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See also: Large-scale structure of the cosmos and Structure formation The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us what the early Stelliferous Age was like. Another Hubble image shows an infant galaxy forming nearby, which means this happened very recently on the cosmological timescale. This shows that new galaxy formation in the Universe is still occurring. Structure formation in the big bang model proceeds hierarchically, with smaller structures forming before larger ones. The first structures to form are quasars, which are thought to be bright, early active galaxies, and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles. Reionization 150 million to 1 billion years after the Big Bang See also: Reionization and 21 centimeter radiation The first stars and quasars form from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma. Formation of stars See also: Star formation The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. However, as yet there have been no observed Population III stars, and understanding of them is currently based on computational models of their formation and evolution.[12] Formation of galaxies See also: Galaxy formation and evolution Large volumes of matter collapse to form a galaxy. Population II stars are formed early on in this process, with Population I stars formed later. Johannes Schedler's project has identified a quasar CFHQS 1641+3755 at 12.7 billion light-years away,[13] when the Universe was just 7% of its present age. On July 11, 2007, using the 10-metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old.[14] Only about 10 of these extremely early objects are currently known.[15] More recent observations have shown these ages to be shorter than previously indicated. The most distant galaxy observed as of October 2013 has been reported to be 13.1 billion light years away.[16] The Hubble Ultra Deep Field shows a number of small galaxies merging to form larger ones, at 13 billion light years, when the Universe was only 5% its current age.[17] This age estimate is now believed to be slightly shorter (see Reference quoted at the end of the previous paragraph). Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago.[18] Formation of groups, clusters and superclusters See also: Large-scale structure of the cosmos Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters. Formation of the Solar System 9 billion years after the Big Bang Main article: Formation and evolution of the Solar System The Solar System began forming about 4.6 billion years ago, or about 9 billion years after the Big Bang. A molecular cloud made mostly of hydrogen and traces of other elements began to collapse, forming a large sphere in the center which would become the Sun, as well as a surrounding disk. The surrounding accretion disk would coalesce into a multitude of smaller objects that would become planets, asteroids, and comets. The Sun is a late-generation star, and the Solar System incorporates matter created by previous generations of stars. Today 13.8 billion years after the Big Bang The Big Bang is estimated to have occurred about 13.8 billion years ago.[19] Since the expansion of the universe appears to be accelerating, the cosmic web is likely to be the largest structure that will ever form in the universe. The present accelerated expansion prevents any more inflationary structures entering the horizon and prevents new gravitationally bound structures from forming. |
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