demagogue on 17/6/2009 at 23:06
Or everywhere is the "center".
Quote Posted by Vivian
Can't find the picture now, so maybe I made it up.
That picture you're talking about might be the (
http://map.gsfc.nasa.gov/media/poster2002/WMAP_poster2002a.jpg) WMAP image of the background radiation. Anyway, that's the image of what's being discussed here.
The beauty of it as the smoking gun for the big bang, which I wrote in my tldr; post, is that it shows the same radiation coming in from every freaking direction in space, from points that are billions of light years apart. It's silly to try arguing there just happened to be billions^billions bursts of radiation at every point in space, all at the same time, all with the same temperature, unless you have something like a single burst from a single "place" in mind.
Vivian on 17/6/2009 at 23:21
Er... so all the energy in the universe springs out of an infinitesimally small space and expands. Where does it expand from if not a center of some kind? I mean, the expansion wasn't instantaneous, was it? It's still happening. So unless I'm thinking about this wrong, there is still a mean point that everything is being flung away from?
...ok, it does seem like the internet is of one mind on this: I
am wrong (an 'explosion
of space, not an explosion
in space', thanks (
http://math.ucr.edu/home/baez/physics/Relativity/GR/centre.html) internet!). Which is why the CMB (ooh acronyms) is constant, right? Weird. I can't get a decent mental image of what that means.
Demagogue, yeah I think that was the one. Anyway, even if it isn't the big bang wasn't there some bright patch on that image people were particularly interested in?
demagogue on 17/6/2009 at 23:37
Quote Posted by Vivian
Weird. I can't get a decent mental image of what that means.
I'm no expert so can't give an "official" answer, but I asked the same question once and the answer I got was at least an instructive visual image ... something like, in general relativity, space really is analogous to the surface of a balloon. It's not just a metaphor. And it helps to think about each point in relation to all the others, rather than try to imagine the whole structure itself.
When you blow a balloon up, every point on the surface (imagine some drawn in with a felt-tip pen) mutually gets farther from the other ones at the same pace (measured across the surface). There isn't a center to the surface of the balloon either; any arbitrary point is just as much the "center" as another (and by definition the inside of the balloon isn't part of that space, so you don't count its center, since you'd need a third dimension which isn't in the "balloon space" itself).
Cosmic space is like that, I was told, except the "surface" is in 3 dimensions spreading out instead of 2 like a balloon. (An expanding sponge, maybe. But that comes with some footnotes.*)
Or another way to think about it, a reductio argument: if there were a center, like when you pour batter on a pan and it spreads out from a center (the 2D analogy), every point would NOT go *away* from each other at the same pace; points would move in tandem with the points around it; or say you're a "north-bound" point (vector); points going in the "northeast" direction from the center would move away from you more slowly than points going in the "south" direction from the center. But the observation in our universe is that every point moves away from every other at the same pace, like a 3D version of the surface of a balloon and not like a 3D version of spreading batter.
What's really supposed to rot your brain is the implication that if you shot a lasar beam in one direction in space, eventually (in theory) the beam will return from the other direction!
Quote:
Demagogue, yeah I think that was the one. Anyway, even if it isn't the big bang wasn't there some bright patch on that image people were particularly interested in?
Yeah, I think I recall something about that, some quirk about the distribution that meant it wasn't so smooth as one might expect, or something, so the theory needed some refinement but it didn't change the core principles. Don't know the details.
edit:
* Ok, went back to see about the sponge analogy. Apparently it's a workable analogy if you're talking about an arbitrary big chunk of space, it expands like a expanding sponge (points in space relative to each other expand like that). The 2D analogy with the balloon would be like looking at a chunk of a balloon surface with a grid drawn on it as it's blown up; looking locally, the squares of the grid would all grow bigger.
Where it gets weird is imagining the whole thing. You have to get your head around the fact there isn't an "end" to the sponge; it's all sponge and wraps in on itself. But if you imagine the balloon with a grid drawn over its entire surface, if you were a 2D guy living on the surface it'd be weird to imagine that grid also would wrap in on itself "somewhere way out there", in just the same way. Their confusion at how weird that would be is just like ours.
And since it's sort of manageable to understand the whole balloon world but still imagine their confusion, that sort of helps get an idea of what's going on for us.
Tocky on 18/6/2009 at 02:24
Huh. I always thought of it expanding from a singularity too. An everywherelarity is still a bit difficult to wrap your head around even when you have a pretty balloon or the help of Spongebob.
demagogue on 18/6/2009 at 03:09
Another thing I found funny about it is that it was neither "Big" (it was infintesimal), nor was there an audible "Bang", nor was it ever an "Empire".
dj_ivocha on 18/6/2009 at 04:02
Quote Posted by demagogue
Another thing I found funny about it is that it was neither "Big" (it was infintesimal), nor was there an audible "Bang", nor was it ever an "Empire".
Well if you look at it this way - the universe is
still expanding, so the "bang" part hasn't finished yet. So, considering the universe is more than a dozen billion light years across now, I'd say that really was (is?) a pretty BIG bang ;).
Tocky - consider this: if the big bang really occurred "somewhere" and we have to look for that one point in the sky, then that means the universe was already there for the big bang to occur "somewhere in it". But since it
created the universe, it can't have occurred anywhere
in it since there was no universe yet at the first moment of the bang.
Or let me give you another example - when a sperm and an egg "meet", a baby is born 9 months later. So at the beginning, all there was were that sperm and egg. Looking at your baby/teen/grownup child,
where are that sperm and egg that created it? Somewhere in the liver? Left foot? Lungs? No, they are everywhere, as they ARE that new person (or, their building blocks and DNA are, if you will).
The mother and father in that example would be that alien guy playing with the galaxy balls in Men in Black :o
heywood on 19/6/2009 at 03:30
Quote Posted by demagogue
What's really supposed to rot your brain is the implication that if you shot a lasar beam in one direction in space, eventually (in theory) the beam will return from the other direction!
That would be true only for a spherical universe (e.g. a 3D hypersphere) which is expanding slower than the speed of light. But red shift observations which indicate the expansion of the universe is accelerating seem to rule out a spherical universe, or any closed universe for that matter. More likely, very distant objects visible today will accelerate away from us fast enough that the light from your laser will never reach them.
Quote Posted by Tocky
Huh. I always thought of it expanding from a singularity too. An everywherelarity is still a bit difficult to wrap your head around even when you have a pretty balloon or the help of Spongebob.
Well, it
is expanding from a singularity. Go back to demagogue's balloon analogy. The balloon started expanding from a single point, but you can only see that by observing from an external frame of reference. Think if you were on surface of the balloon and could only observe within the two expanding dimensions. You would know space is expanding because everything is moving away from you. But you couldn't locate the center of expansion because it cannot be defined in the two dimensional coordinates you observe within.
Quote Posted by DDL
And given that clumps of stuff DO exist, you can actually calculate how they formed, and it transpires that a standard model for universe expansion (i.e. work out the speed stuff is now, extrapolate back) doesn't fit: things couldn't move fast enough to get clumpy before being so distributed they were unclumpable, so you have to bring in inflation theory, where (for some reason) the universe expanded
exponentially for a bit, and it all gets a bit "wait, what? WHY?"
Yes, exactly, why?
The two key pieces of evidence for the Big Bang theory: red shifts and the CMB, are in contradiction with each other unless you postulate a brief period of exponential expansion that starts shortly after the Big Bang. Without this inflationary period, the standard Big Bang model of expansion has a horizon problem which doesn't allow you to explain the uniformity of space time and the CMB. The fact that we can't explain the mechanism of inflation doesn't seem to bother too many cosmologists (?)
In a similar vein, the uniformity of the CMB and gravitational lens measurements indicate that the large scale curvature of spacetime is nearly flat, which means the density of the universe must be very near the critical density. If that were true, the expansion of the universe should be slowing down, but the red shift measurements instead indicate the universe is expanding. So to explain that, we've reinstated the cosmological constant, aka dark energy. Again cosmologists don't seem to be concerned that we can't explain the origin or mechanism. Oh, and its energy density is too small for us to detect it experimentally, so you pretty much just have to take it on faith.
Even dark matter bothers me. To explain the rotation of galaxies, you need cold dark matter ("cold" means not moving fast) located primarily on the outskirts of galaxies. But why would dark matter be confined to the outskirts of galaxies? Shouldn't it mix right in with baryonic (ordinary) matter? Since the only force acting on dark matter is gravity, there is no explanation for the physical separation of baryonic and dark matter. Dark matter should be right in our own solar system, right here on Earth. Hell, there's supposedly 5 times more dark matter in the universe than ordinary matter. How come it isn't here or even in stars that we can observe?
I really think cosmology has become a sick perversion of science. At least Einstein was honest enough to admit the cosmological constant was a fudge factor and reject it. Today's cosmologists don't seem to have that intellectual honesty. When the observational data doesn't fit their theoretical models, they don't rethink the theories, they just add fudge factors and give them cool names. It's like crystal spheres all over again.
Aerothorn on 19/6/2009 at 04:20
Keep in mind that there are two definitions of "theory" floating around. One is the day-to-day use, basically meaning "a hypothesis." The other is the scientific use, meaning "something that has survived numerous attempts to disprove it and is generally held to be true." The phrase "only a theory" applied to scientific theories is often somewhat misleading - it IS a theory, but it's hard to attain the "status" of theory. My understanding, anyway; I'm not a scientist.
Renzatic on 19/6/2009 at 04:59
This is a good thread to bring this up in. I've been reading about 4D space recently. Thought the best way to spend a couple of hours was to learn about something I couldn't possibly apply to real life. I've been studying it the last few days, and I'm beginning to get a very vague idea of the theory.
Thing is, beyond a few nifty little websites describing the concept, I can't find much information on how it actually interacts with the universe as we know it. Beyond wormholes, which from what I understand are basically like 4D holes punched through 3D space (like punching a hole through a sheet of paper in a way), there isn't any information out there beyond the theory. So really, do 4D objects actually exist and intersect with our universe, or is the only proof we have just scribblings on a blackboard?
