  The energy conditions of Einstein gravity (classical general relativity) are
designed to extract as much information as possible from classical general
relativity without enforcing a particular equation of state for the
stress-energy. This systematic avoidance of the need to specify a particular
equation of state is particularly useful in a cosmological setting --- since
the equation of state for the cosmological fluid in a
Friedmann-Robertson-Walker type universe is extremely uncertain. I shall show
that the energy conditions provide simple and robust bounds on the behaviour of
both the density and look-back time as a function of red-shift. I shall show
that current observations suggest that the so-called strong energy condition
(SEC) is violated sometime between the epoch of galaxy formation and the
present. This implies that no possible combination of ``normal'' matter is
capable of fitting the observational data.
