The expansion of the universe is accelerating, and scientists have no idea why. Gravity should be slowing this expansion over time, but billions of years ago it shifted from slowing down to speeding up.
The cause is attributed to an unknown form of energy, and this energy makes up for nearly three fourths of the entire universe’s mass. Its name in scientific circles is dark energy, yet that is simply a placeholder until the truth is discovered. At Brookhaven National Lab, cosmologists study dark energy, as well as its companion dark matter, and conduct experiments in the hope of breaking new ground on the origins and current nature of our expanding universe.
By the end of the decade, these scientists hope that a new tool, the Large Synoptic Survey Telescope, will allow them to plunge even deeper into the depths of the sky. The telescope will allow them to collect enormous amounts of data with the hope that in these troves of information lays the key to understanding what exactly dark energy and dark matter are.
“All we know about them is empirical. We don’t really have any theoretical understanding of it,” says Erin Sheldon, a Brookhaven astrophysicist and cosmologist who works with the Dark Energy Survey, an internationally collaborative study that will begin collecting and analyzing data related to the mystery this fall. They will be using a four-meter mirror telescope with the ability to survey an expansive amount of space.
Dark energy and dark matter are two terms now commonly thrown around in academic circles, and even used by amateur physics and astronomy buffs, because of the alluring mystery they provide and the large amount of research being invested in the field of cosmology, or the study of the creation of the universe. But to the untrained mind, the two terms are easy to mix up. 
Dark energy makes up for between 72 and 74 percent of the universe’s mass-energy density, according to a number of reports from NASA and other organizations that are funded through the National Science Foundation. Its current and generalized definition is the unknown cause for the acceleration of the expansion of the universe.
Dark matter, on the other hand, constitutes for between 21 and 23 percent of the universe’s mass-energy density and is known as an invisible form of matter that is causing a noticeable discrepancy between what is actually present in faraway objects, like galaxies in distant clusters, and what we’re seeing using our current methods. “The problem is that dark matter doesn’t emit light, so we can only see its effect through gravity,” explains Sheldon.
Dark energy is by a wide margin the more complicated of the two. “Frankly, its vague to everybody, even us. There’s lots of other kinds of theories, but none of them are even appealing,” says Sheldon, explaining that the universe is thought to have expanded after the Big Bang, and then pulled inward due to gravity.
“But instead of slowing down, it looks like the universe started to speed up a few billion years ago,” he says. “This is a shock, and no one really has an explanation for it.”
Unlike dark matter, which was discovered in an elementary form in 1934, dark energy arose from the very recent discovery in 1998 of the expansion of the universe. The study of Type 1a (one- A) supernovae by the High-z Supernova Search Team posited this shocking rev- elation, which was then confirmed by Supernova Cosmology Project in 1999 and then numerous other studies that used various techniques in the years that followed. The core of the discovery by the High-z team lies in the fact that the light emitted by supernovas was red shifted, which means that those celestial objects are moving away from us if you analyze a spectrograph that translates light into wavelengths, but at an accelerating rate.
“We know there is something that accelerates the universe. We have the simplest theory, and you put in by hand and it explains the data,” says Anže Slosar, a cosmologist and astrophysicist who works alongside Sheldon at BNL, but in a separate project titled BOSS, or the Baryon Oscillation Spectroscopic Survey.
Slosar is referring to the fact that dark energy is explainable, and only barely so, through the use of a sloppy mathematical constant thought up by Einstein decades ago. It is a term that, once inserted, helps coincide gravity with the obvious discrepancies in the mass-energy density of the universe that comes from its unexplainable acceleration and our lack of knowledge.
“In the late 1990s, people worked out that we need to put in the term in order to make everything work,” says Slosar. “You take your Einstein equation, and it turns out you can put the term in there and you can describe everything.”
“It works mathematically, but it’s not nice. We are hoping the real theory works more beautifully,” he adds. So basically, the scientists can make everything make sense on paper, but very much in the way a lazy physics student could ace a lab by working backward from the right answers and tweaking all the math. The scientists know what’s happening with dark energy, but not why or even where to look to find out.
The LSST telescope is projected to begin scanning the skies in 2019 after serious delays throughout the latter half of the last decade. Sheldon and Slosar will be some of the first scientists to analyze the data through their affiliation with BNL.
“Maybe with data, this breakthrough will happen,” says Slosar. But he also entertains the idea that this is an unreachable goal, that unification, a theory of everything and dark energy are just fleeting utopias in a scientist’s dreams. “It’s also possible that we will never reach this,” he says. “Then we are sort of screwed. If you don’t have more than one clue, then you can’t distinguish between the various ideas.”
“The idea is to get more data. Get more detail about the universe to see how fast it was moving over time and see how it started to speed it up,” says Sheldon.
“From our point of view, since we are experimenters, we’re just going to go and look and measure the best we can and shed some light on it, get some kind of clue.”
