Originally posted by Great King Rat
Alright, but then what is expanding? If the distance between a proton and a neutron in an atomic nucleus is expanding, what is expanding? The vaccuum? Is dark energy making the vaccuum bigger? If the whole universe is expanding, does this mean that matter - being a part of said universe - is also expanding? Are protons getting bigger ever so slightly? Or is it only the vaccuum that is expanding?

If you keep asking, sooner or later you are going to get a completely confusing analogy such as one involving a springy chest expander -then you will wish you hadn't.

Originally posted by Great King Rat
Alright, but then what is expanding? If the distance between a proton and a neutron in an atomic nucleus is expanding, what is expanding?

Yes and no. Yes, in that the distance is expanding, no in that the expansion is so small that at that scale you would never notice and the neutron and proton would have gone back to their former separation before you could spot the difference.
However, if you put a photon in space, and keep it there long enough, it gets stretched. This is known as redshift and is observable.

We also shouldn't forget that at smallish scales, space is not flat anyway, so near heavy objects, space gets squashed up - or spread out, I can't seem to figure it out.

Originally posted by Great King Rat
Alright, but then what is expanding? If the distance between a proton and a neutron in an atomic nucleus is expanding, what is expanding? The vaccuum? Is dark energy making the vaccuum bigger? If the whole universe is expanding, does this mean that matter - being a part of said universe - is also expanding? Are protons getting bigger ever so slightly? Or is it only the vaccuum that is expanding?

Put a marker at each point in space. The distance between two markers is given by a quantity called the metric. As time goes by the physical distance between two given markers increases. A proton does not increase in size during all this. The radius of a proton is a fixed distance. Imagine two markers one at the centre of the proton and one at its edge. After some cosmologically large time the proton will have the same radius, this is controlled by the strong force which dominates over gravity at all scales below the Plank scale, the marker at the centre of the proton will still be at it's centre [1], but the one that was at the edge will have moved away from the centre of the proton. The standard model electron is elementary and point-like and wouldn't increase in size anyway.

In Newton's universe space was just a void containing nothing and having no physical properties of its own. With Einstein, space-time became something dynamic. So, yes it is the vacuum that is expanding. Dark energy is a name for an effect and there is no theory for what it is - there are some guesses, but they can't be dignified with the name theory.

[1] I'm ignoring quantum uncertainty in this discussion.

Originally posted by twhitehead
The total amount of dark energy increases, but current theory suggests its density is uniform (which is why the total increases as space expands).

What current and universally accepted theory? See Quintessence in Wikipedia for a version of dark energy where the density does change with time.

Originally posted by twhitehead
Yes and no. Yes, in that the distance is expanding, no in that the expansion is so small that at that scale you would never notice and the neutron and proton would have gone back to their former separation before you could spot the difference.
However, if you put a photon in space, and keep it there long enough, it gets stretched. This is known as redshi ...[text shortened]... y, so near heavy objects, space gets squashed up - or spread out, I can't seem to figure it out.

In the big rip scenario eventually even Baryons get ripped apart.

That's what DeepThought was saying, eventually the forces will
grow so large [in that scenario] that even sub-atomic particles will
be torn apart by negative energy.

Whether that scenario is true or not....

Originally posted by DeepThought
What current and universally accepted theory? See Quintessence in Wikipedia for a version of dark energy where the density does change with time.

Thanks for that. I was clearly wrong that my description is the prefered one. It is however one possibility.
(I am doing an astronomy course but have not yet got to dark energy.).

Originally posted by DeepThought
[1] I'm ignoring quantum uncertainty in this discussion.

Presumably a fast moving electrons wave function is stretched just like photons. The photon loses energy, I wonder if the same is true for the electron.

Originally posted by twhitehead
Thanks for that. I was clearly wrong that my description is the prefered one. It is however one possibility.
(I am doing an astronomy course but have not yet got to dark energy.).

Preferred possibly, the only idea no. In the version of dark energy you were quoting it's identical to the cosmological constant. Dark Energy is the name for an effect and there's no really good theory, the only real evidence as far as I know is that the expansion of the universe is accelerating. It should tie in with a theory of inflation, since it's basically the same effect - just slower.

Presumably a fast moving electrons wave function is stretched just like photons. The photon loses energy, I wonder if the same is true for the electron.

Umm., electrons are normally quite localised. But yes, I think that your statement is correct, provided it is a free electron and not bound in an atom.

Originally posted by googlefudge
In the big rip scenario eventually even Baryons get ripped apart.

That's what DeepThought was saying, eventually the forces will
grow so large [in that scenario] that even sub-atomic particles will
be torn apart by negative energy.

Whether that scenario is true or not....

Which leads to an intriguing possibility. If one pulls a proton apart then quark-antiquark pairs pop out of the vacuum so one is left with a proton and a meson. But in a big rip scenario this keeps happening so space should get filled with quarks and anti-quarks. The only source of energy is dark energy, which is being used up filling space with matter, the expansion should stop and we have something that looks like a big bang. In case you are worried about entropy - all the original matter is causally disconnected, so the average entropy density is zero and we have the potential for a nice model of a cyclic universe.

Originally posted by DeepThought
Which leads to an intriguing possibility. If one pulls a proton apart then quark-antiquark pairs pop out of the vacuum so one is left with a proton and a meson. But in a big rip scenario this keeps happening so space should get filled with quarks and anti-quarks. The only source of energy is dark energy, which is being used up filling space with matte ...[text shortened]... average entropy density is zero and we have the potential for a nice model of a cyclic universe.

Yes I was wondering about that effect. [and no I wasn't worried about entropy,
the second law of thermodynamics is an emergent law not a fundamental one]

But my question would be does the effect happen fast enough that it would
convert enough dark energy to regular energy to create the big bang.

Because if this model is valid then one would expect that this cycle would
have been happening forever and thus it should be able to produce the
observed big bang.

Otherwise you have to posit that we live in the super special time of the
first universe in an infinite chain.

This would seem in the above scenario to have a near infinitely unlikely
prior probability.

Originally posted by KellyJay
Nope do not understand that, and I'm not trying to be difficult or to jerk
yours or anyone else' chain on this. I'm quite serious I don't see how you
or anyone else can say something is expanding if it is isn't moving into
new areas prior to expanding, hence the word, "expanding".
Kelly

What we call "distance" is an internal property of the universe, which is to say it can be wholly defined without reference to anything "outside" the universe. Both mathematically and physically a universe is expanded by increasing the distances between its points, and still no reference to an external realm is needed. From a simplistic standpoint one could understand this by saying we are increasing the number of spatial coordinates that lie between any two existing spatial coordinates. Say the distance between two neighboring points is defined to be 5 (maybe because it takes us 5 time units to hop from one to the other). In the universe U={0,8} (the universe consisting of two "space points" denoted by 0 and 8), the distance between 0 and 8 is 5 since 0 and 8 are neighboring points. But in the universe V={0,2,4,8} the distance between 0 and 8 is 15, since it is 5 units from 0 to 2, another 5 from 2 to 4, and yet another 5 from 4 to 8. However, we wouldn't say the "expanded" universe V has actually pushed itself out to cover a greater span of the real number line, right? It's still "confined" to the interval of real numbers from 0 to 8, denoted by [0,8]. The universe W={1,2,3,4,5,6,7,8} is even more "expanded," and yet still is confined to [0,8]. This analogy isn't perfect, because here I'm drawing in more space points from an external source (the real number line), but it need not be so in general. I could pull in more space points by giving them arbitrary symbols and defining an order for them.

You can think of an expanding universe as a universe that is adding more space points (in the manner described above) on the line segment between any two points P and Q in the universe, with the time it takes light to travel from one space point to the next remaining a constant.

Originally posted by Soothfast
What we call "distance" is an internal property of the universe, which is to say it can be wholly defined without reference to anything "outside" the universe. Both mathematically and physically a universe is expanded by increasing the distances between its points, and still no reference to an external realm is needed. From a simplistic standpoint one co ...[text shortened]... e, with the time it takes light to travel from one space point to the next remaining a constant.

I think I covered that with my display with increasing resolution analogy 😉 😛

Originally posted by googlefudge
I think I covered that with my display with increasing resolution analogy 😉 😛

It's a long thread. I did not read past the first page.

Originally posted by KellyJay
Nope do not understand that, and I'm not trying to be difficult or to jerk
yours or anyone else' chain on this. I'm quite serious I don't see how you
or anyone else can say something is expanding if it is isn't moving into
new areas prior to expanding, hence the word, "expanding".
Kelly

Instead of visualizing space expanding try to visualize everything in space shrinking with yourself shrinking at the same rate as everything in space. You don't see space getting bigger. Space isn't a thing, it is the absence of things or, more accurately, between things, so it doesn't have to expand into something.