The unkown 96 % of the universe

 20th century Astronomers have made 2 great discoveries that have changed the scientific view of the universe. Some new, unknown sort of matter usually called dark matter seems to make up 23% of the universe. This so-called dark matter is observed solely through its gravitational effects, visible for instance in the way stars orbit the centers of galaxies. Astronomers measuring Doppler Effect of stars far from the center of galaxies discovered deviations from the prediction of standard Newtonian mechanics. This discovery has been interpreted as a signature of dark matter.

Despite a precise determination of how much dark matter there is, its nature remains unknown. One of the major activities of particle theorists in recent years has been the study of possible extensions of their Standard Model that could somehow explain dark matter. Some physicists, for example, posit the existence of new particles, which may be observable in collisions taking place at the Large Hadron Collider (LHC) in Geneva. The fate of such ideas will be determined through data collected over the next few years.

Another great achievement is the discovery of dark energy by two vigorously competing groups of scientists—the Supernova Cosmology Project and the High-Z Supernova Search Team—who are gathering observations of supernovae in distant galaxies. A supernova is a catastrophic explosion of a star that causes it to shine briefly with a brightness greater than that of an entire galaxy. These explosions are fairly well-understood by astronomers, making it possible to use them to estimate the distance to the galaxies in which they occur.

Putting this information together with data on the expansion of the universe has made it possible for the first time to map out how the expansion rate changes with distance. The two groups racing to gather supernovae observations found their answers at about the same time. The expansion of  the universe is accelerating. Such a possibility was inherent in general relativity, using a term in Einstein’s equations that could be interpreted as assigning energy to the vacuum of empty space. It was Einstein who introduced the term ad hoc in order to make the universe static. Later on he set the term to zero after the discovery of the expansion of the universe and admitted the introduction of the term to be his greatest mistake. Until the supernova-measurement results, nearly everyone had assumed that this vacuum energy was exactly zero, but instead it turned out to be a number equivalent to 73% of the total energy of the universe. The universe was not just expanding; the rate of expansion was increasing.

Physicists have long been trying to unify their Standard Model and general relativity. A vacuum energy of such a large size, however, is hard to make sense of by using current models, thus driving theorists to desperate measures. One popular “solution” is the multiverse, in which all possibilities occur and our universe happens to have the exact vacuum energy that allows our existence.

The discovery of both dark matter and energy also makes our familiar world of Baryons, leptons and light to be mere 4 % of the universe. We  know nothing about 96 % of the universe inspite of the great scientific achievements. That is a sobering thought.

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