  Dark energy is a premier mystery of physics, both theoretical and
experimental. As we look to develop plans for high energy physics over the next
decade, within a two decade view, we consider benchmarks for revealing the
nature of dark energy. We conclude, based on fundamental physical principles
detailed below, that understanding will come from experiments reaching key
benchmarks:
  $\bullet\ \sigma(w_a)<2.5\sigma(w_0)$
  $\bullet\ \sigma(w_0)<0.02$
  $ \bullet\ \sigma(\rho_{\rm de}/\rho_{\rm crit})<(1/3)\rho_\Lambda/\rho_{\rm
crit}$ for all redshifts $z<5$
  where the dark energy equation of state $w(a)=w_0+w_a(1-a)$. Beyond the
cosmic expansion history we also discuss benchmarks for the cosmic growth
history appropriate for testing classes of gravity theories. All benchmarks can
be achieved by a robust Stage 5 program, using extensions of existing probes
plus the highly complementary, novel probe of cosmic redshift drift.
