Protogalaxy

In physical cosmology, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which is forming into a galaxy. It is believed that the rate of star formation during this period of galactic evolution will determine whether a galaxy is a spiral or elliptical galaxy; a slower star formation tends to produce a spiral galaxy. The smaller clumps of gas in a protogalaxy form into stars. The term "protogalaxy" itself is generally accepted to mean "Progenitors of the present day (normal) galaxies, in the early stages of formation." However, the "early stages of formation" is not a clearly defined phrase. It could be defined as: "The first major burst of star formation in a progenitor of a present day elliptical galaxy"; "The peak merging epoch of dark halos of the fragments which assemble to produce an average galaxy today"; "A still gaseous body before any star formation has taken place."; or " an over-dense region of dark matter in the very early universe, destined to become gravitationally bound and to collapse." It is thought that the early universe began with a nearly uniform distribution (each particle an equal distance from the next) of matter and dark matter. The dark matter then began to clump together under gravitational attraction due to the initial density perturbation spectrum caused by quantum fluctuations. This derives from Heisenberg's uncertainty principle which shows that there can be tiny temporary changes in the amount of energy in empty space. Particle/antiparticle pairs can form from this energy through mass–energy equivalence, and gravitational pull causes other nearby particles to move towards it, disturbing the even distribution and creating a centre of gravity, pulling nearby particles closer. When this happens at the universe's present size it is negligible, but the state of these tiny fluctuations as the universe began expanding from a single point left an impression which scaled up as the universe expanded, resulting in large areas of increased density.