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Dark Matter in the Universe

Dark Matter in the Universe. Ue-Li Pen 彭威禮. Overview. The observational case for dark matter dark matter candidates dark matter dynamics simulations conclusions. Observational Evidence. CMB-structure formation cluster of galaxies strong gravitational lensing galaxy rotation curves.

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Dark Matter in the Universe

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  1. Dark Matter in the Universe Ue-Li Pen 彭威禮

  2. Overview • The observational case for dark matter • dark matter candidates • dark matter dynamics • simulations • conclusions

  3. Observational Evidence • CMB-structure formation • cluster of galaxies • strong gravitational lensing • galaxy rotation curves

  4. CMB-structure formation • Perturbations grow in matter era after decoupling proportional to scale factor. • Grows by factor of 1100/(1+zm) where zm is the end of matter domination • for open universes, 1+zm=1/10, growth factor is only 100 • BUT COBE perturbations are only 10-5

  5. Clusters of Galaxies • In 20’s, Zwicky pointed out that the velocity dispersion of the Coma cluster is inconsistent with its self-gravity • One expects v2=GM/R, the mass inferred this way is 100 times larger than the inferred stellar mass

  6. The Coma Cluster X-ray image of coma from ROSAT Galaxies in optical

  7. Rotation Curves • Galaxy rotation curves tend to be inconsistent with light distribution. 21 cm gas curves. M33 rotation curve (points) compared with the best-fitting model (continuous line). Also shown are the halo contribution (dot-dashed line), the stellar disc (short-dashed line) and the gas contribution (long-dashed line). Edvige Corbelli; Paolo Salucci 2000: The extended rotation curve and the dark matter halo of M33 , Monthly Notices of the Royal Astronomical Society , 311, 441-447

  8. Dark Matter Candidates • Weakly-Interacting-Massive-Particles (WIMPS): neutrinos, axions, etc • Massive Compact Halo Objects (MACHOs): black holes, planets, brown dwarfs

  9. Kinematic Classification • Hot Dark Matter: v/c or order unity at matter radiation equality: neutrinos • Cold Dark Matter: v/c << 1 at all times in matter domination

  10. Dynamical Effects • Transfer function: spatial clustering of dark matter. HDM does not cluster sufficiently. • CMB  dark matter must be non-baryonic in order to start growing at matter-radiation equality (z100 000) instead of decoupling (z  1000) • most present epoch observations are of non-linear fluctuations, require massive computer simulations to predict quantitatively

  11. Current Research Directions • CMB: sub degree scale, polarization: detailed tracers of linear dark matter distribution • Gravitational lensing: measures total mass distribution, I.e. dark matter • Sunyaev-Zeldovich Effect: inverse Compton Scattering of CMB photons along line of sight: measures unbiased non-linear electron distribution -- requires simulations to model • galaxy surveys

  12. Cosmological Simulations • Mean homogeneous Hubble expansion -- Friedman equation (relativistic) • Most other physics is Newtonian: v/c < 0.01 • 3 fluids: , DM, baryons • non-interacting DM given as collisionless fluid • baryons are ideal Eulerian gas

  13. Place universe in a box • approximate local physics using constant density • Box must be big enough that perturbations are small: L >> 32 Mpc • cell must be small enough to resolve features of relevance • Non-trivial non-linear dynamics through Newtonian gravity • Gravitational clustering moves matter into small scales: adaptive resolution required

  14. Initial Conditions • Adiabatic, scale invariant super-horizon perturbations. • Photon-baryon-dm-etc ratio constant in space and time (super-horizon) • fluctuations in total density and curvature • Harrison-Zeldovich-Peebles: power law fluctuations should not diverge on any scale

  15. k^3 P(k)=10^-5 on all scales: mass fluctuations independent on smoothing scale. • Dark matter perturbations grow in matter domination subhorizon • wave of half the size entered the horizon at 1/4 the scale factor size • perturbations grow as the scale factor • transfer function goes as k^4 in MD • P(k)=k on large, subhorizon scales

  16. Hardware • High resolution simulations now possible with new Canada Foundation for Innovation supercomputers at Toronto: 32 proc GS320 729 Mhz alpha, 64 G, 48 proc alpha cluster, 4 proc NEC SX-5, 48 proc SGI O2K, worth over CDN$ 15 million.

  17. Software: MMH • Moving Mesh Hydro/N-body • General purpose cosmological N-body & hydro code • high resolution TVD characteristic solver • adaptive grid changes • model diffuse X-ray emission, S-Z effect

  18. Irrotational, stable grid

  19. Ray-tracing through expanding universe

  20. Ray-traced gas density

  21. Ray-traced dark matter

  22. Ray-traced SZ effect

  23. Conclusions • Direct ab initio simulations of dark matter and gas now possible, can predict SZ effects. • Full observable map and knowledge of dark matter and baryon distribution possible with upcoming SZ experiments (AMIBA, Planck, CBI) and galaxy/SZ cross-correlations. • Dynamics seems to suggest Cold Dark Matter, underlying nature unknown.

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