Theoretical models of dark energy

Observations of high redshift type Ia supernovae indicate that the universe is accelerating, fueled by an unknown form of ‘dark energy’ having large negative pressure p<0. The simplest example of dark energy is the cosmological constant (p=−ρ≡−Λ/8πG). The cosmological constant arises at a fundamental level from one-loop quantum effects which generate a Λ-term many orders of magnitude larger than the observed value of dark energy 10−47 GeV4. This leads to the ‘cosmological constant problem’. Dynamical models of dark energy include scalar fields with exponential and power law potentials. Dark energy can also be generated in extra-dimensional ‘braneworld’ models. Model-independent methods which attempt to reconstruct dark energy from supernova observations are discussed.