What is Dark Energy?

dark energy
Artist's impression of the expanding universe under the influence of dark energy. SKA Organisation/Swinburne Astronomy Productions

One of the shocking revelations of the late 20th Century was that the universe was expanding at an accelerating rate. Before that mysterious "speed-up" was discovered, people thought that the rate must be slowing down as the universe expanded. What's worse, at the time of discovery, there was no known mechanism to explain how the expansion of the universe could be accelerating.

Guess what! There still isn't a well-explained one.

But, at least whatever it is has a name. 

This mysterious driving force is known as Dark Energy. There are a few possibilities of what it could be. 

Is Dark Energy a Property of Space-Time?

General relativity is often thought of as a theory of gravity, mostly because this is its greatest application as it explains the dynamics of objects in accelerating reference frames (like a gravitational field). However, general relativity is more than that, and it has far reaching implications into the vary nature of the universe.

One of the most amazing consequences of Einstein's theory is that empty space isn't really empty. In fact, empty space can possess its own energy, it is inherent to the very fabric of space-time.

In general relativity this manifests itself as the Cosmological Constant in the Einstein Field Equations. It essentially acts to explain that as more space comes into existence (another property arising out of general relativity) that this new space would appear with this vacuum energy.

The vacuum energy could be the missing dark energy of the universe, causing space-time itself to expand. The problem? It is not understand where the thing this cosmological constant describes comes from, and if it is really even correct. The only supporting evidence is that there is this mysterious acceleration of the universe that may or may not be tied this phenomenon.

Is Dark Energy a Quantum Effect?

Another possibility that has been put forth is that dark energy is the result of virtual particles being created - then annihilating - in the quantum foam of the universe.

These virtual particles, which are caused by fluctuations of the background field of the universe, are also thought to be responsible for carrying the electromagnetic, weak and strong forces between objects. So it seems like a perfect candidate for dark energy.

However, calculations attempting to estimate the total energy of such particles that would be randomly popping in and out of existence throughout the entire universe were much too large. This does not necessarily discount the theory, but clearly there is something that we still do not understand about the nature of when and how these virtual particles are created.

Some New Energy Field?

One possibility, that your author personally does not care for, is that there is some new energy field that permeates the universe that we have, as yet, have not measured.

This new field would be all around us and would not interact hardly at all over small distances. It would only have a measurable effect on anything when you are talking about scales approaching the size of the observable universe.

Some theories assign the name quintessence, after the fifth element described in Greek literature. However, this theory arose simply by looking at what properties dark energy must have, and giving those properties a name. There is no scientific justification of where or why such a field would exist.

Though, admittedly, that does make this theory incorrect. But given that it is not based on our current understanding, only a guess about a possibly energy field that we can't probe with current technology, it makes for a somewhat unsatisfying theory.

Could Einstein Have Been Wrong?

There is a final possibility, one that would have been considered almost unthinkable a few decades ago. Perhaps general relativity is just wrong.

Of course we say this with a few caveats; first of all general relativity has been tested and confirmed through countless experiments over the years.

In fact, it is continually tested every nanosecond of every day, as our communications and GPS satellites would not properly function if we did not take into account the corrections of general relativity.

So any modified version of general relativity would still have to provide the same solutions in the weak gravitational fields and small distances seen in the vicinity of Earth. However, there is room to work on large scales and in very weak or very strong gravitational wells.

A series of modified gravity theories have popped up over the years, but they were primarily based in Newtonian mechanics (where the effects of general and special relativity are considered negligible.. A cohesive theory that includes relativistic effects has been elusive. Those that have been proposed thus far are not very compelling at this time.

Where Do We Go From Here?

At this point in time we are still asking the question: what is dark energy? There is still the distinct possibility that we are missing something more fundamental, and we are instead seeing a flaw in our understanding instead of some mysterious force of nature. Though, if one thinks about it, those could be seen as essentially the same thing. 

Either way, we are still fumbling around in the dark, quite literally, trying to grasp what dark energy (and for that matter, dark matter) really is. It is going to take a lot more data and a lot more thinking to arrive at a solution. One solution will be for astronomers to continue surveying large areas of the sky to detect the distortion of the images of distant galaxies, measure the masses involved and perhaps arrive at a better understanding of mass distribution in the universe and how dark energy is involved.

 

Edited by Carolyn Collins Petersen.