Looking Back in Time

To understand the Universe today, we have to understand the behaviour of particles at the time of its birth. This gives us another reason for building powerful particle accelerators, because increasing energy is equivalent to going back in time, as we're about to see.

Most scientists today believe that the Universe was born about 15 billion years ago in a hot Big Bang and that it has been expanding and cooling ever since. The evidence for this is compelling. When we look out into space, we see galaxies rushing away from each other, and scientists have even measured the afterglow of the Big Bang. No matter where we look in the Universe, the temperature of space seems to be the same, about 3 Kelvin. This is exactly what you would expect if everything began in a dense point, and it means that the Universe was once hotter. Much hotter. Since heat is a form of energy, increasing the energy of particle collisions in accelerators like LEP, is equivalent to probing back in time ever closer to the Big Bang. In other words, particle collisions recreate the conditions of the Big Bang.

Map of the sky photographed by COBE, the COsmic Background Explorer. The different colours represent different temperatures within a range of just +/- 150 micro-Kelvin around ~3 Kelvin.

In LEP's collisions, electrons and positrons could annihilate giving rise to a very high density of energy, or in other words a very high temperature. From this energy, new particles emerged, some of which don't exist naturally in the Universe today but would have existed in the very early stages of the Universe. Under the extreme conditions of high temperature generated at LEP, the particles themselves show us how they behaved shortly after the Big Bang.