These lecture notes intend to form a short pedagogical introduction to the use of typical type Ia-Supernovae (hereafter SNIa) as standard candles to determine the energy density of the universe. Problems of principle for taking SNIa as cosmological probes are pointed out, and new attempts at solving them are indicated including the empirical width-luminosity relation and its possible explanations. Finally, the observations of SNIa at high redshift carried out by two major teams are briefly reviewed and their interpretation as evidence for an accelerating universe is also rapidly discussed.
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