Mystery of the Cosmological Constant

The 2011 Nobel prize in physics has been awarded to S. Perlmutter, A. Riess and B. Schmidt for their path breaking discovery that the rate of expansion of the universe is increasing with time. The trio used Type Ia supernovae (SNe Ia) as standard candles to estimate their luminosity-distances. To appreciate some of the far reaching implications of their work, I have provided an elementary exposition of general theory of relativity, accelerated expansion of the universe, luminosity-distance, SNe Ia and the cosmological constant problem.