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.
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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.