| Krauss and Craig |
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Brief Background
Something called the kalam cosmological argument (KCA) goes something like this:
- Anything that begins to exist has a cause.
- The universe begins to exist.
- Therefore, it has cause.
What about the second premise? Craig argues that the scientific evidence currently makes the universe having a beginning more probable than not, and that the Borde-Guth-Vilenkin (BGV) theorem plays a significant role in the scientific case for a cosmic beginning. Why? The BGV theorem says that any universe that has on average been expanding throughout its history cannot have an infinite past but must have a beginning. This would close off a lot of avenues for those who want to avoid a beginning of the universe, for the theorem holds even if our universe is part of a multiverse, the multiverse would also require a beginning if the “has on average been expanding throughout its history” part holds true.
In a nutshell, here’s what happened: Krauss quoted an edited email of Vilenkin that gave some caveats with respect to the efficacy of the Borde-Guth-Vilenkin (BGV) theorem. Theists like William Lane Craig have often used the BGV theorem in arguing for a beginning of the universe, whereas Krauss denoted some of the limits of the theorem in their discussion in Sydney Australia.
Misleading Editing?
You can see and hear Krauss mention the edited email at around 41:23 of the debate:
In the clip above, Krauss says that the theorem doesn’t hold true in the multiverse, but this misleading. Whether it holds true in the multiverse depends on whether the “has on average been expanding throughout its history” condition is met. If the condition is met, then the theorem does hold for the multiverse, and Krauss doesn’t mention this. Krauss also says that the theorem doesn’t work when there’s quantum gravity, which also isn’t necessarily true (more on that later). For those who want to read the edited email:
Hi Lawrence,Craig wondered what came after the ellipsis of this claim:
Any theorem is only as good as its assumptions. The BGV theorem says that if the universe is on average expanding along a given worldline, this worldline cannot be infinite to the past.
A possible loophole is that there might be an epoch of contraction prior to the expansion. Models of this sort have been discussed by Aguirre & Gratton and by Carroll & Chen. ……
. . . Jaume Garriga and I are now exploring a picture of the multiverse where the BGV theorem may not apply. In bubbles of negative vacuum energy, expansion is followed by contraction... However, it is conceivable (and many people think likely) that singularities will be resolved in the theory of quantum gravity, so the internal collapse of the bubbles will be followed by an expansion. In this scenario, ... it is not at all clear that the BGV assumption (expansion on average) will be satisfied.
...of course there is no such thing as absolute certainty in science, especially in matters like the creation of the universe. Note for example that the BGV theorem uses a classical picture of spacetime. In the regime where gravity becomes essentially quantum, we may not even know the right questions to ask.
A possible loophole is that there might be an epoch of contraction prior to the expansion. Models of this sort have been discussed by Aguirre & Gratton and by Carroll & Chen. ……At around 50:18 Krauss insinuates that Craig quotes Vilenkin incorrectly. With that in mind, notice the interaction between Krauss and Craig here:
For those who prefer reading a transcript, here’s a key part of what transpired in the segment above (I apologize in advance for not getting the transcript perfect below; I was hampered by the fact that Krauss repeatedly interrupted Craig and the moderator had only limited success in toning this down):
Craig: [Reading the edited email] The BGV theorem says that if the universe is on average expanding along a given worldline, this worldine cannot be extended [apparently realizing he misread it slightly] uh or cannot be infinite to the past.A few specific points to notice: Krauss says the entropy thing is something Vilenkin finds “unpleasant.” Also, Krauss actually concedes that the universe likely had a beginning! This is notable (and surprising) since as Craig said the claim is that the “universe begins to exist” premise is more probably true than false.
A possible loophole is that there might be an epoch of contraction prior to the expansion. Models of this sort have been discussed by Aguirre & Gratton and by Carroll & Chen. [Done reading email] Now the thing is Lawrence that in the very paper that I quoted from Alex Vilenkin last April, he shows specifically, by name, that the Aguirre & Gratton model, and the Carroll & Chen model, don’t work. That there—
Krauss: No no, he says that you have to make an assumption about entropy.
Craig: Yes, he—
Krauss: You have to make an assumption about the evolution of entropy at the point of minimum size.
Craig: Those—
Krauss: So you—you have to make an assumption which he would argue that they don’t have any rationale for. That’s not the same as saying that they’re wrong.
Craig: Well, yes it is. I—he—he argues that the—all of the evidence shows that the universe had a beginning and that in this model—
Krauss: I would agree that all the evidence shows—let’s look—let’s accept that fact.
Craig: All rught.
Krauss: All the evidence suggests our universe had a beginning.
Craig: Oh, OK—
Krauss: But we don’t KNOW! That’s what I keep telling you! Knowing and suspecting are two VASTLY different things!
Craig: I—I’m never saying that this is known with certainty. This is a—a mischaracterization on your part. What I argue is that the premise [the universe begins to exist] is more probably true than false. And actually you agree with me on that—
Krauss: Yeah but—
Craig: that the universe began to exist, but I want to focus—
Krauss: Well I say it’s likely that the universe began to exist—
Craig: —on this claim that I’ve somehow misrepresented Vilenkin—
Krauss: Well but the—but the key line is the last one. That the theorem breaks down—
Craig: Wait a minute.
Moderator: Hang on.
Krauss: — at the point it really matters.
Moderator: One at a time.
Craig: Now there’s more here. Because I noticed that at the end of this paragraph where the Carroll-Chen and Aguirre-Gratton models are mentioned that he specifically shows—
Krauss: [my guess is that Krauss might have thought that Craig was about to ask why Krauss omitted what came after the ellipsis] Because it was technical! He said—he talked about the fact that—
Moderator: Lawrence, hang on. Just let, just let him finish.
Craig: That he specifically shows that these models cannot be past eternal, uh and that they involve therefore a beginning of the universe, just like the others. I—
Krauss: You can do the math if you want.
Craig: Now wait—
Krauss: I’ll let you do it.
Craig: Now wait. I couldn’t help notice although it’s [the slide showing the edited email] is down from the screen now that there was a series of ellipses points following the paragraph—
Krauss: Yeah because it was technical! And I thought it was, you know—
Craig: Well I wonder what you deleted from the original letter. Could it—
Krauss: I—I just
Craig: Now wait—
Krauss: I just TOLD you!
Craig: Now wait. Yeah, but you didn’t—
Krauss: I just told you. He says that you assume of entropy at the lowest point for which there is no rationalization in the papers within the context of the model given, and therefore he finds them unpleasant.
To sum up a few key points of the above transcript though, Craig wonders what was after the ellipses points here:
Hi Lawrence,Craig was also evidently trying to say that Vilenkin says that the models he mentioned don’t work with respect to extending them to an infinite past. Krauss assures Craig that the stuff after ellipses is technical. To be sure, there was some technical stuff after the ellipses, but there was also something else. Here’s a fuller part of the quote, with a key part emphasized by yours truly:
Any theorem is only as good as its assumptions. The BGV theorem says that if the universe is on average expanding along a given worldline, this worldline cannot be infinite to the past.
A possible loophole is that there might be an epoch of contraction prior to the expansion. Models of this sort have been discussed by Aguirre & Gratton and by Carroll & Chen. ……
Any theorem is only as good as its assumptions. The BGV theorem says that if the universe is on average expanding along a given worldline, this worldline cannot be infinite to the past. A possible loophole is that there might be an epoch of contraction prior to the expansion. Models of this sort have been discussed by Aguirre & Gratton and by Carroll & Chen. They had to assume though that the minimum of entropy was reached at the bounce and offered no mechanism to enforce this condition. It seems to me that it is essentially equivalent to a beginning.Perhaps Vilenkin does find the entropy assumption “unpleasant,” but he also says the last emphasized sentence in the quote above that Krauss never mentions. So why did Krauss omit the part about it being essentially equivalent to a beginning? Because it was too technical?
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