What is “Dark Energy”?
by Doug Watson
Cosmologists have known for over a century that galaxies all throughout the universe are moving away from us and each other. Space itself is getting bigger! We would expect this expansion to slow over time because gravity causes objects to attract one another; however, a little over twenty years ago we discovered that not only is the universe expanding but the rate of that expansion is speeding up. The expansion of the universe is accelerating!
Einstein showed us that even empty space has energy, so-called vacuum energy or “dark energy.” We say that this hypothetical energy is dark because we are unable to directly observe it; we can only measure its effects. Dark energy acts as a repulsive force, pushing on space. One would think that the density of the universe would decrease as the volume of the universe increases, but the density of dark energy remains persistent even as the universe expands. Weird! This leads to even more bizarre questions, like “What is space expanding into?” and “Will the accelerated expansion ever stop?”
The nature of dark energy still eludes scientists. It may be Einstein’s vacuum energy or it may be something entirely different. One thing is for sure: the quest to understand dark energy is one of the most important tasks of twenty-first century cosmology.