The nature of dark energy: the big challenge in the physics new era

In 1998 two groups of astronomers using supernovae survey in the distant universe made an astonishing discovery, the supernovae were dimmer than cosmologists expectations. This was the first evidence that the expansion of the universe is accelerating. During several years astronomers believed that the expansion is slowing down under the effect of the matter density gravity. What the supernovae groups found, is that the universe is flying apart due not only to the general expansion we known since the famous Hubble discovery in 1929, but that this expansion is going faster and faster! The expansion of the universe is accelerating. According to Newton, physicits believe that gravity acts as a deceleration force. If the expanding universe is accelerating, this implies the existence of a stuff called "dark energy" acting as a repulsive force or a "negative gravity" at the universe scale. But this result was much questioned, because of the quality of the data and the effect of systematics.

The Wilkinson Microwave Anisotropy probe (WMAP)satellite with its present-day data representing the best all-sky measurement of the fluctuations of the cosmic Microwave Background to date offered a remarkable confirmation of the simplest picture in which the universe was indeed dominated by a strange material, the so-called "concordance model" , although it was not impossible to avoid the existence of such undesirable dark energy stuff. The recently Sloan Digital Sky Survey (SDSS) provided us with measurement of the large scale structure of the present day universe in a very good agreement with the "concordance model" prediction. In addition, the WMAP data combined with the Sloan Digital Sky Survey (SDSS) results confirmed the supernovae discovery rending the dark energy unavoidable.

What is causing this cosmic accelaration? What do we know about the dark energy property? Is it a constant just like Einstein's cosmological constant (which he called L) as reported by the Supernova Legacy Survey (SNLS) based on the best high redshift supernovae sample to date, or is it a dynamical dark energy, which changes as the universe expands, if so, how?

The mere existence of dark energy leads to a lot of new questions, why is the acceleration happening during the present epoch of the cosmic evolution or why do we live at the time when both dark matter and dark energy densities are roughly equal?

We can hope that astronomical experiments such as JDEM (Joint Dark Energy Mission ) a space-based telescope and also DUNE (Dark UNiverse Explorer ) an ESA project - which will try to see supernovae at higher redshifts, but also experiments at the interface of cosmology and quantum field theory, will enlighten the dark energy mystery, which could be the deepest problem in the XXIth century physics!