In the past seven years, there have been a number of measurements that seem to indicate that the universe is full of a very strange form of invisible energy. It is invisible because it doesn’t emit, or interact with light. In this sense it is very much like the famous dark matter that we have all heard about. But this energy has a further eerie property and that is that it doesn’t interact through gravity in a way which we are familiar with other forms of matter. Our intuition is that things attract each other through gravity. An apple falls to Earth. The Sun pulls Jupiter towards itself. Furthermore, we have learnt the classical picture that what matters for the gravitational force is the mass of each object taking part in the interaction. This is what Newtonian proposed and it works brilliantly. Einstein showed that it is more complicated than that and that what we perceive as the gravitational force is actually driven by a combination of the energy and the pressure of the objects. This means that two objects with the same overall energy can have very different gravitational effects. And in particular, it is possible to choose a pressure which compensates and may even counteract the effect of the energy. For example if we pick a negative pressure which has an amplitude which is greater than the energy of the object, then the effect it will exert may even be repulsive. It is therefore possible to have forms of energy which repel each other under the effect of gravity. Dark energy is such a hypothetical constituent of the universe. Its repulsive nature and its abundance may be responsible for what seems to be the acceleration in the expansion of the universe. I am being careful in how I choose my words here, because many would argue that the evidence for acceleration is tentative.
Posted by Pedro Ferreira at December 1, 2003 11:37 PM