Announcements

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Announcements

• Projects are graded
• 3rd Midterm Wednesday April 25th
• review session Monday April 23rd, 6pm
• final projects due Monday April 30th

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Lecture 39Dark Matter III structure formation
in the Universe
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Structure formation in the Big-Bang model
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How does structure form ?

• Wrinkles in the CMB regions of higher and lower
  temperature
• Those regions correspond to density fluctuations,
  regions of slightly higher/lower density than
  average
• Gravitational instability
• higher density ? more mass in a given volume
• more mass ? stronger gravitational attraction
• stronger gravitational attraction ? mass is
  pulled in ? even higher density

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Q What is it ?
A MACHOs or WIMPs
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MACHOs ?

• MAssive Compact Halo Objects
• Brown dwarfs (stars not massive enough to shine)
• Dim white dwarfs (relics of stars like the Sun)
• Massive black holes (stars that massive that even
  light cannot escape)
• but if the DM is really in MACHOs, something
  with the nucleosynthesis constraint must be wrong

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How can we see MACHOs ?

• Solution monitor 10 million stars simultaneously

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How can we see MACHOs ?
Magnification due to gravitational lensing
There are not enough brown dwarfs to account for
the dark matter in the Milky Way.
Alcock et al. 1993
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WIMPs ?

• Weakly Interacting Massive Particles
• Massive neutrino
• at least we know that it exists
• we dont know whether it has mass or not
• hot dark matter (hot moving at speeds near the
  speed of light)
• Another (yet undiscovered) particle predicted by
  some particle physicists
• cold dark matter (cold moving much slower than
  the speed of light)

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WIMP candidate I massive neutrinos

• At least we know that they exist
  ? n ? p e-
• We dont know whether they have mass
• In particle physics, masses are expressed in
  terms of their energy equivalent mc2 eV
  electron volt
• 1 eV ? 1.8?10-33 g
• electron 512 keV
• protron 938 MeV

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WIMP candidate I massive neutrinos

• What mass do we need to account for all the dark
  matter ?
• There are 100 neutrinos per cm3
• A mass of 20eV results in ?00.3
• Can we measure their mass ?
• tricky
• use energy conservation. Measure all masses and
  velocities in the ? n ? p e- reaction with
  high precision. Difference between left and right
  hand side ? neutrino mass

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WIMP candidate I massive neutrinos

• Result now clear detection, but an upper limit.
  The mass of the (electron) neutrino is less than
  a few eV ? electron neutrino is ruled out as a
  dark matter candidate.
• BUT There are two more neutrino families, mu
  neutrinos and tau neutrinos (the muon and tauon
  are particles similar to the electron, but more
  massive and unstable)
• a massive mu or tau neutrinos still must be
  considered

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WIMP candidate II the least massive
supersymmetric particle

• Main goal of particle physics to develop a
  theory that unifies the four forces of nature
• Those models predict a whole zoo of particles,
  some of them are already detected, but most of
  them still very speculative. Most of these
  particles are unstable.
• Supersymmetry is a particularly promising
  unifying theory
• The least massive supersymmetric particle
  (neutralino) should be stable

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WIMP candidate II the least massive
supersymmetric particle

• Its mass should be gt 150 GeV, otherwise
• its contribution would be irrelevant
• it should already have been detected
• But how to prove its existence ?

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How can we find cold WIMPs ?

• Cryogenic (ultra cold) detectors
• search for annual modulation of the signal

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Do we have already detected WIMPs ?
DAMA collabor- ation

• Results are still very controversial and
  inconclusive

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Can astronomy help to discriminate between
neutrinos and neutralinos ?

• Neutrinos
• mass in the tens of eV ? very low mass
• very low mass ? high velocities ? hot
• can travel several tens of Mpc over the age of
  the universe
• Neutralinos
• mass in the hundredst of GeV ? very high mass
• very high mass ? low velocities ? cold
• cannot travel significant distances over the age
  of the universe

• Neutrinos Hot Dark Matter (HDM)
• mass in the tens of eV ? very low mass
• very low mass ? high velocities ? hot
• can travel several tens of Mpc over the age of
  the universe
• Neutralinos Cold Dark Matter (CDM)
• mass in the hundredst of GeV ? very high mass
• very high mass ? low velocities ? cold
• cannot travel significant distances over the age
  of the universe

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The spatial distribution of galaxies

• Galaxies are not randomly distributed but
  correlated
• Quantitative measure two-point correlation
  function ?(r) excess probability (compared to
  random) to find a galaxy at distance r to
  another galaxy

Courtesy Huan Lin
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Can astronomy help to discriminate between hot
and cold dark matter ?
CDM
HDM
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Structure formation HDM vs CDM

• Hot dark matter
• initial small scale structure (anything smaller
  than a galaxy cluster) washed out due to the high
  velocities of neutrinos
• clusters and supercluster form first
• galaxies form due to fragmentation of collapsing
  clusters and superclusters
• top-down structure formation

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Structure formation HDM vs CDM

• Cold dark matter
• plenty of small scale structure
• small galaxies form first, clusters last
• larger structures form due to merging of smaller
  structures
• bottom-up or hierarchical structure formation

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Hierarchical structure formation
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Structure formation HDM vs CDM

• CDM fits observations much better than HDM
• high-z galaxies are smaller
• irregular shape of galaxy clusters indicate that
  they formed recently
• there are only a very few clusters at high
  redshift, but many galaxies
• two-point correlation function is much better
  reproduced

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A voyage through a CDM universe
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A voyage through a CDM universe
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Announcements

• Projects are graded
• 3rd Midterm Wednesday April 25th
• review session Monday April 23rd, 6pm
• final projects due Monday April 30th