Is Dark Energy really the best term?

[StarTrekkin]

Is Dark Energy the best term for a phenomenon that shares little resemblance to Energy as we understand it?

Dark Energy seems to be a mysterious force that expands the spatial fabric of the universe over long distances, has little or no effect on gravitationally bound objects (except on a universal scale) and is not observable through any method aside from said effects. It doesn't seem to have its own matter analogue, DE=Dmc² is not a thing as far as we understand and that seems to put it in an entirely different category of phenomenon than Energy doesn't it?

 

[PeroK]

Is Dark Energy the best term for a phenomenon that shares little resemblance to Energy as we understand it?

Dark Energy seems to be a mysterious force that expands the spatial fabric of the universe over long distances, has little or no effect on gravitationally bound objects (except on a universal scale) and is not observable through any method aside from said effects. It doesn't seem to have its own matter analogue, DE=Dmc² is not a thing as far as we understand and that seems to put it in an entirely different category of phenomenon than Energy doesn't it?

It has the units of energy. It's not a force.

 

[StarTrekkin]

It has the units of energy. It's not a force.

I was using force colloquially.

 

[phyzguy]

How about if we call it the cosmological constant. Is that better?

 

[Vanadium 50]

Lots of things don't have the "best term" for them. Why do you park on a driveway and drive on a parkway? If a brass horn is made of brass and a tin horn is made of tin, what is a fog horn made of? It's best not to worry about this.

 

[phinds]

Is Dark Energy really the best term?

As has been discussed here on PF numerous times, there are quite a few terms in science that are unfortunate, but they are what they are and they aren't going to change. Dark energy is hardly the worse of them. SO ... you have a reasonable point, but spending time on it is wasted time.

 

[Helios]

Dark Energy Density would be on the mark. Energy density is energy per unit volume. Energy density has the same physical units as pressure, and in many circumstances is a synonym. So even Dark Pressure would be an interesting word, though in this case it's in the negative.

 

[ohwilleke]

Short Answer

No. It is not the best term. But, none of the alternatives are that great either.

But, unlike @Vanadium 50 and @phinds I don't think that exploring the appropriateness of terminology and how it ended up that way is a waste of time. I think, instead, that it is a nice excuse to explore the rich intellectual history that got us to the place were are in now and to examine the assumptions that are buried in our existing terminology.

Longer Answer

The term "dark energy" implies (or seems to imply) that we understand the cause of the change in the rate of the expansion of the Universe (and that it is basically something along the lines of a "quintessence" hypothesis). But, while we have some plausible hypotheses to explain dark energy phenomena, we don't actually know just what causes this phenomena.

Indeed, the timing of the term's invention, and the individual coining this term suggests that it was invented as something of a propaganda tool to promote quintessence type explanations of dark energy phenomena. The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael Turner in 1998 (Turner is a cosmologist at the University of Chicago who has worked a great deal on cosmological inflation theories and on linking cosmology to particle physics), and the timing is notable because the term, unlike "dark matter" was coined long after the phenomena it purportedly explains was widely known. A lengthy personal history of the development of the modern understanding of dark energy phenomena with numerous citations to the academic literature and a description of Turner's role in the debate, from someone who lived through those developments and saw them first hand as a practicing astrophysicist can be found here.

We know that the phenomena attributed to "dark energy" is real, and we can quantify it in a somewhat model specific way (although different approaches to measuring it are in tension with each other which could be a clue that our model isn't the right one since if the model is sound different ways of measuring the same thing usually produce the same result), but we don't know, for example, if this is an inherent property of space-time, or if it is some substance that exists within space-time. As Wikipedia explains (end notes omitted):

Two proposed forms for dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy. Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

There is a whole sub-set of general relativity theorists who work on modified gravity theories that try to explain dark energy phenomena from a gravitational perspective, as the lambdaCDM Standard Model of Cosmology does (these theories are distinct from theories such as the better known MOND theory devised as a phenomenological explanation of dark matter phenomena in galaxies).

The term "cosmological constant" applied to the phenomena as opposed to the very model specific meaning of the term as a component of Einstein's equations which is used as one approach to explaining this phenomena is also problematic. This is because, while the data are not inconsistent with a cosmological constant as an explanation for the phenomena, the data is also not inconsistent with a cause that varies in magnitude over space and/or time. Distinguishing between those possibilities is an active area of investigation and it is harder to pinpoint exactly what the magnitude of dark energy is at a particular moment in the past than it is to determine its average magnitude over a long period of time.

The best reason to use the term "dark energy" is that it is the most widely used way to describe a set of phenomena that are observed with the caveat that people who use the term shouldn't take it too seriously and should recognize that we really don't know what it is.

The fact that both the term "dark matter" and "dark energy" are used to describe phenomena whose true nature is not understood, however, has helped make the word "dark" in the context of astronomy and cosmology develop a secondary meaning of "stuff we don't understand that seems to act like" whatever is called "dark", so that "dark energy" is almost synonymous with "pseudo-energy" and "dark matter" is almost synonymous with "pseudo-energy" (although "pseudo-" is too strong a prefix because it implies that we know that it is definitely not energy or matter respectively and only seems like it, which we also don't know).

Even on this score, however, "dark energy" is more problematic than "dark matter", because while "dark matter" seems like it acts consistently with conservation of matter-energy laws, "dark energy" generally speaking, does not obey the conservation of energy laws ordinarily associated with all other kinds of energy.

The concept of "dark energy" is arguably closer to the historical 19th century astronomy idea of "aether" (perhaps suggesting the term "dark aether") than to energy in the conventional sense, because it is something with physical effects that permeates space-time that isn't energy (in the conventional sense of a conserved quantity of something that is related in a precise mathematical way to forces), but the concept of aether is so closely associated with a discredited 19th century idea about the nature of space-time that it isn't workable or desirable to use that term either. (Indeed, the name of one class of dark energy theories called "quintessence" shares of common linguistic origin with "aether").

Another example of similar conceptually inappropriate terminology, from the physics of electromagnetism, is the "permittivity of free space" which has its linguistic roots in the outdated 19th century concept of aether, even though the modern definition of this physical constant is divorced from its linguistic roots.

Some Alternatives

"Vacuum energy" is somewhat attractive, as one often thinks of "dark energy" as something that is spread uniformly, or at least widely, throughout the volume of the universe, most of which is a vacuum, but the term "vacuum energy" is too easily confused with an apparently different set of phenomena in quantum physics.

Phantom energy would have been a better term, but has been appropriated for a specific kind of quintessence theory of dark energy, instead of being used as a general term for the phenomena.

Probably the most neutral term for dark energy phenomena is to call it the "accelerating expansion phenomena", but that is a bit clunky and doesn't have wide acceptance as the default term for the phenomena called "dark energy".

Another plausible neutral term would be "Hubble phenomena" since Hubble is the guy who, in 1929, received (rightly or wrongly) most of the credit for the experimental discovery of the phenomena now called "dark energy", but I've never seen that term used in print.