View Poll Results: Is Lawrence Krauss right about the universe being flat?  
Yes  12  41.38%  
No  17  58.62%  
Voters: 29. You may not vote on this poll 

#37
Nov909, 09:42 AM

P: 28

Please can I ask you to elaborate on this, as I am struggling to conceptualize it? To me the property of being closed (angles of a triangle add up to more than 180) implies that the topology will wrap back on itself. Can you give some examples of shapes where this is not the case  closed, but does not wrap back on itself? Also, I seem to remember Krauss talking about flatness implying that it is infinite in spatial extent (or am I just misunderstanding him again?)  doesn't "infinite in spatial extent" rule out a topology that wraps back on itself? Yes, I still don't quite have a handle on all this ;) Thanks :) 



#38
Nov909, 04:09 PM

Sci Advisor
P: 4,721

1. What if the curvature changes before it wraps back on itself? 2. What if it spirals in or some such instead of neatly meeting itself? 3. What if it just ends before meeting itself? I mean, sure, in the simplest case with absolutely constant curvature in all dimensions, we'd be talking about a sphere, which obviously wraps back on itself. But what if it isn't quite so simple? 



#39
Nov909, 08:58 PM

P: 86

I should state that I've met Krauss on several occasions and listened to several of his lectures and colloquiums; I do research under one of his previous colleagues (This isn't to say at all, however, that he remembers me). Krauss is an interesting fellow, to say the least.
Bytheby, he's neither wrong nor right. We don't know if the universe's curvature is zero or not. We know that, right now, however, it is very small and close to zero (and possibly negative, if memory serves). 



#40
Nov1009, 09:53 AM

Mentor
P: 6,044

A flat threedimensional torus is a model for a closed, homogeneous, anisotropic universe. The only flat universe that obeys the Cosmological Principle is one for which space is R^{3}. 



#41
Nov1009, 11:38 AM

Sci Advisor
P: 4,721





#42
Nov1009, 01:01 PM

Mentor
P: 6,044

a space that is closed in terms of the precise, mathematical curvature definition, but that is not compact; a space that is compact, but that is not closed in terms of the precise, mathematical curvature definition. Consequently, this geodesic in the flat torus induced by the geodesic y = pi*x in the plane doesn't close in the torus. 



#43
Nov1009, 03:13 PM

Sci Advisor
P: 4,721

k < 0: open k = 0: flat k > 0: closed This is the terminology usually used in observational cosmology. Clearly these terms were inspired by topology, but bear no direct relationship. 



#44
Dec109, 02:59 AM

P: 28

The only way that I can see these two statements being logically compatible is if the space between the quarks within a proton is no different from the empty space between protons. If the virtual particles 'popping' in and out of existence are a property of empty space whether that space is within a proton or not. Is this incorrect, or is there another way to understand it that is still allows the two statements above to be logically compatible? Thanks 



#45
Dec109, 03:03 AM

Sci Advisor
P: 4,721





#46
Dec109, 03:12 AM

P: 28

Thanks, does that then mean that if we were ever able to verify that the universe has a finite mass, it would imply that the universe must be finite in spatial extent, and therefore not perfectly flat?
Conversely if we were able to verify that the universe is flat on the largest scales, would it imply that the universe's mass is infinite? Does Prof. Krauss' claim that the universe is flat refute Hawking's 4d closed manifold hypothesis? 



#47
Dec109, 04:41 AM

P: 121

This is about the Ex Nihilo right? I too believe in this and will post why when I come back from lectures.




#48
Dec109, 04:49 AM

P: 28

1)
time index: 21:20 "It turns out, most of the mass of the proton comes not from the quarks within the proton, but from the empty space between the quarks. these fields produce about 90% of the mass of the proton, and since protons and neutrons are the dominant stuff in your body, empty space is responsible for 90% of your mass." 3) time index: 25:00 "The universe can be one of three different geometries: open, closed or flat" 4) time index: 25:30 "And an open universe would be infinite in spatial extent as would a flat universe" 5) time index: 29:25 "we know how many protons and neutrons there are in the universe, we can actually measure that" Please show me where I am going wrong: a) (1) AND (2) AND (4) implies that a flat or an open universe would have an infinite mass. b) (a) AND (3) AND (5) implies that our universe can't be open or flat  it must be closed. but (b) cant be true because evryone here including Prof Krauss says that we have observationally shown that the universe is so close to perfectly flat, that it may as well be. this is the exact reason why I cant reconcile the logic, please can someone show me where I am going wrong? 



#49
Dec109, 03:21 PM

Sci Advisor
P: 4,721

Well, I think in point (5) he's actually not talking about everything, just the observable universe. So the problem here is that we're talking about two different uses of the word 'universe'.




#50
Dec209, 03:42 AM

P: 216

He also says in this lecture:
"What’s so beautiful about a universe with total energy: zero? Well, only such a universe can begin from nothing. And that is remarkable because the laws of physics allow a universe to begin from nothing. You don’t need a deity. You have nothing, zero total energy, and quantum fluctuations can produce a universe." Wouldn't that universe have to disapear in under a second? 



#51
Jul2610, 02:13 AM

P: 1

Here's my amateur's take on the implications of a flat universe, having just watched Krauss' lecture and read these comments and done some of my own thinking. Please, let me know whether I'm making sense. In a flat universe, it's possible for there to be infinite mass and infinite space if the universe is expanding, because due to relativity, all the mass that is far enough away would be moving away from us faster than the speed of light and all of that mass would not be observable or have any affect on us in any way. It would be as if that mass doesn't exist for us. That leaves only a finite amount of mass that is not moving away from us faster than the speed of light, which is our observable universe, which is finite in both mass and spatial extent. So it's possible that the 'actual' universe is infinite in space and mass while our observable universe is finite in both ways.
Another thing I was thinking about is Krauss' picture of the future of the universe, when galaxies get so far apart that all galaxies are moving away from each other faster than the speed of light, and so as far as any observer could then tell, only the galaxy that is around the observer exists. Now consider the empty space between the galaxies. In that space, if all galaxies are moving away from the empty space faster than the speed of light, it would be as if there was absolutely nothing in the universe at all from the 'point of view' of that empty space. Now, isn't that the same situation that we were at at the beginning of the universe? So if the conditions are the same, shouldn't the outcome be the same? Shouldn't we expect quantum fluctuations to create universes from nothing? So what we end up with as a picture of our universe is an ever expanding universe in which matter keeps spreading out more and more, but in which more galaxies keep popping up within the empty spaces, albeit perhaps only when they are so far away that we can't notice them. 



#52
Jul2610, 02:36 AM

Sci Advisor
P: 4,721

Unfortunately, the speed of light limitation isn't so simple in General Relativity: most of the galaxies we see today have always been moving away faster than the speed of light, by the most obvious definition of speed (speeds of distant objects are actually arbitrary: there is no one choice of how to define it). If there are "other universes" being produced out of vacuum fluctuations within our own universe, those universes would appear to us as microscopic black holes that fluctuate out of the vacuum, then rapidly decay again via Hawking radiation. They might extend in their own spacetime off into infinity, but are only connected to us via a tiny blip that looks like a microscopic black hole. Forever after, they are disconnected and cannot interact with our universe in any way. 



#53
Jul3110, 05:45 PM

P: 1,620

It seems that too many people make this same implication, which is just an assumption, but doesn't need to be right. It would (IMHO) make more sense to assume that space near the Big bang was already infinite, and thus is still infinite. This certainly makes sense in the context of the inflationary scenario, in which in other parts of the universe, inflation keeps gong on, creating ever more expanding universe bubbles. 



#54
Aug310, 02:35 PM

P: 3

Universe can be flat or not
But there are not only one universe , there are total 14 universes. Out of 14 universes some universes can be flat and some can't I think universe cannot be defined as it is flat or not, Actually it is out of the boundry of the word " flat " 


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