The Dark Matter Problem astrophysical perspectives
陈学雷中国科学院国家天文台
What can we learn from astrophysics?
The data evidence of DM abundance of DM distribution of DM
The questions Nature of DM property of DM (mass, interaction, ...) role of DM in cosmic history origin of DM, and relation with DE
Outline
What we have learned evidence of DM and its abundance DM is not baryonic DM is not hot WIMP: the classic CDM What we are learning cuspy halos and missing satellites alternative models of DM CDM strikes back: the mundane answers
The role of DM (an example) DM decay and reionization
Evidence of DMgalaxy rotation curve
dynamics of galaxy cluster
Virial theorem
U=2K K = mi vi
2
U ~ GM2/R
mass to light ratio (B) typical cluster: 100/h-300/h Sun
stellar pop: 1-10 Sun critical: 1390 h +- 35%
Coma cluster
X-ray cluster
hydrostatic equilibrium
beta model:
Strong Gravitational Lensing
Weak Lensing mass reconstruction
RXJ1347.5-1145 (Bradac et al 2005)
Image ellipticity -> shear->
invert the equation
DM Abundance
• mass to light ratio x light density• cluster baryon fraction/BBN baryon abundance• cluster mass function• evolution of cluster mass function
Bahcall: m=0.2Blanchard: m=1.0
WMAP result
Spergel et al 2003
mh2=0.135+-0.009m=0.27+-0.04
WMAP Combined fit:
Results depend on Supernovae and Hubble constant data.
Can DM be baryons?
If all DM is baryonic, it is in conflict with Big Bang Nucleonsynthesis and Cosmic Microwave Background anisotropy.
MAssive COmpact Halo Objects (MACHO)
The result of MACHO experiment (Alcock et al 1996):20% of halo can be due to MACHO
LMC
Abundance of DM: WIMP?
dark -- weakly interacting?
In early Universe, even weak interaction is effective, abundance given by
freeze out when H = n <Av>, the dark matter abundance is
comparable to weak interaction
Collisional Damping and Free Streaming
Initial density perturbation is damped by the free streaming of the particles before radiation-matter equality
perturbations on scales smaller than rFS is smoothed out.
Kinetic decoupling at T ~ 1 MeV (Chen, Kamionkowski, Zhang 2001)
Structure Formation
at freeze-out if weakly interactinghot dark matter relativistic m< 1 keVwarm dark matter quasi-relativistic 1 keV < m < 10 keVcold dark matter non-relativistic m > 10 keV
at freeze-out
The failure of HDM: clusters form before galaxy, can not account small scale structures.
The first dark halos
Due to collisional damping and free-streaming, the smallest halo (no sub-structure) is 10-6 solar mass (earth mass) for neutralino. Dection of such halo may probe the nature of DM.
Diemand, Moore, Stadel 2005
substructure of DM halo
B. Moore et al missing satellites?
simulated Local Group mass system
Dark matter halo profile
simulation (Navarro, Frenk, white 1996): cusp
observation: core
NFW96, rotation curve
Alternatives to CDM
WDM: reduce the small scale powerSelf-Interacting Dark Matter (Spergel & Steinhardt 2000) Strongly Interacting Massive Particle Annihilating DMDecaying DMFuzzy DM
WDM
From Jing 2000
SIDM
DM strongly interact with itself, but no EM interaction can create an core in hierachical scenario (eventually core collapse -> isothermal profile)
Interaction strength: comparable to neutron-neutron
Difficulty: make spherical clusters: against lensing
SIMP
Motivation:• SIDM may have QCD interaction but not EM• Not detectable in WIMP search, blocked.
CMB & LSS constraint:Before decoupling, photons and baryons are tightly coupled, interaction with baryon will cause additional damping of perturbation
Test DM interaction with CMB and LSS
Chen, Hannestad, Scherrer 2002
missing satellites: CDM solution
• satellites do exist, but star formation suppressed (after reionization?)
• satellites orbit do not bring them to close interaction with disk, so they will not heat up the disk.
• Local Group dwarf velocity dispersion underestimated
• halo substructure may be probed by lensing (still controversial)
• galaxy may not follow dwarf
Rotation curve
• Is density profile really universal? scatter in concentration• What is the real slope NFW: 1.0 Moore 1.5, ..., Power et al, Diemand et al, 1.2• Observation beam smearing? 21cm vs H some agree w/ cusp, but most dwarf slope 0.2 • Cusp cheat as core vg != vc , because of inclination, effect of bulge and bar, gas supported by pressure, star orbit in triaxial halo, ... (Rhee et al 2004, Hayashi etal 2003)
active DM: decaying particle
Reionization Rephaeli & Szalay 1981; Salati & Wallet 1984;
Ionization of Reynolds layer, ISM, IGM Sciama 1982-1996; Melott, 1984; but see Bowyer et al 1999
Resolve the conflict between SCDM model and =0.3 Gelmini, Schram & Valle, 1984; Turner, Steigman, Krauss, 1984; Doroshkevich, Khlopov, 1984
If decay early, can affect BBN Audouze, Lindley, Silk 1985; Starkman 1988, Dimopoulos et al 1988
If decaying particle heavy, may give Ultra High Energy Cosmic Rays Frampton & Glashow 1980; Ellis, Steigman, Gaisser 1981; Berezinsky, Kachelriess & Vilenkin 1997; Birkel & Sarkar 1998
Decaying dark matter & small scale crisis
Cen 2001
Candidates of decaying DM
active neutrino, sterile neutrino, unstable susy particle, crypton, super heavy dark matter, R-violating gravitino, moduli, axino, SWIMP, quintessino, Q-ball, topological defect, primodial black hole ...
decaying DM & reionization
standard picture of reionization
surprise from WMAP: early reionization
Thermal History with Decaying DM
Long lived particle short lived particle
Constraints on decaying DM
Summary
• Observations are in general agreement with LCDM, most data consistent with low DM density (0.2-0.3), but there are different voices.
• Small scale crisis: the problem is complicated, explanations inside/outside LCDM paradigm are available
• Many properties of DM can be studied with astrophysical observation
• Some observations unexplained in simplest version of LCDM (tight Tully-Fisher relation, downsizing in galaxy formation, ...)
• Open questions: role of DM in cosmic evolution? relation with DE?