Dark energy was designed to maintain the critical density.

The most amazing fine-tuning of our braneworld is its average density of matter, which is related to the curvature of space. If the density is close to a critical density, the space is flat; otherwise, it is curved. In a curved space, the Euclidean geometry that we are familiar with would not work. The shortest distance between two points would  not be a straight line. Cosmological observations have confirmed that our space is indeed flat. Therefore, its density must be close to the critical density.  

Based on Einstein's cosmological equation, if at any moment our braneworld's density is slightly greater than the critical density, it will become increasingly greater than the critical density. Eventually, the gravitational attraction will reverse our braneworld's expansion, leading to a big crunch. On the other hand, if the density is slightly less than the critical density, it will become increasingly less than the critical density. Eventually, our braneworld will fly apart and become essentially an empty space. Thus, our braneworld appears to be an unstable system, like a pencil balanced on its point. Because the braneworld's density cannot afford for a slight deviation from the critical density, cosmologists believe that it must be "exactly" (to the accuracy of 1 part in 10⁶⁰) equal to the critical density when our braneworld was created (reference). Otherwise, we would not be here today.

My undergraduate major was in control and system engineering. At that time, my real interest was in philosophy. I did not spend much time learning the engineering stuff. However, after more than 30 years, I still remember the most basic principle: you cannot design an unstable system! Well, that is common sense. You do not need to go to an engineering school to learn this basic principle.

Since the Big Bang, our braneworld has been expanding for 13.7 billion years without a problem. It must be a stable system. God would not design an unstable system. There must be something missing in our understanding of the braneworld. The missing stuff could be the "dark energy" which was discovered in 1998.

To design a stable system, you must have a counteracting force. If somehow the system is pushed away from its stable state, the counteracting force can pull the system back to its stable state. The major force that governs the braneworld's expansion is the gravitational force. For ordinary matter, the gravitational force is always attractive. Then, where could the counteracting force come from? Is there any matter that has the property of anti-gravity?

In 1998, two independent teams of astronomers discovered that our braneworld's expansion was accelerating. This was a total surprise. The gravitational attraction of ordinary matter can only slow down the expansion. There must exist some invisible stuff that can counteract the gravitational attraction. This mysterious stuff is called "dark energy". The physical nature of dark energy is not known.

Currently, there are two leading candidates for the dark energy: cosmological constant and quintessence. The cosmological constant is a parameter describing the vacuum energy density which has the anti-gravity effect (more info). The quintessence, according to the Merriam-Webster dictionary, is "the fifth and highest element in ancient and medieval philosophy that permeates all nature and is the substance composing the celestial bodies". In the quintessence model, the dark energy is assumed to be made up of "exotic" particles that have the anti-gravity property. The mass of a quintessence particle is expected to be much less than an electron. They should permeate through our braneworld.

Experimentally, it is possible to distinguish the above two models because they have different predictions for the evolution of our braneworld. The vacuum energy density is a constant. It does not vary with time as the braneworld expands. All other matter density should decrease with expanding volume of the braneworld. At present, the dark energy makes up about 70% of all matter. If the dark energy is the vacuum energy, its composition in the early braneworld must be negligible. By contrast, if the dark energy is the quintessence particles, it may still dominate the early braneworld. Scientists are actively investigating the physical nature of dark energy. Sometime around 2010, a project under the Joint Dark Energy Mission will be launched to collect data on 300 million galaxies spanning two-thirds of our braneworld's history. This will tell us the matter composition in the earlier braneworld and distinguish the vacuum energy model from the quintessence model.

We expect the vacuum energy model to be ruled out. Our braneworld needs anti-gravity particles for two purposes: (1) to drive the dramatic expansion at the Big Bang, and (2) to maintain the critical density. From the cosmological equations, it can be shown that the braneworld's density will approach to the critical density when the expansion is accelerating (reference). With the help of anti-gravity particles, the braneworld's density can be maintained at the critical density. There is no need for unrealistic fine-tuning at the Big Bang.

(Last updated: May, 2007)