Observations have led us to a picture of the universe which differs dramatically from what we might have expected. In this section we discuss possible ways to come to terms with this situation; the approaches we consider include both attempts to explain a small but nonzero vacuum energy, and more dramatic ideas which move beyond a simple cosmological constant. We certainly are not close to settling on a favored explanation either for the low value of the vacuum energy nor the recent onset of universal acceleration, but we can try to categorize the different types of conceivable scenarios.
Figure 2. A flowchart classifying reasons why the universe might be accelerating. The possibilities include: 1. Misinterpretation of the data; 2. Breakdown of general relativity; 3. Dynamical dark energy; 4. Unique vacuum energy; 5. Environmental selection.
The flowchart portrayed in Figure 2 represents a classification of scenarios to explain our observations. Depending on the answers to various questions, we have the following possibilities to explain why the universe appears to be accelerating:
Let's examine each possibility in turn.