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Understanding Dark Matter: Structure Formation and WIMP Candidates in Cosmology

Join us for Lecture 39 of Astronomy 201: Cosmology, focusing on dark matter, its role in structure formation, and the latest research on Weakly Interacting Massive Particles (WIMPs). We'll cover key concepts such as gravitational instability, MACHOs, and how we can observe them through tools like gravitational lensing. Explore the candidates for dark matter, including massive neutrinos and the least massive supersymmetric particles. This lecture will enhance your understanding of the universe's hidden components and the ongoing search to identify them.

sylvester-stewart
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Understanding Dark Matter: Structure Formation and WIMP Candidates in Cosmology

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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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