Density contrast is a parameter used in
galaxy formation
In cosmology, the study of galaxy formation and evolution is concerned with the processes that formed a Homogeneity and heterogeneity, heterogeneous universe from a Big Bang, homogeneous beginning, the formation of the first galaxies, the way ga ...
to indicate where there are local enhancements in matter density.
:
It is believed that after
inflation
In economics, inflation is an increase in the average price of goods and services in terms of money. This increase is measured using a price index, typically a consumer price index (CPI). When the general price level rises, each unit of curre ...
, although the universe was mostly uniform, some regions were slightly denser than others with contrast densities on the order of 1 trillionth. As the
horizon distance expanded, the enclosed causally connected (i.e. gravitationally connected) masses increased until they reached the
Jeans mass and began to collapse, which allowed
galaxies
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...
,
galaxy clusters
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galax ...
,
superclusters, and
filaments to form.
References
Physical cosmology
Inflation (cosmology)
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