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As we were reminded repeatedly at this meeting, the cosmological concordance model seems to require some form of dark energy in `empty' space, which should make the dominant contribution to the overall energy density of the Universe:

Equation 7 (7)

Attempts to measure its equation of state suggest that it is almost constant:

Equation 8 (8)

ruling out some attractive tracking quintessence models [96].

We heard at this meeting of some interesting new ideas. For example, the existence of large extra dimensions would offer new approaches [97], such as obtaining from self-tuning in five dimensions, or radion quintessence in six dimensions. Other ideas proposed here included a Chaplygin gas [98] and a repulsive force in massive QED: `spintessence' [99]. Certainly new ideas are desperately needed.

However, in my view, it makes no sense to discuss dark energy outside the framework of a complete quantum theory of gravity. Indeed, explaining the presence and magnitude of dark energy is surely the greatest challenge for a candidate quantum theory of gravity such as string theory. The Holy Grail of such a theory should be to calculate the amount of dark energy from first principles.

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