This document discusses dark matter detection. It presents evidence for the existence of dark matter from observations of galaxy clusters, and notes that dark matter is thought to make up around a quarter of the universe. It also mentions an alternative theory that dark matter may not exist and that modified gravity could explain observations instead. The document outlines different approaches to detecting dark matter, including looking for it at different astronomical scales and using multi-wavelength, multi-epoch, and multi-messenger observations.

1. CTICS 2012 Jan 24th, 2012
Dark Matter detection (1)
Sergio Colafrancesco
Wits University - DST/NRF SKA Research Chair
INAF - OAR
Email: Sergio.Colafrancesco@wits.ac.za
Email
Sergio.Colafrancesco@oa-roma.inaf.it
1

2. Dark Matter exists !
Dark Matter Proof Found,
Scientists Say
[F. Zwicky 1933]
A team of researchers has found the first direct proof for the
existence of dark matter, the mysterious and almost invisible
substance thought to make up almost a quarter of the universe.
In this composite image, two clusters of galaxies are seen after a
collision. Hot gas, seen in red, was dragged away from the
galaxies during the collision. That gas makes up more than 90
percent of the mass of normal, or visible, matter. But most of the
mass—and thus matter—is located in the galaxy portions of the
clusters, shown in blue, scientists say. In other words, the bulk of
visible matter in the clusters has been separated from the majority
2
of mass—which therefore must be dark matter.

3. Dark Matter exists ! … or not !?
Dark Matter Proof Found, Dark matter does not exist !
Scientists Say Einstein wins again!
J. Moffat and colleagues
suggest that there is a
good reason dark matter
why has never been
[F. Zwicky 1933] directly detected:
It doesn't exist .
A team of researchers has found the first direct proof for the J. Moffat suggests that his Modified Gravity
existence of dark matter, the mysterious and almost invisible (MOG) theory can explain the Bullet Cluster
substance thought to make up almost a quarter of the universe. observation. MOG predicts that the force of
In this composite image, two clusters of galaxies are seen after a gravity changes with distance.
collision. Hot gas, seen in red, was dragged away from the Moffat thinks that the present day expectation by
galaxies during the collision. That gas makes up more than 90 many that dark matter must exist is similar to the
percent of the mass of normal, or visible, matter. But most of the expectation by many leading scientists in the
mass—and thus matter—is located in the galaxy portions of the beginning of the 20th century that a
clusters, shown in blue, scientists say. In other words, the bulk of "luminiferous ether" should exist. This was a
visible matter in the clusters has been separated from the majority hypothetical substance, in which the waves of
3
of mass—which therefore must be dark matter. light were supposed to propagate

4. Dark Matter Modified G
Fundamental Physics - Astronomy connection
Gµν=8πG(TMµν+TDMµν+ΤDEµν) F(gµν)+Gµν=8pG TMµν
3 GM  exp(− r / L) 
Φ (r ) = −  1 + 
4a1 R 3
4

5. Multi-wavelength
Multi-messenger Multi-experiments
5

6. Multi-wavelength
Multi-epoch Multi-scale
Multi-messenger Multi-experiments
6

7. Outline
Multi-epoch
The Dark Matter Timeline
The present
Multi-Scale + M3
Galactic center
Galactic structures
Galaxy Clusters
The Future
The DM search challenge
7

8. Outline
Multi-epoch
The Dark Matter Timeline
The present
Multi-Scale + M3
DM search at various astronomical scales
• Galactic center
• Galactic structures
• Galaxy Clusters
The Future
The DM search challenge
8

9. Dark Matter timeline
2T + U = 0 Coma
σ V ≈ (1019 ± 360)km / s
M
≈ 400 ⋅ h558
L
Zwicky used the words “dunkle (kalte) Materie”
dark (cold) Matter
which might be regarded as the first reference to
Fritz Zwicky Cold Dark Matter
[Varna (Bulgaria), 1898 – Pasadena (USA), 1974]
… even though not in the modern sense (!?)
9

10. Dark Matter timeline
1936 - Smith noticed that 1939 - Babcock noticed that the
also the Virgo cluster exhibited outer regions of M31 were rotating
a behavior suggestive of an with an unexpectedly high velocity
extremely high mass. indicating a missing mass.
10

11. Dark Matter timeline
1910’s
Ernst Öpik & wife Grigory Kuzmin
Local Dark Matter Global Dark Matter
Öpik (1915) Zwicky (1933)
Oort (1932, 1960) Zwicky discovered what so many scientists find when
Kuzmin (1952, 1955) probing the depths of the current and accepted
knowledge of the times.
Eelsalu (1959)
What Zwicky uncovered was considered an anomaly.
Jõeveer (1972, 1974) Zwicky, who did not particularly belong to the
Bahcall (1985) astronomical community, was making a claim that
could overthrow present knowledge of the universe.
Gilmore et al (1989)
It was not the right time for the astronomical
community to accept such a revolutionary idea.
11

12. Dark Matter timeline
1959 - Kahn & Woltjer published their 1961 - The renaissance of Dark Matter truly began
discovery of a missing mass in the Local with the Santa Barbara Conference on the Instability of
Group (  hot gas with T ∼ 5·105 K ) Galaxies.
Galaxies
Interestingly enough, they did not cite By this time, enough research was done for the
Zwicky’s (1933) paper. community to see that the missing mass anomaly was
not going to go away.
“When... an anomaly comes to seem more than just
another puzzle of normal science, the transition to crisis
and to extraordinary science has begun” (Kuhn).
Vera Rubin working at the Ford spectrograph (1955) 12

13. Dark Matter timeline
1980 - Experimental results on
the neutrino rest mass were
Jim Peebles explains the secrets of galaxy formation announced.
to Scott Tremaine (Tallinn 1977)
1975 – By this time the majority of astronomers
had become convinced that missing mass existed in
cosmologically significant amounts.
Uncertainty on the Dark Matter nature remained !!!
1977 – Rees speculated that “there are other
possibilities of more exotic character – for instance
the idea of neutrinos with small (∼few eV) rest mass” Zel’dovich & Longair
(Tallinn 1977) 13

14. Dark Matter timeline
A high-resolution CDM simulation with small-scale structure
1980’s – The Cold Dark Matter model with
axions or other weakly interactive particles WIMP
was as an alternative to neutrino models (providing
the Hot Dark Matter). 14

15. Dark Matter timeline J. Soldner 1804
A. Einstein 1911
1990’s – Dark Matter distribution in
clusters can explain the gravitational
lensing of background galaxies.
15

16. False Alarms & Diversionary Manouvres
J. Oort (1960, 1965) believed that he had found some
dynamical evidence for the presence of missing mass in
the disk of the Galaxy. If true, this would have indicated
that some of the DM was dissipative in nature. However, 1987 – One particularly interesting dissenter:
late in his life, Oort confessed that the existence of M.Milgrom. He believed that the existence of the DM
missing mass in the Galactic plane was never one of his implied that Newton’s law of gravity must be amended
most firmly held scientific beliefs. Detailed observations, for gravitational accelerations that are very small, such
(reviewed by Tinney 1999), show that brown dwarfs as the gravitational accelerations seen in a galaxy’s
cannot make a significant contribution to the density of outer fringes.
the Galactic disk near the Sun. Bekenstein followed up Milgrom’s idea in TeVeS model 16

17. Dark Matter timeline
1992-99 – Dark Matter is a main
ingredient of the cosmic fluid and its
effect is present in the CMB anisotropy
spectrum .
17

18. Dark Matter timeline
1990-2000 – Naissance of
Astroparticle – Dark Matter
physical connection.
18

19. The Dark Matter Scenario: timeline
2010
Missing mass Rotation N-body simulations Lensing CMB
Cosmology
Astronomy
Dynamics curves
1977
Thermal relics
Particle Astro Particle
(Lee & Weinberg)
Today
Physics Physics
1984
Indirect DM search idea
ν - mass (SIlk Beyond the SM
& Srednicki) !?
theo. & exp. 1985 SUSY
DirectSUSY
DM experiments AXION
(Goodman & Witten)
Sterile ν
∆ m 2ν
19

20. The Present
“The Present and the bubbles of Time" - Oil and acrylic on thin cardboard (B. D’Aleppo)
20

21. Dark Matter Scenario: motivations
WMAP
21

22. DM & CMB
Generic ΛCDM WMAP 7yr
h 2Ω DM = 0.1123 ± 0.0035
0.222 ≤ Ω DM ≤ 0.236
22

23. DM distribution in cosmic time
Dark Matter grows
increasingly 'clumpy'
as it collapses under
gravity.
The map stretches halfway back
to the beginning of the universe
23

24. DM relic density
n σV ≈ H
x
WIMPs
in thermal equilibrium
3 ⋅ 10 − 27 cm − 3 s − 1
Freeze-out Ω χ h ≈
2
σV A
24

25. Formation of DM halos
25

26. DM: the most palpable proof
26

27. DM: the most palpable proof
27

28. DM properties: 5 basic
Very weak e.m. interactions
Dissipationless no radiative cooling
Dark Matter
DM self-interaction only at No constraint on the
Collisionless high ρ and short relative D space of possibilities
MX > 1 keV thermal
Cold MX smaller non-thermal
No DM discreteness
Fluid on galactic scales Upper bound
M X ≤ 10 70 − 71 eV
DM confined
Classical on galactic scales Lower bound
M χ ≥ 10 − 22 eV
28

29. DM candidates
“Fuzzy” CDM Lower possible end of CDM
Bosons with M∼10-22 eV
Chaplygin Gas DM-DE common origin
P=-A/ρ
Non-thermal production
Axions µ eV < Maxion < m eV
Experimental limits
Neutrinos
Dark Matter
Warm DM 0.0005 < Ων h2< 0.0076
Massive neutrinos acceptable
Sterile ν with mν ∼ 10-100 keV
Light (MeV) DM
Spin = 0 supersymmetric particle Gravitinos
1 MeV < MLDM < 4 MeV
SUSY DM Elusive: only e± 511 keV line Neutralinos
Neutralinos
Kaluza-Klein excitations
Extra dimension L-KK (r-parity) particle: stable Sneutrinos
MKK ∼1 TeV
Axinos
Branons String theory brane fluctuations
Mbranon > 100 GeV Q-balls
Mirror Matter Ordinary matter in mirror world
Split-SUSY
Dissipative & complex chemics
MWIMP
WIMPzillas Produced at the end of Inflation
M > 1013 GeV
PBHs BHs @ quark-hadron transition
MPBH ∼Mhorizon(T=102MeV) > M 29

30. Viable DM candidates
Neutralinos Light (MeV) DM Sterile ν’s
Unstable
3 ⋅ 10 − 27 cm − 3 s − 1
Ω χh ≈2
σV A Radiative decay: line
0.09 ≤ ΩInDM h 2 ≤ 0.13 models, all Standard Model
supersymmetry
particles have partner particles with the same
quantum numbers but spin differing by 1/2. Since ν → ν + γ
s α
the superpartners of the Z boson (zino), the photon
(photino) and the neutral higgs (higgsino) have the
same quantum numbers, they can mix to form four
eigenstates of the mass operator called
"neutralinos". In many models the lightest of the
four neutralinos turns out to be the lightest
supersymmetric particle (LSP).
30

31. Viable DM candidates
Neutralinos Light (MeV) DM Sterile ν’s
Unstable
3 ⋅ 10 − 27 cm − 3 s − 1
Ω χh ≈2
σV A Radiative decay: line
Scalar ≤ (spin=0)2 ≤ 0.13 may be DM candidates, provided they annihilate
0.09 Ω DM h particles
sufficiently strongly through new interactions, such as those induced by a new
light neutral spin-1 boson U. The corresponding interaction is stronger than weak
νs → να + γ
interactions at lower energies, but weaker at higher energies. Annihilation cross
sections of (axially coupled ) spin-1/2 DM particles, induced by a U vectorially
coupled to matter, are the same as for spin-0 particles. In both cases, the cross
sections (σannVrel/c) into e+e− automatically include a v2dm suppression factor,
needed to avoid an excessive production of γ-rays from residual DM annihilations.
Spin-0 DM particles annihilating into e+e− have been claimed to be responsible for
the bright 511 keV γ-ray line observed by INTEGRAL from the galactic bulge.
31

32. Viable DM candidates
Neutralinos Light (MeV) DM Sterile ν’s
Unstable
3 ⋅ 10 − 27 cm − 3 s − 1
Ω χh ≈
2
The term sterile A neutrino was coined by Bruno
σV Decay
Pontecorvo who hypothesized the existence of the
0.09 ≤ Ω DM h 2 ≤ 0.13
right-handed neutrinos in a seminal paper (1967), in
which he also considered vacuum neutrino oscillations
in the laboratory and in astrophysics, the lepton
number violation, the neutrinoless double beta decay,
some rare processes, such as μ → e γ, and several Radiative decay: line
other questions that have dominated the neutrino
ν s → να + γ
physics for the next four decades. Most models of the
neutrino masses introduce sterile (or right-handed)
neutrinos to generate the masses of the ordinary
neutrinos via the seesaw mechanism.
32

33. Viable DM candidates
Neutralinos Light (MeV) DM Sterile ν’s
M
a lD Unstable
g ic
olo
Radiative decay: line
sm
Co
o
N
νs → να + γ
Excluded by BBN
WMAP (95%)
SNe
Excluded by BBN
e- anomalous magnetic moment
Bremsstrahlung
in-flight annihilation
33

34. Hunt for the DM particle
DM exists:
we feel its (gravitational) presence
DM is mostly non-baryonic:
we must think of a specific search strategy
DM is very elusive:
we must consider un-ambiguous evidence
Crucial Probes are required !
34

35. Dark Matter probes
Above-the-ground
Under-ground
35

36. DM direct search
χ
δ = 30o
vsun= 230 km/s
χ
vorb = 30
km/s
Elastic interaction on nucleus,
typical χ velocity ~ 250 km/s
χ
χ
ER ~ 10-30 keV
36

37. DM direct search
37

38. The first hint
There is evidence for a modulation in
the DAMA data at 8.2 σ.
Compatible with what would be
Gran Sasso (Italy)
expected from some dark matter
particles in some galactic halo
models.
38

39. Some of the latest results: CDMS II
2 events in the observed signal region.
Based on background estimate, the probability of observing
two or more background events is ~23%.
Ahmed et al. arXiv:0912.3592v1 39

40. Some of the latest results: CoGeNT
40

41. Some of the recent results: CRESST-II
[arXiv:1109.0702]
41

42. DM direct search: criticalities
Experimental techniques Astrophysics
Effect of substructures
Assume a local DM density on the local DM density
and a DM halo structure
[Kamionkowski & Koushiappas 2008] 42

43. DM search @ accelerators
“Well, either we’ve found the Higgs boson,
or Fred’s just put the kettle on.”
CERN - Geneva
43

44. LHC vs. direct detection experiments
Accelerator searches for DM are particularly promising …
but even if WIMPs are found at the Large Hadron Collider
(LHC), it will be difficult to prove that they constitute the bulk
of the DM in the Universe.
44

45. DM - Astrophysical probes
INFERENCE PHYSICAL
+ +
Virial Theorem
Annihilation Decay
Hydro Equilibrium
X+ X→ π 0, ±
, p,... X → xi + γ
Gravitational lensing X + X → e ± , µ ± ,τ ± ,ν i
45

46. DM - Astrophysical search
INFERENCE PHYSICAL
AL
UCI AL
R I
T C
R UC
+
NO C
Vulnerable against:
Virial Theorem
MOND: Modified Newtonian Dynamics Annihilation Decay
Testable against:
TeVeS : Tensor-Vector-Scalar
Hydro Equilibrium
X + X → π 0, ± , p,... X → xi + γ
Ordinary matter feels a transformed metric Electromagnetic signals
Gravitational lensing X + X → e ± , µ ± ,τ ± ,ν
i
DM illuminates thru its interaction
46

47. DM - Astrophysical search
Clean and unbiased location in the sky
 Best Astrophysical Laboratories
NO YES
Clear and specific SED in the e.m. spectrum
 Most specific e.m. signals
47

48. Viable DM candidates: signals
Neutralinos Light (MeV) DM Sterile ν’s
M
Annihilating MeV DM
a lD Radiative decay: line
ic
• Continuum: HXR/γ-rays
νs → να + γ
og
• Line: e± annihilation
ol
sm
Co
o Ms
N
511 keV
Inverse Compton scattering
Bremsstrahlung
π 0 Mχ
Synchr.
Bremsstrahlung
48

49. Viable DM candidates: signals
Neutralinos Sterile ν’s
Annihilation Radiative decay: line
νs → να + γ
Neutralino density profile
~ (nχ · nχ)
~ ρ2DM
Sterile ν density profile
~ nχ
~ ρDM 49

50. Viable DM candidates: signals
Neutralinos Sterile ν’s
Annihilation Radiative decay: line
νs → να + γ
DM annihilation flux DM decay flux
1 ρ DM ( r )
2
1 ρ DM ( r )
F ∝ 2 2
〈σ V 〉 Astro physics F ∝ 2
〈 Γ rad 〉
DL Mχ DL Mv
 dE 
× [ f ann ( E ; χ )]  Particle physics [ ]
 dE 
× Eγ ( M v )  
 dν   dν 
50

51. Dark Matter halo structure
51

52. DM halo profile: constraints
Galaxies Galaxy Clusters
• No evidence for a density
cusp at r < 0.2 kpc
• NGC2976: η =0.27 at r < 1.8 Kpc
[Simon et al. 2003] η ! l. 2003]
on alal et a
η
• Inner steeper profile ? s [D
i nt
tra
[Gnedin & Primack 2003]
NFW ons
BH ?
n oc
[Sand et al. 2002]
MS2137-23
Cluster η1T η2T
A2029 0.5 2
NGC 6822 A1689 1.3 1.6
[Weldrake et al. 2002] A1835 0.9 1.1
MS1358 1.8 1.8
[Bautz & Arabadjis 2002] 52

53. DM density universal profile
Galaxies Clusters
Dwarfs
Numerical simulations (CDM)
Different groups obtained similar results
[Navarro et al. 2003, Reed et al. 2003, …]
Analytical fitting
General DM profile
53

54. DM halo: smooth + clumps + BHs
Cluster of galaxies
[CPU 2006]
[Berezinsky et al. 2006]
dSph Galaxy
54

55. Imagine a …
Galaxy Cluster of galaxies
55

56. An astronomer’s view
Galaxy Cluster of galaxies
56

57. A cosmologist’s view
Galaxy Cluster of galaxies
57

58. An Astroparticle Physicist’s view
Galaxy Cluster of galaxies
58

59. Theoretical description
[Colafrancesco et al. 2006 A&A 455, 21–43]
59

60. DM halo profile
We consider the limit in which the mean DM distribution
can be regarded as spherically symmetric
and represented by the parametric radial density profile
Two schemes are adopted to choose the function g(x) , with x=(r/a)
Scheme 1
Assume that g(x) can be directly inferred as the function setting the universal
shape of DM halos found in numerical N-body simulations. We are assuming,
hence, that the DM profile is essentially unaltered from the stage preceding the
baryon collapse, which is – strictly speaking – the picture provided by the
simulations for the present-day cluster morphology.
[NFW 2004]
[Diemand 2005]
60

61. DM halo structure
Scheme 2
The other extreme scheme is a picture in which the baryon infall induces a
large transfer of angular momentum between the luminous and the dark
components of the cosmic structure, with significant modification of the shape
of the DM profile in its inner region.
Baryons might then sink in the central part of DM halos after getting clumped
into dense gas clouds (see El-Zant et al. 2001), with the halo density profile in
the final configuration found to be described by a profile with a large core
radius (see, e.g., Burkert 1995):
Once the shape of the DM profile is chosen, the radial density profile g(x)
is fully specified by two parameters:
i) length-scale a ii) normalization parameter ρ.
It is useful to describe the density profile model by other two parameters:
the virial mass Mvir and concentration parameter cvir. 61

62. DM halo structure
Concentration parameter
We introduce the virial radius Rvir of a DM halo
of mass Mvir as the radius within which the
mean density of the halo is equal to the virial
overdensity Δvir times the mean background
density
We assume that the virial overdensity can be
approximated by an expression appropriate for
a flat cosmology
[Colafrancesco et al.’94,’97; Bryan & Norman ’98, …]
The concentration parameter is then defined as
with r−2 the radius at which the effective logarithmic slope of the DM profile is −2.
• x−2 = 1 for the N04 profile
• x−2 = 2−γ for D05
• x−2 = 1.52 for the Burkert profile 62

63. DM halo structure
Since the first numerical results with large statistics became available (Navarro et
al. 1996, 1997), it has been realized that, at any given redshift, there is a strong
correlation between cvir and Mvir, with larger concentrations found in lighter
halos.
This trend may be intuitively explained by the fact that mean overdensities in halos
should be correlated with the mean background densities at the time of collapse,
and in the hierarchical structure formation model small objects form first, when the
Universe was indeed denser.
The correlation between cvir and Mvir
is relevant at two levels:
• when discussing the mean density profile
• when including substructures
[Bullock et al]
[ENS]
63

64. DM Halo structure
For a given shape of the DM halo profile we fit of the parameters Mvir and cvir
against the available dynamical constraints for the Coma cluster.
We consider bounds on the cluster total mass at large radii and on density
profile in its inner region.
• Redshift-space caustics (Geller 1999)
• HE condition (Hughes 1989)
• Velocity moments of a tracer population
(Binney & Mamon 1982)
[Colafrancesco et al. 2006 A&A 455, 21–43]
64

65. DM Halo structure
Sub-structures
ρDM ρ2DM
65

66. DM Halo structure
To discuss substructures in a cluster,
analogously to the general picture introduced
above for DM halos, we label a subhalo through
• virial mass Ms
• concentration parameter cs.
• The subhalo profile shape is considered here
to be spherical and of the same form as for the
parent halo.
• The distribution of subhalos in a DM halo is
taken to be spherically symmetric.
symmetric
66

67. DM Halo structure
The subhalo number density probability distribution can then be fully specified
through Ms, cs and the radial coordinate for the subhalo position r.
The sub-halo mas function is
[Diemand 2005]
A(Mvir) is derived imposing
The quantity Ps(cs) is a log-normal distribution in concentration parameters
around a mean value set by the substructure mass.
The 1σ deviation Δ(log10 cs) around the mean in Ps(cs), is assumed to be
independent of Ms and of cosmology, and to be, numerically, Δ(log10 cs) = 0.14
ps(r) is the spatial distribution of substructures within the cluster.
(with a’>a) and normalized as:
67

68. DM annihilation distribution
For any stable particle species i, generated promptly in the annihilation or
produced in the decay and fragmentation processes of the annihilation
yields, the source function Qi(r, E) gives the number of particles per unit time,
energy and volume element produced locally in space:
Npairs(r) is obtained by summing the contribution from the smooth DM component
and the contributions from each subhalo
68

69. DM annihilation distribution
Δ2Ms gives the average enhancement in the source due to a subhalo of mass Ms,
Δ2 is the sum over all contributions weighted over the subhalo MF times Ms
69

70. Viable DM candidates: signals
Neutralinos Sterile ν’s
Annihilation Radiative decay: line
νs → να + γ
DM annihilation flux DM decay flux
1 ρ DM ( r )
2
1 ρ DM ( r )
F ∝ 2 2
〈σ V 〉 Astro physics F ∝ 2
〈 Γ rad 〉
DL Mχ DL Mv
 dE 
× [ f ann ( E ; χ )]  Particle physics [ ]
 dE 
× Eγ ( M v )  
 dν   dν 
70

71. Outline
Multi-epoch
The Dark Matter Timeline
The present
Multi-Scale + M3
Galactic center
Galactic structures
Galaxy Clusters
The Future
The DM search challenge
71

72. THANKS
for your attention !
72