Dark Matter and the Large Scale Structure of the Universe 暗物質及宇宙建構 Lam Hui 許林...

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.