= Quintessence
{disambiguate=physics}
{wiki}
= Quintessence
{synonym}
<Quintessence> models <dark energy> through an evolving <scalar field>. For a homogeneous canonical field in <natural units>,
$$
\rho_\phi=\tfrac12\dot\phi^2+V(\phi),\qquad
p_\phi=\tfrac12\dot\phi^2-V(\phi).
$$
A positive potential dominating the kinetic term gives negative <pressure>. The <Friedmann acceleration equation> requires $w=p_\phi/\rho_\phi<-1/3$ for scalar-dominated accelerated expansion. A nonnegative canonical kinetic term prevents $w<-1$ when $\rho_\phi>0$.
Unlike a strict <cosmological constant>, the field can evolve, so $w$ need not remain $-1$. This is a model class, not an identified substance.<Cosmology by David Tong/1 The Expanding Universe>[§1.5.3, “Dark Energy or Cosmological Constant?”: evolving scalar models and the equation-of-state test]{ref}
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