Dark energy and Type Ia supernovae

Observational cosmology is currently experiencing a revolution. The measurements of magnitude versus redshift of very distant Type la supernovae indicate that the expansion rate of the Universe is increasing. The acceleration of the Universe requires the existence of a 'dark energy' component that overcomes the gravitational self-attraction of matter, such as the vacuum energy density associated with the cosmological constant (A). Understanding the nature of dark energy, weather constant or dynamical, is among the most fundamental questions in contemporary science. Some of the most critical systematic effects involved in the use of Type la supernovae as distance indicators are discussed.