deletions | additions       diff --git a/section_Introduction_subsection_Galaxy_Evolution__.tex b/section_Introduction_subsection_Galaxy_Evolution__.tex  index 09ca11a..1f22c69 100644  --- a/section_Introduction_subsection_Galaxy_Evolution__.tex  +++ b/section_Introduction_subsection_Galaxy_Evolution__.tex 
...
\subsection{Galaxy Evolution: from the Big Bang to present day}  The big-bang was the birth of our universe. The early state of the universe was hot and dense. In this era, matter was coupled to radiation, but as the universe expanded and temperature decreased, at a redshift of z = 1000, matter and radiation became decoupled. As seen in figure 1, this occurred approximately 380 000 years after the Big Bang. Redshift is the increase of apparent wavelength of light coming toward an observer as the result of an object moving away from the observer where 1 + z = λ_obsv/λ_emit [like this: $1 + z = \lambda_{\rm obs}/ \lambda{\rm \lambda_{\rm  emit}$]. The radiation is a relic of the big-bang that is still observed today; it is called the Cosmic Microwave Background (CMB). The CMB is nearly isotropic blackbody radiation which expanded and cooled and fills the universe and is now T0=2 725 0 002 K. (B.F Burke and F Graham-Smith) states that the isotropy of the CMB implies that sections of the universe that were never in communication with one another have similar properties at the time of observations . Observations of the CMB communicate that the post- Big-bang universe is a homogeneous, isotropic expanding or contracting universe, however this is not the reality of the universe. It has been further theorized that current structure formation originated from quantum fluctuations in temperature and density in an isotropic homogeneous universe (Galaxy Formation and Evolution, Houjun Mo, Frank van den Bosch, Simon White). (B.F Burke and F Graham-Smith) further states that the same fluctuations/ irregularities would have been imprinted on the radiation that we now see as the CMB. The quantum fluctuations we regions whose density was slightly higher than the mean density of the universe. These regions of higher density attracted surrounding matter through gravity.  As a result, slightly-denser regions attracted matter towards them and become even denser. Low-density regions on the other hand become even less dense because matter flows away from them. This amplification of quantum fluctuations is referred to as gravitational instability (Galaxy Formation and Evolution, Houjun Mo, Frank van den Bosch, Simon White). The initial fluctuations were the epicenters of the newly formed clumps of matter mainly made from hydrogen helium and free floating electrons. Thus initial perturbations are the building blocks of atomic nuclei and free floating electrons which combined via recombination to form neutral atoms, which later formed gas clouds, stars, galaxies, and the other astronomical structures.  There is a significant amount of matter in the universe, which is not directly detectable. Fritz Zwicky observed using the Doppler shift and the virial theorem that the velocity spread within a cluster of galaxies implied that there was more mass than the luminous matter accounted for. Through further experimentation it was later concluded that our universe’s visible matter is set in a hallo matter which does not interact with light, but whose effects are observed through gravitational interactions. This non-luminous matter is called dark matter. The nature of dark matter is still unknown. (Galaxy formation and evolutions, Houjin Mo et al). Dark matter influences how galaxies form and even their rotation as it is the frame upon which the visible matter is embedded. The distribution of galaxies also depends on the distribution of dark matter in the universe. (B.F Burke and F Graham-Smith) states that dark matter may be hot or cold. The Hot Dark Matter (HDM) theory allows for a hierarchy in structure formation. The theory states that large scale structure such as the distribution of galaxies form first due to the presence of hot dark matter. Cold dark matter is theorized to be the seed for galaxy formation.