Lecture 27: Dark Matter
The most exciting phrase to hear in science, the one
that heralds the most discoveries, is not ‘Eureka!’
but ‘That’s funny...’
-- Isaac Asimov
dark matter caught astronomers by surprise
what is it, and what do we know about it?
start with observations and develop theory
otation
curves
Observation 1: Rotation Curves
M=Rv2/G
Observation 2: Galaxy Clusters
galaxy clusters filled with hot
gas (purple), more mass than
galaxies
assuming gas in hydrostatic
equilibrium inconsistent with
visible mass
velocity dispersion of cluster
galaxies also inconsistent with
visible mass
Theory
different physical law
MACHOS (BH, WD)
unseen baryons (cold molecular gas, ‘bricks’)
‘hot’ (eg, neutrinos) vs ‘cold’ (eg, WIMPS)
dark matter
Observation 3: Bullet Cluster
1E0657-56 (The Bullet Cluster)
•! Supersonic merger
•! z=0.30 (3.35 Gyr ago, or 1.2 Gpc away)
Observation 3: Bullet Cluster (cont.)
Gas
Galaxies
Dark matter (mass)
Gas
Galaxies
Dark matter (mass)
Scenario I: Collisionless Dark Matter
Gas
Galaxies
Dark matter (mass)
Scenario II: Modified Gravity, mass follows gas
Strong lensing
Weak lensing
Theory
different physical law
MACHOS (BH, WD)
unseen baryons (cold molecular gas, ‘bricks’)
‘hot’ (eg, neutrinos) vs ‘cold’ (eg, WIMPS)
dark matter
Limits
from
microlensing
Observation 4: MACHO
limits from microlensing
< 10% of MW halo from
MACHOS
Theory
different physical law
MACHOS (BH, WD)
unseen baryons (cold molecular gas, ‘bricks’)
‘hot’ (eg, neutrinos) vs ‘cold’ (eg, WIMPS)
dark matter
Observation 5: BBN baryonic matter limits
deuterium, helium-3, and
lithium put strong constraint
on amount of baryonic matter
Theory
different physical law
MACHOS (BH, WD)
unseen baryons (cold molecular gas, ‘bricks’)
‘hot’ (eg, neutrinos) vs ‘cold’ (eg, WIMPS)
dark matter
Observation 6: Structure Formation
start with the CMB
as time progresses, density perturbation will grow,
creating some of structure we see today
Observation 6: Structure Formation
WDM
if dark matter ‘hot,’ has high velocity,
likely to leave high density region,
decreasing its size and mass
CDM
high density region of space with dark
matter particles moving around
if the dark matter is ‘cold,’ has low
velocity, less likely to leave high
density region
Observation
6: Structure Formation
Structure
formation
Theory
different physical law
MACHOS (BH, WD)
unseen baryons (cold molecular gas, ‘bricks’)
‘hot’ (eg, neutrinos) vs ‘cold’ (eg, WIMPS)
dark matter