Structure formation

In physical cosmology, structure formation describes the creation of galaxies, galaxy clusters, and larger structures via gravitational and hydrodynamic processes operating on cosmological inhomogeneities. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation applies models of gravitational instability to small ripples in mass density to predict these shapes. The modern Lambda-CDM model is successful at predicting the observed large-scale distribution of galaxies, clusters and voids; but on the scale of individual galaxies there are many complications due to highly nonlinear processes involving baryonic physics, gas heating and cooling, star formation and feedback. Understanding the processes of galaxy formation is a major topic of modern cosmology research, both via observations such as the Hubble Ultra-Deep Field and via large computer simulations. Structure formation began some time after recombination, when the early universe cooled enough from expansion to allow the formation of stable hydrogen and helium atoms. At this point the cosmic microwave background(CMB) is emitted; many careful measurements of the CMB provide key information about the initial state of the universe before structure formation. The measurements support a model of small fluctuations in density, critical seeds for structures to come.