Published: 10 April, 2009

Learning about the early history of the universe

Can particle accelerators accurately simulate conditions that occurred during the Big Bang?

There is indeed a strong connection between the study of atomic and subatomic particles, i.e. particle physics, performed at accelerators and that of the early history of the universe, i.e. cosmology. Let us start discovering each side of the connection at the time before considering their rich mutual interplay.

In an accelerator, small particles such as protons or electrons are smashed against each other at very high energies. As a consequence of the collision, many new particles are created. Particle physicists study the properties of these particles and the laws governing their interactions. As a figure of merit, the energy reached in the largest accelerator, the Large Hadron Collider at CERN, will be of around 10 TeV, i.e. ten billion times the energy of particles in the core of the sun.

On the other hand, cosmologists investigate the formation and evolution of our universe.

Many experimental evidences have been accumulated in favor of the Big Bang theory, according to which the universe has been continuously expanding until now. Just like for a gas in a box, the expansion has been accompanied by the continuous decreasing of the temperature. Hence, if we go backward in time (in fact about 14 billion years) we eventually find a moment in which the universe was so small and hot that every particle had a typical energy of around 10 TeV.

Accelerators can therefore reproduce the very high energy collisions that were happening all time in the early universe. The results of the experiments performed at accelerators allow us to unravel the laws that govern those high energy interactions. We can then try to use these laws to describe the early universe and its evolution.

For example, consider the theory, known as the Standard Model, that explains all particle accelerator experiments performed up to now. According to this theory, nature should appear much more symmetric at energies of 1 TeV or larger than it does at lower energies. One implication is that when the universe had a temperature above 1 TeV the Weak Force, responsible for example for the radioactive beta decay, was not as �weak� as today but in fact comparable with the Electromagnetic Force. Both Weak and Electromagnetic forces were important and formed a unified Electro-weak Force.

Even though the understanding of these issues is crucial for cosmologists, there are other important issues that are not likely to be addressed by ground-based experiments. Clearly, we can reproduce the content of the universe but not the whole universe itself which is composed by space and time as well. According to general relativity, it is the interplay between the matter content and the space-time that governs the evolution of the universe. Ground-based experiments can not tell us so much about this interaction, which is best studied via astronomical observations.

To summarize, accelerators can teach us a great deal about the early universe but they need to be supplemented by other types of experiments and observations in order to reveal to us the early history of our universe.