What evidence exactly? Hoyle later (once he realized that the theory was right) said that he didn’t mean for it to be derogatory, but that just sounds like backpedaling to me. I’m not aware that there’s any more evidence for the story to have to hold up to other than what’s probably backpedaling by a former opponent of the theory.
Unfortunately, it does not. In the balloon, a 2-D surface is expanding into pre-existing 3-D space. In the universe, 3-D space is not expanding “into” anything at all. And “time” is certainly not a fourth dimension that’s analogous to the third spatial dimension in the balloon.
I just had another interesting thought regarding spatial expansion regarding our own solar system. The solar system is about 4.6 billion years old, meaning it’s been around for about a third of the amount of time that the universe has existed. During all this time, has the solar system remained the same size? If so, does that mean that spatial expansion has been occurring at a constant rate for the last 4.6 billion years, and that the sun’s gravity is exactly strong enough to counteract this expansion? What I’m assuming, in this order is this.
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The solar system is the same size today as it was 4.6 billion years ago.
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The sun’s gravitational force hasn’t changed during that time. Therefore
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The universe’s rate of expansion has also been constant over the last 4.6 billion years, or else the expansion would have ripped Earth away from the sun if it was speeding up or the sun would have sucked us in by now if the expansion was slowing.
Again, I’m not a physicist, but I do wonder what the current thinking on this is.
Then I guess I would go with this answer, which probably still won’t please the no-centrists.
Q: Where is the center of the Universe?
A: Good news and bad news. The good news is that if the Universe is finite (and we aren’t sure it is), then there would probably be a point at the center of it. The bad news, and there’s lots, is that this would not be the center point from which it all sprang (that would be everywhere, since everything came from it). There are also spatial geometries that would make it very difficult to define a ‘center’. Then finally, the really bad news. We will actually never know where the center is, because the Universe is larger than the Observable Universe. That is, the part we’re limited to being able to see even with the best telescopes we could ever build. So we’ll never see enough of the Universe to be able to tell where the center would be.
The sun’s gravitational pull doesn’t have to exactly match the expansion of space. It just has to be stronger. Just because you can pull a truck with a rope doesn’t mean the rope will “suck in” any lighter object you try to pull with it.
Our entire galaxy (and the ones that are close – Andromeda, Magellanic Clouds) is held together with more force than the expansion of space can overcome. Galaxies (and clusters) that are far apart aren’t held together at all. That’s why they’re receding from us.
Instead of dots drawn on the balloon, imagine coins glued to the surface. And some are glued together. The coins wont expand, the strings will hold them together, and only the unattached coins will gain distance between them.
The basic gist is that the expansion of space is weak enough for even the tiniest gravitational field to overcome. It’s just that at millions of light years, even gravity is too weak.
Oh, and by the way, the expansion of space is accelerating. That’s what dark energy is.
It might also be noted, FWIW, that an explosion is typically not uniform when examined closely, and the overall effects are not always centered on the point of detonation. If the big bang were a lot like an explosion, I think the dynamics of it would not be like a bunch of stuff moving away from the center point of the blast.
Also, it was a few years ago, so chaoticness has had a while to settle in.
True enough. I was just thinking of the average person’s conception of an explosion, your basic Mythbusters-type boom. What I meant was just that the word “bang” has a lot of unrelated associations which seem to tend to lead people to the understandable misconception that the Big Bang must have had a locus central to the trajectory of – well, everything.
I know expansion without a centre is outside our normal conception, but the idea drops out naturally when you apply general relativity to cosmology, which is why the basic idea of big bang theory only came about half-dozen to a dozen years after Einstein published his theory of general relativity.
This is admittedly a tricky subject, but Noether’s theorem in general relativity depends on the symmetries of the particular spacetime in question that are represented by killing vector fields. So for example as cosmological spacetime does not usually have a timelike killing vector field there is no conservation of energy in cosmology.
There are however usually spacelike killing vector field in cosmological spacetime, which means there is conservation of momentum, but it is conservation of canonical (comoving) momentum not of physical (proper) momentum. Physical momentum density is proportional to 1/a, where a is the scale factor and so decreases with time in an expanding Universe.
So what this means is that in cosmology:
[ul]
[li]It is not usually possible to pick coordinates in which energy is conserved, instead you have to introduce an, arguably objectionable, gravitational stress-energy-momentum pseudotensor if you insist on energy conservation.[/li]
[li]It is possible to pick coordinates in which momentum is conserved, but in the coordinates we think as ‘physical’, momentum is not conserved (again though it can be conserved in these coordinates with the help of an appropriate pseudotensor).[/li][/ul]
This is all very, very interesting.
Thanks for this link! Fascinating. And I see there are other similar lectures by him that are more recent.
In the entry-level general science book I’m (eternally) attempting to write, I’m describing the expansion of space with a “big bread model.”
Imagine that you are cooking a loaf of raisin bread. You take a piece of dough that is 4 inches long and put it in a 12-inch bread pan, and it takes one hour to bake. In that hour, the piece of dough triples in length to fill the pan. Now imagine that you measure the distance between some of the raisins before and after baking the dough. Two raisins that start off 1 inch apart will end up 3 inches apart, moving away from each other at a rate of 2 inches per hour. Two raisins that start off 2 inches apart will end up 6 inches apart, moving away from each other at a rate of 4 iph. Two raisins that start off 3 inches apart will end up 9 inches apart moving away from each other at a rate of 6 iph. And two raisins at the far ends of the dough 4 inches apart will end up 12 inches apart moving away from each other at a rate of 8 iph.
The speed at which the raisins move apart from each other depends on how much dough is between them—the more dough, the faster they move. And no matter which raisin you choose to look at, all other raisins are moving away from it in all directions from it’s own point of view.
Galaxies aren’t flying away from each other like bits of debris in an explosion—galaxies are embedded in space that is expanding—and the more space there is between two galaxies, the more expansion is going to happen, and the faster the two galaxies move apart from each other.
Yeah, the raisin bread analogy is a fairly common one. Its main weakness is that a loaf of bread, unlike the Universe, has boundaries.
And a center.
Why would you think this? A finite universe doesn’t imply that the universe has a center.
Imagine that the universe is like the old Asteroids game. The “edges” of the screen aren’t really edges, when you go off the edge you just appear on the other side. Except you didn’t go off the edge, you just went around the corner.
Where’s the center of a moebius strip? It’s a finite sheet of paper. But if an ant starts walking along it they’ll never reach the end, despite the fact that the strip is a finite surface.