacetime" and "into the singularity", e.g., inside a black hole.

Inextandable in the past, because the rocket materializes "over the edge and onto spacetime" and "off of the singularity", e.g., "at" the Big Bang.

By the Penrose-Hawking singularity theorems, many "reasonable" classical spacetimes must be singular. Very roughly, in any "reasonable" classical spacetime, gravity is so strong that the fabric of spacetime gets ripped, thus creating an "edge".

[Tommohawk]

Right, thanks again for the further input.

If you use the cake with fruit in analogy, then you would have to agree that the fruit on the outside of the cake has travelled further (from the centre) than the fruit which is nearer the centre.

This means either the fruit at the outside has accelerated faster, or it started sooner. If you use this analogy - ie a singular point origin - there is no other interpretation! There are other alternative ideas, eg multiple point origins or a "universal" origin, which would assume more than one starting point. Put that concept on hold for a moment.

Maybe it would be more productive if I use our real world situation, and to try and work toward my key point a little.

When we observe type 1a supernovae, it is apparent using the "standard candle" approach that some are further away from us than others and that those further away have a greater red-shift. Moreover, those with a greater red-shift are further away than they should be if the rate of expansion were constant. From this it is said that not only is the universe expanding, but that the rate of expansion is increasing.

My contention is that if you assume a single point origin to the universe, then the only explanation for one (or more) SN being further away from the centre than any other SN, is that it (or the particulate matter/energy from which it was formed) had a greater acceleration in the first place, or that it started out sooner (or both)

If you assume any kind of multiple point origin, ie anything other than a single point origin, then it becomes impossible to compute the acceleration of an SN (in a single observation) because it isn't possible to know its starting position, and perhaps not even its starting time.

 

[SilverAstro]

56 minutes ago, Tommohawk said:

My contention is that if you assume a single point origin to the universe,

I dont think you have taken on-board what peeps have been saying about there being no "point". It is described as a singularity, that is not the same thing as a singular point.

Nothing has moved away from anything else in a   'cars on a motorway'   sense in this universal expansion.  It is the motorway that is expanding under them and carrying the cars with it. Some of the cars even have their engines turned off. Some are in fact driving along as well, some towards us and some away.

[Ruud]

We're stuck in our metaphors like raisins in a cake. 

[SilverAstro]

How about we try it another way :-

A long time ago in  a singularity not very far away (in fact right here and over there and everywhere else) there was a tiny little very  little Tommohawk, lets call him Th' and there was, ummm a former me, AS'.

Th' says "hello AS', how did you get that close to me". They are close together but they also have really really tiny measuring sticks (cf what I said earlier about dimension)  and can measure a really small distance between themselves.

While they are pondering this strange situation there is an almighty jerk, the rubber ball that they are embedded in   is  becomes suddenly stretched ( I nearly said pulled, but that would have implied or suggested an outside force *  lets not go there for now ) an inconceivable amount and they are all of a sudden a very long way apart and worse, they are getting further from each other as every moment passes, but both will swear he has not moved, 'tis the other fella wot is moving. In fact neither are, it is just the stuff between them that is growing.

* lets not do multiverses and M-branes just yet

But they are not really Th' and AS', they are just hot steamed fish that will soon condense into a plasma which will eventually , ,etc , ,   hot steamed fish is a euphemism for the unknown, just like 'singularity'

Howzat ?

[saac]

2 hours ago, Tommohawk said:

My contention is that if you assume a single point origin to the universe, then the only explanation for one (or more) SN being further away from the centre than any other SN, is that it (or the particulate matter/energy from which it was formed) had a greater acceleration in the first place, or that it started out sooner (or both)

If you assume any kind of multiple point origin, ie anything other than a single point origin, then it becomes impossible to compute the acceleration of an SN (in a single observation) because it isn't possible to know its starting position, and perhaps not even its starting time.

 

I think the terms singularity together with infinity are perhaps the cause of misinterpretation of many a physics theorem.  From what happens at the centre of black holes, to the limit of relativistic mass, the terms singularity and infinity mean different things to mathematicians and physicists. To mathematicians they are a logical expression of equations, to physicist they represent an annoyance - a gap in understanding.  I have always understood infinity and the singularity to exist only as mathematical expressions.  

I'm happy to be corrected but as for the expansion of the universe,  I understood, and it is what we teach now, is that the "bang", inflation and subsequent expansion happened everywhere, there was no locus. The evidence to support this we see in the CMBR and the homogeneous structure of the observable universe. Remember also that matter was not present at the point  of creation (too hot, too energetic) but appeared after inflation and concurrent with expansion.  I don't believe a multiple point origin would therefore explain what we are able to observe and measure.  Again, happy to be corrected but the LCDM model, not withstanding some gaps (acceleration, dark matter/energy) is robustly supported in our observations.

Jim

[saac]

Just as an aside, the fact that we are able to discern a testable model for the origin of the universe simply by observation is utterly amazing.  It's easy to criticise all the stupid things we humans do but we are at the same time capable of some truly amazing things. I am of course talking about the giants past and present on whose shoulders we stand - I certainly can't take any credit, but I do amazement well 

Jim

[SilverAstro]

The trouble with being in awe of what we have been able to test, is that we are running out of the ability to do that !

We have many untestable/undistinguishable exotic theories of how things came to be, we also have many exotic theories that do give testable predictions except there is little chance of building the hardware to test them.

Two examples :

1) was it Brian in a Horizon who gave the example of building the next generation of CERN collider to get a magnitude closer to the BB will (would) require a an instrument the size of Pluto orbit ( and my own addition : consider the synchroton radiation that would give off ! ) No chance even if we had the capability of building a Dyson sphere.

2) The CMB, and time of last scattering, sets a limit to what we can deduce/test about the early universe via electromagnetism, to go a stage further requires gravity wave telescopes and look at how big, and how long it took, to get a result from LIGO.

I dont want to be full of gloom but we are rapidly approaching the stage of " it just happened" ! Because, Ive said this before in these types of discussion, the more we find out about the universe the more bits of it we lose, all that Dark  err Stuff ! not a clue any more about what the greater part of the universe consists of  It will only get worse

 

 

[saac]

Maybe we need to get used to there being a permenant limit to our understanding.

 

Jim