1. The Dark Side of the Universe
Neta A. Bahcall
Princeton University

2. What is the Universe Made of?
10/23/2007
What is the Universe Made of?
Luminous matter: Stars, Galaxies, Gas
Dark stuff:
 Dark Matter: How know? How much?
Where? What is it?
 Dark Energy: How know? How much?

3. 10/23/2007

4. 10/23/2007

5. Weighing Galaxies Motion (v) of Stars and Gas: v2 = GM(<r)/r
10/23/2007
Weighing Galaxies
Motion (v) of Stars and Gas: v2 = GM(<r)/r
[ GM(<r)m/r2 ~ mv2/r ]
Gravitational Lensing: M(<r)
Data show velocity and mass in large halos
around luminous galaxies
 Implies: Dark Matter in Galactic Halos!

6. Flat Rotation Curves
10/23/2007
M(<r) ~ v2r ~ r v r

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12. 10/23/2007

13. Mass Density of Universe
10/23/2007
How much? How distributed?
Mass-to-Light Function
Baryon Fraction
Cluster Abundance and Evolution
Other Large-Scale Structure Obs.
 All yield m ~ 0.25
 Mass ~ Light (on large scales)

14. Mass-Density (Units) b (baryons) (observed) ~ 0.05
10/23/2007
Mass-Density (Units)
Critical mass-density (= density needed to halt the Universe expansion):
critical = 3Ho2/8G
~10-29g/cm3 ~ 6 p/m3
m = m/crit
m = 1 is the critical density
 ‘Flat’ Universe
b (baryons) (observed) ~ 0.05
[Mpc = 106pc; 1pc ~ 3 ly; Mo=2E33g]

15. Mass-to-Light Method m = m/critical
10/23/2007
Mass-to-Light Method
 <M/L>cl Luniv(Lo/Vol) = m(Mo/Vol)
 Weigh cluster mass, Mcl (<R~1Mpc)
<M/L>cl = 300h
m = m/critical
 m ~

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18. Mass-to-Light Function (Bahcall, Lubin & Dorman ‘95; Bahcall and Fan ‘98)
10/23/2007

19. M/L (R) SDSS (Bahcall & Kulier 2010)

20. M/L Function: Conclusions
10/23/2007
M/L Function: Conclusions
 M/L Function Flattens on Large Scales:
 M ~ L (on large scales)
reaching the end of growth of Dark-Matter
 Total Mass-Density of Universe:
 m =

21. Dark Matter How know?  Gravity: Motion, Gravitational Lensing
10/23/2007
How know?
 Gravity: Motion, Gravitational Lensing
Where is it located?
 Large Galaxy Halos, Clusters
How much?
 ~20% of Critical Density
[atoms/luminous: only ~5%]
What is it?
 New particles ???

22. Universe Expansion: Hubble Law
10/23/2007
Universe Expansion: Hubble Law

23. Hubble Expansion of Universe
10/23/2007 d

24. 10/23/2007
Cosmic Acceleration

25. 10/23/2007

26. ‘Dark Energy’ Dark-Energy ‘Stretch’ > Mass-Density ‘pull’
10/23/2007
‘Dark Energy’
Universe acceleration 
Dark-Energy ‘Stretch’ > Mass-Density ‘pull’
(at present time)
Mass-Density: m ~ 0.25
Acceleration: m -  ~ - 0.5
  ~ 0.75
What is it?  ????

27. What is the Universe Made of? The Dark Side of the Universe
10/23/2007
What is the Universe Made of? The Dark Side of the Universe
 Luminous (‘Normal’) matter: 5%
 Dark Stuff: 95%
Dark Matter: 20%
Dark Energy: 75%

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29. Universe’s Baby Pictures (WMAP)
10/23/2007

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31. 10/23/2007
The Universe

32. The Content of the Universe
10/23/2007
The Content of the Universe
Luminous matter (5%): Stars, Galaxies, Gas
Dark Matter (20%): New particles (?)
Dark Energy (75%): Cosmological Constant?

33. The Cosmic Triangle [only 5% atoms/normal/luminous]
10/23/2007
The Cosmic Triangle
 Mass density of Universe: 25% of Critical
[only 5% atoms/normal/luminous]
 Universe will expand forever
 Dark Energy in Universe: 75%
 Universe expansion accelerates
 Space Curvature: 0
 Universe ‘Flat’

34. Fate of Universe  Sparser and Sparser  Darker and Darker
10/23/2007
Universe Will Expand Forever
becoming:
 Larger and Larger
 Sparser and Sparser
 Darker and Darker
 Colder and Colder

36. 10/23/2007

37. Sloan Digital Sky Survey
10/23/2007

38. Baryons in Clusters [Stars and Gas]
10/23/2007
Baryons in Clusters [Stars and Gas]
 Ωb/Ωm(cl)  Mb/Mtot(cl) =
0.13 (gas) (stars) = 0.16 (h=0.7)
Ωb(BBN; CMB) = (h=0.7)
Ωm = Ωb/(Ωb/Ωm) =
 corrected for gas outflow

39. m from Baryon-Fraction
10/23/2007
m from Baryon-Fraction
b/m = h=0.7
(Clusters; CMB)
b = (BBN; CMB)
 m =

40. Cluster Abundance and Evolution
10/23/2007
Cluster Abundance and Evolution
 Powerful method to determine m and 8
8 = Amplitude of mass fluctuations
(initial ‘seeds’)
ncl (z~0)  8 m0.6 ~ 0.35
ncl (hi z)  Breaks degeneracy
 m= and 8=

41. Cluster Mass-Function (SDSS) (Bahcall, Dong, et al ‘02) Best-fit MF: m=0.2 and 8=0.9
10/23/2007
Fit: m=0.2
8=0.9

42. Cluster Abundance Evolution  8 (Bahcall & Bode)
10/23/2007
Cluster Abundance Evolution  8 (Bahcall & Bode) 8

43. Weighing the Universe  Universe Expands Forever!
10/23/2007
Weighing the Universe
 M/L Function m=
 Baryon Fraction
 Cluster Abundance
and Evolution [8 = ]
Supernovae Ia + Flat
CMB + LSS + h + Flat
 m =
 4-5% Baryons + ~20% Dark Matter
Mass ~ Light
 Universe Expands Forever!

44. Accelerating Universe: Supernovae (Kirshner et al)
10/23/2007