“Go then, there are other worlds than these.” -Stephen King, The Dark Tower

Ever since quantum mechanics first came along, we've recognized how tenuous our perception of reality is, and how -- in many ways -- what we perceive is just a very small subset of what's going on at the quantum level in our Universe.

Then, along came cosmic inflation, teaching us that our observable Universe is just a tiny, tiny fraction of the matter-and-radiation filled space out there, with possibilities including Universes with different fundamental laws and constants, differing quantum outcomes existing in disconnected regions of space, and even the fantastic one of parallel Universes and alternate versions of you and me.

But is that last one really admissible? Here's what the best modern evidence has to say, and it isn't necessarily the outcome you're looking for!

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Ethan, you wrote: "As time goes on, because of the dynamics of expansion, no two regions where inflation ends will ever interact or collide". Is this not controversial? I remember reading in the past 2 or 3 years that some astronomers were searching for the signature of cosmic bubble collisions, where our universe *had* in fact collided with another nucleating bubble. Obviously they didn't find any such signature, but there must be some cosmologists who believe that the possibility is not as clearly ruled out as your article suggests. I'm very interested in this, could you follow up?

One of my fave lifetime subjects. Thank you for tackling it. I was taught that as part of the subject, the issue of infinity is a math all its own because there are different rules forced due to indeterminacy. And also, there is an easy way to distinguish your remark that some infinities are bigger than others. Consider the first three orders of infinity (the number of orders are indeterminate too, of course). These three may be envisioned as linear, areal, and mathematical - and these are just the list sorts of infinity.

First order: some list like the list of all real numbers. This is a linear infinity.

Second order: the list of numbers like Pi: This list is not only indeterminate in the linear dimension but has another dimension of indeterminacy beyond the decimal point. It should be noted that you don't have to make these numbers up. They are already inherent in the universe. That is also probably indeterminate.

The third order is operational: Take some second order list and select didgits according to a simple algorithm such as taking adjacent numbers along a diagonal path in the list, which will genrate a new indeterminate list almost certainly never before seen and indeterminately larger, due to the different mathematical operations you can use also being indeterminate.

There are many more orders but the human mind gets baffled not too far up this mathematical sort of indeterminacy.

I am not sure but I think this is Cantor.

Forget parallel universes…alternate universes…whatever. Let’s just think about our universe.

If the universe is infinite, then let’s consider (for example) the cubic parsec of it that’s centered around our solar system. There is only a finite number of possible permutations and combinations of particle types, positions and momenta within that parsec. That is a truly, ungodly large number…but it’s not infinite. If the universe is infinite, then there must be more cubic-parsecs than there are combinations of particle positions, etc within a single cubic-parsec. This means that there must be an infinite number of cubic parsecs that are exact duplicates of each other…that’s an undeniable mathematical fact. It’s still possible that our cubic parsec happens to be unique…especially if the universe is far from homogeneous. But statistically, that has a probability that is infinitely close to zero.

So, if the universe is infinite then there pretty much MUST be an infinite number of you’s out there that are precisely identical to you…some who are identical to how you’ll be 20 seconds from now….identical to you except for sneaking that last kiss.

Another way to think about this is that if I gave you a very large pile of dice and asked you to group them into collections of three and to make the numbers showing on the tops of those dice in each collection to be different from every other collection. If I give you 30 dice and ask you to make 10 groups of three…you can easily make them all unique. If I give you 200 dice, it’ll be harder, if I give you 600 dice, you’ll still be able to make 200 groups of them that are unique…but beyond abut 650 dice, you’ll have one group in every possible combination…and you’ll have no choice but to make duplicates. Well, even if there are googleplexes to the power of googleplexes of combinations of atoms, photons, electrons, etc in a cubic parsec…then you’ll need a lot of cubic parsecs before you run out of different ways to arrange them….but eventually, you WILL run out…then there are duplicates…and that means duplicate earths…and nearly-duplicates…in infinite profusion.

And that’s without invoking parallel/alternate universes or other big bangs. Infinity absolutely requires that there is repetition on the scale of suns and planets.

QED

I got a bit stuck on this sentence: 'And if the Universes are all the same as one another as far as physical laws go, and if the number of these Universes is truly infinite, and if the many-world interpretation of quantum mechanics is completely valid, [...]'

Could someone explain why the many-world interpretation is important here?

I would have thought that it was either/or: so that a) if many-world is true, there are parallel copies of us, or b) if there are infinitely many disconnected regions with the same laws, there are parallel copies of us.

But this might mean I am missing the fundamental connection between expansion and quantum mechanics, which is very likely as I have only a first-year university level of physics education :)

Christopher, Cantor invented set theory and began to explore these different types of infinities, but others built on his work. I wrote a blog article about it here: http://blogbloggerbloggest.com/2013/05/23/infinitely-mad/

Ethan,

What a cool essay and very well written. Thanks for setting the record straight on the crazy unacceptable notions hyped by the popular media on parallel worlds and different versions of you and me.

Philip Gibbs has written an essay for the FQXi Essay Contest (Spring, 2015) titled "A Metaphorical Chart of Our Mathematical Ontology" which is very interesting in which he speaks of Meta-Laws as kind of being the link between the Multiverse (multiplicities of vacuums) and our vacuum solution (our Universe).

http://fqxi.org/community/forum/topic/2290

Below is my comment on that page on the idea that physical-ness is related to 4 dimensional spacetimes (Einstein type) and maybe the reason that there is 'unreasonable effectiveness of maths in physics'. Selection processes for physical type universes are then not necessarily random.

"Professor Gibbs,

The new paradigm shift (from one Universe to multiple Universes) is hard even on Western thought. At first I was reticent but then... . It is actually amazing when one thinks about it compared to the concept of the one singular Universe our egocentric minds have somewhat logically been led to believe in (hard to let go). There are hints to the Meta-Laws. Frank Wilczsek stated something to the order that why are the gauge coupling forces unifying (GUT except gravity) at one point in our Universe (I am assuming SUSY makes the unification more exact)? Thinking about this, is it probable that most successful Universes has this form of unification of gauge forces more or less in some tolerant area near a unifying energy (Planck energy type) with gravity? This cannot be attributed to coincidence. You stated in your essay, "It is known that the combination of quantum theory and general relativity imposes tough constraints on the possible range of consistent space-time models." Also, the GUT point being (how much?)variable with other Universe formations this does rule out 'fine tuning" and maybe one should not look at just one number (say the Higgs boson mass) and say it has some 'unnaturalness' because it looks random and especially 'fine tuned'. It is best just to look at the GUT points in any formed successful Universe that has a 4D space-time. It may be hard to trust just a few numbers whereas the GUT points are more of a gestalt of what is going on. Why 4D? It is well know that 4D manifolds are the most interesting manifolds/topologies in mathematics. Even more so than higher dimension topologies. Recently, the Triangulation Conjecture was disproven. Coverings of simplices (triangles or tetrahedrons) cannot completely cover higher dimension topologies (past the 4D) based on some simple rules. This leaves higher dimension coverings 'foamy' or full of holes. Makes one wonder whether this means that higher dimensional Universes can exist in a 'physical' sense. I am not sure that 'all solutions exists'. It is bound to be super variegated (though with 57 varieties ;)). I am thinking that one should not even consider 'fine tuning' anymore but to look at the tolerance or range of solutions in the hierarchal space as a mathematical structure that can eventually be computed in a Scientific framework. All other Universe that have the 4D space-times would somehow have a computational relation to each other (a dictionary?). The GUT point would also be a measure of how fast nuclear rates (and chemistries) and gravity combine to create the Universe. In some Universes (even if 4D) that stars would burn too fast or gravitational collapse only produces black holes where life could not have enough time to form or exist. Or the GUT point is such that stars cannot ignite and that Universe is a cold dead world with not much going on. Where there is no GUT point for a Universe it perhaps collapses to nothing. I think a lot of people misunderstand the Multiverse especially when it is hyped to the point that suggests 'all outcomes are possible' somewhere in a Universe we will never know. And there is no parallel person that is me somewhere else with a different set of beliefs or lifestyle as that is hogwash. I think that the other Universes have some sort of 4D path integral sensibility which produces some similar outcomes but not the same or nearly the same outcomes found in the other Universes. Finally, here is a direct quote from a white paper "Higher-Order Intersections in Low-Dimensional Topology" by Conant, Schneiderman and Teichner, "The Whitney move, sometimes also called the Whitney trick, remains a primary tool for turning algebraic information (counting double points)into geometric information (existence of embeddings). It was successfully used in classification of manifolds of dimension > 4 specifically in Smale's celebrated h-cobordism theorem (implying the Poincare conjecture) and the surgery theory of Kervaire-Milnor-Browder-Novikov-Wall. The failure of the Whitney move in dimension 4 is the main cause that, even today , there is no classification of 4-dimensional manifolds in sight." I quoted this to make the point that there is a lot of work to be done and that perhaps the unreasonable effectiveness of math in physics is really related to the 4D space-time issue of physical-ness. "

Ethan's essay poses a finite question: "Is (or was) there another you in any universe?" and concludes probably not. This contradicts a simple linear definition of infinity. No matter what amount you cite, infinity is larger. If you specify an “alternate you” even in an infinite or alternate universe, you still have described a finite outcome. Infinity by this definition will include that finite quantity.

Bull Grubb:

English and our intuitions don't work well with Infinities (no surprise, we evolved dealing with sometimes large but finite things). A "simple linear definition of infinity" isn't what we're dealing with here.

What you're saying assumes that every individual item is part of any infinity. A couple of people have already mentioned Cantor and different sizes of infinity. There are an infinite number of positive integers {1, 2, 3, 4....} but the set of positive integers doesn't contain everything. My left shoe is not a positive integer. It doesn't even contain all numbers: one-third isn't a positive integer.

There are provably as many rational numbers (such as one-third) as there are integers. There are also provably more real numbers (all the above plus things like π and the square root of 2) than there are integers.

The "scientists" who get paid to write this fiction should get real jobs, or at least jobs in the Creative Writing section of the English department.

I loved the essay, especially due to the fact that it started me thinking which lead to objections to your premise:

"There are 10^90 particles in each Universe, and we need for all of them to have the exact same history of interactions for 13.8 billion years to give us a Universe identical to our own"

My objection lies in the fact that quantum particles of the same properties ( i.e. electrons with the same momentum and direction.) are essentially identical. Swapping any two and you haven't changed anything. Thus you don't need 10^90 particles having the same history, but a subset of this, i.e. all quantum particles with the same properties replicating that history to have an identical universe.

Another objection I have is that not all particles in the 10^90 count will have the same effect. For example, Most of the neutrinos won't have any interactions on a per particle basis.

Finally, our universe has natural laws that govern which interactions occur. Some interactions are not possible. You won't have an atom with electrons in the nucleus with protons in orbitals around it. Thus you can't count all of the mathematically countable interactions since some of those are not physically possible.

What have I missed and what have I got wrong?

to See Noevo

I totally disagree.

Science involves exploration and exchange of ideas. If these ideas seem nonsensical it's because they are not in our day to day experience. You could say the same thing about quantum mechanics most of which sounds right out of Alice and Wonderland.

What expectations do you have of the Universe? 100 years ago the "Universe" was our galaxy. 60 years ago the universe might have been a Steady State universe. Today those ideas today are quaint. The universe is larger and more complex than that.

what? Real jobs like "post snide comments such as 'the scientists who get paid to write this fiction should get real jobs'"?

Thanks Ethan for a very clear and lucid article on this "infinite multi-universe" topic!

The one thing your analysis omits is the possibility that Consciousness is a universal fundamental force like the 4 known forces so well appreciated in today's physics.

If it is, then the Schrodinger equation collapses even if no one is looking at the moon!

And the arguments about multiple universes is an analysis missing the effect of an important variable.

I don't expect any of the dogmatic scientists in the audience to even consider this though!

You may exist in another universe. It doesn't have to be exactly the same universe. This totally depends on what you mean by you by the way.

Consider the old proverb a million monkeys banging on a million typewriters will eventually type out the works of Shakespeare. While this grossly distorts the actual probabilities, it does convey the idea that within a large enough random set you will find distinct patterns. One of those patterns could be you.

Let's say a subset of all universes are universes where life is possible. Let's say moreover, that a subset of universes where life is possible, there are universes where life like you is possible. We know this is a nonzero possibility since there is at least one. Since we have an infinite amount of time to wait for you to appear within one of these, it's inevitable, even if each universe is completely unique.

By the way, I'm using randomness and it still works. Our universe isn't random at least not on the macrosopic level It's governed by natural laws and initial conditions; Even considering quantum mechanics during the Big Bang the universe is probabilistic. Thus, I'm proposing that the natural laws of the universe lead to life ( certainly not my original idea) , in a non random way. This bolsters the possibility of you being in another universe as universes that can have life like you are highly likely to have it.

Now the big question is what do you mean by you? if by you you mean a being who looks like you, if you allow for small variance such as differing tentacle color or more flamboyant gills, etc. then I would argue yes there are others like you in other universes. If on the other hand you want a being that is your exact clone with the same life experience and thoughts, the probability of this individual existing is vanishingly small.

Let's go on a tangent. Time is a feature of our universe. When talking about other universes, we're using the concept of time despite the limitation of this particular dimension. Apart from entropy which would add confusion to this issue, do we have a word for "time" as it relates to change between universes or if that idea is even coherent.

I don't believe I wrote "Alice and Wonderland"! Uh.

It gets worse. Not only are there an infinite number of copies of you, there are infinite close copies - - - you a femtosecond younger, two femtoseconds younger, older, all variations with freckles shifted a micron, two microns, and eventually you realize it's all a pile of hooey and come to realize it's just a big number we must be dealing with.

I loved the essay, especially due to the fact that it started me thinking which lead to objections to your premise:

"There are 10^90 particles in each Universe, and we need for all of them to have the exact same history of interactions for 13.8 billion years to give us a Universe identical to our own"

My objection lies in the fact that quantum particles of the same properties ( i.e. electrons with the same momentum and direction.) are essentially identical. Swapping any two and you haven't changed anything. Thus you don't need 10^90 particles having the same history, but a subset of this, i.e. all quantum particles with the same properties replicating that history to have an identical universe. This is a much smaller quantity than the 10^90.

Another objection I have is that not all particles in the 10^90 count will have the same effect. For example, Most of the neutrinos won't have any interactions on a per particle basis.

Finally, our universe has natural laws that govern which interactions occur. Some interactions are not possible. You won't have an atom with electrons in the nucleus with protons in orbitals around it. Thus you can't count all of the mathematically countable interactions since some of those are not physically possible.

What have I missed and what have I got wrong?

One more thing on the issue of you in another universe.

One avenue of exploration of the question is living beyond your current life time by having duplicates of you in other universes who are consciously you. Then the concept of "you" allows for a larger number of creatures in terms of physical form. If there was a you that didn't look like you but was you in the sense of sensation and consciousness some continuation of your current you, the specific form your body took would be irrelevant. Unfortunately, I think the conscious you is stuck within the structures of your brain. A twin in another universe will be like a twin in this one.

See Noevo where do I get that creative writing check?

To "Stephen J Baker" comment #3

"If the universe is infinite...This means that there must be an infinite number of cubic parsecs that are exact duplicates of each other"

The Universe is not infinite, it's 92 billion light years in diameter and growing. The density of matter per cubic parsec is dropping.

If I accept your premise that it is infinite, we still have a problem. It can have an infinite number of cubic parsecs that are identical because they are free from matter, with a finite number that contain matter. This doesn't make any more of me than a finite universe.

The argument regarding what is statistically likely ( that there are an infinite number of cubic parsecs with matter) doesn't take into consideration how spacetime and matter came into existence. Since in your scenario there is an infinite quantity of matter and space-time, and the only known mechanism to create these is a big bang , turning the clock back to the big bang, we wind up with an infinite amount of matter in close spacial proximity. This is somewhat of an undefined situation since, we don't have a mechanism for an infinite amount of matter to instantiate but even given that, we might assert that this would create a black hole, since a large enough amount of matter will create a black hole, but I can't jump to that conclusion for an infinite amount of matter because again this is an undefined situation. Thus, your conclusion of what is statistically likely is incorrect. Your argument would be correct in the situation where we have a infinite universe, where space time was not expanding that was randomly populated with matter popping into existence ex nihilo.

So while I don't think your conclusions are correct, I did find your discussion interesting!

Just restating #2 Christopher

I like the example that demonstrates which infinities are larger, mapping.

Take the even integers from 0 to infinity. You can map these to the set of all integers from 0 to infinity. ( draw 2 number lines, one for each set and draw an arrow mapping the two lines. So for example, 0 to 0. 2 to 1. 4 to 2. 6 to 3, etc to infinity. Weird as it seems the two sets are the same because each member of one maps to a member of the other. No number is left unmapped.

Try this with the integers and the real numbers. You can't map them. If you do you have all of integers mapped but many real numbers unmapped. This shows that the real numbers are a larger infinity than the integers.

My objection is that you can't point to something real that's infinite. Our universe is finite, both in the macroscopic and microscopic directions. Matter and energy are finite. The only place where we can point to the infinite is a concept in mathematics. Useful as that it that doesn't mean that it corresponds to something in reality.

That was interesting.

A problem I see for the analysis is the reference to Borde et al paper of 2003:

"The singularity theorem tells us that an inflationary state is past-timelike-incomplete, and hence, most probably did not last a truly infinite amount of time, but rather arose some distant-but-finite point in the past."

Linde's response (in a 2007 paper IIRC) is that such a "blueshift" energy bound on the set of (inflationary) worldlines can be pushed away into infinity. That is, it is entirely consistent with the bound to have a backwards eternal inflating wordline, in other words the maximum affine length of the set is finite but tending to infinity. [If that wording of mine makes sense, I'm not well versed in geodesics.]

More generally, if the quantum field of inflation is somehow instantiated by fluctuations it can have happened any number of times as we look backwards, each spawning its own multiverse fan of universes. Since the universe is zero energy that should be possible. Generally a process should make many instances, if it makes just one (or a few or none) it is terribly finetuned. So I don't see why, if inflation started at least once, unless it is somehow special and finetuned it wouldn't indeed have started any number of times.

@Dr. DWK: "The one thing your analysis omits is the possibility that Consciousness is a universal fundamental force ... I don’t expect any of the dogmatic scientists in the audience to even consider this though!"

You wish.

Of course it has long been considered, and rejected, not out of dogma but from the simple biologist _observation_ that consciousness emerges in well understood electrochemical machinery of brains. (E.g. we can turn consciousness off simply by chemistry: analgesics.)

This isn't rocket science, and you can easily check with Wikipedia on brains and their functionality.

"So I don’t see why, if inflation started at least once, unless it is somehow special and finetuned it wouldn’t indeed have started any number of times."

I was stepping away from the keyboard when I realized that the argument against finetuning of processes is even worse as regards the universe. We must also consider the Copernican principle. (Or else we have learned nothing from the last centuries of astronomy and cosmology.)

That is, it is apriori highly unlikely that if we see inflation start once (if that is what we will eventually see), it will not start any number of times. I.e. our multiverse, if that is what it is, shouldn't be special/unusual either.

You don't think there could be any other reason than that?

You haven't even considered that you haven't defined your consciousness force to apply to quantum dynamical equations such that it can inevitably collapse quantum probability fields.

Or considered that the collapse of the wavefunction isn't necessary since reality isn't going to be necessary since that is only required when running the mathematics and reality isn't *actually* solving the equations.

No, it MUST be that others are closed-minded.

I find it odd how that works for some people...

Bill Poway,

To use your words, “I totally disagree”, with you.

Give me just one possible way science could observe and explore any of the supposedly 10 to the 500th power universes.

To Wow,

You write of me: “what? Real jobs like “post snide comments such as ‘the scientists who get paid to write this fiction should get real jobs'”?”

Absolutely. Hey, you gotta follow your heart, and your head.

Ethan’s essay and the issue of infinity begs the question “what is an alternate you?”. Putting aside theology, biological science suggests that a limited set of directions or blueprint could exist that would exactly duplicate you and your physiology at a given point in time. This would be enormous and we will never see it, but it would still constitute a finite set and exist in our universe.

For purposes of this “fun” article presume that you believe in inflation or any theory that allows for an infinite stretch of time and space. In other words you presume an infinite universe likely exists that is analogous to our known universe. It seems likely this infinite universe would include an infinite subset of information which in turn would have to include the finite blueprint for an alternate you. Using the same rational of a finite quantity in an infinite one, predicts the eventual realization of an alternate you in this infinite universe.

Of course multiverse theory can involve math that describes incomprehensible conditions that do not allow for our existence. However the mathematical concept of infinity can still be applied to “real world” conditions . Cantor’s well accepted descriptions of infinity involve set theory, topology and “well distinguished objects” . This concept of infinity is not limited to a simple number line or numbers. It does demand that a well defined infinite set must include all of its subsets , the cardinality of another “larger” infinity not withstanding.

So do you believe in the existence of this particular defined set consisting of an infinite universe? If so , then strictly from a mathematical perspective, the existence of some alternate you is a given .

If all the branches from the 0-state (x,y,z,t,&c all 0) are quantum processes, then all the possible universes are at most countably infinite with calculations over the Reals then being approximations. Then one needs to ask if the dice that get tossed at each decision point are running a discrete pseudorandom number generator or a real based continuous generator. This is similar to the monist vs dualist argument in philosophy, no?

Infinite is not the same as exhaustive. Even if the conditions for the creation of an infinite number of universes did exist there is no reason to expect that they would be parallel as described.

@Bill - comment number #19.

The thing that you're describing ("92 billion light years in diameter") is the *VISIBLE* universe...limited by the distance light can have travelled since the big bang (45.7 billion lightyears). We can't ever *see* anything beyond that - but I don't think any serious scientists believe that this is all of the universe that there is...that would be a rather surprising coincidence. Also, that distance isn't "growing" at the speed that space itself is growing. The expansion of space is pushing things out beyond 45.7 billion light years, but the distance we can see is only growing by one light year per year. So those things are vanishing from view...we'll never be able to see them again.

Because of that, we know for sure that the universe is larger than 92 billion lightyears - stuff that we could (theoretically) see last year has moved further away than 93 billion and one lightyears over the past year. So for sure there is stuff beyond our ability to see it...which means that the entire universe is definitely larger than the visible universe.

We don't know (and may never be able to know) how big the entire universe is - or whether it's infinite or large-but-finite.

But we do know, for 100% sure, is that it's bigger than 93 billion light years. That means that your assertion that my post cannot be true is incorrect.

Yeah, 'cept you have a problem that they all seem to be a long way away from the bit of you posting here.

Maybe they'll send you a postcard someday.

1) I must have missed something: mundane question.

Last I checked, we didn't have any good way of estimating, much less measuring, what might have occurred before the Big Bang. Now I see that it appears to be mainstream consensus that inflation preceded the Big Bang.

Question: How recently did "inflation preceded the Big Bang" make it into the canon?

(And, how could i have missed that?, though I don't expect anyone to be able to figure that part out;-) (I'm inclined to think that every time I read about "inflation and the Big Bang," I assumed that "Big Bang" referred to the moment of origin of the universe, and phrases such as "the inflationary period" referred to events following that. Oops. Though, that does point to the need for spelling that out more clearly, such as Ethan did in this article.)

2) Tegmark vs. Everett: more-interesting question.

I was originally a complete skeptic about Tegmark, based on the naive assumption that he was basing some of his ideas on Everett, and based on the assumption that Tegmark's theory is unfalsifiable without exception. However I found his article in SciAm entirely reasonable, so now I'm inclined to be friendly to the idea and slightly skeptical. Especially since it appears that some of Tegmark's ideas may be indirectly testable.

Question: I've got it that Tegmark != Everett+, but I may as well ask: where the heck does Everett get the energy needed to split and duplicate the universe at every wavefunction collapse? Or do his splits only occur in the immediate vicinity of each wavefunction collapse, rather than splitting and duplicating the entire universe?

3) Nitpick about cosmic narcissism:

Why all this obsession (every where the Multiverse is discussed) about "infinities of Me"? Sure, it's mildly interesting as a fantasy. But the level of interest in "infinities of Me" strikes me as excessive to the point of narcissistic.

That or it's another grasping-at-straws for immortality, only slightly more well-founded than The Singularity (which is complete rubbish). Bottom line is, while you're alive, your mind lives in your brain, and either there's a hereafter or there isn't, but "infinite-Yous" aren't going to save you.

4) Simulations of alt universe development?

What I do find interesting, intensely so, is how other universes might come out if their core physical values were tweaked just a little bit. What range of variability of those values would produce stable universes? What subset of that range would make protein synthesis (and therefore life) possible?

I understand that we don't have the computational capacity to get at this yet. Question: how soon do the working scientists here think we'll have the capacity to start doing alt-universe simulations that can get us as far as the conditions for development of water molecules? How about proteins? Or am I totally missing the correct framing of this?

5) One cosmic question we'll be able to get at:

Does biology work the same way in other galaxies as in ours? This we'll be able to get in @ 5 billion years (if I recall correctly) when Andromeda merges with the Milky Way. That will bring Andromedan star systems within reach of our interstellar travel capacity at the time (assuming we progress that far: a goal that's worthy of all the effort it will take to achieve).

By that time we should have scoped out enough of the Milky Way to have found other life and other intelligent civilizations, and determined if evolution is generally convergent or widely divergent, in our own galaxy. But that by itself doesn't tell us about other galaxies. Checking out planets in Andromedan star systems will do that, for the only example we can ever expect to reach (at least given what we know now).

So even as the universe expands, reducing our view to our local cluster only, we will still be able to get an answer to another of the most important questions of existence.

It's that the definition of what bit the Big Bang is is changing.

We're discussing a Big Bang that is the bit we've already worked out what can plausibly have happened. Before that, the Big Bang was defined as the bit that started space and time.

Inflation was thought to be after the big bang, but it's turned out to be just as plausible that inflation may have created a time that went different speeds. Therefore a time that would, sans inflation, have been 10^-34 s long may have been, with the effect of inflation, be 1000 years long.

On the other hand, if there are 10^10^10^7 universes according to one of the 'fathers" of inflationary theory

http://phys.org/news174921612.html

then there are 10^10^10^7 (10^10^10^7 divided by 10^90 factorial) copies of us out there.

This essay reads like science fiction. See this bit?

[I]"What we find is that the Universe is most consistent with being spatially flat"[/I]

That's fine. Nothing wrong with that. But see this?

[I]"...with being uniform over a volume that’s much greater than the volume of the piece of the Universe observable to us, and therefore probably containing more Universe that’s very similar to our own for hundreds of billions of light years in all directions, beyond what we can see".[/I]

We just don't know this. People who say the universe is flat and therefore it must be infinite are falling for a non-sequitur that just isn't scientific, and getting bogged down in speculation. They aren't facing up to the other option, which is that the universe is flat and finite.

To See Noevo #25

Give me just one possible way science could observe and explore any of the supposedly 10 to the 500th power universes

Your initial point was that scientists who write this fiction should get real jobs. Now you're jumping to observation.

Conjecture and speculation are important first steps when investigating something this out of our daily experience. At times these go no where and others they lead to hypothesis and experiments. The best people to make this kind of speculations are people who have expertise in the field because they will avoid many avenues that known science indicates are not possible. However, having said that, there has been an experiment that looked at the CMB to see if there was an imprint of another universe cause by quantum entanglement during inflation.

See Noevo ... thanks for the discussion.

to Stephen J Baker #30

The 92 billion light year diameter is straight from Dr. Siegels article. Quibbling over the the exact size is merely a red herring. My arguments regarding your assertion that the universe is infinite stand regardless of the size.

We don’t know (and may never be able to know) how big the entire universe is – or whether it’s infinite or large-but-finite.

That's just not correct. The fact that the Universe is 13.8 billion years old and expanding sets a size limit on the Universe. It is not infinite regardless of the size.For the sake of argument, assuming that the current cosmological model is wrong and the current universe is infinite, your particle position to parsec argument still fails because you need an infinite number of particles. By the way particles don't cut it, we need stars and planets for multiple "you"s, unless you're advocating a form of Boltzmans brains.

However, if by entire Universe you mean the Multiverse, which I'm going to define as other universes that are not within our space time. We can speculate on that. Your argument there must be an infinite number of cubic parsecs that are exact duplicates of each other needs to be fleshed out if you're talking about a multiverse. Here are my objections:

1. An infinite number of finite universes don't guarantee your argument true.

2. Most universes could be empty, or full of only photons, therefore fulfilling your particle requirement yet not the desired outcome, more "you"s out there.

3. Most universes could have different tuning thus full of particles that haven't formed into atoms, again meeting your requirements but failing on the outcome.

4. Most universes could immediately collapse into black holes.

5. Other universes might have fewer, more or different dimensions. What would a universe with 5 spacial dimensions and three time dimensions be like?

Stephen, although I disagree with your conclusions, I do appreciate the exchange. Thanks.

A very good article but I'm still completely baffled about something. If the universe isn't infinite but has a certain amount of matter, spread out over a certain distance, then surely this means that there must be a certain amount of matter in every direction from us, so what does the person sitting right way far out away from us see when he looks in the opposite direction to us.

They see us right on the edge of their universe.

Remember, from their POV, we're retreating very fast from them because we're a long way away in an expanding universe.

@ see neovo #25

imagine two pre-historic cavemen talking, one goes: - tell me how science is supposed to make me talk to a cavern the other side of the world; the other goes: - yeah, that would mean science would have to come up with cell phones

And those other cavemen are not just on the other side of the world. They're in another universe!

@see noevo #41

you missed the point

cris, that's what SN's current attempt to troll and whine at science for not being godly enough is doing.

It's trying to fill this site with its shit so nobody can discuss anything.

See Nowt, get lost and spam this location instead:

http://scienceblogs.com/startswithabang/2012/09/23/weekend-diversion-yo…

stop pretending you're here to make valid complaints.

@Bill #37

No, you're confusing two concepts: The OBSERVABLE universe (which is indeed circa 92 billion lightyears across) and the ENTIRE universe (which is 100% certainly bigger than 92 billion lightyears - but we have no idea how much bigger). I'm not disagreeing that 92 billion is the right number - it's the best estimate science has to offer. Also, some people leave out the word "OBSERVABLE" when they talk about the observable universe - which is why your reference says what it does.

What I'm telling you is that you're discussing the wrong thing. Within the OBSERVABLE universe (sometimes called the VISIBLE universe), there is essentially zero chance of there being another earth with people identical to us walking around on it. Within the ENTIRE universe, it's only possible if the entire universe is infinite...which it may or may not be. NOBODY knows the size of the entire universe - because it is (by definition) larger than we can see or ever know about.

Now, you argue that the speed of expansion of the big bang is a limiting factor for the size of the universe...but that's not true for two reasons:

1) For sure, the universe (even the OBSERVABLE universe) is expanding faster than the speed of light. The age of the universe is established to be about 13 billion years - so if the speed of light was the limit on it's rate of expansion, it would be only 26 billion lightyears across. You admit that it's 92 billion - so already you know that light speed isn't a limit on the rate of expansion. This demolishes your argument and it's trivially demonstrable.

2) Space itself is what's expanding. (That's why light speed isn't a limit)....its not that material is moving apart through space...everything could be standing stock still - and yet moving further apart as space itself expands...and (oddly) it's perfectly possible for something that's infinite in extent to expand. There is every possibility that an infinite amount of space contained the big bang - which then caused space to expand and material to be pushed apart in all directions at once. Sure, it's tough to get your head around - but it's plausible *and* it's the best bet as to what happened.

I recommend that you check out: The WIkipedia article about the shape of the universe:

https://en.wikipedia.org/wiki/Shape_of_the_universe

As always with Wikipedia, don't take their word for *anything* without checking out the references at the bottom that explain where they got their information. To quote that article's introduction:

"The model most theorists currently use is the

so-called Friedmann–Lemaître–Robertson–Walker

(FLRW) model. According to cosmologists, on this

model the observational data best fit with the

conclusion that the shape of the Universe is infinite

and flat, but the data are also consistent with other

possible shapes, such as the so-called Poincaré

dodecahedral space and the Picard horn."

...so they are saying that the ENTIRE universe (not just the OBSERVABLE part of it) fits a mathematical model that, given the data we have predicts that the universe *is* infinite.

However, there is room for doubt - so I continue to say that "We Don't Know".

But you absolutely can't rule out an infinite universe - and indeed the available data says that it's highly probably that it's infinite.

Which means that there are an infinite number of places out there - far, far beyond the VISIBLE universe - where you and I are having this exact same argument. However, they are overwhelmingly (almost) certainly more than 46 million lightyears away...outside the spherical slice of the universe that we can observe - so we'll never be able to see them, detect them, talk to them, send them emails...and to be perfectly honest - that's a good thing!

There are 10 millions universes in Multiverse. Each universe is 13.8 billion light years across.

And you counted them? Or was that just rhetorical a claim?

Some people coming here with drive-by posts actually DO mean their posts literally, so, yes, I DO have to ask.

So, will the Multiverse ever come to an end? Did it also begin with an initial singularity? If and when it ends, does it return to a singularity?

To bastardize Max Planck, "when its proponents eventually die, and a new generation grows up."