In the standard model of cosmology the big bang was followed by an extremely short period of exponential expansion called inflation, after which the universe continued to expand at a slower rate. During inflation quantum fluctuations in the very early universe were stretched into the density variations that eventually led to the clusters of galaxies and other structures we see in the universe today.

The first traces of this structure can be seen in the tiny variations in the temperature of the cosmic microwave background -- the radiation left over from the big bang. Maps of the cosmic background -- which provide a picture of the universe as it was 380,000 years after the big bang -- also lend support to the "concordance model" in which 5% of the universe is made of ordinary baryonic matter, 25% is dark matter and 70% is dark energy. The nature of dark matter and dark energy remains a mystery.

Peebles, now at Princeton University, predicted some of the most important properties of the cosmic background in the 1960s. He also quantified how galaxies clustered together to form large-scale structures over time and played a leading role in developing theories of "cold dark matter".

Gunn, also at Princeton, studied theoretical aspects of galaxy formation and proposed important observational tests for dark matter candidates in galaxies. He has also been closely involved in the Sloan Digital Sky Survey, which is the most extensive 3D map of the universe to date.

In 1968 Rees -- who has spent most of his career at Cambridge University -- was the first to predict, with the late Dennis Sciama, that fluctuations in the microwave background were due to the uneven distribution of matter in the universe. And with Simon White, he was the first to identify the central role of dark matter in the formation of large-scale structure. More recently, he has developed theories about the formation of the first stars and galaxies. He has also written several popular science books.

The three scientists will receive the prize, which is worth US$500,000, from the King of Sweden in September in Stockholm.