Supernova Constraints on Braneworld Dark Energy
  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 $\Omega_m \ggeq 0.3 (\lleq 0.25)$. 
Ujjaini Alam and Varun Sahni