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Fundamental Cosmology 8.Dark Matter
You Dont understand the Power of the Dark
Side. Darth Vader - Star Wars Episode 6.
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8.1 The Fate of the Universe

• It all depends on Omega

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8.1 The Fate of the Universe

• It all depends on Omega

We would like to measure Wo (WM,WL) Supernova
Project constrains Wo1 but doesnt individually
constrain WM WL e.g. Wo (0.3,0.7), Wo
(0,0.4), Wo (1,1.7) all consistent with the
data! Want to measure WM independently

• We would also like to measure the contributions
  to WM
• Stars
• Gas
• Cold Stellar Remnants

• Neutrinos
• Exotic Particles

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8.2 Weighing the Galaxies

• A Story of Mass and Light

• Measured Luminosity Density of stars in visible
  Universe 108Lo,B Mpc-3

• Assume stellar mix of Solar Neighbourhood ? r
  4x108 MoMpc-3

• Density of the starlight in the Universe W r
  / rc 0.004 lt 0.5

Depends critically on assumed M/L Milky Way
90 stellar light from stars MgtMo 80
stellar mass from stars MltMo
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8.2 Weighing the Galaxies

• and Gas
• Galaxy Clusters

• COMA CLUSTER
• Abell 1656
• in constellation of Coma Berenices, near NGP
  pole.
• Distance 150Mpc (350 million light years )
• Size gt1.5Mpc
• estimated gt 1000 cluster member galaxies

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8.2 Weighing the Galaxies

• and Gravity
• Galaxy Rotation Curves

• Newtons Law of Gravitation the force of gravity
  between two bodies -
• increases as the product of their two masses
• decreases as the square of the distance between
  them

• Keplers Laws of Planetary Motion
• Orbital velocity is proportional to the inverse
  square root of the distance

? Motion of stars around the galactic center
should slow down with increasing distance from
the center of the galaxy.
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8.2 Weighing the Galaxies

• and Gravity
• Galaxy Rotation Curves

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8.2 Weighing the Galaxies

• and Gravity
• Galaxy Rotation Curves

1. Rigid body rotation at centre (speed increases
   with distance as if a single object)
2. Curve falls off slightly from centre
3. Curve flattens (Velocity is constant with
   distance ?Mass must be increasing with distance)
4. Galaxy is spinning too fast !! Visible matter is
   not sufficient to hold galaxy together!
5. Flat rotation curve extends beyond the luminous
   matter (21cm, CO)

The Problem of MISSING MASS ? Giant Dark
Spherical Halos
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8.2 Weighing the Galaxies

• Missing Mass ? - Rather MISSING LIGHT !!

• Galaxy Rotation Curve
• The Disk Component
• The Buldge ( stellar halo) Component
• Dark Matter Halo Component

• M/L in Disk 4
• M/L to edge of Disk 10
• M/L to Dark Halo 40 (75kpc)-100(300kpc)
• (estimated from Globular Cluster and satellite
  galaxy motion)
• (discs are unstable and would collapse to bar ?
  require halo)
• ? gt90 of galaxy mass in Dark Halo (WG0.16)
• Rotation curve must fall at edge of galaxy ?

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8.2 Weighing the Galaxies

• Galaxy Cluster Dynamics

• Zwicky 1933 Dispersion of radial velocities of
  Coma Cluster memebers 1000kms-1
• Not enough matter in luminous form ? Cluster
  should be flying apart !!
• Required dunkle materie

Measure the dynamical mass (i.e. gravity not
light) with VIRIAL THEOREM

• Assume
• Cluster stable, self gravitating, spherical
  distribution of N objects, mass m, position x

P.E. of system
K.E. of system
M Total mass of cluster R1/2 Radius of
cluster ltvgt2 Mean squared velocity of cluster
members
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8.2 Weighing the Galaxies

• Galaxy Cluster Dynamics
• - For Coma Cluster

• z 0.023 (from mean redshift of cluster members)
• ? Distance 100Mpc (cz/Ho)
• Mean square velocity 3vr2 (vr Radial Velocity
  900kms-1) ? ltvgt2 2.4x106ms-1
• in practice measure the half light radius (small
  correction0.5), R1.5Mpc

• From optical data
• Optical Luminosity of stars in Coma Cluster LB
  1012Lo,B
• Assume Stellar mix of Solar Neighbourhood M/LB
  4Mo/Lo,B
• Total stellar mass in Coma Cluster M
  3x1013Mo
• From X-ray Data
• ? Total gas mass in Coma Cluster Mg
  2x1014Mo 6 M

Assumed Solar Neighbourhood M/LB 4Mo/Lo,B
WRONG
Correct Mass to Light ratio ? M/LB 250Mo/Lo,B
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8.2 Weighing the Galaxies

• Gravitational Lensing

• General Relativity - Gravity can bend light -
  Gravitational Lens
• Dark Matter effects both the motion of matter
  and light
• Dark matter in intervening space distorts the
  background galaxies - Einstein Arcs
• For a dark matter lens directly along line of
  sight between observer and source - ?Einstein Ring

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8.2 Weighing the Galaxies

• Weak Lensing and cosmic shear

Measure of the distribution of mass in the
universe, as opposed to the distribution of
light (eg. Galaxy surveys)
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8.3 The Need for Dark Matter

• Measured, weighed and found wanting ..

M/L
Solar Neighbourhood 0.004
Galaxy Disk 10
Galaxy Halos 40-100
Galaxy Clusters 250
Mass
Stars M
Neutral Hydrogen (M31) 0.1M
Neutral Hydrogen (DDO 240) 0.1M
Hot Gas in clusters 6M
W
Solar Neighbourhood 0.004
Atomic/Molecular Gas 0.0008
Hot Gas in clusters 0.02
Galaxy Halos 0.08-0.16
Galaxy Clusters 0.2
LUMINOUS MATTER CANNOT ACCOUNT FOR DYNAMICS OF
STRUCTURES ON ALL SCALES !!! WHERE HAS ALL THE
LIGHT GONE ???
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8.3 The Need for Dark Matter

• Limits on Baryonic Matter Density (Wb ) from
  Nucleosynthesis

• Primordial Helium
• depends on ratio of neutrons to protons (25 H)
• ? weak dependence on W (?)
• Primordial Deuterium
• a steeping stone to the formation of Helium
• Efficiency of Helium production depends
  Deuterium
• Denser Universe( high ?)
• ? Deuterium processed more efficiently
• A high W(?)
• ? lower Deuterium Abundance
• Deuterium only destroyed in Astrophysical
  Reactions

The observed abundance of Deuterium today sets
upper limit for primordial abundance
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8.3 The Need for Dark Matter

• Limits on Deuterium Abundance

Detection of Deuterium in absorption spectra of
quasars
DISCREPENCY since Wcluster 0.2. NOT ENOUGH
BARYONS !!!
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8.3 The Need for Dark Matter

• Inflation

During inflation, H is constant W is driven
relentlessly towards unity Inflation can make
the Universe arbitrarily flat
Inflation ? W1
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8.3 The Need for Dark Matter

• CMB
• (WMAP, SDSS, SNP, 2dFGRS)

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8.3 The Need for Dark Matter

• Summing Up !

W
Solar Neighbourhood 0.004
Hot Gas in clusters 0.02
Galaxy Halos 0.08-0.16
Galaxy Clusters 0.2
Baryon Nucleosynthesis 0.04
Inflation 1
WMAP Dark Mass 0.23
WMAP Dark Energy 0.73

• Baryonic matter density consistent with local
  solar neighbourhood and intracluster medium
• Some of Halo mass possibly dark baryons -
  BARYONIC DARK MATTER
• What is this Baryonic Dark Matter ?
• Fraction of Halo and Cluster dark matter ?? NON
  BARYONIC !
• What is the form of this Dark Matter ?

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8.4 Baryonic Dark Matter

• Baryonic Dark Matter ?

• RED DWARF STARS lt 1Mo (To2000K)
• Not enough detected
• STELLAR REMNANTS (Black Dwarf, Neutron Stars,
  Black Holes) 1Mo
• Universe too young for so many remnants to form
• Universe too young for remnants to cool to Black
  Dwarf
• BROWN DWARF lt 0.08Mo (To1000K) - failed star
• Not enough detected
• JUPITERS / PLANETS / ROCKS 0.001Mo
• Not Seen
• Huge Numbers Required
• PRIMORDIAL HELIUM
• Recently detected, scattered throughout the
  intergalactic medium. This primordial matter may
  exceed all of baryonic matter previously
  accounted for.

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8.4 Baryonic Dark Matter

• MACHOS

Baryonic dark matter in galactic halos -
MAssive Compact Halo ObjectS

• Lensinsing Projects - detected several MACHOs,
  each positioned in front of stars in LMC.
• Microlensing events - no information about
  distance to lens (Dont know whether lens is
  close to the source star in LMC or observer in
  our galaxy, or in between.)
• ? Use Hubble - faint red star - distance 600ly
  away M0.1Mo
• Located in disc/luminous main part of our galaxy
  ? not halo.

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8.4 Baryonic Dark Matter

• MACHOS

HST detects too few Red Dwarves in the Milky Way
halo
? Red Dwarves ruled out as significant
contributors to dark matter in Milky Way (? other
galaxies)
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8.5 The Nature of Dark Matter

• Welcome to the Dark Side
• Non - Baryonic Dark Matter

• Even without constraints from Inflation/CMB
• 50-100 of Galaxy Halo must be non baryonic
• gt 80 of Clusters must be non-baryonic
• Adding constraints from inflation and CMB
• 96 of Universe is non-baryonic

• Candidates
• Hot Dark Matter
• Cold dark Matter
• Relics
• Dark Energy

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8.5 The Nature of Dark Matter

• To be born Dark, to become dark, to be made dark,
  to have darkness

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8.5 The Nature of Dark Matter

• Hot Dark Matter Candidates Light Neutrinos

• Neutrinos - The only non-baryonic candidate
  known to exist
• Neutrino background nn 3x(3/11)ng 3.4x108m-3
• Extremely weakly interacting (pass through few
  pcs lead)

• MSW Oscillations in solar neutrinos constrain
  mass difference between 2 Oscillating flavours
  0.007eV
• Observations of muon neutrinos in atmosphere
  constrain m-t mass difference 0.05eV
• Observations from Sanduleak -69 202 ? 22
  neutrinos in 12s ! (must be very light)

However If a mass for the neutrino is detected
then there will be a contribution to the Dark
Matter
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8.5 The Nature of Dark Matter

• Cold Dark Matter Candidates Axions

• Strong CP problem
• CP violation predicted but not observed on order
  of 10-8 (c.f. flatness problem in inflation)
• 1978 Peccei-Quinn Constraint- Introduce Spin 0
  pseudoscalar ? suppress Strong CP violation
• Requires symmetry breaking on GUT scales with
  particle mass ? 1/energy scale Peccei-Quinn
  Scale

Frank Wilczek allegedly was look for an
opportunity to use a washing detergent name
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8.5 The Nature of Dark Matter

• Cold Dark Matter Candidates The WIMPs
• Weakly Interacting Massive Particles (opposite to
  MACHOS !!)
• Supersymmetric Particles

SUPERSYMMETRY particle spin
Particle spin SS partner spin
quark 1/2 squark 0
lepton 1/2 slepton 0
photon 1 photino 1/2
gluon 1 gluino 1/2
W/Z 1 zino / wino 1/2
graviton 2 gravitino 3/2
Higgs 0 Higgsino 1/2
axion 0 axino 1/2
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8.5 The Nature of Dark Matter

• Cold Dark Matter Candidates The WIMPs
• Supersymmetric Particles

• As supersymmetry has a new symmetry, R parity,
• R Parity Conservation ? a new stable particle
• Relic particle will be the lightest
  Supersymmetric partner (LSP) with charge or colour

? CHARGED PARTICLES selectron, squark, smuon,
wino, charged Higgsino RULED OUT
msneutrino gt msleptons
gravitino - self annihilates too slowly
? too high abundance at
present epoch
Photino mass 0.5GeV ? Possible candidate for
LSP
Stranger possibilities - neutralino - mixing
state of photino, higgsino, wino states ??
Linear Collider at CERN - ee- collider 1TeV
Successor to the LHC (LHC too much debris) Should
discover Higgs, Supersymmetry, String dimensions
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8.5 The Nature of Dark Matter

• QUINTESSENCE - The Fifth Element
• Dark Energy

• Rolling homogeneous new scalar field behaving
  like a decaying cosmological constant (i.e. NOT
  CONSTANT )
• Eventually attain the true vacuum energy (energy
  zero point)
• Strange that at this epoch is small but gt0 WL ?
  Wm

• Mechanisms - many ?
• k-Essence (fields from String Theory for driving
  inflation)
• Could contribute to Dark Energy
• Universe is a viscous fluid and dark matter
  modelled by Tachyon field and Chaplygin gas
• Quintessence fields from c, h, G
• ? only fundamental constants
• Quintessence filed turns on at some epoch and
  dominates the expansion of the Universe

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8.5 The Nature of Dark Matter

• Dark Matter Candidates Identity Parade

Candidate Mass
Neutrino 4 eV Hot Dark Matter
Axion 10-5 eV Cold Dark Matter
Photino 0.5 GeV LSP
Neutralino 10GeV LSP
Axino keV LSP
Cosmion 5-10 GeV Created by P annihilation, useful for Solar Neutrino Problem
Quark Nuggets 1015 kg Created in initial stages of Big Bang but predicted flux of 106kg yr-1 not detected
Shadow Matter GeV Predicted by E8xE8 Superstring Theories, Decouples 10-43 s after Big Bang
Primordial Black Holes gt1012kg Collapse of Space time on scales of Horizon due to fluctuations
Relics ?? Monopoles, Strings, Textures
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8.6 Structure Formation in a Dark Matter Universe

• Dark Matter is needed for Structure Formation

• CMB smooth to 1 part in 107
• Baryons coupled to radiation until de-coupling
• NOT ENOUGH TIME TO FORM STRUCTURE
• Need Dark Matter
• Dark Matter Condenses at earlier time
• Matter then falls into the DM gravitational wells

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8.6 Structure Formation in a Dark Matter Universe

• Dark Matter Structure Formation Scenarios

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8.7 The Search for Dark Matter

• Detection of WIMPs

WIMP interact weakly with matter ? WIMP
DETECTION REQUIRES

• sensitive to few keV - GeV energies
• Large Deposition of Mass of detector material
• Superb background rejection (expected event rate
  lt 1 kg-1 day-1)
• Stable over long periods

• Search for 2 asymmetries
• 10 annual modulation of the event rate due to
  the Earth's motion around the Sun
• Asymmetry in the direction of the WIMP flux due
  to the Sun's motion through the galactic halo

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8.7 The Search for Dark Matter

• Detection of WIMPs

• First generation of WIMP experiments were
  rare-event experiments (proton decay, solar
  neutrinos) that were adapted to search for dark
  matter.
• e.g. ultra-low background germanium
  semiconductor experiments developed for double
  beta-decay - modified into dark matter detectors.
  (recoiling Ge nucleus produces -hole pairs that
  are detectable down to recoil energies keV).

• Gas Detectors - Time-Projection Chamber (TPC)
  detectors used in particle physics. experiments.
  To Detect a WIMP ? require enormous volume,
  possibility could detect asymmetric direction of
  WIMP recoil due to the Earth's motion around the
  Sun.
• Superconducting Grain Detectors - 1mm size
  superconducting grains. WIMP recoil ? heating ?
  phase transition. Resultant change in magnetic
  field detected by a SQUID.
• Ancient Mica - WIMP detection requires
  detectors/exposure times of kg/yr. ? Instead of
  100 kg detector use with small amounts of
  material that has been exposed for 109yr.
• Atomic Detectors - Detect inelastic collisions
  of SUSY relics with atoms. X-section for atomic
  interactions smaller than nuclear interactions
  but there is a wider range of usable material.
  (not yet any such experiments to look for
  WIMP-atom scattering)

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8.8 Summary

• SUMMARY

• In the last 100 years the Copernican Principal
  has grown in strength

• Hubble Universe is expanding - all galaxies are
  receding from each other
• Zwicky Presence of Dark Matter - Dark Baryons
• Massive Halos/Clusters Nucleosynthesis
  Existence of Non-baryonic Dark Matter
• COBE Baryonic Matter is not dominant in the
  structure formation process
• WMAP 75 of Universe is in the form of Dark
  energy

• BIG BANG has been very succesful. BUT in truth
• We can still only understand 4 of the Universe
• ? Its a very exciting time to be an
  Astrophysicist

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8.8 Summary

• Summary

?
Fundamental Cosmology 8. Dark Matter
?
Fundamental Cosmology