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Abstract: The discovery of the accelerated expansion of the University is now more than a quarter century old, but the exact mechanism that drives this accelerated expansion has not been definitively established.  While a cosmological constant remains the “standard” theory, a variety of other models have been proposed, and recent observational results have rekindled interest in such ideas.   After reviewing the standard model containing cold dark matter and a cosmological constant, I will examine the most popular nonstandard models, such as quintessence.  Then I will go further afield to explore more speculative ideas such as holographic dark energy and cosmological bulk viscosity.