What is the Multiverse, and why do we think it exists? (Synopsis)

Very nice summary!

I would only change this sentence: "But if you’re seeking a fifth factor of a million, the Universe simply won’t get you there."

Instead of "the Universe", I would put "our observations" or "our observable universe". Because "the Universe" is there, we just can't see it.

(I've said this before, but Ethan you have a major talent for doing this. Cosmos III, dude.)

Layperson questions dep't:

1) Tegmark's name is on some of those pictures, and some of what you've written reminds me of articles he wrote, minus his apparent fondness for a mathematical ground-of-being that's somewhat Platonic. Yet on the other hand there are articles in reputable places saying basically that he's engaged in some kind of pseudoscience. What'up?

a) Is Tegmark's theory viable?

b) How close or far is it from being mainstream? (Majorities don't make (or un-make) facts, but none the less it's useful to know where we stand.)

c) Do you agree with Tegmark and if not, who are your preferred multiverse theorists?

2) I take it that Everett's many-worlds theory doesn't enter into this picture. But while I'm pestering you about Everett, I may as well ask:

a) Does he really mean that the _whole universe_ splits at every wavefunction collapse?, or only that the region encompassed by each wavefunction collapse splits, independently of every other such region?

b) Where does the energy come from for that? I'm inclined to be sceptical of the idea that even the vacuum energy is sufficient to do it.

3) While we're on the subject of vacuum energy, re. your diagram about false-vacuum:

a) Where do you think we are on that curve?

b) How long before we start running down the steep part of it? (I tend to think in multi-billion-year spans of time, so yeah this is relevant even if it's a gazillion years away.)

c) How could one tell empirically, or what measurements could one make, that would differ from one point in time to the next as a function of sliding down that slope? (see "wild stuff" below)

4) What's your philosophical take on the relative merits of:

a) Instances where there are strong theories but no testable hypotheses and no available empirical facts?

b) Instances where there are empirical facts but no viable hypotheses?

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"Wild stuff":

A team of scientists makes certain physical measurements today, and then calls upon their descendants to make the same measurements at some identifiable point in the future. Not in terms of Earth years, but some other metric, such as "when you observe that Galaxy Q's red-shift changes such that the distribution of observable light conforms to the following spectrum, _then_ repeat the measurements we've specified."

At some point in the distant future, observers see Galaxy Q's red shift match the specification left by their ancestors, so they repeat the original set of measurements. The results show different values, and the comparison between the original values from long ago and the new ones just measured, demonstrates that we have moved some measurable degree down the curve from false vacuum to true vacuum.

A subsequent set of measurements at another point distant in time, supports the same conclusions. But now, interestingly, we're starting to observe other changes in physical properties as compared to the two-sets-of-long-ago measurements.

What (if any) kinds of changes in physical properties would you expect under those conditions?

It would seem that the speculation that there are a large number of other universes, each with its own set of fundamental constants and physical laws, would take the starch out of the anthropic principle arguments regarding fine tuning of our universe. With 10^10^10...etc, universes out there, it's no surprise that ONE universe which is amenable to life would arise. In fact, the opposite would seem to be true; it would be unlikely that ALL those universes would be hostile to life.

@ G

re. your 3) ... there is no such plot in the article

if you mean the inflation article from last week, present time is where the energy density is at it's lowest. Inflation happened while the energy was at it's highest, the drop if where re-heating occurs. There is no time to drop, it happened 13.7 billion years ago.

There is a false vacuum theory that says we are just on another plato and the field will drop again... but there is no way to test this unless it actually happens.. in which case, it won't matter :D

Ethan, the following two statements you made don't seem to agree (I interpret them to mean that an electron is larger than our observable universe):

"The part of the Universe that’s unobservable to us — filled with more planets, stars, galaxies, clusters and voids — is at least 150 times the size of the part that is observable!"

"In very short order — far less than a second — a region of space that began as small as a single electron would have expanded to be trillions of times the size our observable Universe is today."

This is the first time I've seen the Multiverse Theory predicated on physical laws. So if the multiple universes can bump into each other then they must be part of something larger, no? I find it interesting that the more discoveries that are made, the more complexity that is seen, the greater my faith in a Creative Mind behind it all. Cosmos has got it wrong. Increased knowledge makes it that much harder to explain away God.

Ethan, where does the "at least 150" come from in the ratio of the size of our pocket universe to the size of the observable patch? Is it from flatness measurements? (I mean: is it based on the assumption that our pocket universe is closed on itself, a 3-sphere, that must be huge because the curvature is smaller than we can measure?) Because I thought the jury was still out on whether our universe was finite-but-unbounded--i.e. wrapped around on itself in a higher dimension--or whether it was a simple 3D volume that actually has an edge (just one that lies outside the observable patch). Please, could you clear this up for me?

@Ethan and @uncleMonty #7: I have the same question.

I thought that the best quantitative limit on the ratio of the universe outside our observable patch was only about five or six -- based on searches for super-Hubble correlations across the CMB fluctuation map (that is, looking for "circles in the sky").

On the other hand, if you take simple inflation at face value (60-ish e-foldings), you end up with some monstrously huge scale like 10^20 or so.

Ethan, if you've got a review article or something that explains the x150, I'd love to be able to read it.

I read the full article. I found it unconvincing. Re this:

"The part of the Universe that’s unobservable to us — filled with more planets, stars, galaxies, clusters and voids — is at least 150 times the size of the part that is observable! The fundamental constants look to be the same at all locations and at all times in our observable Universe, and our true Universe appears to be at least many millions of times the volume of the part we can see."

There's just no evidence for this. The very large universe is based on the non-sequitur that a universe that looks flat must have a very large radius of curvature. But like uncleMonty suggested above, the universe could just be flat. it could be a simple 3D volume with some kind of edge. It doesn't have to be much bigger than the observable universe at all.

The very large universe is based on the non-sequitur that a universe that looks flat must have a very large radius of curvature. But like uncleMonty suggested above, the universe could just be flat.

This is not my area, but AIUI that solution would require some additional force acting on the universe. In the absence of such a force, a finite,static spacetime with matter in it is going to curve in on itself.
if that's the case, then it seems to me that physicists are perfectly warranted in saying that the simplest way to explain observed flatness without invoking new special forces is if the universe is much bigger than what we observe.

"Whether these Universes are similar or different to our own, whether they have the same physical laws and properties, whether they have the same fundamental constants, particles and interactions, we do not know."

I was wondering, how could a different Universe have different physical laws than our own, if they all have a common origin? At a minimum, some basic primordial features like quantum mechanics (quantum jitter in a field especially) as well as the inflation field itself - and associated boson - should be shared by all of them, right?

Of course they might evolve in very different ways, e.g. generating different kind of particles and forces, different constants etc., but I guess all the Universes must share something... and probably our own Universe is even more similar to the others that have survived 13.7 billions years like ours, as I understand there's a narrow window for a universe (e.g. for fundamental constants) to be "stable" for a long time, no? I guess I'm talking about "natural selection of universes" or something similar...

...does this make sense or do I sound utterly crazy? :-)

@ Eric, Uncle, Micheal ...

I think Ethan is basing that size on the minimum of e foldings in inflation to get a homogeneous and smooth universe. But that is a number which can very by quite a bit foldings based on choice of inital conditions. 60 or so foldings being a minimum.

The size depends on the time inflation was happening... that area had almost no observational data. Maybe these new BICEPS measurments will give some answers to how long inflation lasted, that can give some numbers on how how large it is.

@eric #10: it doesn't require some additional force. The scenario is something like flat-earthers in reverse. The flat-earthers could not conceive that the Earth was curved. Cosmologists like Ethan cannot conceive that the universe isn't curved. The simplest way to explain the observed flatness isn't to say the universe is really big and curved. The simplest way to explain is to say it's flat.

Another question about curvature: if our universe (in the eternal inflation model) is a 3-sphere, that is, a 3D volume curved on itself in a higher dimension, does that allow us to conclude that the space-time foam also has 4 large spatial dimensions?

Sinisa Lazarek @ 4, yes I was referring to the previous week's article. In any case thanks for the info.

Mike @ 6: Science is limited to what it can measure, and what it can describe and theorize in quantitative terms (math). Science can't provide a definitive answer to whether there is a Creator, because any such entity could know the possible outcomes and could interfere with any experiment performed to ascertain its existence.

That situation is basically the same as this: If you're taking a test but the answer sheet is visible to you, there's no way for the teacher to know if you got the answers from memory or from the answer sheet.

Working scientists tend to get annoyed with discussions of Creators and deities generally, because that subject is outside the scope of their disciplines. By analogy, hitting a baseball involves your brain performing complex trigonometry at high speed, but you wouldn't ask a baseball coach to teach you trigonometry, and you wouldn't ask a math professor to coach you at baseball practice.

For which reason, if we're interested in discussing the theological implications of various developments in science, we should be seeking a forum hosted by theologians, rather than trying to fit that discussion into this forum, which is only concerned with what science can measure and describe in theory.

Why is this even a question. there's many verses, all together it makes the 'uni' verse.

ffs. #:|

The interweb is a verse, within a verse...

^ its a sound character witness.. ;)

Ethan, tahnk you for the nice post! One question, I am not totally sure I understand how natural inflation, which is one of the few models that sruvives both Planck and BICEPII contraints, falls under the cathegory of eternal inflation. (Your statement, adn I paraphrase her from memory: up to now, all known models of inflation that are in agreement with the data predict eternal inflation. Sorry if the paraphrasing is not perfect, but for some reason when select comment I can no longer see the full article and I am too lazy to go back and cpy paste.)

John @13 - the difference is, flat earthers didn' know about GR, but now we do. AIUI, GR predicts curvature in the sort of case you are talking about (i.e. a relatively small, matter-filled universe). IOW a known phenomenon will cause curvature in the scenario you envision. In order to "flatten it back," you need to posit some unknown phenomena acting against GR. So yours is not a simple explanation. The words may be simple, but because it ignores the effect of real, known, phenomena, the words alone do not actually explain what needs to be explained for it to work. Your 'its just flat' idea It has a lot of work to do before it could be accepted.

Eric: GR doesn't predict curvature. See this. The three options are repeated on this WMAP article. This says "We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent". Only it doesn't. It's a non-sequitur. We cannot "truly conclude is that the Universe is much larger than the volume we can directly observe".

Nice read! I do remember reading in ancient Indian text mentioned that there are universes upon universes. We recently published an article which attempts to satisfy the curiosity that if we are living in a multiverse then how far can be our closest neighbors?

http://www.blueplanetjournal.com/cosmos/mapping-the-multiverse-how-far-…