We have presented the most complete review to date of the many attempts to explain the origin of dark energy, the remarkable energy that many believe to be responsible for the observed acceleration of the Universe in the framework of Einstein’s theory of gravity, general relativity.

“We have presented the most complete review to date of the many attempts to explain the origin of dark energy, the remarkable energy that many believe to be responsible for the observed acceleration of the Universe in the framework of Einstein Theory of Gravity.”

Since this is a review article, it is primarily a synthesis of knowledge. It also describes a methodology used by many in the field, because it concentrates on approaches to model dynamical dark energy. We have also implemented some new results which were not published elsewhere.

This is a huge subject, with over 1,300 papers appearing on the Stanford Linear Accelerator Center (SLAC) database since 1998 with the words "dark energy" in the title. In this situation, it is generally difficult for people new to the field to judge where the main progress has been made. We have tried to make this review comprehensive, so that people interested in dark energy, whether from an astrophysical or particle physics standpoint can easily follow it. Our paper is the most detailed and latest theoretical review on the subject.

Since many dark energy models have been proposed so far, it is very important to classify them and also to understand the significance of these models, both theoretically and observationally. In our article, we have provided the most complete review of the approaches which have been adopted, in order to try and explain the nature of the current accelerating Universe. We have also discussed a number of interesting future directions that could lead to an understanding of the origin of dark energy.

The supernova observations first reported in 1998, coupled with detailed observations of anisotropies in the cosmic microwave background and the distribution of large scale structure have led us to believe that the universe is accelerating and is dominated by a novel form of energy called "dark energy." The simplest candidate for dark energy is the "cosmological constant," but, whereas it fits the data well, there is a real problem explaining its size from a particle physics point of view.

It has led a number of physicists (including ourselves) to investigate alternative models of dark energy, both within the context of particle physics or simply from a phenomenological point of view. These models allow the dark energy to be dynamical evolving to today's observed value. In spite of the tremendous efforts that have been made, we have not yet been able to definitively identify the origin of dark energy. In our opinion, understanding this poses the biggest challenge facing physics in the 21st century.

  Where do you see your research leading in the future?

In the next five years or so, we will have more precise observational data to constrain the dark energy even more, and possibly to determine whether it is dynamical or a simply cosmological constant. On the theoretical side, there will be a continued effort to explain the origin of dark energy from fundamental theory such as string theory.

None, other than the fact that it is a subject of real interest to the general public.