Lecture 5: Matter Dominated Universe Today, matter is assembled into structures: filaments, clusters, galaxies, stars, etc. Galaxy formation is not completely understood. Main mechanism is gravitational instability, i.e. rthen q rnow q

MASS ASSEMBLY V. SMOOTH NO METALS V. LUMPY, METAL RICH

Surface of Last Scattering Before decoupling: matter and radiation tightly coupled. After: radiation propagates freely. The CMB retains an imprint of conditions on the surface of last scattering.

Snapshot of Universe at z = 1100 CMB Anisotropies COBE 1994 WMAP 2004 Snapshot of Universe at z = 1100

Almost perfect CMB isotropy --> almost uniform matter distribution at recombination z = 1100 T ~ 3000K t ~ 3x105 yr Tiny CMB anisotropies. The “ripples” in T --> ripples in density. After decoupling, gravity amplifies these initial density ripples.

Three mechanisms give rise to anisotropies Sachs-Wolfe effect Doppler effect Re-ionization (Sunyaev-Zeldovich effect)

Sachs-Wolfe effect Photons from over-dense region must climb out of the potential well, losing energy --> longer wavelength . --> lower T. last-scattering surface

Doppler effect Gas velocity on the last-scattering surface produces Doppler shifts.

Re-ionisation (Sunyaev-Zeldovich effect) Once stars form, their UV radiation re-ionises nearby gas. Once galaxy clusters form, gas falling in is shock-heated to X-ray temperatures (~106-8 K). Free electrons liberated scatter CMB photons. We see CMB cool spots as silhouettes of the hot gas.

Galaxy Clusters are filled with hot X-ray gas optical (galaxies) X-ray (hot gas)

Sachs-Wolfe effect Doppler effect Sunyaev-Zeldovich effect Mass distribution at recombination. Velocity distribution at recombination. Ionised gas in intervening galaxy clusters.

Angular size (degrees) CMB Power Spectrum 10 1 0.1 S-Z S-W size of anisotropy  Many experiments Doppler peaks

2001 -- Boomerang results Size of the moon

1994 - COBE

2004 - WMAP

WMAP - Power Spectrum

2004 - CMB Power Spectra

2004 - CMB Binned

The CMB anisotropies depend critically on Cosmological parameters: Different parameters predict different CMB power spectra.

Precision Cosmology

Planck To be launched by ESA in 2009?

2003 WMAP, 2004 Planck 2009? Models

WMAP (and Planck) measure cosmological parameters to exquisite accuracy. Anisotropies are the starting point for galaxy formation!

Large-Scale Structure formation Simulations on supercomputers. Typically ~1010 particles randomly placed then adjusted to match large scale anisotropies. Gravitational accelerations computed. Particle positions followed in time. Filaments and voids form. Clusters where voids intersect.

Galaxy Redshift Surveys 100 Mpc z ~ 0.2 z = 0

2DF Galaxy Redshift Survey

Simulations predict structure formation with different cosmological parameters. Large scale surveys measure the structure of the local universe. Agreement requires Dark Matter

Galaxy formation + = Matter anisotropy at CMB Gravitational instability = Galaxy formation

Two main scenarios Initial collapse Hierarchical merging Fragmentation q q Fragmentation r r q q r r Merging q q

Do small building blocks form first and then merge? Do large regions collapse first into giant galaxies and later fragment into small ones ? Both methods can occur. Also: angular momentum is important!

Ellipticals Spirals Irregulars Globular Clusters Hubble Sequence Ellipticals Spirals Irregulars Globular Clusters building blocks? low a.m. high a.m. Galaxy formation is an active research topic.

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