k, be careful what you wish for:
Howdy all, long time lurker, first time poster (sorry for the lack of further introduction), but I might be able to help a bit here.
Quote:
Originally Posted by xelasnave
I cant get my head around the Universe expanding but there is no outside so I dont even try but such a situation would not offend the concept of infinite is so far as you can double infinite but it is still infinite.
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Quote:
Originally Posted by alpal
Dark matter, Dark energy & now dark flow -
it's certainly hard to imagine & then you have to wonder what the universe is expanding into?
It seems to be expanding into itself.
If you can imagine & explain that then you need to get a Nobel prize.
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Think of the surface area of a (perfect) balloon: at different pressures its surface is stretched by different amounts, but never needs to expand into some other area. Don't get too hung up on the balloon as a 3d object with a 2d surface; the point is that the surface forms an unbounded (it has no edge) but finite (anyone with a pi key on their calculator can tell you its area) space, and some underlying property (the air inside the balloon) would cause it to increase, moving points on it further apart, and some other property (the air outside the balloon) would cause it to decrease.
I'm sure most of you have heard this analogy before, but maybe there are some ways in which it seems the analogy simply falls down (it is only a simple analogy after all) and it's easy enough to wave away questions (like "so is the universe expanding because it has one less spatial dimension than the space it resides in?") with an instruction to disregard that part of the analogy, just like I did a few lines ago. But we don't always need to.
The surface is 2D, the volume of the balloon is 3D, and this is what allows the surface to curve back on itsself and have no boundary. Do we need the universe to reside in a container, perhaps with a greater number of spatial dimensions? No. If you imagined yourself as an inhabitant of the balloon surface, maybe the surface is conductive and you're an electron moving freely, but only along the surface lattice, then you could circumnavigate the balloon and end up where you started without finding the edge of your 2D universe. But you'd have traced a circle in the 3 dimensions an observer of the balloon sees, right? Well I don't think its helpful to try and imagine a viewpoint external to the universe, as that's the whole problem we want to avoid (since we keep being told it's not possible). More importantly, we don't have to if we want to relate the 2D analogy to 3D reality: we have another dimension available and that is time. Lets say the universe is not expanding and you set off in one direction to circumnavigate it, like the electron in the conductive balloon. Will you get "back to the same place"? No, because you will have taken so long to go anywhere (relatively speaking) that no part of the universe you travel through will ever bear any resemblance to the place you started out from, even though you may actually wind up meeting the same protons (unless they turn out to have a half life too) several times. Time is the dimension in which the space of our universe is curved.
Does it sound as if I've cheated a bit? I mean, you may be familiar with the lycra-sheet demonstration of "spacetime curvature by massive bodies" where a physics teacher plonks some weights on a stretchy surface and makes marbles "orbit" them, and this is a great way to visualise why, say, massless photons follow curved paths around gravity wells, but extending this (other, but related!) analogy to the "real" universe and saying you can circumnavigate the entire universe but you won't know when you've finished sounds a bit... unfulfilling, right? Well, that's exactly why it's not a bad analogy. The universe, in this respect, is rather mundanely exactly as strange as we imagine (i.e. a little bit). The mass around which the spacetime of the universe is curved is just all the matter in the universe, the paths of all photons form closed orbits of the mass forming this gravity well (a better way to put it is the universe fits inside its own event horizon for the simple reason that the constants and relationships that define an event horizon are the properties of exactly this universe). I don't even need to point out that in a universe expanding as rapidly as ours seems to be the "circumnavigation" part is a total non-issue, as the balloon is being inflated faster than it is possible to travel along its surface (the observable universe is about 93 billion lightyears across but only 13.8bn years old).
I'm going to leave that there, as the "what's it expanding into?" query is all I really wanted to address, but there are a whole host of ways to relate cosmology to strechy fabric that can be quite informative. Distance is one I would gladly touch on in much less detail: galaxies are getting farther away from us as the universe expands, but it is the space expanding and not them moving through space. A distinction is usually made between ("proper") motion due to the object moving through space and an observed receding motion due to the expansion of the universe (which are not always trivial to disentangle): "redshift" can mean many things in different contexts. But if you try to use texta marks on a balloon to illustrate this: how do you know they're "further apart" when the balloon is inflated? with a ruler? c is the ruler? But if space is expanding and space is really spacetime so time is expanding and c should be useless to measure this, right? But there are other rulers, like energy states and binding energies of hydrogen atoms, which don't depend on how long a meter or a second is. In fact if you think of redshift not as a doppler effect due to relative motion, but as a decrease in energy (not that energy is lost, you're just observing less of it) of the photons you can see how its possible to quantize everything, even expansion rates, distance and, ultimately, time, in terms of energy: the one yardstick that's (afawk) invariant under all known universe topology modifications...
OK, so finally to the reason for my post: "Dark". Specifically flow, but generally energy and matter too. It just means as-yet-not-directly-detected-but-predicted, which is important to remember. It made sense with dark matter (we see all other matter by its emitted/reflected/absorbed "light", even if it's just reradiated as IR etc. but there seems to be this matter, without any light)... Now why you'd expect a flow to emit light... I mean if the galactic centre mass excess ("dark matter", pre hypestorm) had *somehow* come before general relativity gravity waves would probably have been called "Dark Waves(TM)". My point is that half of cosmology is predictions of effects that haven't been observed directly (yet), but then the same is true for most of science. Yet the popsci press often gives a lot of weight to ideas and calculations that may be related to other work which gets no press. The one I'm thinking of here is that the fine structure constant is strongly suspected to vary across the universe (spatially and/or temporally) and it's embedded in G, remember, so we don't need to suppose a differently inhomogeneous universe beyond what's observable to get clusters of things behaving noticeably differently to "the norm". That's not to say that discoveries of same wouldn't be momentous achievements, but just that we don't need to go supposing radically different bizzarro universe just beyond what we can see because of them. That's actually as valid a conclusion to draw as that we see every galaxy moving away from us not because the universe is expanding, but because the earth sits exactly at the epicentre of our universe and we have always been the centre of everything. Our patch of sky is big enough to be a representative sample, especially since there are secondary effects of almost any extreme solution to these sort of unknowns which are the reason they remain unknown (so eg dark flow: if it's real and due to vast mass concentration outside our observable universe we should expect everything in the observable universe to be orbiting it, but orbital motion would result in galaxy redshift being polarised: we'd be orbiting in a plane so the galaxies above and below us would be systematically more redshifted than galaxies ahead or behind us and galaxies closer to/further from the mass concentration would have elongation-dependent redshifts relative to us, none of which is observed (afaik), and in fact these sort of systematic trends would be very obvious in things like CMB, which is such an old, low energy signal that anything affecting galaxy speed in an isotropic way should show up there... and we're back to "dark", again...). Personally I think it's just a rather spectacular supercluster in the early stages of formation, quite interesting from a "population statistics on non-rotating black holes" perspective potentially, etc. etc. but not exactly requiring a rewriting of "what we thought we knew about the universe"...
Also, you can't actually double infinity. Mathematical manipulations that involve such things are just placeholders for "so large it no longer changes the result of this calculation (appreciably)"
Can I pick up my prize in January? I'm busy in December.
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It should be interesting to dig this up in 5 years and see if I still agree with myself...