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### Lecture 39:Dark Matter III –structure formation in the Universe

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

Astronomy 201 Cosmology - Lecture 39

Structure formation in the Big-Bang model

Astronomy 201 Cosmology - Lecture 39

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

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

How can we see MACHOs ?

- Solution: monitor 10 million stars simultaneously

Astronomy 201 Cosmology - Lecture 39

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

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

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 1.810-33 g
- electron: 512 keV
- protron: 938 MeV

Astronomy 201 Cosmology - Lecture 39

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 in0=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

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

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

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

How can we find cold WIMPs ?

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

Astronomy 201 Cosmology - Lecture 39

Do we have already detected WIMPs ?

Results are still very controversial and inconclusive

DAMA

collabor-

ation

Astronomy 201 Cosmology - Lecture 39

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

The spatial distribution of galaxies neutralinos ?

- 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

Can astronomy help to discriminate between hot and cold dark matter ?

HDM

CDM

Astronomy 201 Cosmology - Lecture 39

Structure formation: HDM vs CDM matter ?

- 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

Structure formation: HDM vs CDM matter ?

- 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

Hierarchical structure formation matter ?

Astronomy 201 Cosmology - Lecture 39

Structure formation: HDM vs CDM matter ?

- 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

A voyage through a CDM universe matter ?

Astronomy 201 Cosmology - Lecture 39

A voyage through a CDM universe matter ?

Astronomy 201 Cosmology - Lecture 39

Announcements matter ?

- 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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