View Poll Results: Is Lawrence Krauss right about the universe being flat?
Yes 12 41.38%
No 17 58.62%
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'A Universe From Nothing' by Lawrence Krauss, AAI 2009


by Schlofster
Tags: universe expansion
Schlofster
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#37
Nov9-09, 09:42 AM
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Quote Quote by Chalnoth View Post
Er, well, no. A torus is not closed in that sense. It's flat.

Closed is a statement about curvature. Whether or not the universe wraps back on itself is a statement about topology. It is perfectly possible to have a flat universe with a topology that wraps back on itself. Just as it is, in principle, possible to have a closed universe that doesn't extend far enough to wrap back on itself.
Hi Chalnoth,
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 :-)
Chalnoth
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Nov9-09, 04:09 PM
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Quote Quote by Schlofster View Post
Hi Chalnoth,
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.
Well, there are a few different possibilities here:

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?

Quote Quote by Schlofster View Post
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?
I don't think he actually said that. If he did, he's wrong. But yes, if it so happened that our universe was infinite in spatial extent, then it clearly couldn't wrap back on itself.
GoldPheonix
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#39
Nov9-09, 08:58 PM
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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.


Quote Quote by Schlofster View Post
I think that I might be starting to see a pattern emerge - it seems to me that the intellectual abilities that scientists like Krauss are so far beyond what ordinary humans are capable of, he cant even articulate them at all in a way is intelligible by the majority of humans on this planet.

Ordinary humans would understandably respond with incredulity, and since they pay his salary, I suggest that people like him should only make statements that are logically intelligible ordinary people.
I think that public lectures like this just serve to polarize the voting public, against science.

I am rather interested though in what the evolutionary consequences of this cavernous gap between the intelligentsia and the rest of the human population would be.
Is it possible that we eventually see a divergence in the species - this may also be accelerated if people are selected for establishing a human colony on another planet like Mars.
Presumably the criteria for selecting the people for this mission would be heavily biased in favour of intellect.
You should send him an e-mail... He'd probably actually respond to you if you phrased it that way.


Quote Quote by Schlofster View Post
what I said is that most of the mass of protons and neutrons can be attributed to virtual particles.. it is that mass that we can calculate using the theory of the strong interaction and powerful computers..

_______________________________
Lawrence M. Krauss
Foundation Professor
Director, Origins Initiative
Co-Director, Cosmology Initiative
ARIZONA STATE UNIVERSITY
College of Liberal Arts and Sciences
School of Earth and Space Exploration
http://krauss.faculty.asu.edu

On Nov 3, 2009, at 5:04 PM, Jessica Lee wrote:


------ Forwarded Message
Date: Wed, 28 Oct 2009 06:48:23 -0700
Subject: A Question For Prof Krauss.


Hi Jessica,
Please could I ask you to send this question on to Prof. Krauss?

Hi Prof Krauss,
I loved your recent public lecture <http://www.youtube.com/watch?v=7ImvlS8PLIo> at AAI 2009, but I don't understand something,

and I was hoping that you might be able to spare a few moments to clear it up for me.

First of all let me state that I am no physicist, so I am rather certain that my
misunderstanding is due to an incomplete grasp of the mathematical model that you
attempt to describe in natural language. I do understand that it is actually impossible to
do this since the mathematical model cannot be expressed in natural language.

In the lecture you say that you have calculated the mass of the universe, and have calculated that
most of the mass in the universe is as a result of virtual particles "popping in and out of existence" in empty space,
while matter particles constitute a very small fraction of the mass of the universe.
Considering this, and since you are able to calculate the mass of the universe to be a finite quantity,
it seems to me that a conclusion that the universe is infinite in spatial extent would be logically incompatible with a universe of finite mass.
I say this because if empty space itself constitutes most of the mass of the universe, then if the amount of empty space is infinite,
then the mass of the universe must also be infinite.

Thanks very much & kind regards,


------ End of Forwarded Message
Tell me how this goes. I'm interested to see what he replies with.



By-the-by, 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).
George Jones
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Nov10-09, 09:53 AM
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Quote Quote by Skolon View Post
Yes, maybe a torus is mathematically flat shape but is a strange kind: a torus shape Universe is closed (going on any direction you will finally arrive at the same point) but for each direction other "diameter" of Universe exist. So, it seems like each direction have its own properties.
In our Universe all directions seem to have the same properties, at least until now. Or I don't know well?
Yes, this model is closed, and is different in different directions (anisotropic).
Quote Quote by Chalnoth View Post
Er, well, no. A torus is not closed in that sense. It's flat.

Closed is a statement about curvature. Whether or not the universe wraps back on itself is a statement about topology.
I don't think so. "Closed" is a statement about topology, not about curvature. A universe is closed iff its spatial sections are topologically compact. I think that this usage is standard. I cant think of how to use (the) curvature (tensor) to distinguish between "closed" and "open."
Quote Quote by Chalnoth View Post
Just as it is, in principle, possible to have a closed universe that doesn't extend far enough to wrap back on itself.
I'm not sure what this means.
Quote Quote by Chalnoth View Post
But in any case, it's still perfectly possible that the circumference in each direction is identical.
But this model is still anisotropic. Along the axes of symmetry, there are geodesics that close, but there also are geodesics "between" these axes that wind around the torus, never closing.

A flat three-dimensional 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 R3.
Chalnoth
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Nov10-09, 11:38 AM
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Quote Quote by George Jones View Post
Yes, this model is closed, and is different in different directions (anisotropic).
Well, that doesn't have any meaning in observational cosmology, as when we refer to "closed" we exclusively refer to the curvature, not to the topology (which is unobservable). Well, I mean, a topologically closed universe could, in principle, be observed. But the fact that it hasn't so far seems to indicate that if our universe is topologically closed, then we won't ever be able to tell (because the distance is just too great, by current observational limits).

Quote Quote by George Jones View Post
But this model is still anisotropic. Along the axes of symmetry, there are geodesics that close, but there also are geodesics "between" these axes that wind around the torus, never closing.
In what sense? Given that a torus is finite, any geodesic must necessarily come back to itself eventually. Some will take longer than others, of course, but it must eventually occur.

Quote Quote by George Jones View Post
A flat three-dimensional 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 R3.
In the sense that the word "flat" is used in observational cosmology, a torus is quite flat. But I thought you said it wasn't anisotropic? Of course, it is anisotropic in the sense that not all geodesics that wrap around a torus come back after the same distance is crossed (like a sphere). But it is very much flat in terms of curvature.
George Jones
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Nov10-09, 01:01 PM
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Quote Quote by Chalnoth View Post
Well, that doesn't have any meaning in observational cosmology, as when we refer to "closed" we exclusively refer to the curvature, not to the topology (which is unobservable).
If "closed" is defined using curvature, and is not equivalent to topological compactness, I guess I need a precise, mathematical definition of "closed" in terms of curvature, and at least one of:

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.
Quote Quote by Chalnoth View Post
In what sense? Given that a torus is finite, any geodesic must necessarily come back to itself eventually. Some will take longer than others, of course, but it must eventually occur.
Here's what I had in mind. For simplicity, consider only two spatial dimensions. The flat torus is constructed by putting an equivalence relation on the 2-dimensional plane, with (x, y) ~ (x + n , x + m) for all integers n and m. Now consider the geodesic y = pi*x in the plane, which goes the point (0, 0). The only way that this geodesic can close in the torus is if the line goes through a point in the plane that is equivalent to the point (0, 0), i.e., if the line goes through a point (n, m) for some integers n and m. But a line that goes through (0, 0) and (n, m) has a slope that is a rational number, which can't be equal to the slope pi of the original line.

Consequently, this geodesic in the flat torus induced by the geodesic y = pi*x in the plane doesn't close in the torus.
Quote Quote by Chalnoth View Post
In the sense that the word "flat" is used in observational cosmology, a torus is quite flat.
I agree, and I didn't write anything that contradicted this.
Quote Quote by Chalnoth View Post
But I thought you said it wasn't anisotropic?
I didn't writes this, and I didn't mean to give the impression that I believed this. Quite the opposite; in the first sentence of my previous post, I explicitly stated that the flat torus is anisotropic.
Quote Quote by Chalnoth View Post
Of course, it is anisotropic in the sense that not all geodesics that wrap around a torus come back after the same distance is crossed (like a sphere).
Quote Quote by Chalnoth View Post
But it is very much flat in terms of curvature.
Yes.
Chalnoth
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Nov10-09, 03:13 PM
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Quote Quote by George Jones View Post
If "closed" is defined using curvature, and is not equivalent to topological compactness, I guess I need a precise, mathematical definition of "closed" in terms of curvature, and at least one of:
From the parameter k in the FLRW metric:

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.
Schlofster
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#44
Dec1-09, 02:59 AM
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Quote Quote by Schlofster View Post
time index: 19:35
what could contribute a term like this, and we know the answer - nothing - by "nothing", I don't mean nothing, I mean nothing.
If you take empty space, and that means get rid of all the particles, all the radiation, absolutely everything,
so there is nothing there, if that nothing weighs something, then it contributes a term like this.
Quote Quote by Schlofster View Post
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.
I am sorry to bring this up again, but I have just understood why I drew the conclusions I did from what he says here.
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
Chalnoth
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Dec1-09, 03:03 AM
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Quote Quote by Schlofster View Post
I am sorry to bring this up again, but I have just understood why I drew the conclusions I did from what he says here.
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
Yes, that's basically right. The only difference with the empty space within a proton is that you've got these strong nuclear forces between the quarks that cause additional excitations of the vacuum.
Schlofster
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#46
Dec1-09, 03:12 AM
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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?
Unto
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#47
Dec1-09, 04:41 AM
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This is about the Ex Nihilo right? I too believe in this and will post why when I come back from lectures.
Schlofster
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#48
Dec1-09, 04:49 AM
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1)
Quote Quote by Chalnoth View Post
Yes, that's basically right. The only difference with the empty space within a proton is that you've got these strong nuclear forces between the quarks that cause additional excitations of the vacuum.
2)
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?
Chalnoth
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#49
Dec1-09, 03:21 PM
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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'.
Freeman Dyson
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#50
Dec2-09, 03:42 AM
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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?
shoejunk
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Jul26-10, 02:13 AM
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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.
Chalnoth
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Jul26-10, 02:36 AM
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Quote Quote by shoejunk View Post
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.
Well, you actually get the same answer in Newtonian gravity in this case. It's due to the [itex]1/r^2[/itex] falloff of gravity: stuff further away contributes less to the force, so that if you add up all the mass extending to infinity, the result is a finite effect.

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).

Quote Quote by shoejunk View Post
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.
Well, this view of galaxies popping out of the expanding space was actually championed a while ago as an alternative to the big bang theory, termed the steady state universe. The observation of the CMB, though, basically cemented that it's the big bang, not the steady state.

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 space-time 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.
heusdens
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Jul31-10, 05:45 PM
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Quote Quote by Schlofster View Post
I started this thread a while ago talking about Stephen Hawking's "No Boundary Condition".

I think that I finally got a handle on it, but now Lawrence Krauss has discussed here why he thinks that the universe is not closed, but flat, and he says by implication, 'infinite in spatial extent'.
(he also claims to have empirical evidence of the flatness of space-time on the largest scales)

I don't understand how he can reconcile this with the big bang (which he also seems to accept).
If the universe is infinite in spatial extent, at what point did it become infinite, because when the universe was 1 second old, it was not infinitely large (I think that this is the scientifically accepted view).
He is obviously a widely respected physicist, and I am not a physicist, so I expect that I just don't understand what he is saying.

Could anyone explain it to me, or can it not be expressed in natural language?

Thanks,
Schlofster
In the metaphore of the universe being a flat sheet of rubber (the 2D analpgue of the 3D universe) that expands uniformly in all directions, it is true that (in the mathematical sense, not in the physical sense) all space we can observe now can be brought back to a single 0D spot, but that does not claim to say that all of space was just that single 0D dot.

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.
Nihar Shah
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#54
Aug3-10, 02:35 PM
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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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