2:30 pm Monday, 31 March 2003
Observational evidence suggests that our universe is presently dominated by a dark energy component and undergoing accelerated expansion. I will first discuss the upper and lower bounds on the dark energy equation of state $w$ obtained from analysing a combination of all data available. Then I will describe a model motivated by string theory for Planck-scale physics, which may explain dark energy without appealing to any fine-tuning. The idea of the transplanckian dark energy (TDE) is based on the freeze-out mechanism of the ultralow frequency modes, $\omega(k)$, at very short distances, by the expansion of the background universe, $\omega(k) \leq H$. The upper and lower limits on $w$ make the dark energy puzzle worse. On the bright side, such challenges also press the need to search for new physics and a deeper understanding of it at the fundamental level.