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Density contrast is a parameter used in
galaxy formation The study of galaxy formation and evolution is concerned with the processes that formed a Homogeneity and heterogeneity, heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, a ...
to indicate where there are local enhancements in matter density. :\Delta=\frac It is believed that after
inflation In economics, inflation refers to a general progressive increase in prices of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a r ...
, 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 In stellar physics Astrophysics is a science that employs the methods and principles of physics in the study of astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical entity, assoc ...
and began to collapse, which allowed
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
,
galaxy clusters A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity Gravity (), or gravitation, is a list of natural phenomena, natural phenomenon b ...
,
superclusters A supercluster is a large group of smaller galaxy clusters or galaxy groups; it is among the largest known structures of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, ...
, and filaments to form.


References

Physical cosmology Inflation (cosmology) {{Cosmology-stub