Where does space end..

[Zuts]

Except for those galaxies which are gravitationally bound like our local group of galaxies, everything is effectively moving away from everything else. On a large scale, visible universe scale the proper motion of galaxies is far smaller than the expansion of space time. Eventually many billions of years from now, except for a handfull of galaxies we will be alone in space.

Whether the expansion continues, stops or starts to contract depends on the amount of matter in the universe. Currently scientists believe there is not enough matter in the universe to stop the expansion.

Worse still, normal matter only accounts for 5% of the matter and dark matter maybe 25% more of the matter in the universe. People believe the remaining 70% is made up of dark energy and unfortunately this has the effect of negative gravity and is causing the expansion to speed up.

Cheers
Paul

[sjastro]

Originally Posted by jungle11

So (speed of light nonwithstanding) if we positioned a probe a megaparcec away, and could communicate with it in real time, would that probe be moving away from us or sitting still?

If you want the result in space-time velocity, the value is v=H, in spatial velocity assuming the probe is not subject to gravitational forces v=0.

Steven

[Ian Robinson]

Well , this was something we discussed a fair bit when I was studying physics for a while.

My reading of this is while the dominant model suggests space time as we define it (post the "Big Bang") is of a radius about 14-15 bly.

An interesting thing is that with read shift effect due to inflation, once you get beyond about 15 byr , the universe is running away from us at an apparent velocity > c , so the light from these regions will never be visible ...

I don't accept that the universe came out of existance out of " nothingness ' , more that there was something surrounding the monoblock "before" it went "bang" . Don't ask me what , don't know , and it's probably unknowable.

[xelasnave]

Ian said...........

My reading of this is while the dominant model suggests space time as we define it (post the "Big Bang") is of a radius about 14-15 bly.

Ian one would think the radius is determined by how far light could travell given the age of the Universe however the expansion was of space and not limited to C ... I have found estimates for size of the Universe at some 160 billion light years across...
I dont know if that figure sits with others as reasonable nor can I site authority.. but others may be able toadd.
alex

[Ian Robinson]

Originally Posted by xelasnave

Ian said...........

My reading of this is while the dominant model suggests space time as we define it (post the "Big Bang") is of a radius about 14-15 bly.

Ian one would think the radius is determined by how far light could travell given the age of the Universe however the expansion was of space and not limited to C ... I have found estimates for size of the Universe at some 160 billion light years across...
I dont know if that figure sits with others as reasonable nor can I site authority.. but others may be able toadd.
alex

Along a similar line .... as far humanity is concerned, even if somehow we hang on in there for billions of years into the future, as the red shift of distant galaxies reach c , that will be as far as we can ever see irrespective of how big the universe gets beyond that limit.

[wraithe]

Hope you guys dont mind me dropping into this conversation?


Lets assume the "Big Bang" theory is wrong and the universe as a whole is not expanding...

If galaxies move indescriminitely, ie one influences another to either pull towards or push away, just like leaves in a pond, then there is the poissibility that the measurements of an expanding universe may actually be the effect of this part of the universe(visible to us) showing an expansion and another part maybe doing the complete opposite...

There is no way of confirming any theory of the size of the universe, but then the idea of a limit is very human in itself...

[xelasnave]

Originally Posted by wraithe

Hope you guys dont mind me dropping into this conversation?


Lets assume the "Big Bang" theory is wrong and the universe as a whole is not expanding...

If galaxies move indescriminitely, ie one influences another to either pull towards or push away, just like leaves in a pond, then there is the poissibility that the measurements of an expanding universe may actually be the effect of this part of the universe(visible to us) showing an expansion and another part maybe doing the complete opposite...

There is no way of confirming any theory of the size of the universe, but then the idea of a limit is very human in itself...

All input is always welocome and for my part more welcome if a different view can be aired.

Interestingly I think along the lines you outlined.
Humans think they have it all worked out but that does not mean they must be right.
I find the monopolization of observations to suit the big bang theory morosophic... can there be no other explaination for the cosmic background radiation I wonder.
Now there are folk who will say that I ignore the facts as if I have not looked at them... well I look at observations without seeing them as fulfilling a prediction exclusively but also on the basis of what other prospects they may generate.

Keep thinking keep questioning keep your self happy and well

alex

[g__day (Matthew)]

I'm pretty sure gravitationally bound structures aren't expanding - so atoms aren't getting bigger - nor the distance between them, its the spacetime fabric or framework between galaxies - which does not even have to be relativistically bound - which is said to be expanding.

Put simpler - where gravity is extremely weak - the framework of space and time that gravitational bound structures like galaxies and cluster hangs in is in itself getting more diffuse and larger. We don't know for a fact what physical frameworks hold dominance in the spacetime between galaxies - not even relativity is proven to be the dominant force across the vast emptiness of spacetime.

So when physicsts say 95% of matter is missing - the provisors should be given:

1) assuming relativity holds as the dominant framework for non gravitationally bound (empty) spacetime between galaxies
2) given what we observe is typical of the entire Universe (assumes homogenity in every direction - or that our corner of reality isn't an odd one)
3) it is not an exotic after effect of distorted geometry for spacetime itself arising from the aftershocks of the big bang and inflation

They are three very big, very untested provisors that we have no way of knowing hold true. Change a single one and our understanding of reality and missing matter changes totally. Einstein only postulated the second so the maths would be workable - with out that hugely simplifying constraint the field equations couldn't have been solved with only 20th century maths. Our mathematically frameworks have only been powerful enough to model two black holes interacting and merging in the last two years! Go back to the 1920s and 30s and there was no chance to solve the general field equations without alot of simplfying limits that were simply guessed at as reasonable.

Food for thought at the very least!

PS

The edges of our Hubble sphere (light cone) for the Universe, given a calculated age of 13.8 +/- 0.1 billion years - varies from 40 billion to 200 billion light years depending on what variant of the laws of physics you model as interacting to shape the geometry of expanding / unfolding (originally inflating) space time itself. Change how matter and energy curve spacetime, and the fall of rate (e.g. MOND) and you get wildy different geometry. Make spacetime 10 or 11 dimensional or allow super symmetric particles (s-particles as a form for instance of cold dark matter e.g. super massive, uncharged, slow s-neutrons) and you get different geometries again.

The key to what we have to unlock is the geometry of spacetime and what framework of laws shape it. To know that we need to study high energy events and postulate every more complex laws of physics. The best two I have seen lately is 1) scale relativity and 2) a variant of loop quantum gravity which has a tight constraint on matter and time - that being that time can only flow foward. Adding that second rule alone allows for the creation of large scale structures (galaxies) we see today in super computer simulations in a majority of cases. Without that rule (that no branch of physics currently enforces as a boundary condition or law) simulations of creation end in a jumbled mess of dimensions without structure - you can't it seems even force it to remotely create a universe like which we see today (Scienctifc America - July 2008 pg 24 - 29, the principles of casuality - The Self-Organising Quantum Universe- Jan Ambjorn, Jerzy Jurkiewicz and Renate Loll).

So that leaves theoretical physicists asking what governing principle in the framework exists - that we don't know about yet - is it that forces time to be one dimensional and directional (forward). Once we know what causes causility to be an organising principle of our Universe at a Qauntum level - we may understand its geomorty and topology alot better!