46 An introduction to the physics of cosmology
Figure 2.7.This figure, taken from Zlatevet al(1999), shows the evolution of the density
in the ‘quintessence’ field (top panel), together with the effective equation of state of the
quintessence vacuum (bottom panel), for the case of the inverse exponential potential. This
allows vacuum energy to lurk at a few per cent of the total throughout the radiation era, but
switching on a cosmological constant after the universe becomes matter dominated.
in the inverse exponential potential is shown in figure 2.7, demonstrating that the
required behaviour can be found. However, a slight fine-tuning is still required, in
that the trick only works forM∼1 meV, so there has to be an energy coincidence
with the energy scale of matter–radiation equality.
So, the idea of tracker fields does not remove completely the puzzle
concerning the level of the present-day vacuum energy. In a sense, relegating the