In which case, the center would be the center of the inflated balloon. There must be an equivalent point in the universe. If we’re capable of calculating galactic distances and directions of movement, we can figure out that common center point.
No. The balloon example is supposed to work in 2 dimensions, not 3. See the video I posted above.
ETA: a 3 dimensional example would be bread dough with raisins in it. If the bread is large enough, you won’t be able to determine the center just from the position or movement of the raisins when measuring from any particular raisin. The universe is large and old enough that if it has any edges, we can’t detect them.
No. In particular, the surface of the balloon needs no reference to the balloon’s centre in that analogy. In other words the properties of the balloon’s surface don’t depend on the balloon having an “inside” or “outside”. In the same way, if our universe corresponds to the balloon’s surface, the universe doesn’t have to have an outside. More to the point, there is no need to “imbed” the universe in a higher dimension, or to view it from outside, in order to talk about things like its curvature or shape. It is self-contained. In general, the properties of a “space” are intrinsic to that space and don’t require any outside perspective for them to be manifest.
Missed the edit interval.
The bottom line is that I should have phrased the balloon analogy as:
Our 4-D universe can be legitimately likened to the 2-D surface of a balloon. In the same way that as the balloon inflates, dots placed on the balloon’s surface will all move apart from one another, with no stationary point, so too does the Universe expand in all directions. And, in the same way that the balloon expands by stretching itself, and not by stretching “into” something else, so too does the Universe expand itself, and not by expanding into something else. The balloon is not expanding “into” anything. Likewise the Universe.
Just to add my voice to the chorus, the Universe is likened to the surface of the balloon in that analogy. Just as there is no point on the surface of the balloon that has a claim to being the “center” (or at least, no better claim than any other point), so too, there is no point in the Universe that has a claim to being the center of the Universe.
I never said the universe was expanding “into” anything. All I said was that there is a genuine center, from which point things are all moving outward. And both analogies (the balloon and the dough) do nothing to change that impression.
In fact, I thought the fact that the universe seems to be expanding from a single point is how the Big Bang theory was proven in the first place, over the Solid-State Universe theory.
That point is the Universe.
The universe used to be a single point, presumably, at least according to the consensus model these days. But that point expanded to become all points. If you were to travel anywhere in any direction, no matter how far, the rest of the universe, on the large scale, would look the same as it does now. The universe is isotropic. That’s key to all theories of cosmology.
And if a center could be identified it would be a privileged reference point. Einstein says that no such thing can exist, and thus far all evidence agrees. That’s why relativity trips people up as much as QM does. Neither corresponds to the everyday “common sense” understanding of how stuff works that we normally rely on. You have to accept that all your thinking is wrong and adjust it to events that seem loopy. (And not Loop quantum gravity-y. Scientific pun.)
If matter expanded into space, then you would be correct. But space, actually space-time, itself expanded. Space-time is a different animal than matter. You can’t think about it the same way.
Color me stupid, but my understanding of cosmology tracks cmkeller’s. Intuitively, almost by definition, if the universe (presently estimated at about 14 billion years old) is expanding and you run the tape backwards, it will converge to a point. I’m happy to have my ignorance fought, but would appreciate a cite. A page cite to Hawking’s Brief History of Time will do, as I have that in my library. Otherwise, an internet cite, please. I’ve watched the Krauss lecture linked above (fun and informative), but didn’t see anything on this nuance.
Yes, it would converge to a point, but the entire Universe would converge to that point. So basically what you’re saying is that every point is the center. Alternately, you could say that “the center of the Universe” is not a place, but a time: The moment of t = 0.
First of all, you probably mean steady-state, there, not solid-state. Second, expansion is a feature of the steady state model as well: The difference is mostly in the fact that the steady-state model assumes that matter is continually created (somehow) to keep the density constant. And the big piece of evidence that distinguishes between the two is the cosmic microwave background radiation, which can be easily explained in terms of the big bang model but not in terms of the steady-state model.
Pick a point in space. Let’s say for example the point where you happen to be sitting right now. Ok, now let’s place an imaginary marker at that point that doesn’t move, so now you are hurtling away from that point at something like 70,000 mph (that’s how fast the earth moves).
Now pick another point oh, say, 400 light years away from here. Again, put a marker there that does not move. It is absolutely 100 percent fixed to that exact point in space.
Even though neither marker is moving, they are both moving away from each other. It’s not that the markers are moving, because they aren’t. The space between them (and all space for that matter) is expanding.
So, if you rewind it to the start of the universe, both of those markers are at exactly the same spot, even though neither one of them moved. Where is that spot now? It’s right here, where you are sitting. It’s also 400 light years away. And it’s over there 200 light years in the opposite direction. They were all once the same spot.
I say the spot where I am sitting is the center of the universe. Everything else expanded away from it. Zaphod Beeblebrox over in Betelgeuse says that his spot is the center of the universe, and everything else expanded away from it. We’re both right. In fact, any point you can pick in the entire universe was once the point from where it all expanded outward.
So which spot do you want to pick?
Well, you could think of the universe as this little bamboo weave. To make the whole matt bigger, you just spread out the weave so that the spaces get bigger. Each space would expand relative to its own center, not the center of the whole matt. If you play the tape backwards you’d see the bamboo strips tightening until the whole matt became solid with no spaces. Sorta.
I think some of you guys are a bit out of date. According to the links I posted above (to the blog of an actual astrophysicist), space expanded first, very quickly (it is called inflation these days), then the big bang happened, filling it, or partially filling it, with matter and energy.
And, of course, our universe may only be a 4 dimensional slice of a larger, higher dimensional universe filled with clashing branes, or something.
I don’t claim to understand it, it makes my brane hurt.
At least according to the Wiki link on Cosmic Inflation, this one from Absolute Astronomy, and literally dozens more, inflation followed the big bang. (I recognize that these may now all, in fact, be out of date in view of what’s said in your links). But, isn’t the justification for invoking an inflationary scenario the need to smooth out the Universe in terms of the "Horizon Problem, “the Flatness Problem”, and “the (Missing) Monopole Problem” all three of which result from a Big Bang that isn’t followed by inflation?
Inflation occurring after the Big Bang, would eliminate these problems by insuring that what are now widely separated parts of the Universe were causally connected immediately after the BB, by stretching out the matter/energy of the immediate-post Big Bang Universe, and by diluting out magnetic monopoles, respectively, as I know you are aware.
I have looked at the the links you provided (thank you - VERY provocative) but still need to ask, if inflation preceded the Big Bang, wouldn’t the these problems be resurrected?
I don’t think that’s whats being said at those links.
I think what he’s saying is that originally the Big Bang was conceived as an expansion from a singularity – a dimensionless point. But the fact of inflation hides the singularity from us. Essentially all we know is that the universe began as a very hot, dense space. There may or may not have been a singularity before that space came into existence but we have no way of knowing.
He’s not saying that space expanded first and then the Big Bang occurred to fill it. What he’s saying is that the beginning of the Big Bang was a space rather than a point. But the Big Bang still happened everywhere in that space simultaneously.
Chronos:
But that doesn’t make sense. You’re essentially re-iterating the balloon metaphor, but you’re saying that any place on the outer surface of the balloon is equally considered the “center.” Which I suppose is true if for some reason you’re trying to locate the center of the balloon’s surface. But the balloon as an entirety HAS a center. That center is sitting in the air in the middle of the balloon.
Isn’t the universe a spheroid? Haven’t scientists calculated or estimated its measurements, including its radius? You can’t have a radius without a center point.
Even if the universe were spheriod, that doesn’t mean it’s expanding outwards from the center. When you inflate a balloon, the balloon doesn’t inflate from the center outwards, even though yes, the balloon does have a center.
Again, it’s only the surface of the balloon that’s analogous to the Universe. The balloon has other features that the Universe does not appear to have, such as an interior and exterior which are divided by the surface, and that just means that the analogy breaks down there.
Current evidence points toward the Universe being flat, not spheroidal (or at least, if it is spheroidal, it’s much too big for us to be able to detect the curvature). If it is spheroidal, then you can interpret it as being the surface of some object embedded in a higher number of dimensions, with a center located at a point in those dimensions (though note that even though you can interpret it that way, there’s no need to do so, and it’s simpler to not assume the existence of those extra dimensions). In such a case, it’d be possible, at least in principle (though not, as I said, in practice, since it’s so big) to determine what the radius is, and hence how far away that center is. But that’s still no help in saying where it is, because we don’t even have a way in which we can describe the direction. By analogy, you might take an inflated balloon and try to draw an arrow on it using a marker which points towards the center.
I certainly believe the universe has a center from which the big bang happened, yes space was then created around it but it will still be a point in space today.
When we see other big explosions like supernovae, we know also there is always a big remnant left, yet I have never once heard anyone ask about a big remnant black hole at the point of the big bang today? Maybe this is in fact the missing matter everyone is looking for?
Well, there you are, solved another big mystery. Find the place where it happened in todays space and I bet there is one huge black hole there no one can see since nothing is around it but space. There is your missing matter and all that. Thank you.
Incidentally, what sort of EM background would we expect to see in a steady-state universe?