The term Quintessence was used more than 2000 years ago by the Greeks to indicate a hypothetical fifth "element" of nature, after earth, air, fire and water. Today this term has been adopted as a synonym of dark energy. But what is dark energy?

In 1998 two groups of astronomers published the results of their studies into possible anomalies in the expansion of the Universe. They looked at very distant galaxies and studied in particular Supernovae type 1a(SN1a), which are very bright and which all have the same luminosity for a certain period of time. For this reason they can be considered as standard sources, as "standard candles". From the luminosity observed, scientists can work out their distance, because the luminosity observed decreases with the square of the distance from Earth.
This is a difficult thing to study for two reasons: firstly, Supernovae are very rare, and secondly, the measurements are difficult and require the use of a variety of extremely powerful telescopes.
Furthermore, the speed at which these distant galaxies move away from us can be measured by looking at the displacement towards the red of the light that we receive.

Fig. 1: Images relating to the sighting of one of the most distant supernovae ever seen, the supernova 1997ff, using the Hubble Space Telescope. The top photo shows the myriad of galaxies recorded by Hubble in the region of space which is around 10 billion light years away from the earth. The white square indicates the area in which the supernova 1997ff is found. The bottom left photo is a close-up of this region. The arrow indicates the galaxy which hosts the supernova. The red color is due to the billions of old stars which live there. The bottom right photo shows the supernova itself (white dot in the centre). This image was obtained using software techniques that compare photos taken at different times. (Credit: NASA, Adam Riess (Space Telescope Science Institute, Baltimore, MD))

Fig. 2: The drawing shows the expansion of the Universe from the moment of the Big Bang. Until about 8 billion years ago, gravity was the dominant force in a still sufficiently small Universe, and expansion was slowing down. In the following period, the size of the Universe increased until a mysterious repulsive "dark" force came to dominate the weaker force of gravity, thus producing an accelerated expansion of the Universe. The red concentric circles show how, in the first half of the life of the cosmos, the relative speed at which galaxes separate from each other was slower compared to the speed in the second half of its life (green circles). (Credit: Ann Feild (STScI))

What can this repulsive force be which is accelerating the Universe and which, with its density of energy, contributes to the total density of the Universe? It is worth remembering that in the theory of Special Relativity matter and energy are equivalent: mass and energy are in fact linked by the relation E=mc².

Some physicists believe that this repulsive effect can be traced back to the energy of vacuum. In fact, in modern physics, an empty space does not correspond to the "philosophical void". In the physical vacuum, thanks to Heisenberg’s principle of indetermination pairs of particles-antiparticles appear and disappear – they are virtual, but they can have tangible effects. It is thanks to them that the vacuum can have an invisible, "dark" energy density, different from zero, and can have a repulsive gravitational effect. In other words, the vacuum is not something inert; its effects seem to dominate the behaviour of the Universe, and our understanding is limited by our ignorance of the subject.

In 1916 Einstein introduced a constant (the cosmologic constant ) into the equations of General Relativity) in order to avoid an expanding Universe, but rather try to have a static Universe. More general solutions (without a cosmologic constant) were obtained by the Russian Alexander Friedmann in 1922 and, then by the Belgian Georges Lemaître in 1927. Today, a positive value of the cosmologic constant can help us to recognise the accelerated expansion of the Universe.

It has been estimated that dark energy dominates the energetic content of the Universe contributing a long 65%. The remaining 35% is due to mass. These measurements seem to find confirmation from the measurements of the microwaves background cosmic radiation