Dark Energy Part 7

Dark Energy - Gone Yesterday, Here Today

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)

The first expansion acceleration took place right after the birth of the universe, and it is covered by the theory of cosmic inflation. This inflationary period is represented by the outward curve in the yellow region at the bottom of the "goblet" above. Cosmic inflation theory is still very much a work in progress. Even so, this theory also has many strengths. It accounts for the homogeneity and isotropy of the universe, the universe's flatness (we'll be exploring this in a moment) and its lack of magnetic monopoles. It also agrees very well with the latest WMAP data. However, the mechanism responsible for it is not yet well understood. It may be the inflation of the Higgs field or the inflation of a grand unified field. Some theorists hope that supersymmetry or string theory will be able to explain inflation as part of a unified theory of everything.

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.