What will happen to the Universe in the future depends on the average density of its matter. If the average density of the Universe is too low, it will continue to expand forever because the matter’s gravitational forces will never be enough to stop the expansion. Such a Universe we call “open”. If the average density of the Universe is too high, the gravitational forces of the matter will stop the expansion and replace it with contraction. In this case we talk about “closed” universe.

Most probable, however, is a scenario about the future of the Universe in which it is neither open, nor closed. In this case, the average density of the matter is neither too high, nor too low, and the cosmological expansion accelerates with time. This latest discovery completely changed our perceptions about the state of the Universe. Before that, it was accepted, that the whole history of the cosmological expansion is in fact a history of its damping after the Big Bang. Now, we have data showing that it is at the present epoch that the dynamics of the expansion has switched from a stage of slowing down towards a stage of new speed up. This acceleration is due to the prevalence of the dark energy in the Universe. The dark energy creates a cosmic “anti-gravity” which governs the dynamics of the Universe in the present epoch and its expansion is accelerating.

The study of the past and the present of the Universe is one of the most complicated tasks that the human mind has ever faced. Georges Lemaître, one of the finest cosmologists of the 20 century says the following about these difficulties: “The evolution of the Universe can be compared to a firework that has just ended – what is left are a few remnant scarlet sparks, ashes and smoke. Standing on the cooling coals, we stare at the slow extinction of the suns and try to imagine the vanished magnificence of the beginnings of the worlds.”