Journal of Al-Nahrain University Vol.16 (1), March, 2013, pp.84-90 Science 84 N-Body Simulation for Dark Matter Clustering at Different Epochs of the Universe * Mariwan Ahmed Rasheed and Mohamad Ali Brza Department of Physics, College of Science, University of Sulaimani. * E-mail: mariwan rasheed@yahoo.com. Abstract In this study the cold dark matter particles is simulated at different epochs of the universe using Gadget-2 code for lambda cold dark matter model (ΛCDM). Firstly, the dark matter particles distributed in homogenous form then with time because of the gravity effects the particles collected gradually to construct sub clumps of halo and then big clumps of halo. This process is shown in the simulation from redshift z17.7 (700Myr.) after the Big Bang of the universe to z0 (13.3Gyr.). During these epochs of the universe the clustering of dark matter halo, filaments, and voids are constructed. These clusters of halo which were found from the simulation are the basic parts of galaxy clusters. In the simulation also the velocity scale of each epoch shows the displacement of the dark matter particles at different directions to construct the halo. Keywords: N-Body Simulation, Dark Matter Clustering, lambda cold dark matter model. Introduction Luminous matter is matter that we can detect at any wavelength using any of the telescopes on the earth. This matter, everything we see, seems to make up only about 10 percent of the universe. Gravity gives away the presence of the other 90 percent, what we call dark matter because it is non luminous [1]. Historically, Jan H. Oort 1932[2] explained that there are some missing matter in our galaxy, then this matter discovered by Zwicky 1933[3] during studying the motion of galaxies within Coma cluster of galaxies. In spite of this, the idea of dark matter was not accepted till extensive observational results of Rubin and Ford 1970[4]. After that the existence of a dark matter got further support by the theory of inflation introduced by Guth 1981 [5]. Telescopes are probing the very earliest galaxies and computer simulations can extend our knowledge behind the real world observations. When we notice a pattern in nature, we are curious about it and attempt to investigate it. The numerical tool is often the only one available to the researcher studying the long-term evolution of galaxies. When we perform a computer simulation of galaxies, we hope to learn why real galaxies have the features we observe [6]. According to the current paradigm of structure formation inflation amplifies quantum fluctuations, which are present in the dark matter density field of the early universe, to cosmic scales; at the time of matter and radiation density equality, these fluctuations start growing by gravitational instability. After radiation and ordinary baryonic matter decouple, radiation pressure stops supporting the baryonic density perturbations against self- gravity and against the pull of the dark matter gravitational potential wells that have already formed the dark matter in homogeneities thus accelerate the collapse of ordinary matter. At later times, the dark matter particles, which have sufficient momentum to stream out of the denser regions, set in a characteristic scale in the power spectrum, which is proportional to the typical velocity of the dark matter particles themselves [7]. An N-body treatment of the evolution of the dark matter component is gravitational. The importance of this method is that it tracks galaxy and dark matter halo evolution across cosmic time in a physically consistent way, providing positions, velocities, and other physical properties for the galaxy populations, so this method is used by many researchers for simulating dark matter[8][9].