"Every true, eternal problem is an equally true, eternal fault; every answer an atonement, every realisation an improvement." -Otto WeiningerThe best measurements of the distant Universe -- out beyond our galaxy -- have led us to the current picture of exactly what our Universe is doing: expanding and cooling, with its galaxies progressively getting farther and farther apart.
(Image credit: Molly Read for the University of Wisconsin-Madison.)
But what does that mean for our past?
If we're expanding and cooling, that means our past was less expanded and less cooled, or as we like to think of it, denser and hotter.
Now, if you're thinking like a scientist, you don't just want to know what it's doing. You also want to know -- if it's expanding -- both what's causing the expansion, and by how much it's expanding. In other words, we'd like to determine the rate of expansion.
And the answer is actually straightforward: if general relativity is your theory of gravity, the Universe's expansion rate is determined by what type of energy dominates your Universe.
(Image credit: NASA, ESA, and J. -P. Kneib (Laboratorie d'Astrophysique de Marseille) et al..)
In our relatively recent past, while the Universe has been filled with galaxies, stars, planets, and all the objects we've ever discovered, the Universe spent most of its time dominated by matter, both normal and dark.
And when you have a Universe dominated by matter, here's how it expands.
(Image generated by me, as are all the subsequent ones unless otherwise noted.)
Note that the expansion rate, H, drops over time. This means the Universe was hotter, denser, and expanding faster in the past.
But if we go back far enough, because the Universe was hotter and denser, at some point it will have been too hot to form neutral atoms! There's another thing that happens, and it's obvious if you think about it. If the Universe was hotter in the past, that means the radiation in the Universe was more energetic.
(Image credit: Lawrence Berkeley National Laboratories, retrieved from physorg.com.)
And if we continue to extrapolate backwards, the energy from radiation will eventually overtake that due to matter. And that causes the Universe to expand differently! How does a Universe dominated by radiation expand?
It expands similar to matter -- in the qualitative sense -- as the numbers work out only a little differently. But we can't go back to arbitrarily high temperatures, or all the way back to a singularity; there's a limit as to how hot the Universe was in its past, as constraints from the Cosmic Microwave Background tell us.
So what came before that? What came before our hot, dense, full-of-matter-and-radiation Universe?
(Image credit: Ned Wright.)
As best as we can tell, there was a period where the Universe was inflating. Stretching it flat and giving it uniform properties everywhere, cosmic inflation sets up the initial conditions that lead to the Universe we observe today.
Rather than being populated with matter or radiation, the Universe could also be dominated by vacuum energy. (After all, the energy of empty space doesn't have to be zero, and in fact, isn't even zero today!)
When a Universe is dominated by vacuum energy, its expansion history looks very different. Let's take a look...
Notice how the expansion rate doesn't drop over time! This means that instead of growing like some power-law of time, the Universe inflates exponentially, and in very short order can stretch itself to be not only larger than you can fathom, but googols of times larger than the entire observable Universe!
Now, you might want to know how big the unobservable Universe is. That is, there are very likely parts of the Universe that are more than 46.5 billion light years from us; we simply can't see the light from them!
So what determines how much the Universe inflates? Let's take a look at the standard picture of inflation.
(Image credit: Narkilar (Update: Whoops!) Narlikar and Padmanabhan, retrieved from Ned Wright.)
The y-axis here represents energy. That is, in particular, the amount of vacuum energy intrinsic to space. Obviously, the amount of vacuum energy in space today is tiny: some 28 orders of magnitude less than we think it was during inflation!
If we want the Universe to inflate a large enough amount to account for the flat, roughly uniform Universe we observe today, we need it to remain in this inflating state for a long enough amount of time. As far as our graph above goes, that means we need to start out on the flat part of this curve.
So long as we can roll or slide slowly enough down this curve, we'll get enough inflation to produce our Universe. At some later time, you'd expect we'd eventually start to slide closer towards that valley.
And eventually, we'd fall in. That part of it, where we fall into that valley, is where this vacuum energy gets dumped into matter, radiation, and all the stuff that produces the hot big bang that gave rise to our Universe. And, if the idea of inflation is correct, this certainly happened in our region of the Universe; moreover, it happened about 13.7 billion years ago.
But you've got to remember, this field that causes inflation -- whatever it's true nature is -- is likely to be a quantum field/particle, like everything else in the Universe.
Now, what happens to an electron -- a well-studied quantum particle -- in something we can study, like a simple atom? Well, you can measure it, and know where starts out at some given time. But give it a while.
(Image credit: unknown Update: likely Carlos Stroud and Zagorka Gaeta.)
If it's a quantum particle, its wavefunction spreads out over time, freely occupying a superposition of whatever states it's allowed to.
So, how does this apply to our inflationary field above? When we allow it to spread out over time, what do we get?
We get that part of this quantum field, if it's rolling slowly enough, actually spreads out back past where it started, up towards a state where it will continue to inflate! So, remember our classical inflation picture, that we showed you, above?
In this picture, inflation happens for some time, and then it ends everywhere, all at once.
But if we allow inflation to be a quantum field instead -- and of course it must be one -- you have to calculate how quickly it spreads vs. how much the Universe inflates vs. how quickly it rolls down the hill. If it rolls down the hill too quickly, or it inflates too slowly, it won't have enough time to spread out in enough regions of the Universe. But if it rolls slowly enough, inflates fast enough, and spreads out sufficiently quickly, what do we get?
We'll start with an inflating region, shown in blue. If the potential rolls sufficiently close to the valley, inflation will end, and we can mark it with a red X. But if it continues to inflate, we'll leave it blue, and generate more inflating spacetime before we check in on it again. And here's what we find.
Although inflation will end in more than 50% of the Universe at any given time, enough of it will spread back up the hill that inflation lasts an eternity. And this is true for every model of slow-roll inflation we've concocted!
In other words, there are regions of the Universe where it inflated in the past, that false-vacuum energy got turned into radiation and matter, and those parts of the Universe had a history very much like our own. But in between those regions, there are other parts that keep on inflating, and so on, and so on, and so on...
(Image unknown; retrieved from here.)
And that's why there's a multiverse, and not just our Universe!
Now, the story I've told you is a conservative one. In this version of the story, the fundamental constants are the same in all the different regions of the multiverse, and the other Universes have the same laws of physics -- with the same quantum vacuum and all -- as our own. But most of what you hear about the multiverse these days are from people who have speculated much farther than that.
The ideas that you hear -- multiple false vacua, the landscape, connections to quantum gravity, etc. -- are ones that people have speculated upon in recent years. These are mostly driven by including connections to string theory, and they present a whole host of difficulties as well as a great many interesting avenues to investigate. I will not touch upon them here, but when you hear those words, this is the basic story that they all take for granted.
So this basic concept, while it likely isn't the entire story, is just simple quantum mechanics applied to our best working model of the Early Universe. And what we get out of it is a Universe that, in most regions of it, will continue to inflate for all eternity: our Multiverse.
Physical Science
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Posted by: Sagar Gorijala | October 29, 2011 1:04 AM
Which of the nine kinds of Multiverse that physicist Bryan Greene discusses (in "The Hidden Reality") do you find credible?
Posted by: David Pearce | October 29, 2011 2:12 AM
David,
I am unfamiliar with Greene's nine (really? NINE?) kinds of multiverse; I am much more familiar with Tegmark's classification of four types.
What I've presented, above, is the argument for the first type of Multiverse, which I think is correct, and hence, which I believe in.
I think that anything beyond that is too speculative to be believed at this point, at least with any sort of confidence. At least, by me.
Posted by: Ethan Siegel | October 29, 2011 2:29 AM
Ethan, can I ask something?
As far as I - an interested non-physicist - can tell, there's no ruler outside the universe, so you can just as easily take the view that the universe is staying the same size while everything in it is shrinking. Is this in fact an equally valid perspective? Or is there some (non-psychological) reason for favouring the universe-expanding perspective? I've always wondered.
Posted by: dbhb | October 29, 2011 6:07 AM
Yes, Universe will have a finite size. Creation and destruction are impossible and therefore Universe and everything in it/with it remains finite. The Rectangular or Equilateral Hyperbola equation XY=1 or XY=8 or whatever suggests that. How? If X and/or Y equals 0 then we get creation and/or destruction but that's impossible. Universe is finite.
Posted by: Sagar Gorijala | October 29, 2011 6:47 AM
Thanks, it is always good to see people grounded in the conservative view. The other day Sean Carroll started with a bang, of jumping to the non-conservative "how do we get to (and parse) initial conditions" which isn't a necessary part of any theory. Baby steps.
@ dbhb, Gorijala:
You can't measure (and compare) size here, I think. (Layman here, so hopefully experts jump in.) Bubble universes can be of infinite size AFAIK, since the local time evolution is set by expansion.
Think of parabolas, that can diverge out to infinity in finite measure along "x" (as a limit of stretched ellipses with minor axis constant). They are infinite along the expanding curve (spacetime as measured by expansion). You can pack an infinite number of these constructs along x.
In other words, our local measure of spacetime doesn't tell us squat about the generic measure. And universes are most simplest infinite in space and time.
Posted by: Torbjörn Larsson, OM | October 29, 2011 7:47 AM
Oops. I haste to add that I am not implying the generic measure is one of spacetime at all.
If I understand Linde, some results in inflation is derived from using semiclassical particle worldlines, since we are analyzing a quantum field (of inflation).* Who knows if the multiverse has a spacetime at that?
---------
I am not qualified to make any claims here though, not having studied QFT. Or GR for that matter.
Posted by: Torbjörn Larsson, OM | October 29, 2011 7:52 AM
Another oops, suddenly realizing we were discussing from the conservative view - a pervasive spacetime is the simplest model, I guess. I leave that to the experts; the problem of comparison remains, I think.
Posted by: Torbjörn Larsson, OM | October 29, 2011 7:57 AM
It might be a good idea to include some note that distinguishes this from the "many-worlds" use of the term "multiverse", which is certainly the more popular one outside of physics circles.
Posted by: John Armstrong | October 29, 2011 8:26 AM
Think about it: we don't know what dark matter is, and somehow it overwhelms the matter we can manipulate and experiment with. But we pretend to know what the history of the universe is. Can't you feel how enormously speculative the whole thing is?!?
Posted by: Bertrand Ducharme | October 29, 2011 8:35 AM
Thank you for a very nice & easy explanation of multiverse theory etc. Even understandable for a layman (I'm a medical student, so far away from the physics classroom). ;)
Your post got selected by the Zite-app on my iPad, so I found it totally at random.
Posted by: Jan Söderlund | October 29, 2011 9:44 AM
Ethan Siegel:
Until there is a good theory of wave function collapse, it seems to me that QM implies an MWI-style multiverse. Quantum mechanics is counter-intuitive. But I don't see reason yet to say that we people aren't also quantum systems. Once that leap is made...
Posted by: Russell | October 29, 2011 9:56 AM
It could be good to point out for people outside of science that none of this presented on this page is a proof of multiple universes. This is pure speculation.
Posted by: Scientist | October 29, 2011 10:48 AM
Bertrand, it's not speculative if you can account for any problems with the model vs. reality with another layer of dark side of the force. If the great attractor pans out, we get the dark multiverse attraction, followed by dark manyworld compression to maintain the balance. With the usual factor of 5 in energy for each new level of darkness, baryonic matter will be down to less than 0.1% of the universe and we can finally drop the observable stuff from the theory, and ultimate enlightenment will be within reach.
Posted by: Mu | October 29, 2011 10:51 AM
This is not hard data, science rely on experiments, not solely on theoretical models. If conclusions which supports the multiverse models would come out of LHC or OPERA, that would be something completely different.
Posted by: Scientist | October 29, 2011 11:28 AM
Enjoyable. We seem to be In need of a modern Thomas Aquinas to harmonize physics and metaphysics, especially when discussing the points at which they seem likely to be shaking hands.
Posted by: michael | October 29, 2011 11:30 AM
Hey, "Scientist" (sure you are...!),
Ethan is actually a real scientist and therefore never, ever, claims proof of anything - and hasn't in this post either.
The clue is in the title; "Why we THINK there's a multiverse." Did you miss that?
You accuse him of something he hasn't done, and never does.
Your post is therefore pure speculation, without any hard data, etc...
Posted by: Mark McAndrew | October 29, 2011 11:53 AM
"Scientist":
All of physics occupies the realm between proof and pure speculation.
Posted by: Russell | October 29, 2011 12:28 PM
I should only like to point out that by replacing the concept "universe" with a bigger more hypothetical concept "multiverse" you have solved many of the problems with the "Big Bang universe" model by moving to a "Steady State multiverse" model.
I would suggest to just call the whole shebang "Universe" capital "U" and move on to the necessary ideas of a "Steady State Universe". And while you're doing this please clarify how many spacetime dimensions the "Universe" (i.e. your multiverse) has in your conservative opinion. 4 or 11 or?
I prefer to think of an 11-Dimensional spacetime "Universe" and dispense with the new jargon associated with the word "multiverse". In either case, there are more core ideas than you've listed above that need to be clarified by the direct or indirect evidence. e.g. baryon assymetry, dark matter, dark enrgy, inflation, quantum gravity, string theory, differentiation of the 8 gluons, etc, etc.
I prefer to think of the "Universe" as a never ending pattern that can be approximately understood by our mind; but that ultimately all of our scientific thought, all of our excellent models are but descriptions of an appearance that we call the "Universe". The reality of the "Universe" is an infinitely more complex thing than our must complicated mathematical models and calculations can fathom.
Further like 19th century scientists, I suggests that concepts are better suited to explore the realities of the "Universe" first; and that use of mathematical models is a secondary (necessary and supportive) step. The 20th century preference for mathematic first brings a newer set of problems, e.g. the 10^500 possible string theories; that can only be solved with clearer concepts all based of course upon and in agreement with experiment and observation. The grand achievements of 20th century physics (i.e. general relativity and quantum mechanics) were first conceptual theories and secondly mathematical.
Posted by: Thomas Neil Neubert | October 29, 2011 1:15 PM
Multifaceted discussion.
I wonder if Ethan can explain when a scientist is satisfied, then what makes him satisfied with just one theorem?
Is it the chance of getting funding, or something exclusively scientific?
Is it the result of a procedure? What thought process guides a scientist, in contrast to the many non-scientists here?
Posted by: idealist707 | October 29, 2011 1:58 PM
michael:
Where, in any of this post, did you see any metaphysics?
In even speculating about a multiverse, science has exceeded the wildest imaginations of any metaphysical belief system, and shown the world how petty they are.
Posted by: James | October 29, 2011 1:59 PM
idealist707:
I've never known a scientist who was satisfied with one theorem.
Posted by: Russell | October 29, 2011 2:33 PM
TYPO
>> Narlikar and Padmanabhan
Posted by: Pratik Deoghare | October 29, 2011 2:35 PM
James, technically you are correct that metaphysics is an arbitrary distinction as everything that is real connects together in an observable way. That is why the study of the nature of God is in the broadest sense the same discipline as the study of matter.
You are free to insist otherwise but it limits your chance of finding the TOE.
Posted by: Michael | October 29, 2011 3:15 PM
Sorry, you are correct if there is no distinction, but if you insist on one it's otherwise.
And I enjoy this post very much even on your terms if you choose to limit its possible implications. I hope you understand that.
Posted by: Michael | October 29, 2011 3:18 PM
Okay SUPER-layman, here, so bear with me...
You're saying that there are pockets where the vacuum energy has collapsed into radiation and matter--and thus expansion is no longer accelerating--distributed within a larger field where there exists only vacuum energy?
How can that be, when we've observed accelerating expansion in our local, observable universe?
Posted by: David McDermott | October 29, 2011 3:38 PM
dbhb@4 and Torbjörn @6: If the sizes of everything (atoms, rulers, humans, etc) were changing over time, we would observe changes in the fundamental constants of the Universe (Planck's constant, speed of light, etc.) that determine those things. The constraints on their constancy rule that interpretation out.
John @9 and Russell @12: MWI is an interesting and oft-talked about interpretation. There are many reasons to doubt that it is an advantageous (or even physically reasonable) viewpoint when compared to the standard (wavefunction collapse) interpretation of quantum mechanics, but the arguments I would make to state my case for that are too lengthy for a comment here.
Scientist (at many points) and the responses: this is -- and I make no pretensions otherwise -- theoretical physics. It's not airy-fairy pie-in-the-sky speculation, but it also isn't proven the way you would prove that a*b = b*a. You may want to read this recent guest essay that does a wonderful job of explaining, with some brilliant metaphors, what this sort of theory is (and isn't).
Thomas @19: although there are many interesting things one can do with even one extra dimension, four is certainly enough to describe and explain everything I've written above.
idealist @20: I try not to make statements like "it is true..." or "it is proven..." or "it is obvious that...", and instead I remain dubious of anything until I can convince myself that this is, physically, what actually happens under these circumstances. And when I'm sufficiently convinced, and all of my major objections have been answered, that's when I'm satisfied with the solution to a particular aspect of a problem. Your mileage may vary.
Pratik @23: Yikes! And thank you; fixed.
and David @26: The Universe is only just very recently becoming dominated by dark energy. The vacuum energy of the Universe today is at least 18 (and quite possibly many more) orders of magnitude smaller than it was during inflation, and the Universe certainly went through a period of around ten billion years where it was dominated by matter and (before that) radiation. (I mean, just look at it!) There is no reason that the "true vacuum" in the diagram above must be at zero energy, just at a much smaller value than the "false vacuum" shown.
Posted by: Ethan Siegel | October 29, 2011 3:51 PM
Well, interesting, but:
First, I don't think that simple separation of causality rightly constitutes "separateness" of a universe, because the space-time should still be contiguous all in between all those regions (each region is accessible to each adjacent region, even if you can't get all the way over to a distant region by traveling there - you get my meaning, right?
Second, about differing laws of physics:
Not to imply the post was focussed on this issue, but since it relates: the popular idea that string theory "leads to" multiverses with various laws is an unwarranted stretch. Basically, they are saying, that if we can imagine a certain "ultimate structure" at the heart of things, well logically it could have been otherwise. Sure, I very much do get that, which is at the heart of the question "why does this possible world 'exist' (in a trans-Platonic, sweaty materialistic way) and not others instead?" as e.g. Paul Davies lays out so well in "The Mind of God" and "Cosmic Jackpot." Some, like Tegmark, go ahead and say that *all* possible universes (definable as he considers "structures" at least) exist.
But, it's a long way from that well-put metaphysical complaint about our universes exclusivity or privileged existential status, to saying that the other types of universes can actually *form* out of something other than pure ideas (or even, why did ours - if it did!) Note that virtual particles are coming "out of space" all the time, the rules have to be embedded *in the apparent vacuum* (and it isn't really then a true vacuum is it - space is a plenum.)
There needs to be a *dynamics* of not only how this machinery operates, but how it can turn into something else, say when space contracts under enough pressure etc. Otherwise you just have the inclusive metaphysical Platonism of Tegmark and other modal realists but no way to "generate" the other universes from a "real" substrate. Does anyone actually know how to derive such a dynamic of string *change*? How could you connect it to space itself to get the right virtual particles, not just a way to e.g. have two electrons collied so hard they might end up with different physical laws? But laws seems an integrated system of a space-time as a whole, what shaking of "machinery" could turn that into something else, especially an integrated and harmonious one at that?
(BTW, read Paul Davies on that foundational question stuff like why should one or some possible worlds exist and not others, etc. - he is the best.)
Posted by: Neil Bates | October 29, 2011 4:13 PM
@19 "I should only like to point out that by replacing the concept "universe" with a bigger more hypothetical concept "multiverse" you have solved many of the problems with the "Big Bang universe" model by moving to a "Steady State multiverse" model."
One of the most ironic implications of modern (or post-modern) cosmology is that some sort of Steady State Infinite-Universe/Multiverse seems to be implied by many of the new theoretical constructs. Indeed, in the latest of these theoretical extensions, it's a whole Universe that comes barreling out of the false vacuum, and not simply a single hydrogen atom out of a "true vacuum," as per Fred Hoyle's concept.
But because Hoyle was so wrong -- and not just wrong, but wrong-headed, arrogant, and obstinate -- in his dismissal of the "Big Bang," I wonder if resurrecting a similar Steady-State will-of-the-wisp under a larger (as in googolgoogolplex larger) guise, is not likewise ultimately doomed to failure -- though it will likely take alot longer to prove that failure.
I know that alot of scientists -- Einstin included -- found the idea of a Universe, (and now Infinite Universe/Multiverse), in cosmic equilibrium with itself to be "beautiful," and hence almost a goal worth proving. But chaos seems to leak into all these beautiful theories, from around the edges, and to demand something still more profound.
Which is what happened when "the Big Bang" theory was added to, and in a sense supplanted by, inflation.
Posted by: Jack Dawe | October 29, 2011 6:48 PM
@27 Thanks for taking the time Ethan.
What... even if the photons and the Planck lengths were shrinking with everything else? I'm really surprised. I would've sworn that those two perspectives were mathematically equivalent and only psychologically different. Gah I'll never get physics! But I'll keep trying, and I'll keep reading.
Posted by: dbhb | October 30, 2011 6:23 AM
I thought saying "multiverse" and saying "wave function of the universe" were the same thing. Is that not most of your explanation?
Stephan
Posted by: Stephan Wehner | October 30, 2011 1:42 PM
Ethan
Thanks for the clarification on the 4 dimensions.
Well enough. Lacking any new physical insight; I agree that it is better to remain conservative in speculations. And yes, Tegmark level 1 is a conservative multiverse.
The problem with even a conservative Tegmark level 1 multiverse is that it is not based on new physical insight and creates one more layer of assumptions that must be overcome.
A Tegmark level 1 multiverse only apparently solves theoretical problems (e.g. thermodynamic initial conditions, consciousness, etc ). Without new physical insight, such as equivalence principle or the uncertainty principle, and without any new verifiable predictions of a direct or indirect (e.g. Dirac's infinite sea of negative energy electrons) nature; a Tegmark level 1 multiverse theory doesn't have even the minimium use of a scientific theory as a description of nature. Tegmark level 1 multiverse defines naked ignorance as scientific hypothesis. A scientific hypothesis must not be gratuitous; it must be arguable in terms of evidence. Saying that the emperor's new clothes are beautiful is a gratuitous hypothesis that undermines common natural knowledge by promoting naked metaphysics as natural phenomenon. Similarly multiverse metaphysics undermines observation of natural phenomenon.
Posted by: Thomas Neil Neubert | October 31, 2011 11:05 AM
What's the difference between matter and radiation?
Posted by: sg | October 31, 2011 8:51 PM
E=m(c2) ("radiation")
m= E/c2 ("matter")
I thimk....
Posted by: Jack Dawe | October 31, 2011 11:40 PM
Matter has rest mass. e.g. proton, electron, neutrino.
Radiation has zero rest mass. e.g. a photon, gluon, graviton.
E=mc2 means that mass and energy are equivalent.
Posted by: OKThen | November 1, 2011 10:19 AM
Nothing is ever at rest.
Therefore nothing ever has rest mass.
If they're equivalent, what's the difference?
Posted by: Wow | November 1, 2011 11:02 AM
The problem is, really, sg, what context do you ask the question?
Because "radiation has zero rest mass ... " is correct if you're talking about matter under special relativity terms. But if you're talking about under general relativity, then the answer would be different. If you're talking about lithography or medical imaging, the answer is "none", except for the trivial case of "wavelength".
Posted by: Wow | November 1, 2011 11:09 AM
I believe that the wire grid image of a spreading electron wave packet credited to an unknown source was taken from a paper that I co-authored with Zagorka Gaeta entitled "Classical and quantum mechanical dynamics of a quasi-classical state of a hydrogen atom," which appeared in Physical Review A, 42, 6308 (1990).
Posted by: Carlos Stroud | November 1, 2011 11:19 AM
For the purposes of this discussion, "matter" is any object whose rest mass energy (the E = mc2 part) is much greater than its kinetic energy, or the energy associated with its temperature (of order ~kBT).
And therefore "radiation" is anything whose rest mass is negligible when compared with its kinetic energy. For photons, they always act like radiation; neutrinos act like matter now but acted like radiation when the Universe was only a few hundred thousand years old; electrons act like matter now but act like radiation at temperatures over about 10 billion Kelvin (when the Universe was less than 1 second old), etc.
For a good graph illustrating the transition from a radiation-dominated Universe to a matter-dominated one, check out this post: http://scienceblogs.com/startswithabang/2010/12/whats_in_the_universe.php
Posted by: Ethan Siegel | November 1, 2011 11:19 AM
"So this basic concept, while it likely isn't the entire story, is just simple quantum mechanics applied to our best working model of the Early Universe. And what we get out of it is a Universe that, in most regions of it, will continue to inflate for all eternity: our Multiverse."
I get the feeling you use "universe" both of the utter entirety of all, and of individual surviving "bubbles" or "islands" within the "multiverse". Is there an inconsistency of usage here?
Posted by: Arthur Shippee | November 1, 2011 5:32 PM
Ethan (or anyone who might still be reading these comments),
Is there a mechanism that keeps inflationary regions from being contained entirely within non-inflationary regions? If not, what would happen if this occurs?
Also, over what kind of area is the probability curve (wavefunction?) for inflationary strength defined? A sphere of 1 meter diameter? 1 parsec? Is there a mechanism that keeps that probability from being widely different for areas that are close together?
Sorry if these questions don't make sense, I'm coming from a BS in math and PhD in economics, so...yeah.
Posted by: Aaron | November 1, 2011 5:54 PM
@40: I think you're having trouble with semantics (ie, word association and value).
For instance, you seem to have trouble distinguishing between the idea of a "universe" (which could be a bubble infinity) and the "multiverse", (formerly called the "omniverse")which could be a set of identical bubble infinities, or a set of infinities of varying types, governed by different laws, etc. Or it could be all of the above, infinitely and eternally generated by inflation falling out of the false vacuum,(Tegmark's Level IV MultiVerse {?}).
If you want a quick scan of the different levels of possible multiverses, check out http://en.wikipedia.org/wiki/Multiverse. This entry, written largely in layman's language, might provide an easier approach to Ethan's more comprehensive coverage, above.
@41: vis a vis "mechanisms" controlling inflation, etc. This comment I feel is likewise hobbled by the language. The mechanism I think you mean is false vacuum.
I'm stumped by your idea of a "probability curve" throughout a given volume for inflation. I don't think such a curve would have a one-to-one correspondence in spatial values. But statistics and graphs aren't my strong point (I was an "A" student in algebra, but a "C" student in calculus - twice).
Anyway, I think Ethan could provide better insights in regard to both these posts, and also correct any misapprehensions I myself might have, on this difficult subject matter.
Posted by: Jack Dawe | November 1, 2011 7:17 PM
But what about the influence of the Supreme Goat Overlords, who care for us so much and built the universe for our needs? Haven't you read their obtuse and contradictory tome of salvation? Repent! LOL
Posted by: Mike | November 2, 2011 6:34 AM
We had enough politics on the global warming comment page (see "I AM A SCIENTIST"). For the love of... (name your deity, including yourself, the Cosmic Crow, and/or the Absence of God Almighty Damn)... let's not get into religion as well.
Even so, #43, that was fairly funny. "Supreme goat overlords..." Heh heh.
Posted by: Jack Dawe | November 2, 2011 10:32 AM
Thanks for the explanations on matter vs. radiation.
Posted by: sg | November 3, 2011 1:03 PM
Ethan: the charts showing H vs t for radiation or matter dominated Universes are wrong. Ht = constant for any power law model. So for t = 1,2,3 & 4 times t_o, the Hubble parameter is H = 1, 0.5, 1/3 & 0.25 times H_o. This is true for both a ~ t^{2/3} or a ~ t^{1/2}.
Posted by: Ned Wright | November 4, 2011 1:38 AM
@Scientist, posting lucky #13
"It could be good to point out for people outside of science that none of this presented on this page is a proof of multiple universes. This is pure speculation."
I agree. My opinion is that Hawking, with the understanding that the BB model breaks down in the beginning mathematically, looks to an outside cause of our universe. He does not seem to like the idea that the BB entity had the self perpetuated energy to have been the initial cause by itself or that time was created by a BB beginning.
Based upon what I have read from his books and writings, he also does not think that the beginning of our universe could have happened from the ZPF alone, without the same possibility to continue for another universe.
I think it's all wrong including the Big Bang model, but that's just my opinion. Hawking probably has a lot of mentally entertaining ideas concerning the theoretical physics of multi-verses.
I think he is the most well-known proponent of multi-verses.
Posted by: forrest noble | November 4, 2011 2:26 AM
Ned @46,
Thanks for the correction of the (elementary) mistake in the post, above. While the times and relative sizes of the Universe are correct in all the diagrams above, the evolution of the Hubble constant are accidentally given for when the scale of the Universe is two, three, and four times the original value, not for when the time is two, three, and four times the original value.
In the case of an inflating (exponentially expanding) Universe, there is no difference, but in the cases of matter- and radiation-dominated Universes, there is a difference. The correct values, as Ned notes above, can be found by noting that the product of H*t at any given point in a matter- or radiation-dominated Universe must be fixed.
Moreover, H*t for a radiation dominated Universe must always equal 1/2, while for a matter-dominated Universe it must always equal 2/3. For our Universe right now, which is transitioning from a matter-dominated one to an inflating state, it just so happens that H*t = 1.00, to the best of our measurement capabilities.
Thanks again for the catch!
Posted by: Ethan Siegel | November 4, 2011 3:11 PM
I, for one, welcome our Supreme Goat Overlords.
Posted by: Woof | November 4, 2011 4:59 PM
I'm glad somebody does. They've earned our welcome, and we should give it.
On the other hand, if you've seen the murmuration of starlings on the yahoo "animals are funny" video page, I think you have to recognize who the true Lords of Creation are -- and they ain't no billygoats, much less a bunch of flatfooted apes with opposable thumbs.
Birds: last survivors of the dinosaurs, or the dinosaurs back in black, and this times it's for keeps? The answer you give may determine your place in eternity --
Posted by: Jack Dawe | November 4, 2011 5:11 PM
Thanks for that very lucid presentation, Ethan. Does this approach have anything to say about whether inflation is eternal in the direction of the past as well as the future? And does it raise the possibility that we might still be in a false vacuum, since vacuum energy appears to be non-zero?
Posted by: Nick Gotts | November 5, 2011 7:20 AM
Just clarifying a point.
So ‘new’ universes can only bud-off before the ‘parent’ region’s energy becomes matter and radiation ?
That is, we cannot now expect a new universe to happen in our present day universe/region.
What about overlapping of universes what happens at the boundaries?
Posted by: Duncan | November 6, 2011 6:42 AM
Universes "budding off" from each other is one model of a multiverse, though I don't think it has much currency now. Particularly the black hole/white hole scenario, whereby a black hole in this universe becomes a Big Bang in another.
"Budding" via inflation, the way I understand it, works more like an instant grapevine where false vacuum is the stem and universes are the grapes and inflation is what makes the grapes pop out. That's kind of a crude way of looking at it, but -- Correct me if I'm reading it wrong, scientists --
"Overlapping" (again, my reading of it) would have the appearance of a collision, and would leave circular (or elliptical?) traces in the cosmic microwave background. I think the jury is still out on this one.
Posted by: Jack Dawe | November 6, 2011 9:35 AM
"So ‘new’ universes can only bud-off before the ‘parent’ region’s energy becomes matter and radiation ?"
I didn't really address that question in my above comment. I would say that, from my understanding of it, once time starts then inflation has fallen out of the false vacuum and the creation event has occurred (even if it isn't complete) in a pre-baryonic universe.
"Thereafter," creation continues on, from what I think should be designated as a timeless or super-temporal realm, like the Goddess Vishnu doing her dance, and throwing out worlds (universes) on all hands.
Kinda mind-boggling, no?
Posted by: Jack Dawe | November 6, 2011 12:45 PM
Here is a link to a paper which postulates that eternal inflation is not actually eternal.
http://arxiv.org/abs/1106.3542
Posted by: Andy Bower | November 6, 2011 5:35 PM
http://www.youtube.com/watch?v=oT7bAuOz8ao
Posted by: j.davis | November 13, 2011 4:41 PM
I'll just leave this here.
I think Occam's Razor applies here. But, you say my 'logical analysis' is silly pedantry? I'll give you that. But then, let us think about more of the common meanings of meanings of the word...
1. Absolutely everything that exists, including existence.
For the sake of completeness, I'll reiterate: If this was the definition that's used, 'multiverses' are logical nonsense, because something can't exist outside of everything. I would use those fancy logic symbols, etc, but I don't know them. I've always wanted to learn though. Anyone have a link they can give me?
2. A self-sufficient set of existences that don't act with any external existences.
If the second version holds true, then it is, by the scientific method laid down so long ago, unprovable, since such a 'alternative universe' would not act upon us (Unless the protagonist decided to used a foobar device to teleport here and cause havoc) Perhaps many people would say that being unprovable is the same as being religious. I might agree. And I just had an epiphany about a certain implication. Assuming all that jargon with 'accelerating universe' is true, there is matter, that objectively exists, but we will never act upon it, making it, complying with the second definition, its own universe. Although, it has already acted upon us, some time, since we know of its (i.e. the other side of the 'Big Bang') existence... This is a stream of consciousness thing, and most likely complete nonsense. This black section, that is. And irrelevant. To get back to my earlier point:
3. What is observed.
And with the third possible definition I've found, I postulate that, to say there are multiple universes is to say they have been observed. If they have been observed, then they are part of one universe, namely, the one that we observe. Perhaps this is circular and makes no sense. You're welcome to make another interpretation.
So, that's two more, increasing lenient definitions. However, of course this isn't the full set. What do you think 'universe' means, exactly?
Wikipedia wrote:
The Universe is commonly defined as the totality of everything that exists,[1] including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space.[2][3]
Posted by: Nihility | December 29, 2011 8:28 PM