
#127
Jul808, 01:37 PM

P: 15,325

In the 2D "balloon" analogy the universe is finite (its width is the circumference of the balloon) yet unbounded (there is no edge, and no point on the surface is "privileged"). It also means no point on the surface is privileged with having a different mass distribution, and that means no point on the surface has a gravitational gradient differnt from anywhere else. In the equivalent 4D universe, it has a finite size, yet, because it wraps around there is no boundary. This means no point is privielged with an different mass distribution because no point is "nearer an edge" than any other. 



#128
Jul808, 03:35 PM

P: 2,043

Perhaps this is all confusing due to terminology but under GR a closed spacetime is always finite while an open spacetime is always infinite. In addition such spacetimes are also resp. spatially finite and spatially infinite.




#129
Jul808, 03:54 PM

P: 32

Dave thats great as you have a way with words. Well said. Now as I see it space time is curved and exspanding even though the light we see looks like it is traveling a straight line. Light travels at C due to space exspanding at C so space time started first so it is the cause and light speed is the effect. I maintain this is not like either but space time is something and has a quantum foam were particles appear from space time and then disappear. It has a zero point energy. It seems to be that everything inside our universe is something and there is nothing outside not even nothing. Maybe there is an outside but it would have to be in the 5th demension. Okay who are you voting for and how much money do you make and is there a God? Please don't answer that as I was just kidding.




#130
Jul808, 05:17 PM

P: 2,043





#131
Jul808, 06:56 PM

P: 32

Jennifer, hmmm if space time doesn't expand, why is the universe getting bigger? Also it is accelerating as time goes on. Does not the space between the galaxys make the universe bigger now than in the past? Maybe I'm using the wrong words? Please let me know were I'm going wrong.
I know that the speed of light C is not due to space time exspanding. So please explain how that works. Why is the c just about 186,000 mps and not 200000 mps? I'm truly happy as I think I'm going to learn something. Thankyou; milt 



#133
Jul808, 07:46 PM

P: 2,043





#134
Jul808, 09:45 PM

P: 32

Thankyou for your help. milt 



#135
Jul808, 11:26 PM

P: 32

I'm so new here I didn't figure out how to post with a quote.




#136
Jul808, 11:44 PM

P: 267





#137
Jul908, 01:05 AM

P: 32

MeJenniferAs far as I can tell when I think about it space time is 4 demensions and space is 3 dementions but it exists only in my head. No jokes please. How can space exspand without bringing the 4th demension along with it? You do think that the universe is getting bigger and "exspanding"?




#138
Jul908, 01:47 AM

Sci Advisor
PF Gold
P: 9,182

An observationally finite universe makes sense to me. Mainly because there is no observational evidence to the contrary. Nominate the observation[s] you have in mind that refute this proposition.




#139
Jul908, 03:32 AM

PF Gold
P: 513

Dave, thanks for the response in #129. However, if I could just push on a couple of points before I go away and do some more reading. The point of my questions is to try to understand what is generally accepted fact, albeit still subject to verification, and what is still speculative:
1) The universe is said to be expanding, based on redshift measurements, cepheid luminosity etc. This position is generally supported by CMB verification? Therefore, didn’t really understand the comments about space/spacetime not expanding in #132/135. 2) When people talk about curved spacetime, are they referring to gravitational spacetime curvature or the open/closed issue? 3) Given a density in the order of about 20 particle/m^3, a largescale homogeneous universe would appear to be locally gravitationally flat, although this ignores the issue of whether there is an overall gravitational centre. See specific comments to #129 below? 4) The description of the universe being opened, closed or flat seems to be based on models of the universe using GR and the cosmological principle, e.g. homogeneous and isotropic, but ultimately dependent on the total energymass density assumptions of the universe. Today, the general assumption is that k=0, even though the level of speculation regarding the real nature of the energydensity is still high. 5) The case, k=0 is a special form of an open universe described as ‘flat’. Such a universe will expand forever, albeit at an everdecreasing rate? Ok, just wanted to get some assumptions in the open for clarification. I have snipped the following quotes from #129 for reference: While I think I understand the implication that the balloon analogy closes the curvature of spacetime, hence avoids any ‘gravitational gradient difference’ on the scale of the universe, it would seem that you must physically link the gravitational effects on one side of the universe to the other? Finally, one last question with respect to the following quote in #140 P.S. Just for the record, I am actually agnostic on the question raised in this thread, simply because there does not seem to be sufficient evidence, as yet, to be conclusive. However, somebody in the forum may be able to correct me on this assumption. 



#140
Jul908, 11:33 AM

P: 3,966





#141
Jul1008, 05:04 AM

PF Gold
P: 513

Hi Kev, I started to take a look at the gravitational implications of black holes and noticed the poll raised by Marcus in this forum. Therefore, I decided to raise a few basic questions on cosmology although, at the moment, I am simply trying to get a better handle on what is substantiated theory and what is still speculative.
 As a slight aside, many texts explain the expansion and age of the universe via reference to the Friedmann equation set. This leads to different rate of expansion due to the dominance of matter and radiation in the universe. However, it appears that the actual timeline now being associated with standard model are derived on a different, or refined, set of assumptions, which I have not yet got a clear picture. Hence the similar nature of the discussion in the following thread: http://www.physicsforums.com/showthread.php?t=243968  http://arxiv.org/abs/0711.4810 At this point I should really apologises to the originator of this thread because I recognise that I have wandered off the topic originally posted, although I believe that it does have some relevance to the question raised. However, I would still like to better understand what evidence supports the 2D balloon analogy that leads to the suggestion of a 4D universe that wraps around and, in doing so, avoids any ‘gravitational gradient difference’ on the scale of the universe, especially if current observations imply an essentially a flat universe. 



#142
Jul1008, 09:56 AM

P: 3,966





#143
Jul1108, 02:33 AM

Sci Advisor
PF Gold
P: 9,182

Blurring the lines, I think. Observational evidence still implies a finite universe, IMO.




#144
Jul1108, 07:11 AM

PF Gold
P: 513

Hi Chronos: Could I ask what you would briefly list as the top observational evidence that supports a finite universe and what % confidence you have in this evidence?
o As I understand it, dark matter has been speculated based on the observation of a number of gravitational anomalies, i.e. rotation anomalies in spiral galaxies to gravitational lensing around unseen objects? o The first candidate were called MAssive Compact Halo Objects (MACHOs) which consisted of Jupitersized planets, brown dwarf stars, faint lowmass stars, white dwarf stars and even black holes. However, this idea would only account for a fraction of the dark matter required to explain all the previous anomalies? o I understand that Weakly Interacting Massive Particles (WIMPs) is now the frontrunner for the missing matter, which corresponds to the description of Cold Dark Matter? However, the existence of WIMPs has not yet been verified o Would dark matter or dark energy affect the radius of the speculated event horizon? o Would this conceptual black hole universe have a centre of gravity? o Would your caveat to Newton Shells apply, i.e. do we really understand the meaning of time within a black hole to evaluate the scope of time dilation? Apologises if these questions appear to becoming overly speculative, but they are not being forwarded as any sort of proposal and they did seem to be a logical extrapolation of some other discussions already taking place in this forum. 


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