Multiverse cosmology

Otherkin

I really know nothing whatsoever about cosmology although I find it very interesting. It seems that a lot of physicists nowadays reckon there's a multiverse. I don't particularly want there to be a multiverse. WHAT DO YOU FOLKS THINK. Also, if there was a multiverse, would the laws of physics be the same for all of the universes? And would every single possibility be actually occurring in some universe out there? Like, in one universe am I being cut into bits from the toes up without anaesthetic and then having my body regenerated by some piece of advanced technology and then being cut up again OVER AND OVER FOREVER? All the while having faeces smeared in my face?

Chronos

The problem with multiverses is we cannot observe them. So yes, you may be getting run through a wood chipper, resurrected and recycled in another universe. Fortunately, you can never observe this happening to yourself in this universe.

Chalnoth

Well, the universe doesn't much care what you or I or anybody else wants.

Three points:
1. The universe is big, much bigger than what we can observe. It is very likely absurdly, unbelievably larger. We know it has to be much bigger than the part of it we can observe because the universe we observe is very, very uniform: if the universe weren't much bigger than what we can see, then we should see some sort of change in the universe as it approaches this boundary. But we don't, so we expect it must be vastly larger.

2. Our understanding of high-energy physics indicates that at least part of the laws of physics which we are familiar with isn't fundamental, but was rather determined at random early in our region of the universe. If this is the case, which seems very likely today (and is something which the LHC may provide further insights into), then it stands to reason that regions of the universe far away from our own will have different laws of physics in operation at low energies.

Note that in this scenario everything would still be based upon the same underlying laws of physics. But the behavior of objects at low energies (such as we experience) would be very, very different, due to these different events in these regions' pasts.

3. I see no reason to expect that the laws of the universe should be such that life must be possible. So it is philosophically appealing, to me, that the universe is big and variable and mostly devoid of life. We know this is true within our own universe. It seems natural to expect that this is also likely the case between widely-disconnected regions of the universe as well: some parts will be capable of supporting life. Most won't. Unfortunately this particular argument is very weak, much weaker than the above two, because we don't actually know what laws of physics are possible, let alone their relative probabilities, so we can't actually say with any certainty exactly how likely life is.

This is a somewhat different idea, that of the many worlds interpretation of quantum mechanics, an interpretation that is almost certainly accurate. However, it appears you are misunderstanding its implications somewhat. The many worlds interpretation of quantum mechanics does not say, "anything and everything happens," it rather says, "lots of stuff happens." That's a big difference. First, the impossible never happens. It isn't necessarily the case that everything we can imagine actually turns out to be possible. Second, the many worlds interpretation of quantum mechanics is a fully-deterministic theory: everything is a component of the same wavefunction that is just evolving through time.

The world we observe is one component of said wave function, and there are other components that represent other worlds, some similar, some very different. It is difficult to say a priori what is or is not happening in those other worlds, so we can't say, for instance, whether or not there's a world out there where it turns out that I am having sex multiple times a day every day and loving every minute of it. That's certainly not my life in this world. And even if there is a world out there where something like that is happening, would that person's life be so different from mine that they could even count as being "me" at all?

edgepflow

Just to add a little bit to the many things happening in a multiverse.

The number of things that may happen in the multiverse may be infinite but still not include "everything that can happen." Consider an infinite sequence of odd numbers:
1, 3, 5, 7... to infinity. This sequence is infinite but not every number is included.

Chalnoth

Yup. And "everything that we might possibly imagine" also doesn't even constitute everything that can possibly happen. There are lots of things that we can imagine that are quite impossible.

Chronos

Consider, however, our universe is so vast that just about everything physically possible has probably occured at some point in its history. This is not to say that other universes could not have different laws of physics. I doubt, however, such universes are viable. Tweak a few properties of this universe and it either flash fries almost immediately after forming, or never burns at all.

Austin0

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A hearty second on that viewpoint.

How is it that cosmologists have come out with various quantitative calculations of the overall mass? And decided that a universe which may be absurdly bigger still doesn't have enough in it and needs a transfusion of hypothetical invisible mass??
I thought the current view was that we dont see a difference because the space itself was dimensionally convoluted [i should say inconceivably convoluted] and without any boundary???


The world we observe is one component of said wave function, and there are other components that represent other worlds, some similar, some very different. It is difficult to say a priori what is or is not happening in those other worlds, so we can't say, for instance, whether or not there's a world out there where it turns out that I am having sex multiple times a day every day and loving every minute of it. That's certainly not my life in this world. And even if there is a world out there where something like that is happening, would that person's life be so different from mine that they could even count as being "me" at all?[/QUOTE]

Well you might be missing some good sex but at least you won't be getting stuck with their bills.

Chalnoth

The two ideas have nothing to do with one another. The invisible mass comes from the fact that we see lots of mass out there (from the orbits of stars in galaxies, from the orbits of galaxies in galaxy clusters, from the deflection of light, from the behavior of matter early in the universe, and from the relationship between structures in the early universe and those in the late universe). In short, dark matter comes about because we see copious amounts of evidence for it.

And by the way, invisible mass isn't such a strange thing in physics. Neutrinos have mass and are quite invisible (they don't interact electromagnetically). They can't make up the dark matter, because they don't have enough mass. But all we need is another neutrino-like particle with more mass, and the dark matter is explained.

I have no idea what you're talking about here. Dimensionally convoluted? What do you mean by that?

As for a boundary, we don't know. Certainly there is no boundary nearby.

zapher134

I believe both the multiverse and that everything and anything must happen are not real but artificial problems that arise due to the nature of our symbol systems. *For example, in phyisics, we often can arrive at the correct answer in at least two ways, however we throw out the nonsensical answer, having a negative mass at a negative speed for example, though we can imagine it, we understand it may not exist in actuality. In the same way our minds can imagine the infinate producing infinate occurances and think nihilistically that nothing matters as everything must occur. *However, nothing could be further from the truth. *Though we can imagine anything, even impossibilities, evolution will rule them out in actuality. *Further, as our imaginary Universe expands, the possible variations expand even faster, meaning that though some things may indeed occur repeatedly, an exponetially larger number will not. *Finally, since Actuality destroys the past to make the future, what actually is is always much smaller than what has already occurred, only preserved though in our symbol systems. So, anything can exist, but only in your mind, and in actuality many things may repeat, hopefully they are the good things. Let's make it so.*What we do does make a difference.

Tanelorn

I agree with a Multiverse of observable universes if that is the right word. I have trouble with them having different laws of Physics, I would like to learn more about that. We could also argue that the universes of the past present and future are so different as to be different universes where different things are possible.

I dont think it is worth stating that the Universe cares about anything any more than the stone at the bottom of the my garden caring or the 2nd law of thermodynamics giving a toss. I suppose it might not be obvious to some though.

cephron

Hey Chalnoth, I was under the impression (From the FAQ, and from recent discussion here) that according to current models, the universe would have no boundaries even if it were finite, but rather "wrap back in on itself" 3-sphere style (corresponding to "positive spatial curvature"). So, for clarity, is it certain that there are no boundaries, or do you mean it when you say, "we don't know", and would a bounded finite universe be consistent with current models? Thanks!

Cosmo Novice

From my understanding it is pretty clear there are no physical boundaries even given a finite model. A boundary would also need to include a center and this would invalidate the key principles of isotropy and homogeneity.

There is an edge to the universe in a sense: a temporal edge, and this is wherever you stand you are at the temporal edge (the furthest time away from the BB).

You are quite correct in assuming no spatial edge and that finite models are structured in such a topological way to not invalidate this core principle.

Chalnoth

Well, I don't know of a way to completely rule out the concept of some sort of boundary, especially if we are rather vague as to what we mean by the boundary. It is certainly true that we don't observe any boundary. And it is also true that it isn't something that we can describe in our equations just yet (so far as I am aware). But just because we can't describe it mathematically just yet doesn't necessarily mean it can't exist. How likely it is, then, just depends upon what you think, "have a hard time describing it in the mathematics," means.

Finally, as to Cosmo Novice's statement about homogeneity and isotropy, the fact of the matter is that these assumptions are known to be not completely accurate. They are approximations to the true behavior, and we know that these assumptions break down on small scales. It is entirely conceivable that they also break down on scales much larger than the cosmological horizon.

Cosmo Novice

I can concede that they are assumptions, the difficulty is obtaining a burden of proof. If we proved space was anisotropic then job done, while the sizeable chunk we see is isotropic there will likely always be more to see and so the concept of isotropy can never be fully established.

That being said I think the empirical evidence and the sizeable chunk of homogenous U we can see gives a good indication of this being a cornerstone cosmological principle.

Chalnoth I am interested so can you please give further information on how "we know that these assumptions break down on small scales"?

Cosmo

Chalnoth

The Earth itself is proof of this. Things look differently when we look in the direction of the Sun, and when we look away from it (because the Sun is pretty darned bright). And the Earth is a heck of a lot more dense than the space that surrounds it. So clearly both homogeneity and isotropy break down at small scales.

Cosmo Novice

I thought you were reffering to things smaller than this!

I thought this was the point though - isotropy and homogoneity exclude local variance - so yes individual galaxies may look different, clusters and superclusters etc. Overall as a whole though the OU complies with the principle of isotropy and the geometric expansion due to the scale factor being the mechanism that moderates said isotropy. As long as the scale factor is uniform which we know it is, then I am failing to see how larger scales will be anisotropic, in fact I would expect the MORE Universe we take into consideration then the more isotropic U would be. Evidence from this is seen in the 1/1000 uniformity of the CMB?

As always anything I have misunderstood I am welcome to correction.

Cosmo

cephron

@Chalnoth: thanks, that's helpful to know.

Perhaps slightly more on-topic; here's something I've been wondering. Do any of the multiverse models propose ways in which any other universes are actually observable? Do any of the multiverse models make falsifiable predictions? Because I haven't heard of any of these yet, I have always thought that the multiverse hypothesis was just a god-of-the-gaps sort of explanation for the "fine-tuning" of the cosmological constants in this universe. Am I wrong?