Models with accelerated expansion are now an important subject in cosmology. They have been important for a long time in connection with the early universe, where inflation plays a key role. More recently they have acquired a new significance in view of accumulating observational evidence that the expansion of the universe is accelerating at the present epoch. For these reasons it is particularly interesting to consider solutions of the Einstein equations which are appropriate for modelling cosmic acceleration. The last four Sections 7.3 to 7.6 are devoted to various aspects of this topic. An introductory account which describes some of the physical background can be found in .
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