International Journal of Modern Physics D

In canonical quantization of gravity, the state functional does not seem to depend on time. This hampers the physical interpretation of quantum gravity. I critically examine ten major attempts to circumvent this problem and discuss their shortcomings.

Modern cosmology began in 1917 when Einstein published his model of a static Universe built on general relativity. A few months later de Sitter came forward with a competing, but also static model which contained no matter but had the intriguing quality that the spectrum of a test particle appeared redshifted to a distant observer. It was thought that de Sitter's model might explain the redshifted spectra observed by Slipher in spiral nebulae. However, in 1927 Lemaître showed that de Sitter's model violated the principle of homogeneity. He then formulated a dynamical cosmological model and combined it with the available observations, showing that our Universe is expanding. He theoretically derived the linear distance–velocity relationship which today is called the "Hubble-relation." Hubble confirmed the relation in 1929 on purely observational grounds. 80 years ago, in 1931 in a letter to Nature, Lemaître suggested that the Universe had a definite beginning in a rapid expansion out of a highly condensed state: the primeval atom. This event became later known as the Big Bang.