Could Dark Energy be an (unknown) dark force acting at great distances?

[mister i]

TL;DR Summary

Could the dark energy be associated with an (unknown) dark force acting at great distances?

I am not a physicist but if someone can answer this question: It seems that there is a dark energy in the universe. Could it be associated with an (unknown) dark force acting at great distances?: I don't quite understand that when we talk about the approach of galaxies we talk in terms of force (gravity) and when we talk about the moving away of galaxies we talk in terms of terms of energy (dark)

 

[Ibix]

Basically, if you look out at the universe you can see galaxies receding from us, and the further away they are the further back in time you see them. Thus this gives you a view into the history of the development of the universe, and that gives you a view on the mix of matter and radiation in the universe since their densities change at different rates and this affects the expansion rate history.

However, it does not matter how you mess around with the matter and radiation mix in our models, you cannot get the models to match the observed history. You can do so if you add some energy and an associated negative pressure that has yet another different rate of density change. This is dark energy - something that isn't matter or radiation that we have to add to our models to make them match observation. We know very little more than that, but ultimately it's all part of the same General Relativity model that explains every day effects of gravity like what happens when you drop something.

So the simplest idea is just that there's a type of "stuff" in the universe that we weren't previously aware of and it has effects that we've only spotted through gravitation and is utterly negligible on the human scale. We call it dark energy. But is it possible that we're wrong about that and there's actually an unknown fifth force causing the effect? Or that General Relativity is subtly wrong in some way? Or that our measurements aren't quite right somehow? Sure. The problem is coming up with a mathematical model of any of those things that explains the observations and isn't immediately ruled out by other observations.

 

[PeterDonis]

Reference: https://www.physicsforums.com/forums/cosmology.69/post-thread

Your reference is the page for posting a new thread. It isn't an actual thread.

Could it be associated with an (unknown) dark force acting at great distances?

You are looking at things backwards. "Dark energy" is just a name for a particular kind of stress-energy (or cosmological constant) that produces a particular kind of spacetime geometry. In GR, nothing associated with spacetime geometry is a "force" in the usual sense (something that "pushes" or "pulls" and produces nonzero proper acceleration, i.e., makes an object feel weight). Sometimes it is useful to take a more Newtonian viewpoint and call something due to the spacetime geometry a "force" the way gravity is called a "force" in Newtonian physics; but any use of the term "force" in this sense is derivative and does not mean there is any "real" force present. The spacetime geometry is the fundamental thing.

I don't quite understand that when we talk about the approach of galaxies we talk in terms of force (gravity) and when we talk about the moving away of galaxies we talk in terms of terms of energy (dark)

Where do you see these two different kinds of "talk"?

Note that dark energy is not required for the universe as a whole to be expanding ("moving away of galaxies"). It is only required for the expansion to be accelerating.

 

[berkeman]

Your reference is the page for posting a new thread. It isn't an actual thread.

(That's due to a forum bug -- it happens every once in a while. I've deleted that reference line out of the OP now.)

 

[ohwilleke]

TL;DR Summary: Could the dark energy be associated with an (unknown) dark force acting at great distances?

I am not a physicist but if someone can answer this question: It seems that there is a dark energy in the universe. Could it be associated with an (unknown) dark force acting at great distances?: I don't quite understand that when we talk about the approach of galaxies we talk in terms of force (gravity) and when we talk about the moving away of galaxies we talk in terms of terms of energy (dark)

In the paradigmatic LambdaCDM a.k.a. ΛCDM model of cosmology, "dark energy" is described in terms of the cosmological constant, Λ, of general relativity, which is part of the equations of general relativity that usually appears on the left hand side of Einstein's field equations shown below:

In Einstein's field equations, above, the Lambda term (Λ) which stands for the cosmological constant, is immediately to the left of the equals sign, and is part of what are basically the force equations for gravity in general relativity. The cosmological constant Λ is a single, dimensionful number (without an imaginary number component) which has a measured value on the order of 10⁻⁵² per meter squared.

So, what you are suggesting is, in one sense, the status quo.

The language we use is inconsistent. We conventionally put the cosmological constant on the "force" side of Einstein's field equations, even though we call the effects it gives rise to "dark energy" which implies something that would be on the "stuff" side on the right hand side of Einstein's field equations.

But these conventions are based on little more than what "looks pretty" in the equation, and what "sounds nice" when we say it aloud. Sometimes it is more useful, intuitively, to think of the cosmological constant as part of Einstein's field equations. Sometimes is is more useful, intuitively, to think of the cosmological constant as "stuff" that is spread evenly throughout the universe.

Of course, equations are just equations and only have the labels we attach to them. You can call the cosmological constant part of a "force equation" but you could also call it part of the distribution of "stress-energy" to which Einstein's field equations are applied and hence part of "stuff" rather than a "force".

It is trivial, with simple algebra, to show that we can move the cosmological constant term from the left hand side of Einstein's field equations, which we conceptually think of as the "force formula" that tells us how stuff on the right hand side gives rise to gravitational effects, to the right hand side, which we conceptually think of as "stuff" that causes gravitational effects. Both versions are correct. These are just different ways of presenting the same equations. So, ultimately, the question of whether "dark energy" is a force or is "stuff" is an arbitrary distinction in the current paradigm.

"Dark energy" gets its name because the cosmological constant, Λ, in the equation above, is physically equivalent to a field of energy, with a constant amount of energy per volume of space, everywhere in the light cone of the Big Bang. If you wanted to, you could replace the cosmological constant term with a slight, almost infinitesimal, increase to the T⁰⁰ element of the stress-energy tensor shown below, everywhere.

As an aside, the mass-energy of "dark matter", "ordinary matter", and photons, as well as linear momentum, angular momentum, "shear stress", and pressure (which can be thought of as different kinds of energy), are all "inside" the term T_μν, in the Einstein's field equations. This term is called the stress-energy tensor. The stress-energy tensor is a four by four matrix, conventionally arranged as follows (image from the link), although there is nothing sacred about that arrangement:

Another part of the reason that the force-stuff distinction for dark energy is an arbitrary labeling issue is that, as Peter Donis notes in post #3:

In GR, nothing associated with spacetime geometry is a "force" in the usual sense (something that "pushes" or "pulls" and produces nonzero proper acceleration, i.e., makes an object feel weight).

In order words, thinking in terms of "stuff" and forces caused by "stuff" is really a subtle category error.

This particular category error is often harmless. Even professional GR physicists sometimes use those terms as a shorthand, recognizing their technical limitations. But, in the context of the question you are asking, this terminology is problematic.

An effort to make this force-stuff distinction that your question asks about illustrates the fact that your question is as much a question about how we label reality, as what it actually is in a real substantive sense. In truth, "dark energy", as currently understood, isn't exactly either a "force" or "stuff".

Modifications to Einstein's field equations that reproduce "dark energy" phenomena, without a cosmological constant, in a manner consistent with available observations (which aren't terribly precise), have been proposed by professional GR theory specialists. But no one alternative has gained wide acceptance. Still, even Einstein thought that the cosmological constant was ugly and possibly a mistake in the first place, so it is understandable that theorists would look for more elegant alternatives.

One area of active research is to consider variations on the cosmological constant that are more "force-like" or more "substance-like" than Einstein's field equations which are ambivalent about that question.

For example, some researchers consider the possibility of matter-dark energy interactions in a way that resolves this ambiguity in favor of a "substance-like" over a "force-like" understanding of it. These "substance-like" interpretations are easier to tweak and modify freely to fit observational astronomy data that isn't a perfect fit for GR with a cosmological constant, in beyond GR models, than "force-like" interpretations of the cosmological constant. But this freedom to tweak the model is both a blessing and a curse, because with a rigid single number description of dark energy (which is still a decent fit to the data although high precision data is showing some tensions with it) you don't need to come up with a theory about why implicit dark energy densities might be different in one part of space-time than it is in another.

 

[Ibix]

Still, even Einstein thought that the cosmological constant was ugly and possibly a mistake in the first place

AFAIK, he introduced a non-zero cosmological constant with no real justification beyond disliking the implications of Friedman's work. And his usage (to balance expansion and get an eternal universe) turned out to be unstable. And then Slipher and Hubble observed cosmological redshift that falsified his model. So he had reason to dislike it and think of it as a mistake - but the modern usage is data-driven, a very different value, and the notion of "an energy of the vacuum" has precedent now, so I'm not sure his objections are relevant any more. It may all still be wrong, but not for the same reasons Einstein was.

 

[PeterDonis]

AFAIK, he introduced a non-zero cosmological constant with no real justification beyond disliking the implications of Friedman's work.

No, Einstein introduced the CC about 5 years before Friedmann did his work. But Einstein knew enough then (1917) to realize that without a CC there was no static solution for the universe as a whole (he assumed a spatially finite universe with a uniform distribution of matter), and he didn't like that, so he added a CC to allow a static solution.

 

[PeterDonis]

I've seen speculation that the so-called dark matter is an effect of gravitation "leaking" between branes

Discussion of speculations like this (with an appropriate reference) belong in the Beyond the Standard Models forum, since they involve proposed new physics not contained in our current models.

Discussion in this thread is about what our best current models say.