That title is somewhat facetious, of course, but I do think the multiverse is far more than an idle speculation. I think it is an idea that is sufficiently well-supported that it is those who deny it who should be on the defensive.
I would make an elaborate argument in defense of that claim, but Coel Hellier, a physics professor at Keele University in the UK, has saved me the trouble. He recently posted a two-part article on the subject over at Scientia Salon: Part One, Part Two. Hellier writes:
The multiverse concept is often derided as “unscientific” and an example of physicists indulging in metaphysical speculation of the sort they would usually deplore. For example, commenters here at Scientia Salon have said that the multiverse is “by definition not verifiable and thus outside the bounds of empirical science,” and that “advocates of multiverses seem to be in need of serious philosophical help” .
Critics thus claim that the multiverse amounts to a leap of faith akin to a religious belief. Indeed, the religious often accuse atheistic scientists of inventing the multiverse purely to rebut the “fine-tuning” argument that they say points to a creator god, though the fine-tuning argument is readily refuted in several other ways, and anyhow physicists really don't care enough about theology these days to let that worry them; further, the concepts leading to a multiverse were developed well before theologians started taking note of the issue .
Quite right, especially that part about physicists not caring about theology. I would only cavil that I am not interested in whether the multiverse meets some arbitrary definition of what is science and what is not. What I care about is whether or not the idea is reasonable and well-supported.
The “demarcation problem,” as philosophers of science refer to the problem of distinguishing science from nonscience, is mostly just an asinine distraction from more serious questions. You cannot avoid it entirely, since it does become relevant when questions of science education are being litigated. Outside of that context, however, trying to draw clear lines around science is not a worthwhile use of time.
End of digression. How does Hellier support his claim?
The purpose of this article is to argue that the multiverse is an entirely scientific hypothesis, arrived at for good scientific reasons and arising out of testable and tested cosmological models. To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds.
Several different concepts could be labelled a “multiverse”, but I am advocating one particular multiverse concept, that arising from what cosmologists call the “eternal inflation” version of Big Bang cosmology , which was developed to explain observations of our universe and predictions from this model have since been verified, putting it on a sound footing.
I am arguing that if a scientific theory predicts consequences A, B, C and D, and if we then verify that A, B and C are indeed the case, thus giving us confidence in the theory, then we have sound reasons for accepting D even if D cannot be directly verified. Indeed, we would be obliged to accept D unless we can construct another equally good explanation of A, B and C.
Quite right again, though “obliged to accept” seems a bit strong. I would want an extra assumption that A, B and C are really good evidence for the theory. That's just a quibble, though, and the principle here is correct. If well-supported cosmological theories have the multiverse as a consequence, then it is perfectly reasonable to accept that the multiverse exists. Max Tegmark has been especially eloquent on this point, emphasizing that the multiverse should not be seen as a theory in itself, but is rather a consequence of other theories.
I was also cheering when Hellier wrote this:
The multiverse model is often criticized as “unscientific” for invoking universes that can never be seen and thus making claims that can never be verified. But this applies just as much to all cosmological models, which are usually presumed to extend to infinity. All of them are thus postulating that the universe stretches well beyond the observable horizon, from where (owing to the finite speed of light) we can never obtain information to verify any hypotheses. This feature does not make the model unscientific. If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.
Hat trick! One of the more annoying tropes of this topic is the idea that Occam's Razor militates against the multiverse. This seems entirely wrong to me. It is the people who claim there is only one universe who have some explaining to do. Multiverse proponents are simply saying that whatever created our universe, a quantum fluctuation or whatever, created other universes as well. Given that there is some sort of mechanism that created our universe, why is it parsimonious to assume that the mechanism only operated once?
The Occam's Razor argument is especially obnoxious when it comes from theists. They will deride the multiverse as just a crude and desperate attempt to avoid the conclusion that God exists. Here's theologian John Haught, from his book God After Darwin:
Of course, if you are truly addicted to the idea that our life-bearing universe is a purely random, undirected, and unintelligible occurrence, and that life within it must in no sense be the product of divine intelligence and wisdom, you may then imaginatively conjure up an endless series or proliferation of other “universes,” so as to increase the probability that randomness rules.
And here's John Polkinghorne expressing similar thoughts in an interview:
The multiverse theory in its more extreme forms is the idea that there are these vast portfolios of different universes, disconnected from ours, unobservable by us. It's a metaphysical guess. It has mostly been popular and mostly been invented in order to explain away the fine-tuning of our particular universe.
What nerve! Among the many things wrong with these quotes is that Haught and Polkinghorne's preferred explanation is that an omnipotent superbeing created the universe. That is what they put forward as the natural, parsimonious explanation for the universe. I demur from that conclusion. Given the choice between more of the same on the one hand, and hypothesizing into existence a God who can simply bring universes into being with acts of His will on the other, it seems clear which one Occam would choose.
Anyway, go have a look at Hellier's essays. There is far more to them than what I have quoted here.
If our universe was finely-tuned by its designer then the designer was either vicious or just an incompetent apprentice. 99.9% of all species that have ever existed on Earth are now extinct. For goodness sake, fine-tuning the mass-energy efficiency of food metabolism at 1.9E-10 was guaranteed to result in predators, prey, large-scale extinctions, and an endless struggle for survival. How very benevolent, not!
"If well-supported cosmological theories have the multiverse as a consequence, then it is perfectly reasonable to accept that the multiverse exists."
This is what convinced me that the Multiverse is a sound scientific theory/hypothesis, whatever you want to call it. Before I was as skeptic because of the "not falsifiable" argument too.
The way theists use Occam's Razor is just dishonest. It only applies when we have to choose between two theories which correctly describe the same set of data. So if a theory with one Universe indeed describes all the data the theory of the Multiverse describes we have to choose that one. Haught and especially Polkinghorne (a former physicist) know that. So they also know they should adopt the Multiverse the moment it describes more data than one Universe.
"The Multiverse is a Done Deal" What does "Done Deal" mean. Most people would interpret that as there is no question about it. And yet your argument includes this quote " To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds." Not exactly what most people would consider a done deal. In fact your argument is about the question of the multiverse being a scientific hypothesis, not whether its a done deal. But then hyperbole is common, even in science blogging.
Just to add to your line of argument -- If the universe was really designed for humans, why is it that so much of it is essentially uninhabitable by humans? The vast majority of the universe is empty space, far from any star. Certainly those areas are uninhabitable. Even the majority of planets are likely uninhabitable. Consider our own solar system just as an example. Out of all the planets, satellites and dwarf planets in the solar system, only one is capable of supporting human life. There's no reason to think our solar system is in any way special, so it's very likely that the majority of the planets, satellites and dwarf planets in the universe are uninhabitable. Not what you'd expect from a universe that was designed specifically for us, is it?
I think that there's a conceptual problem --whenever infinities are invoked and anything "eternal" (i.e. without beginning or end) constitutes an infinity concept--in positing "eternal inflation."
Since "inflation" is, by definition and, thus, unavoidably, a process no matter how described, it is in inherent contradiction to "eternal" or any other variant of "infinity."
At most, one can posit an eternal expansion-collapse series--by which neither the one or the other alone goes on without end but both, in alternating fashion, succeed in an endless series. This is of course beyond the bounds of scientific observation's experimental reach. But "infinite" and "eternal" are already matters for science's theoretical / philosophical end, not the experimental side of things.
Briefly, any process is time-bound at least at one "end" if not at both (inception / conclusion). To inflate admits a state-change. But, thinking carefully about it, nothing properly infinite or eternal can undergo a state-change. This is simply a contradiction in terms or, put differently, a mistake in the conception of "eternal", "infinite" as somehow subject to limiting conditions. No limiting condition can apply to "things" alleged to be infinite or eternal. Indeed, it seems to me that "things" themselves, as countable, severable, as "entities" are, again, by nature, contrary to the very idea of infinity or eternity as I've tried to understand that very vague concept.
I see one and only one way out of this mess: nothing (except perhaps, as above, an alternating (self-cancelling) series) is infinite or eternal. Everything is bounded-- energy/matter fields, and the space-time construct in which they are theorized to exist.
RE Infinity, See, e.g.
Singh, Jagjit :
(1959) Great Ideas of Modern Mathematics (Dover)
and Great Ideas and Theories of Modern Cosmology (Dover)
Once Before Time: A Whole Story of the Universe (edition 2010)
"Given that there is some sort of mechanism that created our universe, why is it parsimonious to assume that the mechanism only operated once?"
But "that there is some sort of mechanism that created our universe" is not a "given." Whether "once" or more than once, it's an assumption, not a given.
I bit my tongue very hard as I wrote my comment. I totally agree with your addition. I'd like to add that I have replaced my inefficient tungsten light bulbs with CFLs to save a few watts, but just our Sun amongst the trillions of stars is emitting circa 3.8E26 watts so my energy saving attempts seem to be little more than a futile obsessive-compulsive disorder :-)
Thanks for drawing attention that article, it is really a nicely argued and a good explanation of scientific practice. Parsimony is under-appreciated by many and misunderstood by most others.
That being said, there are different types of postulated multiverses. The inflation version makes sense - even if I cannot claim to really understand the physics, it does boil down to the observation that if such a process condensed into one universe then there is no reason why it should condense into many, ad infinitum.
However, and yes, very unrelatedly, there is also the many worlds interpretation of quantum physics. Again, I do not really understand the physics, but the idea that a complete new universe comes into existence for every particle being in a slightly different state does appear a tad unparsimonious.
It appears then "Multi-verse" as Hellier intends that is indeed after all an infinite series of expansion-collapse-expansion rather than the type envisioned by others as concurrently existing but space-time existentially distinct (i.e. non-communicating) realms of existence.
By these terms, then, "eternal expansion" is not a very accurate description to put it mildly. The expansion is not per se "eternal" but, rather "periodic" and always preceeded and succeeded by another periodic stage, "collapse." The only "eternal" phenomenon is the serial expansion-collapse--and, as I tried to argue above, if this is in fact eternal, then there was never an "original" expansion prior to any "original" collapse.
In its character then, what Hellier calls a "multiverse" is in my view just a universe after all. All experience occurs in a single universal environment which is either in a collapse phase or an expansion phase but never both---either at once or in sequence. No experience from one phase "survives" to the next and this puts the phenomena beyond the reach of experimental investigations so far in our capacity to imagine or conduct. By this view, "multiverse" applied to a physical existence which is never more than experienced as one single finite phase of a putative infinite series is simply playing with terms in one phase encompasses all physical existence as this is understood in scientific terms.
MnB @ 2: Much the same for me. I was initially inclined to be more than slightly skeptical, on the grounds that multiverse theories are untestable. I read a couple of articles by Tegmark with that sense of skepticism in mind. But in the end I was persuaded that multiverses are an implication of existing theories, and found Tegmark's ideas in general to be interesting.
Here's another example of how I think Hellier is mistaken in his representation of these issues:
In the comments section of part one, (See link here: http://scientiasalon.wordpress.com/2014/06/03/the-multiverse-as-a-scien… ) Hellier responds (in part) to a comment by "labnut"
“If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.”
No, you are indulging in guesswork. I would never call that being scientific.
This is what Sean Carroll said in his article ‘How Did the Universe Start’, 2007-04-27:
“Personally, I think that the looming flaw in all of these ideas is that they take the homogeneity and isotropy of our universe too seriously. Our observable patch of space is pretty uniform on large scales, it’s true. But to simply extrapolate that smoothness infinitely far beyond what we can observe is completely unwarranted by the data. It might be true, but it might equally well be hopelessly parochial. We should certainly entertain the possibility that our observable patch is dramatically unrepresentative of the entire universe, and see where that leads us.”
And Sean Carroll is one very shrewd physicist with some knowledge of philosophy. As he says, your observation is completely unwarranted by the data."
by arguing that Carroll's views support his (Hellier's) own:
"If you read that snippet in context you see that Sean Carroll is arguing exactly what I am, the next sentences go on to argue for eternal inflation. Again, this is in line with parsimony of *information content*. (Interpreting things like parsimony and Occam’s razor in terms of information content is all-important here.)"
But, reading Carrol's cited comment further, Carroll goes on to say,
" Inflation makes it plausible that our local conditions don’t stretch across the entire universe. In Alan Guth’s original scenario, inflation represented a temporary period in which the early universe was dominated by false-vacuum energy, which then went through a phase transition to convert to ordinary matter and radiation. But it was eventually realized that inflation could be eternal — unavoidable quantum fluctuations could keep inflation going in some places, even if it turns off elsewhere. In fact, even if it turns off “almost everywhere,” the tiny patches that continue to inflate will grow exponentially in volume. So the number of actual cubic centimeters in the inflating phase will grow without bound, leading to eternal inflation. Andrei Linde refers to such a picture as self-reproducing.
"If inflation is eternal into the future, maybe you don’t need a Big Bang? In other words, maybe it’s eternal into the past, as well, and inflation has simply always been going on? Borde, Guth and Vilenkin proved a series of theorems purporting to argue against that possibility. More specifically, they show that a universe that has always been inflating (in the same direction) must have a singularity in the past.
According to this view, then, Hellier it seems does somehow subscribe to what is apparently literally eternal expansion without any complete and overall (i.e. "universal") interruption of it--even if the continuing expansion is "only localized" rather than multiverse-wide.
I'm now in doubt about how exactly Hellier intends the term "multiverse"-- concurrent and serverable physical realms of existence-- whether "communicating" or not ? or a single universal realm with either multiple stages, phases or with various distinguishable sections in concurrent existence but discontinuous in space-like or in time-like dimensions?
If the existence of multiple universes could somehow be proven, so what?
What exactly does the multiverse believer gain?
Why does jrosenhouse appear to want MT to be true?
I might be somewhat cynical here but this looks more like a cosmologist employment initiative. If there's more than one cosmos, then more cosmologists are needed to work out the details. There's always more research to be done -- especially if you can invent whatever concepts you like that explain other concepts you like. Shades of Scholasticism, I'd say.
If a model EXPLAINS (measurements existed before model) A, B, and C, that’s nice. If it PREDICTS (BEFORE MEASUREMENT) A, B, and C, that’s a model that must be reckoned. Unfortunately, the words explain and prediction are used interchangeable. What is the meaning here? Further if other models also explain 3 of the four, these models should not be rejected and are also possible. The trick is to find one model that predicts something (D) that all other models fail to predict.
That a model fails to explain or predict D (the measurement that was measured was contrary to the prediction) then a new model is needed. But the new model should reduce to the model that predicted (but not explain) A, B, and C.
This blog did not discuss other possible models. I understand that many models reduce to the Schrodinger equation. Therefore, they are all possible.
I suggest “D” is the double slit experiment and the Ashfar experiment.
What is the MWI explanation of these?
Why NOT try to find out about the multiverse? What we gain is better knowledge and understanding of how the universe works. Besides, it certainly is not a matter of anyone WANTING the multiverse to be true; that was not the point of this post. The point of the post is that the multiverse is, despite assertions to the contrary, a scientific hypothesis, and one that seems to be well-supported by the currently available evidence. Whether Jason or anyone else wants the multiverse to be true is utterly irrelevant.
Why would more cosmologists be needed? The ones we have were the ones who developed the multiverse hypothesis, so I would think they'd be perfectly capable of gathering additional evidence to support or refute their hypothesis.
Indeed, they are already at work on the task, and to date, the hypothesis seems to hold up. I don't claim to understand all the technical details, but the multiverse arises from inflationary big bang cosmological models. Inflation was invoked primarily to explain the horizon and flatness problems in standard big bang cosmology.
In case you are unaware, the flatness problem refers to the lack of spacetime curvature observed in the universe. In a big bang model, a perfectly flat spacetime curvature represents a very unlikely state of affairs. There is no theoretical reason to a priori expect spacetime to be flat. The fact that it is presents a problem to standard big bang cosmology.
The other problem is the horizon problem. The cosmic microwave background shows that conditions in the early universe were VERY homogenous. That is also a fairly unlikely state of affairs according to standard big bang models. The reason is that it takes time for spatially separated points to equilibrate with each other. Energy can travel no faster than the speed of light. In the early universe, points that are now observed to be homogenous would not have had sufficient time to equilibrate with each other.
Inflation solves both of these problems. For the flatness problem, consider a thought experiment. Take a small rubber ball. Assume it's made of infinitely stretchable rubber. Mark two points on it. The curvature of the surface between these points is obvious. Now stretch that ball until it's the size of the earth. Those two points now reside on a surface that is for all intents and purposes locally flat. Inflationary models work in the same way; any points that we can observe in the universe correspond to closely spaced points on the original rubber ball. When spacetime was inflated, what we now observe is locally flat spacetime between us and any other observable point.
Inflation obviously also works well for the horizon problem. Inflation tells us that all points in the observable universe were once very close to each other. They were close enough prior to the inflation that they did have time to equilibrate, thus taking care of this issue.
Now, if that were all that there is, this would be a nice idea, but one that really cannot be said to have been well tested. However, there are other observations predicted by inflationary models that have been made subsequently to their proposal that lend additional support. Polarization features of the CMB and the power spectrum of the CMB are a couple of examples. No other model has been able to account for these observations better than the inflationary model. Given that, we provisionally accept the inflationary model. In turn, the inflationary model entails the multiverse. Since we have accepted the inflationary model, it certainly makes sense to accept the multiverse as well.
Of course, it's always possible that some new observation that refutes the inflationary model will be made. If that happens, there will undoubtedly be a new model proposed. If that model does not entail the multiverse, then we will drop the idea of the multiverse. But until then, the multiverse is certainly much more than just a concept invented to support other concepts that we like.
I love the multiverse. Every time I make some trivial decision, KAPOW!! another universe comes into being, along with all its galaxies, stars, tramcars and goldfish. How awesome is that?
I think it is an idea that is sufficiently well-supported that it is those who deny it who should be on the defensive.
I'm sure Peter Woit will be fascinated.
uh.. that is the muti-worlds hypothesis.. . not the same..
AIUI, the eternal inflation multiverse has nothing to do with fine tuning because the universes arising out of it are all expected to have the same fundmental properties. There are other types of multiverses where you can get fundamentally different universes, but this isn't one of them because the mechanism of inflation has no method of altering or fixing fundamental constants.
So, it doesn't really answer the fine tuning question one way or the other...except to say that if some theolgian thinks scientists are pushing the eternal inflation model because in order to avoid the fine tuning argument, that theologian doesn't know what he's talking about. ;)
By these terms, then, “eternal expansion” is not a very accurate description to put it mildly. The expansion is not per se “eternal” but, rather “periodic” and always preceeded and succeeded by another periodic stage, “collapse.”
No, this is incorrect; inflationary expansion can lead to multiple simulaneous universes, and no collapse in any one of them is needed before another pops up. In fact, the moment some region of the inflating space enters into its "standard big bang" phase, it because causally disconnected from the rest and could not possibly be affected by any event in any of the other regions.
In one of your later sentences you say that you do not view this as a multiverse at all. Well, there are different types of multiverses. You might look up Max Tegmark's four levels, which I believe have become somewhat standard and accepted. At least in terms of that nomenclature, this is a Type 1 Multiverse.
Why NOT try to find out about the multiverse?
Well, they're causally disconnected...you can't get there from here. :) Of course our current theories could be wrong and there may be a way to observe them, but for now, according to current theory, that doesn't appear possible.
AIUI, the eternal inflation multiverse has nothing to do with fine tuning because the universes arising out of it are all expected to have the same fundmental properties.
This is exactly backward. Simple inflation can give you uninteresting multiverses. Eternal inflation gives you multiverse mania, which, among other things, is a cover for the fact that string theory predicts exactly nothing, because particle physics becomes irrelevant in a Theory of Anything.
You might look up Max Tegmark’s four levels, which I believe have become somewhat standard and accepted.
The only thing new in this (here, for those who don't want to bother with the trade book) is "Level IV," which is aimless speculation that "the universe is a mathematical structure." I haven't the slightest idea how one could arrive at the notion that it is either "standard" or "accepted."
I thought I remembered reading that inflation theory had lots of problems. So I googled a bit tonight and found an example of what I recollected.
“Inflation has destroyed itself. It logically self-destructed.” – MIT’s Max Tegmark
“Inflation is still the dominant paradigm, but we’ve become a lot less convinced that it’s obviously true.” – Cal Tech’s Sean Carroll
Carroll* and Tegmark are not exactly the most objective parties here. If you'd like a complete destruction of Tegmark, I'd suggest you start with Luboš Motl, e.g., here (particularly the reply to Tegmark's comment).** This dismantles the Many Worlds–Multiverse confection.
Non-anthropic inflation is doing just fine.*** It's not particularly explanatory in and of itself in its current form: it's a prescription for a scalar field with a certain (simple) potential. Originally, the idea was that one could, on top of the horizon and flatness problems, get symmetry breaking of a grand unified theory (GUT) out of it, but GUTs haven't panned out. Especially string unification.
Eternal inflation, which is purely speculative, and all of the Multiverse burbling that follows, is a lost cause, as noted at length in the Tom Banks paper linked above.
The most optimistic goal is that one somehow connects the origin of the fundamental parameters of the Standard Model to cosmological theory. The Multiversers, in the face of the failure of SUSY and string theory, would prefer to toss all this out the window. It's a flight from physics that pops in occasionally to try to piggyback on actual results in order to claim that the Theory of Anything is actually testable.
I strongly suggest waiting for B-mode data from Planck before trotting out quotes from Tegmark about the death of inflation.
* The New Scientist item is paywalled.
** Including the possiblity that uninteresting isolated Hubble volumes may also be out.
*** If the game reduces to whose theory sucks less, straightforward inflation wins.
I'm not a physicist, but I think Both Alex SL and Mike Cope misinterpreted many-worlds (which is distinct from the multiverse being discussed anyway). For one, it has nothing to do with "decisions" or human-level events like turning left or right at a stoplight, or winning or losing an election. As Alex SL said, it's about the states of subatomic particles.
But I think it's also wrong to frame it as "whole new universe" coming into being, as if some kind of duplicator machine makes a "copy" of every star and bluebird and xylophone in existence whenever certain quantum states change. The point is that the universe diverges, and the two different quantum states occur and exist simultaneously, the result being different universes.
There's a very good argument that it's the best and simplest explanation if quantum phenomena. Shroedinger's cat doesn't "become" dead (or stay alive) solely because the "potential" effects of the quantum phenomena cross some arbitrary macroscopic boundary, after which an arbitrary state is chosen. The quantum effects simply happen, both being true simultaneously, and hence there are two worlds (each immediately followed by an infinite number of additional universes).
@20 (and variously to other points raised)
Don't build your extra-Universal shopping mall just yet.
It seems to me that the principle of parsimony isn't getting the respect it once enjoyed--back before string theory blew out all the stops on unbridled ad hoc rescue of faltering theoretical constructs. If scientists are going to have multiple universes,then, for the greater popular respect of science, scientists should "create that bridge" only when, from deficiencies of theory, logic or a Ford Prius finally drive us to it, they find that there's no other equally servicable route available. In the meantime, fascinating flights of fancy--which I'm all for--should be marked clearly as such rather than declaring flat out things such as that, "inflationary expansion can lead to multiple simulaneous universes." (emphasis added)
Something in all this discusssion reminded me of Harvey Brown's discussion found at Section 2.1 (pages 11-14) of his Physical Relativity (2005, Oxford University Press) -- which involved, in part, the problem of where exactly within the 4-space manifold the universe ought to be located.
Here's an excerpt:
... Thus, Liebniz considered two material universes differing only by the locations in space God decides to put them. Einstein, roughly two centuries later, found himself likewise considering two empirically indistinguishable space-times that differ only by the metric tensor field gμv in each relates to the background four-dimensional point manifold. (...) Liebniz (...) dismissed his pair of cosmological alternatives as a fancy, on the grounds that it violated both the Principle of Identity of Indiscernables and the Principle of Sufficient Reason. ...Einstein, in tackling what was later called the Hole Problem, came to reject the reality of the space-time manifold essentially on the grounds that such a position allowed his field equations to avoid the spectre of underdetermination--the analogue of Liebniz's spectre of divine indecision. 
[footnote [ 6] : It might be more correctly said that when Einstein realized that the "diffeomorphically related" spacetimes were physically indistinguishable, he ceased to believe them physically (as opposed to mathematically) distinct, thus adopting a stance with clear echoes of Liebniz.... ]
[* though, here, it is true, what are proposed aren't "alternatives" in the strict sense but, rather, just some number n discrete universes ]
@ 22 : I wish I'd said that.
I agree--especially since, as it seems to me, this (cited from Carroll, @ 11, above)
“If inflation is eternal into the future, maybe you don’t need a Big Bang? In other words, maybe it’s eternal into the past, as well, and inflation has simply always been going on? "
is a logical self-contradiction on several grounds.
It seems evident to me (though, somehow, not so to many others) that "eternity" as a concept is irreconcilable with notions of "future" or "past" occurences "within it"--- one, because, eternity, being unbounded, cannot encompass events, which are necessarily discrete spacetime-bound and located phenomena--even if that spacetime is relative in appearance from all potential points of observation.
If inflation is meant to refer to a "volume increase," then, logically, it cannot occur "enternally"-- or, that is " ever since forever" (that phrase, again, is a logical self-contradiction.) because volume increases are logically states in which a process occurs from a past point in time through a present point to (putative) future points. By our very terms of reference, we refute enterity as being in any meaningful sense an operating state in which any kind of conceivable phenomena may "unfold"-- and, speaking of physical phenomena, "inflation" is a term which is the epitome of an operative process, an "unfolding."
Logically, an eternal status-- status being a key term in that phrase--also logically precludes other concepts; locus, for example is one and that relates to the logical impossibility of eternity "co-existing" with any singular identity since that would presuppose that identity's "locale" which eternity cannot physically "admit."
OOops. It should have been Leibniz, not Liebniz. Apologies for my repeatedly misspelling Leibniz's name.