Certificate programme in Science: Astronomy Core Module 4 Cosmology

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Certificate programme in Science Astronomy (Core
Module 4)Cosmology

• Dr Lisa Jardine-Wright,
• Institute of Astronomy, Cambridge University

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Lecture 10 Dark Matter

• Why do we think it exists?
• Observational Evidence
• Theoretical Evidence
• What do we know about it?
• What isnt it?
• What could it be?
• How are we trying to find it?

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Review of Observational Evidence

• X-ray clusters
• Temperature of the gas is so high that gas should
  not be bound in the cluster
• Gravitational Lensing
• Shapes of the lenses produced by galaxies and
  galaxy clusters do not match the visible mass
• Rotation of spiral galaxies
• Calculation of mass density curves
• Sizes of objects containing dark matter
• Globular clusters are the smallest objects in
  which observational evidence of dark matter
  exists.

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Review of Theoretical Evidence

• Galaxy formation simulations
• Using various forms of dark matter can we
  reproduce the galaxies that we observe in the
  Universe?
• Mass distribution of galaxies
• Number of galaxies of a given mass in a specific
  redshift band
• Density of dwarf galaxy cores.
• Density profile of dwarf spiral galaxies.

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Testing theories of dark matter

• Velocity with which an object orbits depends on
  the strength of force of gravity pulling on the
  object

r1
r2
r3
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Consider the Solar System

• Draw mass against radius AND
• Plot the velocity of each of the planets in the
  solar System against their radius using the
  following equation

r1
r2
r3
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Planet Data
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Galaxy Data

• Consider the following cases for a spiral galaxy
• As you move out from the centre of the galaxy in
  equal steps the mass increases in equal steps.
• r 10kpc M 2,000,000 Msun
• r 20kpc M 4,000,000 Msun
• r 40kpc M 8,000,000 Msun
• The density for the whole galaxy remains the
  same.
• Density Mass / Volume
• Mass Density x Volume

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Galaxy Rotation Curves
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What do we know about it?

• Hot dark matter
• Neutrinos
• Cold dark matter
• WIMPS
• MACHOS
• Warm dark matter
• A happy medium between cold and warm.

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What do we know about it?

• The particle properties of dark mater determine
  the size of the first structures to form in the
  Universe.
• Where m is the mass of the dark matter
  particle.
• Note mass of a galaxy is 1011 MSun
• mass of an electron 511,000 eV

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Hot Dark Matter

• Big Bang Nucleosynthesis indicates the production
  of neutrinos among the reactions which form the
  light elements.
• Provides some prediction of the observed light
  element abundances, and also some indication of
  the neutrino abundance in the Universe.
• Much as the radiation field which forms today's
  CMB had "decoupled" from matter a few minutes
  after the Big Bang, so too did neutrinos decouple
  from matter -- but at a much earlier time than
  the photon background.
• Hence, there is a predicted cosmic neutrino
  background as dense as the photons which comprise
  the CMB.

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Hot Dark Matter

• However,
• Neutrinos would have emerged from the Big Bang
  with such highly relativistic velocities (i.e.
  close to light speed) that they would tend to
  smooth out any fluctuations in matter density as
  they streamed out through the Universe.
• In the early Universe, the neutrino density was
  enormous, and so most of the matter density could
  be accounted for by neutrinos.
• Given their great speeds, neutrinos would tend to
  smooth out overdense regions--that is, regions
  with densities greater than the average density
  in the Universe.
• This process implies that density fluctuations
  could appear only after the neutrinos slowed down
  considerably. (i.e. As the Universe expanded, its
  temperature decreased, thereby resulting in
  neutrino "cooling.")
• Therefore if dark matter was HOT the first
  objects to have formed would have been very large
  - superclusters.

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Cold Dark Matter

• MACHOs
• Massive Compact Halo Objects
• Dark stars
• Supermassive Black Holes
• WIMPs
• Weakly interacting massive Particles
• Neutralinos

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Detection Techniques

• Most powerful method of discovering new particles
  is with particle accelerators, so if I had to
  guess, I would guess that discovery of
  supersymmetric dark matter will come from CERN.
• The new LHC at CERN is an important step for
  discovering new types of particles.
• If these particles are detected, from there
  masses and energies we will be able to determine
  their contribution to dark matter.
• M more satisfying to actually detect the
  particles in our halo as they move past and
  through the Earth. This would also allow
  measurement of the local density of dark matter
  and establish beyond doubt that the dark matter
  is non-baryonic cold dark matter.
• Currently there are two main methods being
  aggressively pursued.

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Detection Techniques

• Since we roughly know the speed and the density,
  we can say that for a Wimp of mass of order
  10-100 GeV, roughly 100,000 dark matter particles
  a second pass through every square centimeter of
  the Earth, including our bodies.
• If they exist, these are very weakly interacting
  particles, so it is quite rare that one of them
  will interact at all.
• In addition, if one does elastically scatter off
  a nucleus, the deposited energy is usually in the
  keV to 100 keV range, so extremely sensitive
  devices must be used.

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Detection Techniques

• Boulby Mine, Yorkshire
• Scintillation
• AMANDA, antartic
• Cerenkov light

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Boulby Mine