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\subsection{Background}  Cosmology is the branch of Astrophysics which studies the  universe on the largest scales.   Cosmology is primarily  concerned with studying understanding  the universe as a whole. Key dynamical  entity, the key  questions include: Where did being: what is  the universe  come from? Does current state of the universe?  What did  it go on for ever? Will look like a long time ago?  What will  it end some day? Many physicists look like in the future?  While these  are shocked very difficult questions to answer, in recent decades there has been  incredible progress in observational cosmology that has allowed us great insight into  the dynamical evolution of the universe.  For instance, measurements of the cosmic microwave background  by the poor quality WMAP and Planck  satellites have given us an extremely accurate picture of the state  of the data; if you don't throw up your  hands in disgust entire  observable universe just 380,000 years after the Big Bang,  and scream ``you can't assume that!'' at least once  during this prac, you won't show us the tiny  fluctuations in matter density in the early universe that  have understood what you're doing. Yet grown into  the truly remarkable thing is galaxies and clusters we see today.  The discovery in the 1990s  that cosmology the Universe's expansion  is possible at all; that accelerating fundamentally  changed our understanding not just of how the Universe will evolve but also its  composition.  Large galaxy surveys such as the 2dF Galaxy Redshift Survey and  the microscopic inhabitants Sloan Digital Sky Survey   have mapped the positions in the sky  of this insignificant speck millions  of dust called galaxies in the nearby Universe,   and show that rich, complex structure exists on even  the Earth can dare largest scales.  Our inability  to try replicate the Universe within the confines of a laboratory makes  doing accurate Cosmology inherently difficult,  and understand it is hoped that this  practical exercise will give you an appreciation for  the nature power and limitations  of using astronomical data to learn about  the universe. Universe as a whole.  In this practical, you will use observations of two galaxy clusters to  estimate several key cosmological parameters. The observations were  made with the UK Schmidt Telescope at Siding Spring Observatory in NSW;  see the asteroid hunting exercise for details of the telescope and  the photographic plates it produces \ -- \ photos of this telescope are displayed in the lab. Most galaxies live in groups. Our own galaxy, the Milky Way, lives in  a rather insignificant cluster we call the Local Group. This cluster