The Universe in a Computer

"Ordinary matter," says Frenk, "is made of atoms, particles that we're familiar with on Earth, protons, electrons, neutrons... [With] dark matter, although we don't yet know exactly what it is, we have every reason to believe it's a kind of elementary particle which is in essence different from ordinary matter. And that's why it doesn't shine, because it doesn't interact. It doesn't make atoms, it doesn't produce light."

Photo courtesy of Carlos Frenk From the Millennium Simulation, the top image shows the galaxy distribution for a rich cluster of galaxies whereas the bottom one shows the corresponding dark matter distributions.

It's long been understood that if there was no dark matter exerting a powerful gravitational attraction, galaxies would simply fly apart like disintegrating wheels. But did this mysterious stuff have an even more important role to play in the evolution of the universe. Was it the invisible sculptor at work?

Frenk set himself a challenge. Could he programme the computers with the laws of physics, 10 billion particles of matter, plus a very big dollop of dark matter and get something that looked like the universe? After all, it's not possible - yet - to speed into the universe itself and set up a laboratory. The results of the experiment were a stunning success: perhaps best illustrated by the two galaxies shown here. One is 'real' - the other was made by the computer. It powerfully suggests that the model we have of how the universe evolved is right.

The universe as an expanding balloon

The story told by the Millennium Simulation is a spectacular one in which the very large is a consequence of the unimaginably small. The universe starts as a blazing fireball many times smaller than an atom and we can understand its structure by imagining an expanding balloon covered with spray paint. As the balloon expands, the paint cracks. Something similar happened early in the history of the universe: it wasn't born perfect. According to Frenk, the baby universe was born with a 'rash'. No one is quite sure why this happened but according to MIT physicist Max Tegmark: "One of the foundations of quantum physics, the laws that govern the micro-world, is the so-called Heisenberg Uncertainty Principle, which basically says in plain English that you can't have something completely uniform. And therefore, early on, when everything was tiny, there must have been these fluctuations."