  From an observational perspective cosmology is today in excellent shape -
advances in instrumentation and data processing have enabled us to study the
universe in detail back to when the first galaxies formed, map the fluctuations
in the cosmic microwave background which provide a measure of the overall
geometry, and reconstruct the thermal history reliably back to at least the
primordial nucleosynthesis era. However recent deep studies of the Hubble
expansion rate have suggested that the universe is accelerating, driven by some
form of `dark' (vacuum) energy. If true, this implies a new energy scale in
Nature of order 0.001 eV, well below any known scale of fundamental physics.
This has refocussed attention on the notorious cosmological constant problem at
the interface of general relativity and quantum field theory. It is possible
that the resolution of this situation will require fundamental modifications to
our ideas about gravity.
