5. Tracker fields and k-essence
The energy density of the matter (blue), radiation (red) and tracker fields (green) all change with time or, equivalently, redshift. If the energy is plotted on a logarithmic axis, the energy density of the radiation falls as a straight line as the universe expands. The energy density of matter also falls, at a slower rate. In the early universe the energy density of a tracker quintessence field can start from a wide range of initial conditions, which all join a common path before the matter density overtakes the radiation density at the onset of the matter era. In the case of k-essence, matter domination triggers a change in the behaviour of the tracker field that causes its energy to decrease and then freeze at a value that remains nearly constant with time. Ultimately, k-essence overtakes the matter density and initiates the cosmic acceleration.