  We use the joint measurement of geometry and growth from anisotropic galaxy
clustering in the Baryon Oscillation Spectroscopic Survey Data Release 9 CMASS
sample reported by Reid et al. to constrain dark energy properties and possible
deviations from the General Relativity. Assuming GR and taking a prior on the
linear matter power spectrum at high redshift from the cosmic microwave
background (CMB), anisotropic clustering of the CMASS DR9 galaxies alone
constrains $\Omega_{\rm m} = 0.308 \pm 0.022$ and $100\Omega_{\rm k} = 5.9 \pm
4.8$ for $w = -1$, or $w = -0.91 \pm 0.12$ for $\Omega_k = 0$. When combined
with the full CMB likelihood, the addition of the anisotropic clustering
measurements to the spherically-averaged BAO location increases the
constraining power on dark energy by a factor of 4 in a flat CDM cosmology with
constant dark energy equation of state $w$ (giving $w = -0.87 \pm 0.05$). This
impressive gain depends on our measurement of both the growth of structure and
Alcock-Paczynski effect, and is not realised when marginalising over the
amplitude of redshift space distortions. Combining with both the CMB and
Supernovae Type Ia (SNeIa), we find $\Omega_{\rm m} = 0.281 \pm 0.014$ and
$1000\Omega_{\rm k}=-9.2\pm5.0$ for $w = -1$, or $w_0 = -1.13 \pm 0.12$ and
$w_{\rm a}=0.65 \pm 0.36$ assuming $\Omega_k = 0$. Finally, when a $\Lambda$CDM
background expansion is assumed, the combination of our estimate of the growth
rate with previous growth measurements provides tight constraints on the
parameters describing possible deviations from GR giving $\gamma = 0.64 \pm
0.05$. For one parameter extensions of the flat $\Lambda$CDM model, we find a
$\sim 2\sigma$ preference either for $w > -1$ or slower growth than in GR.
However, the data is fully consistent with the concordance model, and the
evidence for these additional parameters is weaker than $2\sigma$.
