The BGV Theorem

[ParaclitosLogos]

It sounds like the model you are describing starts at a finite time in the past t0, then has a contracting phase that approaches a singularity, then an expanding phase that becomes our universe.

Am I understanding you right?

The LQC model posits that there is a contraction phase from infinite past "past"  then at around planck density gravity becomes repulsive and there is a bounce into an expanding phase


 So it goes

t--> - oo      contraction    "  planck scale-repulsive-gravity t=0   "    inflation-BB -expansion-   t-->oo

What I am saying is


t= -tn  nucleation-contraction  "  planck scale-repulsive-gravity t=0   "  inflation-BB -expansion-   t-->oo


-tn: finite past time
Nucleation: Vilenkin´s model type quantum nucleation.

Vilenkin´s quantum nucleating model allows for a universe to tunnel into existance in a contracting phase, also (not necessarily in an expanding phase).

[grosso]

Okay that helps, thanks. I have a question though.

1. On LQC, I understand a "contraction from infinite-past" escape the expansion-on-average criteria of BGV. But can it achieve the same goal with a more modest proposal? Say, if they postulate the universe has been contracting from 100 billion years, doesn't it achieve the same effect?

[ParaclitosLogos]

Okay that helps, thanks. I have a question though.

1. On LQC, I understand a "contraction from infinite-past" escape the expansion-on-average criteria of BGV. But can it achieve the same goal with a more modest proposal? Say, if they postulate the universe has been contracting from 100 billion years, doesn't it achieve the same effect?

I think it does, but -100 billion years seems more arbitrary than -OO, and, how they go about showing that is the case ? at least with -oo they have an excuse.

Though, I think they have philosophical and practical arguments against -oo. (not to mention, that I have no clue what t-->-oo means (that´s not an actual infinite, is it?))

I think what my argument seems to show is that conceptually LQC can not guarantee an eternal universe.

[grosso]

About the ad hoc nature of -100 billion years: That's true, you're right.

About t-->-00, it sounds like an actual infinite. I don't know how it can be anything else. Maybe redtilt1 can enlighten us on this.

[ParaclitosLogos]

About the ad hoc nature of -100 billion years: That's true, you're right.

About t-->-00, it sounds like an actual infinite. I don't know how it can be anything else. Maybe redtilt1 can enlighten us on this.

As far as I know these equations are usually worked using limits , not sets.

[grosso]

I guess the question is "What happens when you translate a limit from an equation to reality?"

[ParaclitosLogos]

I guess the question is "What happens when you translate a limit from an equation to reality?"

You get unlimited membership to Disneyworld, but, it is only valid for yesterday.


I either ran out of creative answers, or sucumbed to the incapacity of making sense of what seems to me a total absurd.

It is best to wait for redtilt, probably.

[redtilt1]

you asserting something doesnt make it so. On what basis do you say i misunderstood it? What point do you think Craig was trying to make by using that quote?

How is a contraction prior to expansion not a description of the physical universe? You havent any justification for that statement, again you asserting something doesn't make it so.

Your quotes about quantum cosmology are irrelevant. He's discussing his own "tunnelling from nothing" model. That has nothing to do with other quantum models such as LQC or string gas cosmology.

Your statement about empirical support is irrelevant. WLC clearly stated that even if the models were true then they are not eternal into the past. That is simply false. You can watch the people that made these models discuss them here:
here:https://www.youtube.com/playlist?list=PLANt-1sb3M3o82YWDDm3oiFwToHVj07Vo

Furthermore, there is no evidence for anything that happens in the planck regime , that includes models with a beginning and models without. So to pretend one conclusion is backed by the evidence is just wrong.

I just pointed out half a dozen ways in which you misunderstood aspects of the issue.

But at the risk of repeating myself,

When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.

They are referring to the very nature of the model within which the universe is conceived. Broadly we have two available models that can serve as underlying physical descriptions: relativistic cosmology and quantum cosmology.

This is what is meant by physical descriptions. Instead of doubting me, just look it up yourself and follow the evidence. You'll see.

On top of Vilenkins quote, as for LQC, the only salient feature that can help the LQG model avoid a beginning is if the contraction prior to the expansion is so massive that the universe is contracting on average. This feature, which makes it exempt from the BGV is an ad hoc feature.

It's an interesting model, but doesn't touch the Kalam.

Delicate flower you say:
“When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.”

This is simply not the case, a contracting universe is valid solution to the Einstein equations and there is no reason at all not to call it a physical description. You have  provided no reference, so I see no reason to take what you are saying seriously. It seems like a desperate dodge in order to excuse WLC from saying something that is so obviously wrong.

At my university the cosmology course is taught with the title “physical cosmology” it is defined here on wikipeida as
“Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate”

So of course a contracting period is included as a physical description.

You are wrong yet again to say that the bGV is only violated if the universe is contracting on average, not so. H has to be greater than zero, so in order to violate the theorems assumptions , H can be negative ( the case you describe ) or it can be zero , or it can be undefined.
I would say that as long as you can show that there was a contracting period prior to the big bang then H is undefined. Unless of course you assume from some symmetry principle than the universe is expanding to infinity on both side of the bounce point then h=0). i.e. if you drew a little dot representing the bounce point then there would be two cone shapes expanding to infinfity either side of this point, but one is contracting before the bounce from our point of view.
I think the simplest thing to conclude is nothing about the ultimate length of time of the contraction. Therefore H is undefined and the BGV theorem cannot be used as a proof of the beginning.

So when you say that the lQC doesn’t touch the kalam you are simply wrong. Even Vilenkin admits that a contracting universe prior to expansion does violate the theorem. Of course it isn’t just LQC that does this,  string cosmology model and horava gravity also finds the same conclusion. In string cosmology its common to use the dualtities of string theory to show a symmetry around what they call “the self dual point”. This is derived in a different manner to LQC but ultimately the conclusion is the same, a mirror image of our universe lies on the other side of the self dual point i.e. a contracting period of the universe. Again if this is true then it violates the BGV theorem. Now recall what WLC said in his debate with Peter Milican. He said even if these theories are true then his conclusion (universe had an absolute beginning) still holds and that is blatantly false.

[grosso]

you asserting something doesnt make it so. On what basis do you say i misunderstood it? What point do you think Craig was trying to make by using that quote?

How is a contraction prior to expansion not a description of the physical universe? You havent any justification for that statement, again you asserting something doesn't make it so.

Your quotes about quantum cosmology are irrelevant. He's discussing his own "tunnelling from nothing" model. That has nothing to do with other quantum models such as LQC or string gas cosmology.

Your statement about empirical support is irrelevant. WLC clearly stated that even if the models were true then they are not eternal into the past. That is simply false. You can watch the people that made these models discuss them here:
here:https://www.youtube.com/playlist?list=PLANt-1sb3M3o82YWDDm3oiFwToHVj07Vo

Furthermore, there is no evidence for anything that happens in the planck regime , that includes models with a beginning and models without. So to pretend one conclusion is backed by the evidence is just wrong.

I just pointed out half a dozen ways in which you misunderstood aspects of the issue.

But at the risk of repeating myself,

When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.

They are referring to the very nature of the model within which the universe is conceived. Broadly we have two available models that can serve as underlying physical descriptions: relativistic cosmology and quantum cosmology.

This is what is meant by physical descriptions. Instead of doubting me, just look it up yourself and follow the evidence. You'll see.

On top of Vilenkins quote, as for LQC, the only salient feature that can help the LQG model avoid a beginning is if the contraction prior to the expansion is so massive that the universe is contracting on average. This feature, which makes it exempt from the BGV is an ad hoc feature.

It's an interesting model, but doesn't touch the Kalam.

Delicate flower you say:
“When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.”

This is simply not the case, a contracting universe is valid solution to the Einstein equations and there is no reason at all not to call it a physical description. You have  provided no reference, so I see no reason to take what you are saying seriously. It seems like a desperate dodge in order to excuse WLC from saying something that is so obviously wrong.

At my university the cosmology course is taught with the title “physical cosmology” it is defined here on wikipeida as
“Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate”

So of course a contracting period is included as a physical description.

You are wrong yet again to say that the bGV is only violated if the universe is contracting on average, not so. H has to be greater than zero, so in order to violate the theorems assumptions , H can be negative ( the case you describe ) or it can be zero , or it can be undefined.
I would say that as long as you can show that there was a contracting period prior to the big bang then H is undefined. Unless of course you assume from some symmetry principle than the universe is expanding to infinity on both side of the bounce point then h=0). i.e. if you drew a little dot representing the bounce point then there would be two cone shapes expanding to infinfity either side of this point, but one is contracting before the bounce from our point of view.
I think the simplest thing to conclude is nothing about the ultimate length of time of the contraction. Therefore H is undefined and the BGV theorem cannot be used as a proof of the beginning.

So when you say that the lQC doesn’t touch the kalam you are simply wrong. Even Vilenkin admits that a contracting universe prior to expansion does violate the theorem. Of course it isn’t just LQC that does this,  string cosmology model and horava gravity also finds the same conclusion. In string cosmology its common to use the dualtities of string theory to show a symmetry around what they call “the self dual point”. This is derived in a different manner to LQC but ultimately the conclusion is the same, a mirror image of our universe lies on the other side of the self dual point i.e. a contracting period of the universe. Again if this is true then it violates the BGV theorem. Now recall what WLC said in his debate with Peter Milican. He said even if these theories are true then his conclusion (universe had an absolute beginning) still holds and that is blatantly false.

ontologicalme, can I tag you in here? I could use some clarity.

When the claim is made that the BGV applies independent of the physical description of the universe, does it mean:

a) Independent of whether you're assuming classical spacetime or quantum spacetime?

or

b) Independent of properties of the type: "having a contraction prior to expansion"?

My hunch is (a), redtilt insists on (b). What do you think?

[ParaclitosLogos]

you asserting something doesnt make it so. On what basis do you say i misunderstood it? What point do you think Craig was trying to make by using that quote?

How is a contraction prior to expansion not a description of the physical universe? You havent any justification for that statement, again you asserting something doesn't make it so.

Your quotes about quantum cosmology are irrelevant. He's discussing his own "tunnelling from nothing" model. That has nothing to do with other quantum models such as LQC or string gas cosmology.

Your statement about empirical support is irrelevant. WLC clearly stated that even if the models were true then they are not eternal into the past. That is simply false. You can watch the people that made these models discuss them here:
here:https://www.youtube.com/playlist?list=PLANt-1sb3M3o82YWDDm3oiFwToHVj07Vo

Furthermore, there is no evidence for anything that happens in the planck regime , that includes models with a beginning and models without. So to pretend one conclusion is backed by the evidence is just wrong.

I just pointed out half a dozen ways in which you misunderstood aspects of the issue.

But at the risk of repeating myself,

When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.

They are referring to the very nature of the model within which the universe is conceived. Broadly we have two available models that can serve as underlying physical descriptions: relativistic cosmology and quantum cosmology.

This is what is meant by physical descriptions. Instead of doubting me, just look it up yourself and follow the evidence. You'll see.

On top of Vilenkins quote, as for LQC, the only salient feature that can help the LQG model avoid a beginning is if the contraction prior to the expansion is so massive that the universe is contracting on average. This feature, which makes it exempt from the BGV is an ad hoc feature.

It's an interesting model, but doesn't touch the Kalam.

Delicate flower you say:
“When cosmologists talk about physical description they have something very specific in mind, much narrower than individual properties of the universe like a contraction prior to expansion.”

This is simply not the case, a contracting universe is valid solution to the Einstein equations and there is no reason at all not to call it a physical description. You have  provided no reference, so I see no reason to take what you are saying seriously. It seems like a desperate dodge in order to excuse WLC from saying something that is so obviously wrong.

At my university the cosmology course is taught with the title “physical cosmology” it is defined here on wikipeida as
“Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate”

So of course a contracting period is included as a physical description.

You are wrong yet again to say that the bGV is only violated if the universe is contracting on average, not so. H has to be greater than zero, so in order to violate the theorems assumptions , H can be negative ( the case you describe ) or it can be zero , or it can be undefined.
I would say that as long as you can show that there was a contracting period prior to the big bang then H is undefined. Unless of course you assume from some symmetry principle than the universe is expanding to infinity on both side of the bounce point then h=0). i.e. if you drew a little dot representing the bounce point then there would be two cone shapes expanding to infinfity either side of this point, but one is contracting before the bounce from our point of view.
I think the simplest thing to conclude is nothing about the ultimate length of time of the contraction. Therefore H is undefined and the BGV theorem cannot be used as a proof of the beginning.

So when you say that the lQC doesn’t touch the kalam you are simply wrong. Even Vilenkin admits that a contracting universe prior to expansion does violate the theorem. Of course it isn’t just LQC that does this,  string cosmology model and horava gravity also finds the same conclusion. In string cosmology its common to use the dualtities of string theory to show a symmetry around what they call “the self dual point”. This is derived in a different manner to LQC but ultimately the conclusion is the same, a mirror image of our universe lies on the other side of the self dual point i.e. a contracting period of the universe. Again if this is true then it violates the BGV theorem. Now recall what WLC said in his debate with Peter Milican. He said even if these theories are true then his conclusion (universe had an absolute beginning) still holds and that is blatantly false.

ontologicalme, can I tag you in here? I could use some clarity.

When the claim is made that the BGV applies independent of the physical description of the universe, does it mean:

a) Independent of whether you're assuming classical spacetime or quantum spacetime?

or

b) Independent of properties of the type: "having a contraction prior to expansion"?

My hunch is (a), redtilt insists on (b). What do you think?

As far as I understand, it applies as long as plack density is not reached (GR and classical quantum levels).

It does not apply in the planck regime, I understand, this is what most physicist believe today, that at such stage quantum cosmology is needed, this is  most probably true (though, maybe not), from what we seem to know.

It could turn out that the BVG applies in that stage too, but I don´t know how plausible or implausible that is, and, I think neither does anyone else (here, I would take redtilt word for it, if he gives reasons why it does not).

I think the most interesting argument Redtilt is giving lately is that most models even if implausible or false, on such regime seem to show that there is no geodesic incompleteness into the past, I tend to disagree, but, I do not have the desire to enter a long argumentation on something that will be speculation over speculation on both sides, and, I feel a little inadequate to be debating physics with redtilt, given that he is a profesional, and, I am only an amateur, I prefer to take Vilenkin´s analysis as good enough, for the time being.

Hope that helps clarify.

[grosso]

I think you're right: BGV says nothing explicit prior to planck-time.

Aron Wall points out, however, that there's no reason to think the BGV wouldn't apply prior to planck-time. Rather, he believes, there is a much more potent escape from the BGV. I would pull it up, but Wall's blog is down.

[redtilt1]

I think you're right: BGV says nothing explicit prior to planck-time.

Aron Wall points out, however, that there's no reason to think the BGV wouldn't apply prior to planck-time. Rather, he believes, there is a much more potent escape from the BGV. I would pull it up, but Wall's blog is down.

I had a look at Wall's blog and it says clear as day:
"However, Carroll's secondary point that the assumptions of the theorem might not hold seems even more devastating.  It says that there must be a beginning if the universe is always expanding.  So maybe have it contract first, and then expand.  That's an easy way around the BGV theorem, and (as Carroll points out) there are a number of models like that. On this point I agree with Carroll that the BGV theorem is not by itself particularly strong evidence for a beginning."

i would put it stronger than that, most cosmologists agree that a quantum theory of gravity is needed to understand the big bang. The most common solution found in various different quantum gravity approaches is a bounce i.e a contraction followed by expansion.

[ParaclitosLogos]

I think you're right: BGV says nothing explicit prior to planck-time.

Aron Wall points out, however, that there's no reason to think the BGV wouldn't apply prior to planck-time. Rather, he believes, there is a much more potent escape from the BGV. I would pull it up, but Wall's blog is down.

I had a look at Wall's blog and it says clear as day:
"However, Carroll's secondary point that the assumptions of the theorem might not hold seems even more devastating.  It says that there must be a beginning if the universe is always expanding.  So maybe have it contract first, and then expand.  That's an easy way around the BGV theorem, and (as Carroll points out) there are a number of models like that. On this point I agree with Carroll that the BGV theorem is not by itself particularly strong evidence for a beginning."

i would put it stronger than that, most cosmologists agree that a quantum theory of gravity is needed to understand the big bang. The most common solution found in various different quantum gravity approaches is a bounce i.e a contraction followed by expansion.

Having it contract first and then expand does not in itself evade the theorem, it only needs to be on average expanding to fall under the theorem, that means it could contract for some meassure as long as it is on average expanding.

Let´s say that you have a model that is not expanding on average, then, indeed the model circunvents the BGV theorem.

So what? what does that show? it does not show the universe has no beginning.

It does not proof that it is the case that the model describes our universe.


You can have it contract from infinity past so it does not have a beginning, but how is that a real universe?
when does a contracting universe from infinite past ever get to the planck regime?

How much is t=-oo + 3000 trillion aeons?  answer t= -oo


basically never.

[grosso]

I think you're right: BGV says nothing explicit prior to planck-time.

Aron Wall points out, however, that there's no reason to think the BGV wouldn't apply prior to planck-time. Rather, he believes, there is a much more potent escape from the BGV. I would pull it up, but Wall's blog is down.

I had a look at Wall's blog and it says clear as day:
"However, Carroll's secondary point that the assumptions of the theorem might not hold seems even more devastating.  It says that there must be a beginning if the universe is always expanding.  So maybe have it contract first, and then expand.  That's an easy way around the BGV theorem, and (as Carroll points out) there are a number of models like that. On this point I agree with Carroll that the BGV theorem is not by itself particularly strong evidence for a beginning."

i would put it stronger than that, most cosmologists agree that a quantum theory of gravity is needed to understand the big bang. The most common solution found in various different quantum gravity approaches is a bounce i.e a contraction followed by expansion.

I agree with Wall as well. As would Craig, I imagine.

The BGV by itself is not particularly strong evidence for a beginning.

But I don't think it's presented by itself. So while everybody is correct in this statement, I'm not sure who the criticism is supposed to apply to.

[redtilt1]

Wow , have you not been listening to any WLC's debates in the past few years?
here is an example, WLC versus Stephen LAw. Note the only piece of scientific evidence WLC brings up in this debate is the BGV theorem:

"his purely philosophical conclusion has been confirmed by remarkable discoveries in astronomy and astrophysics. We now have pretty strong evidence that the universe is not eternal in the past but had an absolute beginning a finite time ago. In 2003, Arvin Borde, Allan Guth, and Alexander Vilenkin were able to prove that any universe which is, on average, in a state of cosmic expansion throughout its history cannot be infinite in the past but must have a past spacetime boundary. What makes their proof so powerful is that it holds regardless of the physical description of the early universe. Because we don’t yet have a quantum theory of gravity, we can’t yet provide a physical description of the first split second of the universe. But the Borde-Guth-Vilenkin theorem is independent of any physical description of that moment. Their theorem implies that the quantum vacuum state of the early universe—which some scientific popularizers have misleadingly and inaccurately referred to as “nothing”—cannot be eternal in the past but must have had an absolute beginning. Even if our universe is just a tiny part of a so-called “multiverse” composed of many universes, their theorem requires that the multiverse itself must have an absolute beginning.

Of course highly speculative scenarios, such as loop quantum gravity models, string models, even closed timelike curves, have been proposed to try to avoid this absolute beginning. Now these models are all fraught with problems; but the bottom line is that none of these theories, even if true, succeeds in restoring an eternal past. At most they just push the beginning back a step."


See? he didnt use any other scientific evidence , only the BGV theorem. He also totally mispoke when he said even if other models like loop or string cosmologies are true then they dont avoid the beginning (perhaps  as the BGV is independent of any physics? lol). These models may or may not be right, but if they are right they certainly dont contain any beginning event in them. Most of these models dont have singularities and do  have prior contraction thus violating the BGV.

in fact I would go as far to say WLC simply doesnt understand LQC. he says here:
"The third class of Quantum Gravity models, Loop Quantum Gravity theories, features versions of a cyclical universe, expanding and contracting. These models do not require an eternal past, and trying to extend them to past infinity is hard to square with the Second Law of Thermodynamics and seems to be ruled out by the accumulation of dark energy, which would in time bring an end to the cycling behavior."

Read more: http://www.reasonablefaith.org/contemporary-cosmology-and-the-beginning-of-the-universe#ixzz3aTzL3POL

However Craig has it totally wrong yet again. LWC does not predict a cyclic universe. it predicts that singularities are replaced by bounces. This means if dark energy is a cosmological constant then there is just a one time bounce, an mirror image of our expanding branch is matched by contracting phase of the universe on the other side of the bounce. Look a bit like this:
http://science.psu.edu/news-and-events/2011-news/Ashtekar8-2011