Cosmology, Particle Physics, and the Big Bang

The universe has not always been as we find it now: very cold in deep space at a temperature of 3K (−270◦C); about 300K here on Earth, warmed by sunlight; 10 million degrees at the centre of the Sun where nuclear fusion reactions power its furnace. Long ago the entire universe was like our sun, and even hotter. Collisions among particles in high-energy physics experiments reproduce some of the conditions of that early hot universe, and it is in that sense that the physics of the smallest particles and the cosmology of the universe at the largest scales come together.