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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Observing Dark Matter
http//pancake.uchicago.edu/carroll/universelab05
/
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Observing Dark Matter
http//pancake.uchicago.edu/carroll/universelab05
/
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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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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 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.
• 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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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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Cold Dark Matter

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

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Lecture 11 Dark Energy

• Why do we think it exists?
• What are the possibilities?

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Review of Observational Evidence

• Measurements of
• Distant supernovae
• Cosmic Microwave Background
• X-ray Clusters
• All indicate that the universe is
• ACCELERATING
• How is this possible?

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Review of Theoretical Evidence

• What would make the Universe accelerate?
• An additional force pushing against the
  attractive force of gravity
• Dark Energy
• A form of energy that exists even in the vacuum
  of space vacuum energy.

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What forms could this energy take?

• Constant density with time (Cosmological
  Constant)
• The energy is not diluted by the expansion
• What will happen to the gravitational energy as
  the Universe expands?
• How will this affect the balance between gravity
  and dark energy?
• Generic Dark Energy
• A vacuum energy whose density does change over
  the history of the Universe
• Examples include quintessence, scalar fields and
  even more strange concepts

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The model of the Universe?

• Problems with dark energy
• There isnt enough
• If dark energy is a property of the vacuum
• Every where in the vacuum of space particles
  should be being spontaneously created and
  annihilated

• However, there is so much space that this means
  that the amount of vacuum energy that we predict
  is
• 10120
• that observed!
• The effect of that much energy would be?

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The model of the Universe?

• Constant -vs- Changing Dark Energy
• Equation of state
• Pressure pushing outwards, P, depends on the
  density, ?.

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Observations ? or ??

• Can we determine whether the dark energy is
  constant or evolving over time?
• NO
• Not from observations of SN 1as with cluster and
  CMB observations.

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Observations ? or ??
Main goal of the SNAP satellite (launch 2010?)