1. 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

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

3. 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.

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

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

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

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

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

9. 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.

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

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

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

13. Boomerang results
Size of the moon

15. COBE

16. WMAP

17. WMAP - Power Spectrum

18. CMB Power Spectra

19. CMB Binned

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

21. Precision Cosmology

22. Planck
To be launched by ESA in 2009?

23. 2003
WMAP, 2004
Planck
2009?
Models

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

25. 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.

28. Galaxy Redshift Surveys
100 Mpc
z ~ 0.2
z = 0

29. 2DF Galaxy Redshift Survey

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

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

32. Two main scenarios Initial collapse Hierarchical merging Fragmentationq q
Fragmentation r r q q r r
Merging q q

33. 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!

34. 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.

35. fini