An Unseen Force - The Mystery of Dark Energy

The galaxies of our universe are accelerating apart. But why?

The universe is expanding. Not only that, but its expansion seems to be speeding up. For that to happen, there must be something adding energy to the universe’s expansion. The problem is, we don’t see anything like that. We know there must be some unknown form of energy out there, but we have yet to understand what it is. We call this mysterious force dark energy.

Like dark matter, dark energy is a general name we have given to one of the greatest mysteries of the universe. It’s something that we can observe the effects of, but not something we can see or study directly. Not only is dark energy present, but it’s common. Across the entire universe, dark energy makes up 68% of all mass and energy.

The Mystery

We know dark energy exists thanks to our observations of the universe. In 1929, Edwin Hubble, namesake of the Hubble Telescope, proved that the universe was not static, but rather expanding, leading to our modern theory of the Big Bang. Initially, it was assumed that as the universe grew older, its expansion would slow. This is the logical assumption, as the gravitational attraction between galaxies would over time overcome the initial momentum imparted to them by the Big Bang.

However, in 1998 astronomers realized that supernovae at a certain predicted distance from us were dimmer than expected. Type Ia supernovae act as standard candles: because their intrinsic brightness is known, measuring how faint they appear reveals their true distance from Earth. By comparing a supernova's brightness to its redshift, which is how much its light has been stretched by the expansion of space, astronomers can map cosmic expansion over time. If gravity was slowing the universe down, distant supernovae should have appeared brighter than their redshifts implied. Instead, they were dimmer, meaning they were farther away than predicted and that the universe was accelerating. But acceleration requires a source of energy. That added energy is what we now call dark energy.

The expansion of the universe over time since the Big Bang. The curved edges show how the universe expands faster as time goes on.

Einstein's Biggest Blunder?

In 1917, Einstein had suggested introducing the cosmological constant to his field equations, which described the mechanical workings of the universe. At the time, Einstein, like most of the world before Hubble’s discovery, assumed the universe was static. It neither expanded nor contracted. The purpose of his cosmological constant was to explain why the static universe didn’t collapse in on itself due to gravity. The constant represented a force inherent to the fabric of spacetime itself with the effect of a constant negative pressure across the vacuum of space, which would work to push against the attractive force of gravity, balancing them out.

Einstein would eventually remove the cosmological constant from his field equations. He called its contrivance his greatest blunder, as he allowed himself to be misled by his preconceived notions, rather than trusting the math. What Einstein didn’t know at the time was that a cosmological constant would end up being needed after all in the form of dark energy.

What is Dark Energy?

Upon realizing the universe’s expansion was accelerating, scientists thought to reintroduce “Einstein’s greatest blunder.” What was once used to explain why a static universe didn’t collapse could now be used to explain why an expanding universe would accelerate. The idea is that the vacuum of space is not a true vacuum, and is filled with its own inherent energy. This energy would push outwards in all directions, stretching space and causing the accelerating expansion.

Another dark energy theory is called quintessence. As opposed to dark energy being an inherent trait of the vacuum of space, it could be an actual substance or field that’s invisible to us, yet permeates the universe. This invisible substance would have the exotic properties opposite to those of regular matter and energy, meaning its gravity would repel matter, rather than attract it. Where the cosmological constant would be, as expected, constant across the universe, quintessence could vary from one region or time to another.

It’s also possible that it’s our understanding of physics that’s wrong. While the Theory of General Relativity has held up all these years as our best way to explain the universe and how it works, there’s always a chance that Einstein missed a part of the picture and that the universe doesn’t require a contributing force to speed up its expansion. Perhaps that’s simply how the physics works, and we just don’t know the proper equations to describe how yet.

Dark energy is one of the greatest mysteries in modern astrophysics. Understanding it will help us to understand the ultimate fate of the universe. We are entering what some suggest will be a golden age of cosmology, as new telescopes and observatories, such as the Roman Space Telescope, will give us new insights into the universe, its structure, and its mechanics. Perhaps we’re not far off from the breakthrough that will help us to solve the mystery of dark energy once and for all.