10 constrained by consideration of Big-Bang Nucleosynthesis and possibly by precision data relating to the equivalence principle. V. CONCLUSIONS ACKNOWLEDGEMENTS We have proposed a theory underlying the accelerated expansion of the universe based on entropy and entropic force. This approach, while admittedly heuristic, provides a physical understanding of the acceleration phenomenon which was lacking in the description as dark energy. The evidence and general arguments supporting our hypothesis were presented in § III. In addition we considered an interesting phenomenological model, loosely based on surface terms in § IV, and showed the models are capable of providing a good fit to the supernova data. Following the above arguments to their logical conclusion, the accelerated expansion rate is the inevitable consequence of the entropy associated with the holographic information storage on a surface screen placed at the horizon of the universe. An interesting question [14] is: how does this entropic viewpoint of cosmic acceleration impact on inflationary theory? We each thank our colleagues for their contagious energy and enthusiasm and IPMU for providing the venue that encouraged this work. This work was supported by the World Premier International Research Center Initiative (WPI initiative), MEXT, Japan. The work of D.A.E is supported in part by a Grant-in-Aid for Scientific Research (21740167) from the Japan Society for Promotion of Science (JSPS), by funds from the Arizona State University Foundation and by the National Science Foundation (KITP, UCSB) under Grant No. PHY05-51164. The work of P.H.F. was supported in part by U.S. Department of Energy Grant No. DE-FG02-05ER41418. G.F.S. work supported in part by by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, by WCU program of