Dark Energy: Mysterious and Fascinating

Is Dark Energy Real?

expansion of the universe
The accelerating, expanding universe, showing the influence of accelerated expansion in the most recent epochs of cosmic history. NASA/WMAP

As we seek to understand more about the universe around us we study the mass of material in it. That includes all the stars, planets, and nebulae in our own galaxy, plus the same objects in countless other galaxies. Yet, it seems that there's more to the universe than we can detect. There are also other aspects to the universe that challenge our understanding. Dark energy is one such aspect. It's related to the expansion of the universe, an action that has proceeded since the Big Bang some 13.7 billion years ago.

 

An Expanding Universe: How and Why?

The universe is filled with mass.  Masses attract other masses due to the force of gravity. In a universe filled with mass in the form of galaxies, galaxy clusters and superclusters (and their associated stars, planets, and gas and dust clouds), scientists first assumed that that the expansion of the universe was slowing down due to the action of gravity. It's not an unreasonable assumption to make. 

Why worry about the expansion rate of the cosmos? It turns out there's a lot at stake. There's nothing less than the fate of the universe at stake in understanding how and why it expands. And, whether or not it has expanded at the same rate in the past and will do so in the future.

The universe could slow down more and more, never quite coming to a complete stop. Or perhaps it would, like a spring, reach some maximum size and then begin to collapse back in on itself.

All that would depend on how much mass there is in the cosmos and the gravitational attraction it has. So, understanding the expansion rate also means we need to know how much mass there is "out there".

A Twist in the Expansion Rate Tale

At the end of the 20th century, work by researchers studying galaxies and, separately, by supernova specialists using the Hubble Space Telescope, found that the expansion of the universe is not slowing down, but is actually speeding up.

Nobody is quite sure why the universe's expansion rate is increasing. However, to even understand what's happening, physicists have been reworking their calculations by adding a cosmological constant to Einstein's field equations of general relativity. That could account for whatever physical process is speeding things up. The term dark energy was applied to whatever it is that's causing this speeding-up That raises a big question: is dark energy real, or are we simply getting something wrong in our current framework we use to explain the dynamics of the universe?

Evidence That Dark Energy is Real

There is some indirect evidence that dark energy is real. First of all, surveying galaxies throughout the cosmos reveals some irregularities in the way they're distributed throughout space. You would not expect to see a somewhat "lumpy" assortment of galaxies if the expansion of the universe was constant or even slowing down.

The Nobel Prize winning discovery of dark energy came in the form of a Hubble Space Telescope measurement of distant supernovae. These stellar explosions were found in galaxies that were receding more slowly than they should have been. This indicates that the universe was once expanding more slowly than it is today.

More accurate measurements taken since that 1998 discovery have only confirmed this theory. But we still don't know what the driving force behind the acceleration, only that direct measurement and characterization of it remains elusive; hence the moniker Dark Energy.

Could it Still Be Something Else?

So the evidence is pretty convincing the the expansion of the universe is accelerating. That really only leaves two options: either there is some still-unknown force or mechanism that is driving this acceleration, or perhaps our understanding of the forces of gravity is wrong.

There have been attempts to modify gravity before, but they have, almost universally, focused on non-relativistic forms of gravity theory. A cohesive reformulation of Einstein's relativity theories that could account for the observed acceleration, without the need of Dark Energy, remains elusive.

But it is certainly possible.

In the end it seems a pretty safe bet that something is missing from out current understanding of the universe, and neither option is very satisfying. Either we have completely missed some major driving force in the universe.  Or, one of the foundational theories of physics - one that gets tested and confirmed every nanosecond of every day - is somehow incomplete or wrong. Either way, there's still a missing piece (or more than one!) from the puzzle that shows how and why our universe is expanding. Stay tuned! 

Edited and updated by Carolyn Collins Petersen.