Theoretical Models of Dark Energy
  Mounting observational data confirm that about 73% of the energy density
consists of dark energy which is responsible for the current accelerated
expansion of the Universe. We present observational evidences and dark energy
projects. We then review various theoretical ideas that have been proposed to
explain the origin of dark energy; they contain the cosmological constant,
modified matter models, modified gravity models and the inhomogeneous model.
The cosmological constant suffers from two major problems: one regarding
fine-tuning and the other regarding coincidence. To solve them there arose
modified matter models such as quintessence, k-essence, coupled dark energy,
and unified dark energy. We compare those models by presenting attractive
aspects, new rising problems and possible solutions. Furthermore we review
modified gravity models that lead to late-time accelerated expansion without
invoking a new form of dark energy; they contain f(R) gravity and the
Dvali-Gabadadze-Porrati model. We also discuss observational constraints on
those models and on future modified gravity theories. Finally we review the
inhomogeneous Lemaitre-Tolman-Bondi model that drops an assumption of the
spatial homogeneity of the Universe. We also present basics of cosmology and
scalar field theory, which are useful especially for students and novices to
understand dark energy models.
