It’s not. Basically, the gravitational field in general relativity *is* spacetime: it’s what determines distances in both space and time, and hence, lengths and durations. But it enters the stress-energy tensor (the generalization of mass-energy, i.e. the cause of the gravitational field) just as ordinary matter- and force-fields do. Spacetime does not ‘push’; there are solutions of Einstein’s field equation that describe an expanding universe (e.g. de Sitter space), but there also are solutions (for instance the appropriately named anti-de Sitter space) that describe a contracting space, solutions that do neither, and solutions that twist and turn in all kinds of interesting ways.

Yep in basic GR the vacuum is no more or less spacetime than a non-vacuum region. The geometric difference between a region of spacetime representing a vacuum and one representing space filled with matter, is that a certain tensor, derived from the Ricci curvature tensor, vanishes.

GR is a very complicated and very, very general theory (arguably TOO general as large classes of perfectly valid solutions to the EFEs are usually rejected as being unphysical). Gravity as an attractive force between two masses only really emerges within certain limits.

Again, thanks. This helps steer my ontological thinking in areas I haven’t quite grasped, and get a feel for what the scientific consensus is right now in what I find the more fascinating aspects of modern physics and cosmology.

If anyone has any further thought along these lines, or can point me to some interesting papers, books or abstracts, I’d love to soak it in. I’m currently reading *Why does the World Exist: An Existential Detective Story.* Interesting read on subject.

That said, how does something like Twister Theory hold up to QFT, for example?

Not to beat a dead horse, but would you (or anyone) mind elaborating a bit more about on this?

I realize this is an area in physics and cosmology that’s incomplete and fervently researched on many different theoretical fronts, which is why I started the OP in GD. The standard model is a marvel, despite the arguably growing notion that it’s describing the shadows on Plato’s proverbial cave wall — and once one starts to become ever more convinced of it, it’s all the more tempting to imagine what might be casting these “shadows” because our vantage is frustratingly forced one way.

Gravity doesn’t have stress-energy just like the other forces, if we’re strictly talking within the formalism of GR. However there is a non-linear aspect to GR which means that, unlike Newtonian physics, the gravitational field itself is a source of gravity and you can in fact express it’s stress-energy in terms of a pseudo-tensor (stress-energy is expressed in the form of a tensor).

So far most of the Universe’s expansion has been due to it’s initial conditions/conditions very early on in the Universe, why it should have such conditions we can’t really say as we only infer them by evolving the current state of our Universe back in time. In the present era though, according to our best cosmological models, the expansion has become dominated by dark energy which is causing it to accelerate. The cosmological constant was originally a negative pressure term introduced by Einstein to counteract the expansion of the Universe (a positive pressure or zero pressure in a non-empty Universe always leads to a Universe with either expanding or contracting dynamics). One of the problems though with the introduction of the term is that it is inherently unstable: either the Universe is exactly static or you get runaway expansion or contraction. The term was re-introduced and labelled dark energy though when it was noticed the Universe’s expansion is in fact running away (i.e. it is accelerating).

If you’re asking about the energy density of the vacuum/cosmological constant, here’s a nice writeup by John Baez.

Asymptotically fat, so why the the relabeling of Λ to Dark Energy? Just to give it a less clumsy name?

And thanks for the link HMHW, I’ll read it soon.

One day, you’re all coming over, and we’ll play Yahtzee, and I’m buying the pizza.

It’s more general to view it as a form of energy than as a constant of spacetime: Einstein added it as a geometric property of spacetime which pretty much decided its form, whereas seeing it as the property of a field *within* spacetime allows it to have different forms without fundamentally altering general relativity. Also when Einstein put in the cosmological constant he was trying to achieve a static Universe, so for example it didn’t matter if it varied as the volume of a region of space varied.

It does seem to vary though according to data from the Baryon Oscillation Spectroscopic Survey

That article is about the Universe’s expansion rate varies and how the density parameter of dark energy (which is sometimes confusingly called the cosmological constant because it has the same sign) varies rather than how the cosmological constant varies. AFAIK it generally assumed (though alternatives like quintessence are certainly on offer) that the effect of dark energy is the same as the effect of introducing a cosmological constant (which is constant) because of the assumption that it is vacuum energy which behaves like a cosmological constant in the dynamics of the Universe.

It’s quite possible that I’m missing a fine distinction, but in the next paragraph the article says “The measurements so far lend support to the leading theory that dark energy is a natural property of empty space: The more the universe expands, the stronger dark energy becomes.” So that sounds to me like it is a basic property of the fabric of the universe. IDK.

What it means is that as the Universe expands space becomes more empty so dark energy becomes more dominant.

Actually, I interpret it to mean that there is more space so there is more zero point energy.

It’s the dark energy density that is important (and in fact as in the standard cosmological model the Universe is infinite, I’m not sure that there’s any way to say there is more space), for vacuum energy that doesn’t change so you end up with the cosmological constant.

OK, so you’re saying that the energy density of any cubic cm of space remains constant. No problem there.

But isn’t the expansion of the universe normally described as an expansion of space itself? I think one of the popular examples is to imagine that you’re a raisin in a loaf of raisin bread. As the bread rises, all of the other raisins move farther away. But that’s because the bread is expanding, not because the other raisins are moving.

Yes the expansion of the Universe is for all intents and purposes the expansion of space, but the cosmological constant doesn’t change with the expansion of space and neither if we choose to view it as dark energy then the associated energy density doesn’t change either. That’s of course not true for all dark energy models.

OK, then I suppose I’m missing the point. If we agree that each cubic unit of space has the same energy and that as space expands you get more cubic units then the bottom line is that you have more total energy.

Isn’t that the bit that matters for cosmology?

Is there even enough data on this matter (heh) to draw a solid consensus on the nature of dark energy/cosmological constant?

Huge pieces of the puzzle are missing, and everyone from Penrose to Weinberg have several theories.

I look at a star, and the vacuum of space surrounding it, and I wonder how errant it is to think that there’s any less (or more) “stuff” in the fabric of space, than there is in the hydrogen of the star.

Then I’ll wonder, is this “stuff” of space, and this “stuff” of matter, really all that fundamentally different from each other?