How Things Work: Dark Energy

No, “dark energy” is not a reference to Star Trek or any other syndicated sci-fi series. Dark energy is not the same as your excuse for your completely immoral behavior, and it is certainly not the reason your dog ran away. With the elimination of these possibilities, we now have a similar understanding to that of the scientific community as to what dark energy is. The concept is so new that in July 2003 former CMU professor Robert Nichol appeared on The Late Show with David Letterman to discuss the research that he and the Sloan Digital Sky Survey have done in regard to dark energy.

Dark energy is a theoretical form of energy that is currently thought to make up about 70 percent of the universe, and is causing the universe to expand at a more accelerated rate than previously expected. This accelerated rate is the result of a strong negative pressure which is able to overcome the forces of gravity.

Take for example a balloon. Put two dots on the balloon representing two arbitrary points in the universe. As you blow air into the balloon, the dots begin to move apart as the skin of the balloon expands. This is the same manner in which universal expansion works. Bodies in the universe are accelerating away from each other due to the expansion of the fabric of the universe itself.

However, a new dynamic was added in 1998 when two groups of astronomers studying type 1A supernovae (exploding stars) noticed that these supernovae were dimmer than expected. This discovery revealed that the supernovae were farther away than they should have been in the absence of dark energy.

Now take your balloon from before, deflate it, and add some dark energy. As you blow into it, the density of dark energy becomes smaller and smaller. In turn it exerts an increasingly large amount of negative pressure, causing the balloon to expand at an even higher rate. I know what you are thinking — where was dark energy when I was spending all that time blowing up my air mattress? True, in theory, the negative pressure would have helped, but the fact is it only works in large doses, too big for the purposes of this Earth.

Further evidence for its existence is the so-called “missing mass” problem. By observing the cosmic microwave background radiation, evidence for the Big Bang, astronomers are able to account for only 30 percent of the matter in the universe. Dark energy would be this missing 70 percent.

There are many theories as to what dark energy is, but currently the two most compelling are the “cosmological constant” and “quintessence” theories.

According to the cosmological constant theory, space has its own energy. The cosmological constant, Lambda, represents the energy density of empty space. The constant is defined in terms of the energy present in a vacuum. Because the cosmological constant has negative pressure, according to general relativity a positive cosmological constant would allow for empty space to have positive energy, therefore causing the expansion of empty space to accelerate. This would account for universal expansion at a faster rate than expected.

One problem with this theory is that its predicted energy from the cosmological constant is as much as 120 magnitudes too large, resulting in what is known as the cosmological constant problem.

“Quintessence” is a theoretical, undiscovered substance that makes up the vast majority of the energy in the universe. Quintessence has a density and equation of state that varies through time and space. Think of it as an oscillation similar to that of a pendulum. The cosmological constant theory, on the other hand, has a fixed energy density with no varying terms. The fact that quintessence is dynamic makes it difficult to observe. Currently there is no evidence supporting it.

No matter which theory is true, if dark energy continues to behave as it is, it will eventually become the most dominating force in the universe, resulting in the “Big Rip.” Once it dominates, it will tear apart all gravitationally bound structures, such as solar systems and galaxies. The final deed will be when dark energy overcomes nuclear and electrical forces, causing atoms to be torn apart.

So what have we have learned today? We do not know what dark energy is, but eventually it is going to tear our universe apart! So, there’s one theory as to how the end of the world will come about.