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Large Scale Structures, Simulation

Moving-Mesh Moving-Mesh Evolutiob A computer simulation of the large scale structure was unveiled in May 2014. It has successfully reproduced many observations from clusters of the galaxies to the types of galaxies. The simulation involves solving the equations of gravity and hydrodynamics as functions of time. It empolyes a novel numerical scheme called "Moving-Mesh". As shown in Figure 03-05a, each cell (or mesh) is obtained from the intersection of half-spaces among the closet points. Resolution of the cosmic structure depends on the size of such cell, nothing smaller than such size will show up in the simulation. Figure 03-05b portrays the evolution for these

Figure 03-05a Moving-Mesh Construction [view large image]

Figure 03-05b Moving-Mesh Evolution [view large image]

cells for the case of stationary vortex flow. It shows that the mesh is changing shape and moving in the direction of the velocity field.

Illustris Large Scale Evolutiob The cosmic model is called Illustris (see logo in Figure 03-05c). It traces the evolution of both visible and dark matter starting just 12 million years after the Big Bang. It ends up at the present epoch showing the large structure in clusters of galaxies as well as smaller details in individual galaxies. The simulation contains 12 billion cells in a cube of (106.5 Mpc)3 across the universe. The smallest size scale over which the hydrodynamics is resolved, is 48 pc (about the distance from the Sun to its nearer neighbour stars). It took about 16 million CPU hours on stat-of-the-art desktop computers to complete the simulation. Some of the results are shown pictorially in Figure 03-05d. Many of the simulated features are summarized in Table 03-01. The work is published in the 8 May 2014 issue of "Nature" and presented in videos under "Moving Mesh Cosmology".

Figure 03-05c Illustris Logo [view large image]

Figure 03-05d Large Scale Evolution
[view large image]

    The input parameters are (see cosmological constants) :
  1. matter density m = 0.2726,
  2. dark energy density = 0.7274,
  3. baryon density b = 0.0456,
  4. Hubble expansion rate H0 = 100h km/s-Mpc, h = 0.704 for the present epoch,
  5. spectral index of the primordial power spectrum ns = 0.963,
  6. root mean squared amplitude of mass fluctuations in 8h-1 Mpc spheres s = 0.809,
Initial Conditions : at z = 127 in a periodic box with a side length of about 106.5 Mpc and gas temperature 245 K.

Cosmic Feature System Scale Simulation Observation
Cluster of Galaxies ~ 100 Mpc Super-clusters in the form of Cosmic Web Same
Intergalactic HI Clouds ~ 10 Mpc Number of absorbers as function of column density In good agreement
Cluster Satellites ~ 200 kpc Number of satellites from halo center In good agreement
HI Gas in Galaxy ~ 50 kpc Mass of HI gas as function of galactic mass Discrepancy in elliptical galaxies
Metal Content in Galaxy ~ 50 kpc Metal content as function of galactic mass In good agreement
Galaxies Morphology ~ 50 kpc Mixture of elliptical, spiral, irregular galaxies In good agreement
Low-mass Galaxies < 1010 Msun ~ 5 kpc Build up too early ~ 3 times later in observation

Table 03-01 Features in Cosmic Simulation (click underlined text to see pictorial illustration)

The large scale features in the table are the result of processes within size scale of a few tens pc from gas cooling; stellar evolution; supernova explosion; chemical elements creation; AGN feedback; supermassive black holes formation and accretion.

The simulation demonstrates that the current knowledge on the large cosmic structures is essentially correct. It offers a tool to cross examine observation and theory in future development.

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