The case for the cosmological constantVARUN SAHNI Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007, India Abstract. I present a short overview of current observational results and theoretical models for a cosmological constant. The main motivation for invoking a small cosmological constant (or L-term) at the present epoch has to do with observations of high redshift Type Ia supernovae which suggest an accelerating universe. A flat accelerating universe is strongly favoured by combining supernovae observations with observations of CMB anisotropies on degree scales which give the `best-fit' values øL @ 0.7 and øm @ 0.3. A time dependent cosmological L-term can be generated by scalar field models with exponential and power law potentials. Some of these models can alleviate the `fine tuning' problem which faces the cosmological constant. Keywords. Cosmology; supernovae; cosmic microwave background; cosmological constant; vacuum fluctuations. Pacs No.98.80.-k; 98.80.Hw; 98.70.Vc |