Updated: Sep 28, 2020
What is the best way to explain dark energy? We know it is somehow driving the universe’s accelerating expansion, but is it a mysterious form of energy? Is it a property of the fabric of spacetime itself? Is it an invisible force field? We come across these sorts of answers when we search for answers on the web. You can take my word when I say this, scientists are just as confused about dark energy as you are! When exploring the mysteries of the universe, the more we learn, the more we realize that much more is unknown than is known. The same applies when learning about the nature of dark energy.
Here, I attempt to explain what dark energy is in a formal fashion. From my experience, it’s a much better idea to focus on understanding one standard explanation instead of multiple possible explanations that currently exist, at least for the sake of not losing any sleep at night. After all, even Albert Einstein was puzzled, seeing the strange expanding behavior of the universe!
Einstein had touched upon several fields of physics, and I think it is safe to say that he considered himself quite successful with his accomplishments. His most groundbreaking work (which can vary depending on who you ask) was coming up with the general theory of relativity, which made everyone realize Newton’s law of gravity was only an approximate theory and not the theory of gravity. The equations (it’s plural because it’s a set of equations) describing the theory is complicated, but it can be explained in words in a straightforward way: Things cause spacetime to curve, and the curved spacetime, in turn, tells those things how to move in it (John Wheeler / Geons, Black Holes, and Quantum Foam). One side of the equations had the things, and the other side had the curvature of spacetime. Everyone was happy with the theory, except that there was one small problem with it (you will come to learn later that it wasn’t really a problem, but it was, aahhh, it’s complicated..).
It was widely believed at the time that our universe is static, meaning it is stationary and neither expanding nor contracting. Since the sky is full of stars and galaxies, wouldn’t their cumulative gravitational forces result in the universe to collapse? It was a big problem, indeed. Einstein came to the rescue and introduced a lambda term in his equations. He had placed the term on the left-hand side of the equation to fight against the forces of gravity caused by the planets, stars, galaxies, etc., in order to keep the universe stationary. Later, it was found by Edwin Hubble that the universe was never static; it is actually expanding! Einstein immediately removed his lambda term from his theory and called introducing it the biggest blunder of his life.
Of course, we now know the universe is expanding at an accelerated rate. The discovery was made fairly recently in 1998 by observing a bunch of supernovae. How do we explain this late cosmic acceleration? Einstein is no longer here to come up with a new solution. He did, however, find a way to come to the rescue once again because the solution that had gained the most popularity involved reviving Einstein’s lambda term back to his equations! It is currently present (again) in the equations, and we can even place it on the right-hand side of the equations with the other things. Instead of keeping the universe stationary, it is now fighting (and winning!) against the gravitational forces, resulting in the universe to expand at an accelerated rate.
Now, going back to the original question of what dark energy is. The lambda term that had to be added to Einstein’s equations to explain the accelerated expansion of the universe needed a (catchy) name, and somehow the coined phrase that we all settled on was dark energy!
You may or may not be satisfied with this explanation. Still, you can probably see how this is the origin of dark energy, and all the other possible explanations that you’ve seen are only trying to tell you what is the source of the lambda term. You can also probably see how the coined term “dark energy” is a misnomer since it has nothing to do with energy! It can be interpreted as another component of the universe (68.9%) in the same way ordinary matter and dark matter (31.1% combined) are. What makes dark energy special is that its density weirdly doesn’t decrease with the universe’s expansion.
The diagram above can be confusing at times. How can the density of dark energy not change at all as the universe is expanding? Here is a set of figures from Ethan Siegel’s Beyond the Galaxy that makes it much clearer!
What is the source of this lambda term in Einstein’s equations? That’s the million-dollar question! As of now, we only know the present value of this constant quite accurately, but we are unfortunately very far from determining its source. It could be due to the energy of empty space, which is a consequence of the infamous Heisenberg’s uncertainty principle, perhaps a new undiscovered particle like the Higgs boson (popularly known as the “God Particle”) that permeates all of spacetime is responsible, or it could even be that Einstein’s equations are only valid in smaller scales and they no longer apply when we go up to the humongous cosmic scales!
The puzzle here is to be able to identify the source of the lambda term in Einstein’s equations. No one would be able to tell you definitely what dark energy is made of, but you can certainly understand the history behind it and the current problems. Only after having a good grasp of the current issues will we be able to ask the right questions, which will lead to a better understanding.
If you’ve made it this far (I hope you’ve learned something new, but more importantly), now is the time to ask yourself: can you make a bit more sense of dark energy?
1. Einstein, A., 1917, Sitzungsber. Preuss. Akad. Wiss., p. 142 [English translation in The Principle of Relativity (Dover, New York, 1952), p. 177].
2. Cormac O’Raifeartaigh, Michael O’Keeffe, Werner Nahm, and Simon Mitton. Einstein’s 1917 static model of the universe: a centennial review. The European Physical Journal H, 42(3):431-474, Jul 2017.
3. Norbert Straumann. The history of the cosmological constant problem, 2002.
4. Adam G. Riess, Alexei V. Filippenko, Peter Challis, Alejandro Clocchiatti, Alan Diercks, Peter M. Garnavich, ... John Tonry. Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. , 116(3):1009–1038,
5. Y. Akrami et al. Planck 2018 results. I. Overview and the cosmological legacy of Planck. 7 2018.