Dark Matter and the Large Scale Structure of the Universe
暗物質及宇宙建構
Lam Hui 許林Columbia University
This is the second in a series of 3 talks.
July 5: Dark energy and the homogeneous
universe
Today: Dark matter and the large scale
structure of the universe
July 18: Inflation and the early universe
Outline
Basic mechanism: gravitational instability
Basic ingredient: dark matter
Review: the inhomogeneous universe
Basic problem: mass
Basic statistical tool: correlation function
Blandton & Hogg
Hubble deep field
CfA survey
Sloan Digital Sky Survey
WMAP website
recombination
CMB: cosmic microwave background
Sloan Digital Sky Survey
Gravitational Instability
Interested quantity: overdensity
LANL
Scale factor a(t)
x=0 x=2x=1
x=0 x=1 x=2
time=t
time = t’
distance = a(t)Δx
distance = a(t’)Δx
Fundamental equations
mass conservation:
momentum conservation
Poisson’s equation:
i.e. F = ma :
From:
Obtain:
Growing mode:
Gravitational Instability
Interested quantity: overdensity
Problem:
There is not enough time for structureto grow from recombination to today!
p+
e-γ
Solution:
γ
H
Dark matter - no/little interaction with photons; pressureless.
Blandton & Hogg
Credit: Richard Pogge
Evidence from rotation curve (Vera Rubin)
WMAP
What makes up dark matter?
Most plausible candidate: Weakly Interacting Massive Particle (WIMP)
If annihilation cross section naturally get the right left over
abundance.
,
credit: U. Zaragoza
Direct detection of WIMPs
Current structure formation paradigm:Pressureless (cold) dark matter, which makes up about 20% of the universe,forms structure via gravitational instability.
To understand the quantitative successof this model, we need to learn one statistical tool: the 2 point correlationfunction.
Two-point correlation function ξand power spectrum P
r
Common to plot:quantifies amplitude of
fluctuations at scale 1/k.
threshold
Hierarchical clustering: small things form first
Problem: usually don’t observe mass directly.
CMB: observe temperature Galaxies: observe counts
Want: mass
Rauch
☼
n=2
n=11216 Å
Gravitational lensing
☀lens
observer
source
☀
☀image
image
deflection angle = 4GM/r
Jain
Tegmark
Summary- Structure formation occurs via the gravitational instability of cold dark matter.- Small galaxies form first, which then merge to form bigger things.- A major goal of observational cosmology is to measure the mass fluctuations.- We still don’t know what dark matter is.