tag:blogger.com,1999:blog-7164603649660539619.post2437496964839991665..comments2016-11-30T02:14:35.682-05:00Comments on Somewhat Abnormal: BGV and KCMRobert Oerterhttps://plus.google.com/105312983168950964618noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-7164603649660539619.post-37212142975099316142014-06-16T09:46:01.976-04:002014-06-16T09:46:01.976-04:00Thanks for your comment, Obsidian. As far as your ...Thanks for your comment, Obsidian. As far as your "straw man" claim, note that I didn't accuse Craig of claiming BGV was a singularity theorem. In his writings, at least, he seems to be careful to avoid this mistake. Still, I think some of the things he says/writes might mislead people into thinking that BGV requires a singularity. <br /><br />For instance, on <a href="http://www.reasonablefaith.org/contemporary-cosmology-and-the-beginning-of-the-universe" rel="nofollow"> this page</a>, Craig writes<br /><br />"But the Borde-Guth-Vilenkin theorem is independent of any physical description of that moment. Their theorem implies that even if our universe is just a tiny part of a so-called “multiverse” composed of many universes, the multiverse <b>must have an absolute beginning</b>."<br /> <br />And later in the same article,<br /><br />"The first of these string cosmologies, Ekpyrotic cyclic models, is <b>subject to the Borde-Guth-Vilenkin theorem and so is admitted to involve a beginning of the universe.</b> The second group, Pre-Big Bang models, cannot be extended into the infinite past if they are taken to be realistic descriptions of the universe. The third group, the string landscape models, feature the popular multiverse scenario. They are also <b>subject to the Borde-Guth-Vilenkin theorem and so imply a beginning of the universe</b>."<br /><br />[Emphasis added.]<br /><br />It is clear that Craig is claiming that the BGV theorem requires a beginning of the universe. This is simply false: as I explain in the OP, all the BGV theorem implies is a beginning to the region of space-time that satisfies the expansion assumption. Craig has either misunderstood the implications of the theorem, or is begin deliberately misleading. <br /><br />I agree that BGV is a remarkable theorem in that it doesn't rely on Einstein's equations. This does not mean, however, that it applies in the quantum gravity realm. Quantum gravity is not a well-understood area, but many physicists expect that in the quantum gravity regime, not only will Einstein's equations be violated, but also the assumptions of classical spacetime will no longer hold true. BGV of course relies on the assumptions of classical spacetime, so it would not apply in this case.Robert Oerterhttp://www.blogger.com/profile/09708981993708509662noreply@blogger.comtag:blogger.com,1999:blog-7164603649660539619.post-45870609161138383332014-06-16T08:45:58.873-04:002014-06-16T08:45:58.873-04:00Hey Dr Oerter.
I think you're attacking a str...Hey Dr Oerter. <br />I think you're attacking a straw man. Where did someone say BGV was a singularity theorem? To my knowledge no proponent of Kalam has claimed that.<br />Kalam does not depend in any way on the universe having a singularity rather than some other beginning see for example<br />William Lane Craig & James D. Sinclair ,On Non-Singular Space-times and the Beginning of the Universe<br />where Craig discusses non-singular models<br />I might be mistaken about this but I think Vilenkin says that BGV will hold whatever modifications to Einstein's gravity we make with quantum gravity.<br />“A remarkable thing about this theorem is its sweeping generality. . . . We did not even assume that gravity is described by Einstein’s equations. So, if Einstein’s gravity requires some modification, our conclusion will still hold. The only assumption that we made was that the expansion rate of the universe never gets below some nonzero value” [Vilenkin,Many Worlds in One pg 175]<br /><br />I agree that the BGV theorem does *prove* the universe had a beginning , but its a very general theorem that applies to a wide class of models , so it does have teh implication that many contemporary models of the universe (including ones like the Turok cyclic model) which were once thought to avert a beginning , do have a beginning.Obsidianhttp://www.blogger.com/profile/07492809766606634642noreply@blogger.com