Before we get caught up in trying to label dark energy, it might be best to remind ourselves that dark energy, like dark matter, exists only as a theoretical solution to a set of observations. For example, while dark matter, according to one theory, may be an artifact of our incomplete understanding of gravity, dark energy might point to an incomplete understanding of space as well, as we will see. In either case, the label might be updated once the underlying mechanism is figured out. The following 4-minute video by the American Museum of Natural History offers us a good introduction to what dark energy is.
Here is another 4-minute video that introduces us to dark energy. Created by SciShow, it focuses on the concepts and theories underlying this phenomenon (it's good but I recommend some pre-caffeination to keep up with your host).
The universe has been expanding since its birth, but the rate of its expansion has changed over billions of years. Here is the NASA diagram once again.
|Credit: NASA: Ann Field (STScl)|
Following inflation, the expansion rate of the universe decelerated as the influence of gravity, attracting both matter and dark matter together, became more prominent while the influence of inflation decreased. Then, starting about 5 billion years ago, the expansion rate began to accelerate once again and it continues to the present. The density of both dark and ordinary matter is decreasing as the density of dark energy remains nearly or exactly the same. This means that the influence of dark energy is increasing. Its action is the increasing influence of outward pressure on the universe. What is this mysterious outward pressure?
Before we tackle this central question, let's examine the evidence for dark energy, in the next article, Dark Energy Part 8.