What is Dark Energy

A search for understanding dark energy is an exploration into the fundamental nature of the universe. What is it all made of and where is it all going? Humans are able to build amazing machines in attempts to find answers. However, thus far, there are still more questions than answers and more theories than proven knowledge. This article examines the current state of the investigations into the mysteries of dark energy.

Throughout history, people have consistently mistaken rapid advances in science and technology as progress towards completely understanding the universe. At the end of the 19th century, physicists the world over declared that they possessed all the important knowledge, with Newton’s laws reigning supreme. Einstein and other experts in quantum mechanics had a bit to say about that and revolutionized this arrogant thinking. Topics such as dark energy bring everyone back to the true reality; there is so much that we have not even begun to understand. The mysteries of science will continue to riddle generation after generation into the foreseeable future.

Dark energy is really a placeholder term for energy that scientists believe should exist but cannot fully explain. Like most interesting aspects of physics, the problem arises due to a conflict between what theory says the universe should do and what observational data actually says the universe is doing. One hundred years ago, Einstein’s theory of general relativity and gravity toppled the near godlike Newton because discrepancies were observed in Newton’s predictions of Mercury’s position, due to small precessions of its elliptical orbit caused by its proximity to the Sun. Today, dark energy may rub salt in the wounds of the infamous Einstein’s self-proclaimed biggest mistake, his inclusion of a cosmological constant in a gravitational equation to force a static universe. Everyone at the time believed that the universe was neither contracting nor expanding, and Einstein was not prepared to argue that point. The problem, of course, is that the universe is, in fact, expanding.

The cosmological constant has stormed back on the scene in the last decade due to the bewilderment of astrophysicists at data showing that the expansion of the universe is not slowing, as many have believed for more than three decades, but is accelerating. The further away stars and planets get, the faster they are spreading apart. What is causing this repulsion? For now, physicists believe that some mysterious force, termed dark energy, must be at work steadily pushing the universe outward ever more rapidly. As gravitational forces decrease, from matter spreading further apart, the repelling force of dark energy becomes dominant and apparent. Scientists think this energy may be a property of space itself. Data from the NASA Wilkinson Microwave Anisotropy Probe and Hubble Space Telescope suggest that around 72% of the universe is this dark energy, 23% is dark matter, and only 5% is observable matter. The inability to observe or take direct measurements makes it very difficult to assess, understand, and experiment on theories related to dark energy. Yet, it is fascinating that the unknown could make up such a large percentage of the universe.

Another problem is that everyone, including Einstein, decided that the cosmological constant was wrong due to his poor reasoning when working it into his equations. Now, it seems to be helpful in predicting how energy in empty space could cause faster and faster expansion of the universe. It will take some time for someone to work out a testable theory for why this is happening. As of late, scientists are trying to tease out whether dark energy is a property of empty space, an energy field or fluid that behaves differently than matter, or just an indicator that Einstein’s theory of gravity is wrong at very large distances. 

The Joint Dark Energy Mission, based out of the NASA Goddard Space Flight Center and in partnership with the Office of High Energy Physics at the U.S. Department of Energy, aims to gather more precise measurements of the universe. Hopefully, this mission will help shed light on the cloudy subject of dark energy and the expansion rate and contents of the universe.