tag:blogger.com,1999:blog-1433428682510068517.post1897272964788715956..comments2024-02-29T19:21:32.831-05:00Comments on Possible Worlds: Guest Post: An Open Letter to Sean CarrollRandy Everisthttp://www.blogger.com/profile/06870605678781409126noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-1433428682510068517.post-31158412758620819862014-03-24T11:21:15.471-04:002014-03-24T11:21:15.471-04:00It seems to me that the primary motivation behind ...It seems to me that the primary motivation behind positing a primordial vacuum is simply to avoid the absolute beginning implied by the Standard Big Bang Model. Let me explain. We can say with virtual certainty that classical spacetime (i.e., our observable universe subsequent to 10^-43 seconds) has been in a state of cosmic expansion throughout its history -- as Vilenkin affirmed, *all* of the evidence we have suggests this. Moreover, the BVG Theorem [http://arxiv.org/abs/gr-qc/0110012] says that *any* spacetime that satisfies this sole condition cannot be geodesically past-complete (i.e., eternal). Thus, the primordial vacuum state is proposed as an attempt to evade the BVG Theorem by avoiding Hav > 0 due to there being an infinite amount of time spent in the vacuum state. <br /><br />However, even if this scenario could in principle avoid BVG, it's not out of the woods yet; it still faces a deep internal incoherence: according to such models, it is impossible to specify precisely when and where a fluctuation will occur in the primordial vacuum which will then grow into a mini-universe. Within any *finite* interval of time there is some non-zero probability of such a fluctuation occurring at any point in space. Thus, given *infinite* past time, mini-universes will eventually be spawned at *every* point in the primordial vacuum, and, as they expand, they will begin to collide and coalesce with one another. Therefore, given infinite past time, we should by now be observing an infinitely old universe, not a relatively young one. As WLC stated in the debate, if the vacuum were sufficient to produce the universe, it would have done it infinitely-long-ago. As a last resort, one could posit an expansion in the vacuum itself so as to remedy this incoherence. After all, de Sitter spacetime is the exact solution of the Einstein equations Gµν + Λgµν = 0 for the empty universe with positive vacuum energy density. But then we're right back where we started! Namely, the universe, *including* the vacuum state, would satisfy Hav > 0 and therefore necessarily have a beginning.Anonymoushttps://www.blogger.com/profile/11435687056650503559noreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-3592412906013950682014-03-22T22:28:15.970-04:002014-03-22T22:28:15.970-04:00Hmm...OK. I was aware of other models out there, b...Hmm...OK. I was aware of other models out there, but I'm just a little baffled as to why some feel a need to posit a vacuum at all? After all, I guess there's no way to scientifically prove a vacuum existed pre-universe (or is there?) so I can't see why they even posit it, apart from maybe what i said already about it producing fluctuations which theoretically might have the capability of creating a universe?. Hope that makes sense & thanks for the original answer anyhow :)Jamesnoreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-91866712992393419782014-03-22T08:37:18.556-04:002014-03-22T08:37:18.556-04:00Thanks Jack for coming on here and answering that!...Thanks Jack for coming on here and answering that! :)Randy Everisthttps://www.blogger.com/profile/06870605678781409126noreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-32791023194417271722014-03-22T07:57:55.890-04:002014-03-22T07:57:55.890-04:00Hi James,
Actually vacuum fluctuation models are ...Hi James,<br /><br />Actually vacuum fluctuation models are just one of the asymptotically static cosmogonic scenarios purporting to describe what happened 'prior' to the Big Bang; there are also eternal inflationary models, cyclic models, contracting models, and other asymptotically static models. Like vacuum fluctuations models, these others are also plagued with problems and none of them, even if true, succeed in restoring past-completeness. Thanks for your question and I hope this helps.Anonymoushttps://www.blogger.com/profile/11435687056650503559noreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-70043172164346809252014-03-21T11:13:01.489-04:002014-03-21T11:13:01.489-04:00So there is actually something you don't know ...So there is actually something you don't know after all :). The only thing I could come up with is that maybe they choose to have the universe come from a pre-existing vacuum because vacuums produce fluctuations and they are, theoretically, capable of producing a universe like ours, whereas maybe universes filled with matter like our own don't have this capability for some reason. Just a thought. Hope somebody else can help :)Jamesnoreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-45965443972438781382014-03-20T22:44:21.105-04:002014-03-20T22:44:21.105-04:00Not a dumb question at all! I can honestly and una...Not a dumb question at all! I can honestly and unashamedly say I have no idea! I may have to ask Jack to weigh in here and get an answer for ya.Randy Everisthttps://www.blogger.com/profile/06870605678781409126noreply@blogger.comtag:blogger.com,1999:blog-1433428682510068517.post-39666918336684387822014-03-20T22:38:50.891-04:002014-03-20T22:38:50.891-04:00Hi Randy,
This is probably a dumb question (and I...Hi Randy,<br /><br />This is probably a dumb question (and I hope it makes sense), but I'll go ahead anyway: When scientists talk about what may have happened before the big bang, why is it they (or some of them anyway) talk of our universe coming out of a vacuum state as opposed to coming from another universe filled with matter much like our own? Why does it have to come from a vacuum?Jamesnoreply@blogger.com