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-   -   Is the whole Universe expanding, or just the Observable Universe? (http://www.physicsforums.com/showthread.php?t=211849)

astrocat Jan29-08 04:28 PM

Is the whole Universe expanding, or just the Observable Universe?
 
I know the Observable Universe (OU) is Expanding, and I think this is (by now) beyond dispute. However, there is no evidence at all to show that every part of the Universe is expanding.

Perhaps the Universe as a Whole (UAW) is not expanding at all. What if it was contracting?

What if the OU was Expanding and the UAW was contracting? What if it just 'looked like' the UAW was Expanding, perhaps the way it just 'looked like' the Sun went around the Earth to the old timers?

If the UAW was contracting, there would have to be a reason - like for example, if the Universe came already expanded (not even requiring a Big Bang) and Gravity alone was pulling it together. There's a reason.

And evidence. There should be some evidence that Gravity is pulling us all together, of course. Well maybe there is.

Now, if Gravity was pulling us together, say from a centralised singularity - then wouldn't we be Speeding Up? Well, guess what , we are. And wouldn't the center be Speeding Up more than the outer edges?

Well, that might result in some Expansion, don't you think, with the middle collapsing faster than the edges?

Any expansion leads to cooling, I believe, just as any contraction leads to warming.

Okay, so now - politely please - tell me what You think....

jonmtkisco Jan29-08 05:25 PM

Hi Astrocat,

Obviously quite a few exotic scenarios are possible with respect to the parts of the universe which are beyond our particle horizon, but there is little reason to believe that the rest of the universe is any different than our part. The extremely high degree of homogeneity and isotropy of our universe out to the largest observable scales (such as the CMB) tend to make it extremely unlikely that at even larger scales the universe is very inhomogeneous in the way you suggest.

However, having said that, I should point out that David Wiltshire's current cosmological model (See "Dark energy is furphy" thread) proposes that our observable universe is located within an underdense perturbation within the larger flat universe. He suggests that the scale of our perturbation is so large that it extends beyond the horizon. He does NOT suggest that the universe as a whole is overdense (collapsing); he is trying to explain why our observable universe might be underdense while the CMB indicates that the universe as a whole is close to flat. Wiltshire's model proposes a large (or infinite) number of such both underdense and overdense perturbations, and not that our observable universe is at all unique in that respect.

One might speculate that, in the unlikely event that the universe as a whole were collapsing while our observable universe is not, it would most likely be because the whole universe is overdense (more mass/energy than the critical density omega) on average, rather than because of any one structure such as a black hole. That's because the inverse square law for gravitational strength; it would just drop off too quickly from an single point unless it literally had infinite mass. Even with a more conventional overdensity, this hypothetical universe would have to be extremely inhomogeneous.

Lots of modeling has been done about what would happen if the universe collapses. Some models include singularities, others do not.

The end of your note refers to "middle" and "edges", but it is believed that the universe cannot have a "middle" or an edge. That's called the Copernican Principle, if you want to check it out, for example on Wikipedia. There's lots of good stuff there you can learn from.

Jon

Loki Mythos Jan29-08 09:56 PM

Astrocat, I don't think it is beyond dispute. As I understand it the whole argument for an expanding universe is based on the red shift. There are those that argue that the red shift is merely the effect of gravity waves the propagate much faster then the speed of light causing galaxies to appear red shifted when in fact we are in a static universe. Most argue that gravity propagates at the speed of light and the red shift does mean an expanding universe. However, In the same camp, are others that say the red shift somehow indicates collapse. Anyway I hardly think the dispute is settled, regardless of any matter of fact type answers you might get.

astrocat Jan30-08 01:57 PM

Quote:

Quote by jonmtkisco (Post 1589466)
Hi Astrocat,

Obviously quite a few exotic scenarios are possible with respect to the parts of the universe which are beyond our particle horizon,

Well thank you very much, Jonmtikisco, if I can call you that, for agreeing with me. It's such a big Universe, after all, and perhaps only one of many, for all we can know. If we could see every part of the Universe, the Universe as a Whole (UAW) then we wouldn't even need an Observable Universe (OU) But fortunately for me we do.
Quote:

...but there is little reason to believe that the rest of the universe is any different than our part. The extremely high degree of homogeneity and isotropy of our universe out to the largest observable scales (such as the CMB) tend to make it extremely unlikely that at even larger scales the universe is very inhomogeneous in the way you suggest.
Whoa there! The best study of the Cosmos that I know of was done by Dr. Allan
Dressler and his team of the Seven Samurai, in the 1980's who discovered a 'large scale streaming' effect at work. They said that this 'streaming' was dragging the entire Virgo Cluster, along with our own Milky Way Galaxy toward the Hydra Centaurus Super Cluster, but that we could never reach the Hydra Centaurus Super Cluster, because it, the SuperCluster, was being dragged off in its turn, towards some Super Massive Entity they called 'The Great Attractor', faster than we are approaching it, the SuperCluster.

Sandra Faber, one of the team, described a Universe made up of Clumps and Filaments, seperated by voids, a Cosmos structured in some ways 'like a sponge'. What's so homogeneic about all that!

Quote:

However, having said that, I should point out that David Wiltshire's current cosmological model (See "Dark energy is furphy" thread) proposes that our observable universe is located within an underdense perturbation within the larger flat universe.
Yes, the Cosmos is indeed flat. It's also round. Pancake Shaped if you want to get technical. Confined, largely to the lateral plane.
Quote:

He suggests that the scale of our perturbation is so large that it extends beyond the horizon. He does NOT suggest that the universe as a whole is overdense (collapsing);
Ah yes, but I do. Claim the Cosmos is collapsing. There is nothing holding it up, you know.
Quote:

...he is trying to explain why our observable universe might be underdense while the CMB indicates that the universe as a whole is close to flat. Wiltshire's model proposes a large (or infinite) number of such both underdense and overdense perturbations, and not that our observable universe is at all unique in that respect.
I doubt that our Universe is unique in any rerspect.
Quote:


One might speculate that, in the unlikely event that the universe as a whole were collapsing while our observable universe is not, it would most likely be because the whole universe is overdense (more mass/energy than the critical density omega)
Yes, I see what you're saying. Is this Omega the Omega of Alexander Friedman?
Quote:

... on average, rather than because of any one structure such as a black hole.
Few people seem able to grasp just how massive some of these Black Holes really are. I think the range of their 'growing' gravitational attractions is infinite.

Quote:

That's because the inverse square law for gravitational strength; it would just drop off too quickly from an single point unless it literally had infinite mass. Even with a more conventional overdensity, this hypothetical universe would have to be extremely inhomogeneous.
I don't think Dr. Dressler described the Universe as being
homogeneous. Perhaps you wish it was, but it isn't.
Quote:

Lots of modeling has been done about what would happen if the universe collapses. Some models include singularities, others do not.
It's not really one single Black Hole, it's the 'zone' of Black Holes at the Center that is doing all the attraction.

You do realise, don't you, that the Cosmos and we in it, as we evolve, are Clumping Up? From the Early Days that COBE and W-Map showed us, there were only 'gravitational imbalances'; the 'seeds' of Galaxies. That was all there was. Since then, of course, we have 'Clumped Up' and the process continues. Now we see Black Holes all arounds us, including the bunch of them at the Galactic Center. Because they are both 'Black Hole' driven, there is not much difference in the evolution of either the Cosmos or, for example, M51,the Whirlpool Galaxy, or even our own Milky Way.
Quote:

The end of your note refers to "middle" and "edges", but it is believed that the universe cannot have a "middle" or an edge.
There is a lot of clap-trap said about the Universe. We have to be able to think for ourselves.
Quote:

That's called the Copernican Principle, if you want to check it out, for example on Wikipedia. There's lots of good stuff there you can learn from.
I love Wiki too!
Quote:


Jon

astrocat Jan30-08 02:25 PM

Quote:

Quote by Loki Mythos (Post 1589799)
Astrocat, I don't think it is beyond dispute. As I understand it the whole argument for an expanding universe is based on the red shift.

That's the Observable Universe (OU), Loki, if I can call you that. Yes the Observable Universe is Expanding. It's also Speeding Up and Cooling Down. Why don't you put them together? What happens when you fall into a vacuum cleaner?
Quote:

There are those that argue that the red shift is merely the effect of gravity waves the propagate much faster then the speed of light causing galaxies to appear red shifted when in fact we are in a static universe.
Um, I think the red shift shows the OU is Expanding. I think you should be able to see that?
Quote:

the Most argue that gravity propagates at the speed of light and the red shift does mean an expanding universe.
Yeah, basically, I suppose.[QUOTE] However, In the same camp, are others that say the red shift somehow indicates collapse.
Quote:

Anyway I hardly think the dispute is settled, regardless of any matter of fact type answers you might get.
Isn't Science great?


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