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Announcements

Announcements. Projects are graded 3rd Midterm: Wednesday April 25th review session: Monday April 23rd, 6pm final projects due: Monday April 30th. Lecture 39: Dark Matter III – structure formation in the Universe. Structure formation in the Big-Bang model. How does structure form ?.

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Announcements

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  1. Announcements • Projects are graded • 3rd Midterm: Wednesday April 25th • review session: Monday April 23rd, 6pm • final projects due: Monday April 30th Astronomy 201 Cosmology - Lecture 39

  2. Lecture 39:Dark Matter III –structure formation in the Universe Astronomy 201 Cosmology - Lecture 39

  3. Structure formation in the Big-Bang model Astronomy 201 Cosmology - Lecture 39

  4. 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 Astronomy 201 Cosmology - Lecture 39

  5. Astronomy 201 Cosmology - Lecture 39

  6. Q: What is it ? A: MACHOs or WIMPs Astronomy 201 Cosmology - Lecture 39

  7. 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 Astronomy 201 Cosmology - Lecture 39

  8. How can we see MACHOs ? • Solution: monitor 10 million stars simultaneously Astronomy 201 Cosmology - Lecture 39

  9. 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 Astronomy 201 Cosmology - Lecture 39

  10. WIMPs ? • Weakly Interacting Massive Particles • Massive neutrino • at least we know that it exists • we don’t 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) Astronomy 201 Cosmology - Lecture 39

  11. WIMP candidate I: massive neutrinos • At least we know that they exist: + n  p+ + e- • We don’t 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 Astronomy 201 Cosmology - Lecture 39

  12. WIMP candidate I: massive neutrinos • What mass do we need to account for all the dark matter ? • There are~100neutrinos per cm3 • A mass of20eVresults in0=0.3 • 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 Astronomy 201 Cosmology - Lecture 39

  13. 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 Astronomy 201 Cosmology - Lecture 39

  14. 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 Astronomy 201 Cosmology - Lecture 39

  15. WIMP candidate II: the least massive supersymmetric particle • It’s mass should be > 150 GeV, otherwise • its contribution would be irrelevant • it should already have been detected • But how to prove its existence ? Astronomy 201 Cosmology - Lecture 39

  16. How can we find cold WIMPs ? • Cryogenic (ultra cold) detectors • search for annual modulation of the signal Astronomy 201 Cosmology - Lecture 39

  17. Do we have already detected WIMPs ? Results are still very controversial and inconclusive DAMA collabor- ation Astronomy 201 Cosmology - Lecture 39

  18. 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 Astronomy 201 Cosmology - Lecture 39

  19. 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 Astronomy 201 Cosmology - Lecture 39

  20. Can astronomy help to discriminate between hot and cold dark matter ? HDM CDM Astronomy 201 Cosmology - Lecture 39

  21. 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 Astronomy 201 Cosmology - Lecture 39

  22. 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 Astronomy 201 Cosmology - Lecture 39

  23. Hierarchical structure formation Astronomy 201 Cosmology - Lecture 39

  24. 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 Astronomy 201 Cosmology - Lecture 39

  25. A voyage through a CDM universe Astronomy 201 Cosmology - Lecture 39

  26. A voyage through a CDM universe Astronomy 201 Cosmology - Lecture 39

  27. Announcements • Projects are graded • 3rd Midterm: Wednesday April 25th • review session: Monday April 23rd, 6pm • final projects due: Monday April 30th Astronomy 201 Cosmology - Lecture 39

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