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Author Topic: Why is the universe expanding?  (Read 5438 times)

Rev Siviwe

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Why is the universe expanding?
« on: 20/01/2011 00:30:02 »
Rev Siviwe  asked the Naked Scientists:
   
Dear Chris,

I am interested in knowing what causes the universe to expand?

I understand that the galaxies are stationery, and yet I hear that the space between them is growing. My only imagination here is that if the galaxies are not moving, then the universe must be stretching.

If so, what is stretching the universe when in fact stars collapse to form blackholes? Shouldn't the universe be shrinking therefore? If stretching there must be some force pulling it beyond itself. What is that force and where does I come from?

In anticipation of your informed response and clarity, I remain grateful.

Regards,

Rev Siviwe 'Sva' Waqu

What do you think?
« Last Edit: 20/01/2011 00:30:02 by _system »


 

Offline Atomic-S

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Why is the universe expanding?
« Reply #1 on: 21/01/2011 06:06:38 »
Well, I would like to know precisely what the distinction is between "space expanding between two stationary objects" and "two objects moving apart with a uniform velocity difference between them, each appearing to be at rest in its own reference frame while the distance between them increases."  Are or are not these two expressions equivalent, and if not, what exactly is the distinction?
 

Offline Bill S

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Why is the universe expanding?
« Reply #2 on: 21/01/2011 14:23:26 »
Before tackling the OP, it would perhaps be best to look at the question from Atomic-S.

As I understand it, before Einstein, space was thought to be empty (more or less), it was “nothing”, but now it is “something”.  It seems that this something was created at the Big Bang, and has been growing (good non-tech term  [8D]) ever since.  It follows, therefore, that there is a difference between two objects that are moving through space, and two objects that are embedded in space, and being carried along by it as it expands. 

That’s a non-scientist’s view of the situation.     Enter the experts!
 

Offline imatfaal

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Why is the universe expanding?
« Reply #3 on: 21/01/2011 16:10:32 »
Atomic - If two massive objects are moving apart from each other then a light beam will eventually bridge the gap, if space is expanding by a fixed multiple per unit AND if the distance between them is large enough to start with they will move apart from each other at such a speed that they can never communicate as light cannot bridge the gap.
 

Offline yor_on

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Why is the universe expanding?
« Reply #4 on: 21/01/2011 19:55:50 »
We do not know 'why' it expands but we have a constant describing it :), ah, maybe. Einsteins cosmological constant namely, also called his biggest blunder (by himself). But I think he was wrong there myself. As for if it's 'stretching'? Depends on how you define a 'distance'. Can a 'distance' Lorentz contract? If it can, does that mean that distances just are variables, not anything 'fixed'? Does a photon measure a 'distance' if/when 'propagating'?

I find both speed and distance being questionable. But 'Compression', that one is a really interesting subject to me. It does things speed can't and somehow it also redefine 'particles' state, from 'matter' to ? light? Gravity? Whatever..
 

Offline Bill S

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Why is the universe expanding?
« Reply #5 on: 21/01/2011 20:32:07 »
Quote from: Rev
If stretching there must be some force pulling it beyond itself.

You might find D. Hamilton's "falling galaxies" theory interesting, if somewhat outside mainstream science. (Hamilton. Donald L.  A New Cosmology for the 21st Century. http://novan.com/cosmol).  ???
 

Offline Bill S

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Why is the universe expanding?
« Reply #6 on: 21/01/2011 22:41:05 »
A thought arising out of a similar discussion in another forum.

If a projectile is fired into space it continues unrestricted until acted upon by an external force.  Space offers no resistance.  Does it not seem strange that, in view of this, we are assured that galaxy groups are carried along by this same un-resistive  space. 
 

Offline yor_on

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Why is the universe expanding?
« Reply #7 on: 22/01/2011 02:49:48 »
It's not that they are 'carried away' as much as the expansion is working in between galaxies. Are you thinking some sort of inertia through the gravity?

If I take two galaxies A and B and imagine the topology between them I would expect it to represent a hill, with the 'slopes' getting steeper the closer you moved to any of those galaxies. If you then 'stretch' the hill, as I presume the expansion does with the 'space' inbetween, the galaxies still will be placed at the bottom of their respective 'hill-side'. To do anything else should expend energy, I think?

But what will happen if you add objects all around them I'm not sure?
 

Offline Soul Surfer

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Why is the universe expanding?
« Reply #8 on: 23/01/2011 10:39:15 »
To answer the original question The universe is expanding because we live in a largely three dimensional universe and a dynamic (that is one that is capable of change) three dimensional universe cannot be stable in the VERY long term. It must either be expanding or contracting.  We happen to be expanding as a whole but note of course many smaller parts of the universe are contracting to produce planets, stars and most importantly black holes.

To continue with the explanation.  We live in a largely three dimensional universe because that is the only number of dimensions that a universe can have and make reasonably long term large stable structures like orbits for stars and planets and orbitals for electrons.

The reason for this is that if we are to have a conservation of energy law (the only sort of law that allows the physical law to be consistent over a large region of space  see Noethers theorems) the change of force and distance must be a reciprocal law that is one less than the number of dimensions,  That is a three dimensional universe has an inverse square law of force.  Note we also need a conservation of angular momentum law to ensure that the universe is consistent in different directions.  An inverse square law is the only one that allows stable orbits and oscillations to form all others are unstable.

It follows then that to exist for a long time (the hidden fourth and possibly other dimensions) a universe must be three dimensional and expanding.  We could not exist in any other sort of universe because of the long time and relative stability needed to form complex structures that lead to life (as we know it!).

The precise processes that create this expansion are at the moment subject to argument.  Some of the theorists and cosmologists introduce arbitrary fields and processes to explain them in ways that they favour.  I would much prefer it if a few got together and thought of things based on processes using the physical laws that we have already seen and measured  because there are ways in which this I believe could be explained using currently well established physical laws but to go into this would take this explanation into the area of new theories.
« Last Edit: 23/01/2011 11:02:23 by Soul Surfer »
 

Offline QuantumClue

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Why is the universe expanding?
« Reply #9 on: 23/01/2011 19:06:53 »
What soul surfer said is true. I will make an extension. During the very first instant of reality, all matter was infinitely stacked on top of each other - as you might know, two particles can never occupy the same space because this is forbidden by the uncertainty principle. In order for the universe to escape this catastrophy, space between particles had to expand to allow extra degrees of freedom.
 

Offline Bill S

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Why is the universe expanding?
« Reply #10 on: 23/01/2011 22:45:25 »
Quote from: yor_on
If I take two galaxies A and B and imagine the topology between them I would expect it to represent a hill, with the 'slopes' getting steeper the closer you moved to any of those galaxies. If you then 'stretch' the hill, as I presume the expansion does with the 'space' inbetween, the galaxies still will be placed at the bottom of their respective 'hill-side'.

If you extend this analogy to a large number of galaxies, and consider a section through, you have something like an egg box, with a 2-dimensional spread of humps and depressions.  The problem I see is that it is not just the humps (your hills) that are expanding, so are the depressions, in every direction, so why would the galaxies move in an ordered direction?   
 

Offline CliffordK

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Why is the universe expanding?
« Reply #11 on: 23/01/2011 23:29:33 »
In space, nothing can be static.  I.E.  if you had 2 large bodies with no relative movement, they would fall toward each other.

So, there are 3 possibilities:

  • The Universe is expanding
  • The Universe is contracting
  • The Universe is in a large orbit, possibly around a center of mass that is unknown to us

My vote would be a combination of all 3.

There are essentially two definitions of the "Universe".
The first being earth-centric.  "Everything that can be seen or observed from Earth", describing roughly a sphere around Earth.
The second being Everything that is out there whether or not we can, or could ever see or observe it from Earth.  Still, there could be non-local effects of regions of the Universe that are beyond what we can see with visible light.

Not seeing the actual "boundaries" of where matter is in the Universe, it may be difficult to tell for sure whether what we're seeing as expansion might in fact just be an artifact of a local view.

Perhaps one should also ask.
If the most distant star we can observe is 12 to 13 billion light years away.
How do we know that the Universe is still expanding TODAY?
 

Offline Bill S

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Why is the universe expanding?
« Reply #12 on: 24/01/2011 03:17:38 »
Quote from: CliffordK
Perhaps one should also ask.
If the most distant star we can observe is 12 to 13 billion light years away. How do we know that the Universe is still expanding TODAY?

I thought it had more to do with calculations based on red-shift than how for we could see.
 

Offline CliffordK

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Why is the universe expanding?
« Reply #13 on: 24/01/2011 04:17:17 »
Quote from: CliffordK
Perhaps one should also ask.
If the most distant star we can observe is 12 to 13 billion light years away. How do we know that the Universe is still expanding TODAY?
I thought it had more to do with calculations based on red-shift than how for we could see.
Yes & No...
Yes, it is "red shift" which is assumed to be related distance, but not necessarily precise. 

But...  When we are peering to the edge of the visible universe, we are looking at very small specks of light representing events that happened a long time ago. 

So, up until recently the most distant "object" seen was an explosion/supernova GRB 090423, with a redshift of 8.0 ≤ z ≤ 8.3, and an estimated distance of about 13 billion light years.  I.E.  We were seeing the explosion of a star that happened billions of years ago.  Thus, whole new solar systems could exist there, and we might not know for another few billion years.

"Red Shift" is likely an indication that the stars were receding from us a the time the light left the star, or perhaps that the light has essentially travelled through a "curvature of space" for lack of a better term.  But, we still have to assume a constant in events that had occurred billions of years ago.

In fact.  If we can see space about 26 billion light years in diameter.  Then if there was a big bang, we are assuming during the first billion years that space expanded from a point source to at least 26 billion light years in size.  At which point the expansion would be less than the speed of light for it to be visible to us now.  That would indicate potentially a very rapid deceleration in what was at one time a very fast expansion rate.

But, since this all happened billions of years ago, it would be hard to judge the exact state now. 
 

Offline Soul Surfer

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Why is the universe expanding?
« Reply #14 on: 24/01/2011 08:46:59 »
Clifford the "orbiting " universe does not need a central mass to make it do that.  If the universe as a whole has angular momentum it could be like a Kerr (rotating) black hole which has a ring or toroidal structure.
 

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Why is the universe expanding?
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