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?