Structure formation
From Biocrawler, the free encyclopedia.
Structure formation refers to a fundamental problem in cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 11 billion years ago. However, looking in the sky today, we see structures on all scales, from stars and planets to galaxies and, on much larger scales still, galaxy clusters, and enormous voids between galaxies. How did all of this come about from the nearly uniform early universe?
Cosmological Fluctuations
Structure formation includes two key ingredients. The first is the realization that while observations indicate that the cosmic microwave background radiation is nearly uniform, there are very small temperature fluctuations at one part in 100,000. To put this in perspective, the same level of fluctuations on a topographic map of the United States would show no feature higher than a few meters high. However, these fluctuations are critical, because they provide the seeds from which the largest structures within the universe can grow and eventually collapse to form galaxies and stars. COBE(Cosmic Background Explorer) provided the first detection of the intrinsic fluctuations in the cosmic microwave background radiation in the 1990s, and confirmed that these fluctuations were consistent with the predictions of the inflationary model of the universe.
Dark Matter
The second key ingredient is dark matter. One of the key realizations made by cosmologists in the 1970s and 1980s was that the majority of the matter content of the universe was composed not of atoms, but rather a strange mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but does not emit or absorb radiation, and hence interacts only very feebly via the weak interaction (at best) with itself or with ordinary matter.
Dark matter turns out to play a key role in accelerating the process of structure formation precisely because it feels only the force of gravity. As a result, dark matter begins to collapse into a complex network of dark matter halos well before ordinary matter, which is impeded by pressure forces. Without dark matter, the epoch of galaxy formation would occur substantially later in the universe than is observed.
