Dark matter/energy, and the Big Bang

Factual Questions
1

I’m not a great physics teacher, and I need help from time to time, especially on this subject. We watched a documentary on dark matter and dark energy and the big bang, and these are the questions we came up with that we still had. Any help on these would be appreciated.

Isn’t there a center point theoretically from which all the universe is expanding? Isn’t there nothing left there at that point?

Is there any explanation of how the farthest away things are accelerating faster?

Is the outside of galaxies traveling at the same linear speed as the inside? What was discovered to be the case, and how does that prove the existence of dark matter?

If we can find that different parts of the universe are moving away from us at different speeds, isn’t it possible to make a guess at the center of the universe?

If the universe is accelerating, this takes force, but does it literally take energy, and if so, is there any idea of where that’s coming from?

Can we prove that space itself is expanding?

2

Most of those are fairly basic so I’ll take a crack at it.

No, and this is REALLY basic. The analogy often used is the expansion of a balloon with polka-dots on it. This is a 2D analogy of a 3D phenomenon. If you were a very flat ant living on one of those dots, you would observe all the other dots moving away from you, at a rate proportional to their distance. Where is the center for the ant? It always appears to be the ant’s own dot, but in reality, there isn’t any.

ETA: In the real 3D universe space is expanding the same way the rubber of the balloon is stretching. It’s stretching/expanding everywhere, not “from” any particular point.

As above.

The dark matter hypothesis isn’t based on linear motion, but on orbital motion within galaxies, and the absence of the requisite [visible] mass to support that motion.

No, because there isn’t one.

No, because there is no acceleration seen in any local frame of reference, so no real acceleration is occurring, and no force is being applied.

I don’t know what specific empirical evidence exists, I’ll leave that one to others.

3

To add a couple of things:

It’s a little unclear what this question means. More distant objects are receding faster, not accelerating faster. That seems like a fairly intuitive result of uniform expansion: if you draw 3 dots on a rubber band and stretch it, dots that are far apart will move away from one another more quickly than dots that are close together.

There’s a separate issue of whether the overall expansion of space is decelerating, constant, or accelerating - the provisional evidence is that it’s currently accelerating - but that’s the change in the overall rate of expansion through time, it does not refer what more distant objects are doing relative to less distant objects at any given time.

Wikipedia has an excellent explanation of how Galactic Rotation Curves provide evidence for dark matter.

4

One visual aid that might help with the “no center” thing: Get an image with a bunch of black dots scattered over a white background, and make it into a transparency (you know, like we used to use for overhead projectors). Then take the same image, and blow it up by about 10%, and make a new transparency. Put the two transparencies on top of each other, pick one dot, and line them up on that one dot. It will look like all the other dots are radiating away from that point, with dots further away moved more than dots close to that one, and the dot you chose clearly in the center. But now, re-align the transparencies on some other dot, and now that new dot looks like the center, and everything is moving away from it. And this works no matter which dot you choose.

5

Lawrence Krauss shows just this in his lecture (and book) A Universe from Nothing:

6

No, there isn’t a center point from which the universe is expanding. The entire universe is expanding, from any point you will see the rest of it moving away from you. There isn’t a single central point in space with a bunch of stuff that exploded out, there was a bunch of hot, dense matter and everything including space expanded. All of the evidence points to space expanding and no one has a good alternate theory that explains the evidence, so that’s about as much ‘proof’ as you can get. It doesn’t take force or energy in the conventional sense for space to expand, and distance increasing between objects doesn’t mean they’re moving in the classical sense.

7

Is there a simple explanation of “inflation”? If these objects are moving in the classical sense, then the rate of expansion should be slowing down due to gravity. My understanding is that’s not what’s observed …

8

The first time I heard this analogy I was confused by it, and I know others have been confused in the same manner. That this is a 2D analogy is not helped by the fact that a balloon is so evidently a 3D object. I now prefer to describe it as being like stretching a rubber sheet instead. A simple 1D visual demonstration can be made with dots drawn on a rubber band. In both these examples I think the dimensionality of the analogy is more apparent than with the balloon.

9

That’s different from inflation, though possibly related. Yes, if the expansion were Newtonian, then the rate should be slowing down (it may or may not be slowing down enough to eventually stop, but it should be slowing down at least some), but as you say, that’s not what we observe. So, why is the expansion accelerating? The answer is dark energy, but that’s really no answer at all. “Dark energy” is just a label we put on the unknown cause of the acceleration.

What is it actually, and how does it work? On that point, we’re worse than clueless. I say “worse than clueless”, because we do actually have a clue, it’s just a horrendously bad one: Particle physics models predict that there should be something called the vacuum energy, which should behave qualitatively in the right way to be the dark energy. Qualitatively, but not quantitatively: Our particle physics models aren’t yet good enough to calculate the exact strength of the vacuum energy, but we can at least estimate it… And those estimates turn out to be 120 orders of magnitude larger than what we observe. Note: That’s not a factor of 120, that’s a factor of 10^120. So, is the dark energy vacuum energy? Maybe, but if so, it’s got a lot of 'splainin to do.

What of inflation, then? Well, our best cosmological models say that in the very early stages of the Universe, it should have been expanding (and accelerating) at a much greater rate than it is today. In fact, the early rate of acceleration really does seem like it’d be consistent with the predictions for vacuum energy. And then, for reasons that we don’t entirely understand, that super-fast early expansion stopped.

So, are the dark energy and inflation two different manifestations of the same sort of phenomenon? If so, why did its strength change so radically? Are they two completely unrelated phenomena? Then what’s the dark energy, and why did inflation end? Are they separate phenomena, but somehow related? How? We really do have a lot more questions than answers, here.

10

That’s why I think it’s a good example. We’ve reduced everything by one dimension here, so while you can see that the balloon has a shape in 3D space, the universe exists in 4D spacetime. We perceive three spatial dimensions just like the ant on the balloon sees two spatial dimensions.

11

Thank you, Chronos, clear as mud … [giggle] … sounds like we need a bigger particle accelerator …

12

Actually I think this comment only adds to the confusion because it implies that the expansion is somehow happening in an unobservable extra spacial dimension, but in the case of the universe (and the rubber band and the flat projector demonstration) it isn’t.
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13

I’m going to go ahead and take a chance and make a comment here … there’s a rolled up newspaper somewhere around here in case I get out of hand again …

There’s a problem with the spotted balloon example. Yes, we can see that if we pick a dot, then from that dot it “looks like” everything is moving away from us. Pick another dot and it “looks like” the same thing. No matter which dot we pick, the universe will always “look like” we’re at the exact center.

However, if we step back and look at the balloon from outside, then clearly we see there is in fact a center, a single point within the balloon from which everything is moving away from.

Here’s the paradox that confuses me, there is no “outside” the universe for us to look from. The Big Bang isn’t just matter and energy expanding into empty space, the Big Bang is space itself expanding. To say there is “nothing” beyond the edge of the universe isn’t even wrong, it’s nonsensical.

I just find that very difficult to warp my mind around …

14

So, I am not really qualified to answer the question, but I will take a crack at it. Warning: this may still be nonsensical.

The key point to the balloon analogy is that the *surface of the balloon *is 2-D. In the same way that the surface of the Earth is 2-D, in that any point on the surface of the Earth can be identified by 2 numbers–a latitude and a longitude. The Universe that we see is 3-dimensional–we can locate galaxies with 3 numbers. The balloon analogy maps a 2-D example to a 3-D reality.

The *center of the balloon * is not on the surface, and so it requires a third number to locate it–it is 3-D. There is a single point that everything in the Universe is moving away from–but that point requires another dimension to locate it, a point in time. That point is what we call the Big Bang.

15

You’ve just highlighted the point made earlier by ticker – the balloon analogy is a poor one. For the balloon analogy to work, you must imagine that nothing exists but the 2-D surface of the balloon. In a real balloon, that 2-D surface is expanding in 3-D space. But it is not true that the real (spatial) 3-D universe is expanding into 4-D space (or into anything). A lesser problem is that the 2-D ballon surface has closed finite geometry, and that the real universe probably does not (although it might in principle).

I think the best analogy is the 1-D rubber band. But it’s not beyond our intuition to imagine an infinitely long rubber band, and I think it’s better to do so, because it avoids certain misconceptions, such as the idea that a finite stretching rubber band is just expanding into pre-existing 1-D space at each end. It’s not beyond our capacity to imagine an infinitely long rubber band stretching or contracting, while remaining infinite. This is the best conception of the metric expansion of space. The universe may not be infinite, but it may be - and I think it leads to better intuition to first grasp what an infinite universe would look like in the analogy.

Consideration of marked points (or ants) on the rubber bands leads naturally to a realization that the notion of a “center” to the universe is wrong. Every point looks the same from the perspective of an ant at that point - everything is receding.

With a little mind expansion, we can imagine that the rubber-band universe may be infinitely long, and remain always infinitely long, as it stretches or contracts. The analogy then extends quite well to show that when we talk about the “size” of the universe we are usually talking about the observable universe, something that corresponds to points on the rubber band at a certain relative distance to the observer’s position -but that this does not delineate any real boundary, it’s simply the bounds of the “knowable” region from the observer’s perspective. The universe may continue indefinitely outside the observable region.

Thus, if we can imagine a rubber band that’s infinite and always remains infinite, it’s not stretching “into” anything, since it remains infinite at all times while nevertheless stretching or contracting. This also leads to a better intuitive grasp of the fact that the big bang happened everywhere in space. The big bang is simply the state of the entire infinitely long rubber band as the scale reaches zero, and we reach the mathematically intractable state of the state of the rubber band being inifinite length multiplied by scale zero.

16

Picture the classic Nova TV series type illustration, where they depict gravity with a 2D rubber sheet distorted by heavy spheres creating dimples in it. Now start stretching the sheet itself. The sheet is the medium (spacetime) through which gravity is effected. Hence, gravity does not affect the breadth of the entire medium, it only creates local distortions.

The stretching of the medium is infinitesimal on a local level. Essentially immeasurable to us right here, right now. But when you sum this tiny expansion over the hundreds of yottameters of the visible universe, it becomes observable.

One might infer that stretching spacetime would rarify it, such as stretching the rubber sheet would make it thinner and thus weaker, resulting in an increase in gravity. If the spacetime/gravity relationship is elastic in this way, we should in theory be seeing an increase in the strength of gravity. Except, the stretching is again very slow and gravity is an incredibly weak force: it is entirely conceivable that an elastic stretching would be increasing spacetime distances faster than it rarification is increasing net gravitational potential.

17

While on this topic, I have heard the balloon analogy over and over. I assume its premise is that everything is running away from everything else. This conclusion is drawn by observation of the redshift - a resultant Doppler effect observed in the spectrums of heavenly bodies. And, as such, I believe it has been said the universe is homogeneous - the same everywhere you look. Yet, it is also known that some objects exhibit a blueshift. Wikipedia mentions the Andromeda Galaxy and Bernard’s star as two examples. How do scientists explain this and make this behavior still fit the balloon model?

18

Andromeda is only 2 million light years away and is part of a gravitationally bound group of galaxies. Gravity, locally, can overcome expansion.

Bernard’s Star is 6 light years away and is heading towards us.

19

I’ve always thought that is kind of a bullshit explanation and avoidance of the question.

Yes, it all expanded out from a single point and you can’t point to that ‘point’ because it doesn’t exist - it is everywhere. But that isn’t really the question, is it. :dubious:

If space is finite, then there is a point which is in the center of it. It is just far too vast for us to even have a clue where that might be.

20

Some of the dots on the balloon are tied together with a rubber band called gravity. The rubber band is contracting with more force than the balloon is expanding – keeping those particular dots together even while every other dot is receding away.