  Braneworld models of dark energy are examined in the light of observations of
high redshift type Ia supernovae. Braneworld models admit several novel and
even exotic possibilities which include: (i) The effective equation of state of
dark energy can be more negative than in LCDM: $w \leq -1$; (ii) A class of
braneworld models can encounter a `quiescent' future singularity at which the
energy density and the Hubble parameter remain well behaved, but higher
derivatives of the expansion factor ($\stackrel{..}{a}$, $\stackrel{...}{a}$
etc.) diverge when the future singularity is reached; (iii) The current
acceleration of the universe is a {\em transient feature} in a class of models
in which dark energy `disappears' after a certain time, giving rise to a matter
dominated universe in the future. Since horizons are absent in such a
space-time, a braneworld model with {\em transient acceleration} might help
reconcile current supernova-based observations of an accelerating universe with
the demands of string/M-theory. A maximum likelihood analysis reveals that
braneworld models satisfy the stringent demands imposed by high redshift
supernovae and a large region in parameter space agrees marginally better with
current observations than LCDM. For instance, models with $w < -1 (> -1)$
provide better agreement with data than LCDM for $\Omega_m \ggeq 0.3 (\lleq
0.25)$.

Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology;High Energy Physics - Theory