Abstract
In this first introductory lecture, the observational evidence for the existence of dark energy or a cosmological constant is reviewed and explained: the discovery of distant type Ia supernovae, and their greater-than-expected distance by their redshift, but also gravitational lensing. Dark energy has only recently been detected (5 billion years). Just after the Big Bang, its influence was negligible, but in the future it will completely dominate the Universe, which will then resemble the De Sitter model, expanding exponentially. The cosmological constant has a long history. Albert Einstein introduced it in 1917 to represent a static Universe. His model of the Universe was a sphere of finite mass. Shortly afterwards, the expansion of the Universe was discovered, and the cosmological constant was assumed to be zero for eighty years! However, in 1920, Wolfgang Pauli had tried to interpret the cosmological constant as being due to the energy of the quantum vacuum, and had come up against an incompatibility of several orders of magnitude. The calculation was taken up again in the 1960s-1970s, with field theory and quantum chromodynamics. The energy of the vacuum is always an order of magnitude greater than what is observed. It would be necessary to invent a cosmological constant that exactly cancels out the contribution of the quantum vacuum, and this fine-tuning appears highly unlikely. Yet vacuum energy is a confirmed reality: the Casimir effect, discovered in 1948, has been precisely measured for over thirty years. Experiments are also being used to test the force of gravity, currently down to scales of 10 microns. The various solutions proposed for dark energy will be developed in subsequent lectures: various models of quintessence, or modified gravity, models of multi-dimensional Universes, branes, etc.
