Dark Energy is a something that depends on which theory you subscribe to. For instance, it is either a cosmological constant that is a property of space; a new and evolving scalar field named “quintessence”; gravitationally active zero-point photons with the ability to curve space and time; or proof that Einstein was wrong (again). Whatever it is it’s not a form of energy we can detect; so known particles, radiation, and fields can’t explain it.
Supposedly 70% of the universe is dark energy, and this invisible something is thought to be responsible for the observed cosmic acceleration; i.e. speeding up of the universe towards infinity and beyond. For this latter finding, three scientists were awarded the 2011 Nobel Prize in Physics. One laureate in particular, Brian Schmidt, lectured an astronomy class at the Australian National University attended by yours truly. Professor Schmidt is a good orator, above political average to be sure.
How the density of dark energy behaves as space expands is thought to provide some clues as to what it might be, or at the very least point towards which current theory best explains its nature. Two scenarios are of mainstream interests. First, is if the density remains unchanged with expansion; indicating that dark energy is part of space (constant). Second, is if the density becomes diluted with expansion; implying that dark energy is in space (field). Present observations suggest that dark energy is part of space; however that fact is not discouraging researchers from exploring the field explanation further, or other more radical possibilities for that matter.
Speaking of skeptics, there is a small but resilient group of theorists that have gone so far as to argue that dark energy is just “an illusion”; a product from when physicists apply the Friedmann-Lemaître-Robertson-Walker (FLRW) metric from general relativity to calculate the amount of expansion at any time. The problem with this approach, they point out, is that the model assumes that space is homogeneous, i.e. uniform, same density everywhere. Thus, in their alternative inhomogeneous simulation, the observed accelerated expansion is a result of “backreaction” that occurs when mass and energy warps space and time; which is in accordance with the prediction made by Einstein’s theory. So he was right! sympathizers cheer with relief. Not so fast! critics are quick to interject with gusto; pointing out the many quantitative studies that have found the effects of inhomogeneities on cosmic expansion to be negligible or very small. So no, “dark energy isn’t an illusion.”
Suffice it to say for now and for the foreseeable future, what dark energy is or is not will likely remain as mysterious as the mystical Force in “Star Wars,” and will continue to facilitate the formation or dissolution of coalitions; defending a theory on the one side or advancing its refutation on the other. Though if there is one thing all groups can agree on, and this is evident from the last paragraphs in each of their respective published papers, is that further study is recommended to unlocking the mysteries of this mysterious something.