Editing Galaxy formation and evolution (section)

=== Top-down theories ===
[[Olin J. Eggen]], [[Donald Lynden-Bell]], and [[Allan Sandage]]<ref>{{cite journal | last1 = Eggen | first1 = O. J. | last2 = Lynden-Bell | first2 = D. | last3 = Sandage | first3 = A. R. | title = Evidence from the motions of old stars that the Galaxy collapsed | bibcode = 1962ApJ...136..748E | date = 1962 | journal = [[The Astrophysical Journal]] | volume = 136 | pages = 748 | doi = 10.1086/147433}}</ref> in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. The distribution of matter in the early universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum. As the [[Baryon|baryonic matter]] cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation. Then, like a spinning ball of pizza dough, the matter forms into a tight disk. Once the disk cools, the gas is not gravitationally stable, so it cannot remain a singular homogeneous cloud. It breaks, and these smaller clouds of gas form stars. Since the dark matter does not dissipate as it only interacts gravitationally, it remains distributed outside the disk in what is known as the [[Dark matter halo|dark halo]]. Observations show that there are stars located outside the disk, which does not quite fit the "pizza dough" model. It was first proposed by [[Leonard Searle]] and Robert Zinn <ref>{{cite journal | last = Searle | first = L. |author2=Zinn, R. | date = 1978 | journal = [[The Astrophysical Journal]] | title = Compositions of halo clusters and the formation of the galactic halo | bibcode = 1978ApJ...225..357S | volume = 225 | pages = 357–379 | doi = 10.1086/156499| doi-access = free }}</ref> that galaxies form by the coalescence of smaller progenitors. Known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted.