Dark energy or cosmological constant

The first results obtained from the SNLS (Supernova Legacy Survey) international collaboration – in which CEA-Dapnia and CNRS (IN2P3 and INSU) are participants – are showing that the mysterious “dark energy” assumed to be responsible for the acceleration in the Universe’s expansion could be Einstein’s cosmological constant. The results were published on Monday 21 November in the journal Astronomy & Astrophysics.

A few years ago, astrophysicists thought that the Universe’s expansion, discovered by Edwin Hubble during the 1920s, was slowing down under the effect of gravitation . But in 1998, researchers observed that distant supernovae seemed less bright than would be expected in a Universe whose expansion was decelerating. In fact, far from decelerating, the Universe’s expansion is accelerating from the effect of a mysterious energy known as “dark energy”.

The Universe is now thought to consist of around one quarter matter and three quarters dark energy, which acts on the Universe’s expansion like a repulsive force. Matter and dark energy behave differently with respect to the Universe’s expansion: matter becomes diluted; dark energy does not, or does so only a little.

Supernovae are stars exploding at the end of their life. They are very bright and can therefore be used as “milestones” in the Universe because their apparent brightness tells us how far away they are. So when we look at supernovae, we can measure the speed and distance at which they are moving away from us (through their shift towards red) and, from this, work out the speed at which the Universe is expanding.

The SNLS has measured the distance of 71 supernovae, the furthest of which exploded when the Universe was less than half its current age. The aim of the project is to measure dark energy precisely and to determine its nature, which currently remains unknown. However, by measuring the flux of distant supernovae, it is possible to work out whether it behaves like Einstein’s cosmological constant or according to a number of other theoretical hypotheses. What sets these theories apart is whether or not the density of dark energy becomes diluted with the expansion of the Universe. The measurement being published today is the most precise so far and favors an absence of dilution.

French researchers are currently working in close collaboration with European and North American (Canadian and US) research teams to find out more about supernovae, using photometry with Megacam and spectroscopy with the largest ground-based telescopes. At the end of five years’ observation, the results published today could be two to three times more precise.