What Is Dark Energy, and Why Is It Important?

If you’re wondering what dark energy is, that’s understandable; scientists are still wondering that very same thing.

It’s probably the most mysterious idea in all of physics, and a good deal less understood than dark matter—a substance which is itself hidden in the shadows. Dark matter can at least be mapped and understood as physically distributed in the universe, much like ordinary matter. But dark energy has only recently been able to be investigated at all.

So, what is dark energy? Well, it’s been a few things over its lifetime.

How We Got Here

After Einstein, physicists were fairly certain about how the universe was evolving: the Big Bang threw all the matter outward, as an explosion does, after which gravity naturally attracted that matter to itself and slowed the outward expansion.

This was, itself, a big admission. Einstein initially proposed a static universe, but later accepted Edwin Hubble’s evidence that the universe was expanding and called the static-universe hypothesis his “greatest blunder.”

Einstein introduced the “cosmological constant,” denoted by the Greek letter lambda (λ), which was introduced to support a static universe. He set this idea aside, along with his error, and moved on. So did the rest of physics.

Faced with the fact of an expanding universe, it remained unclear whether that expansion would ever fully stop, or perhaps even reverse, causing the universe to contract back in on itself. This idea was referred to as being an eventual Big Crunch. The Big Crunch hypothesis found a lot of backers for the same reason as the static universe hypothesis before it: it felt good.

The idea of Big Crunch allowed the further idea that this compression of all matter and energy would cause a new ultra-singularity and, perhaps, a new Big Bang, creating the nice and neat idea of a cyclical universe that was infinitely expanding, contracting, and expanding again. It was the only thing everybody could be sure of: Universal expansion was definitely slowing down over time.

Except it isn’t. Studies looking at Type Ia supernovae, so-called “standard candles” in astronomy, provided the first-ever evidence that the expansion of the universe is not decelerating, but actually accelerating—moving outward more and more rapidly over time. Arriving in 1998, this was one of the biggest scientific announcements of the century.

Later, evidence from things like maps of the Cosmic Microwave Background (CMB) radiation showed that this was correct: In defiance of the gravitational influence of matter, the universe is indeed expanding faster every year.

However, another problem was introduced by the CMB maps and other cutting-edge observations: According to the best evidence at the time, there was simply too much matter and energy in the universe. Physics had already concocted a value called the “critical density,” which was a prediction of the total amount of matter and energy in the universe.

The actual observable amount didn’t remotely equal this figure, however. Even with dark matter taken into account, the universe still only seems to have about a third as much stuff in it as expected.

Physicists realized that these two observations could potentially have the same explanation. There could be a whole lot more energy in the universe than we had previously realized, and that energy could be the thing driving the unexpected, accelerating expansion we observe in the universe.

So, What Is Dark Energy?

Crucially, one of the other names you might run across for dark energy is “the energy of space.” This refers to the modern working definition of dark energy, which considers it an intrinsic property of space.

In other words, if you have some dark energy in some space, and then you expand that space by some amount, you don’t dilute the dark energy you had because the new space you just created will have dark energy all its own. This dark energy would seem to exert an outward force, and thus space itself has an intrinsic tendency to expand.

Credit: NASA

So, as the universe expands, this energy can keep driving that expansion, since it doesn’t get diluted. If it did, as regular matter and energy would, its influence would be smeared out to the point that gravity would eventually dominate, and expansion would slow or stop.

This doesn’t have to break the idea of conservation of energy, since dark energy is an attribute of space, not really a quantity within it; one of the issues may be the name “dark energy,” which is a holdover from before this idea existed. In general, dark energy is represented by the same symbol as Einstein’s old cosmological constant, λ, and that’s probably a more intuitive way to refer to it.

The big problem with this energy-of-space idea is that we can calculate what the value of λ should be—that is, the strength of the hypothetical repulsive force of dark energy—and that calculation leads to figures much higher than we actually observe. If the cosmological constant were what it is supposed to be, the universe would be flying apart at a much faster rate than we actually observe, unless some other unknown force is counteracting it.

The Cosmic Microwave Background (CMB) radiation map is one of the most important images ever created, in particular to the study of dark energy.
Credit: NASA

This leads to the so-called “Hubble tension” between the observed expansion and the expansion predicted from the early universe.

Still, evidence for the idea of dark energy as a universal constant is mounting. The Dark Energy Survey (DES) observed light from 26 million galaxies to determine how the universe has evolved over the past 7 billion years. There’s also the Dark Energy Spectroscopic Instrument (DESI), which was able to measure the “redshift” of galaxies to determine how quickly they were moving away from the Earth, and directly measure universal expansion.

This work found that λ appears to be reasonably accurate. That doesn’t tell us anything about the nature of dark matter, but it does tell us that we can at least fairly accurately quantify its impact. Some aspects confound theory, such as the finding that dark energy may have evolved over time and may continue to evolve; that would undermine the idea that it is a “constant” and remove the necessity that the universe will continue to expand into eternity.

Other surveys, such as Planck, eBOSS, and Pantheon+, largely converge on the conclusion that dark energy behaves very much like Einstein’s cosmological constant. The new Nancy Grace Roman space telescope is about to start contributing incredibly high-resolution data, as well.

The language used for new observations is that they “constrain” the possibilities for dark energy’s attributes, narrowing the range of possible values as observations falsify all but a progressively smaller set of possibilities. In the past decade, scientists have constrained dark energy enough to take it from a vague placeholder idea to a real theory with increasingly concrete tendencies.

We’ll keep you up to date on the latest in the years to come.