Figure 1

The $w\mathrm{\text{−}}{w}^{\prime }$ phase space occupied by thawing and freezing fields is indicated by the shaded regions. No strong constraints on this range of dark energy properties exist at present. The fading at the top of the freezing region indicates the approximate nature of this boundary. Freezing models start above this line, but pass below it by a redshift $z\sim 1$. The short-dashed line shows the boundary between field evolution accelerating and decelerating down the potential. Future cosmological observations will aim to discriminate between these two fundamental scenarios.Reuse & Permissions

Figure 2

The evolutionary tracks in $w\mathrm{\text{−}}{w}^{\prime }$ phase space are shown for a variety of particle-physics models of scalar fields. The two broad classes are clear: those that initially are frozen and look like a cosmological constant, starting at $w=-1$, $w^{\prime }=0$, and then thaw and roll to $w^{\prime }>0$, and those that initially roll and then slow to a creep as they come to dominate the Universe. The sample of thawing models shown have potentials $V\propto {\varphi }^n$ for $n=1,2,4$ (short-, dot-, and long-dashed curves) and a PNGB with $V\propto {cos}^2(\varphi /2f)$ (solid curves). The rightmost point of the tracks corresponds to the present. For variety, the $n=4$ model has ${\Omega }_{\mathrm{de}}=0.6$, and the $n=1$ model ending at $w=-0.8$ has ${\Omega }_{\mathrm{de}}=0.65$. All other models end with a fractional energy density ${\Omega }_{\mathrm{de}}=0.7$. The sample of freezing models shown have potentials $V\propto {\varphi }^{-n},{\varphi }^{-n}{\mathrm{e}}^{\alpha {\varphi }^2}$ (solid and dashed curves). The line $w^{\prime }=1.5w(1+w)$ indicated by the light, dotted line is a possible lower bound on the freezing models. The leftmost point of the tracks corresponds to the present; the rightmost point is at $z=1$. For variety, upper and lower close pairs of curves have ${\Omega }_{\mathrm{de}}=0.7,0.8$, respectively. All other models end with a fractional energy density ${\Omega }_{\mathrm{de}}=0.7$.Reuse & Permissions