DARK MATTER & GALACTIC

ROTATION

2012ASTRO SUMMER SCHOOL

In 1933, Fritz Zwicky calculated the mass of the Coma Cluster

More evidence was discovered by Vera Rubin who measured the rotation curves of many galaxies during the 1970s.

The expected distribution of dark matter in the Milky Way

Cosmic Microwave Background (CMBR)

CMBR Power SpectrumThe first peak gives us information about the curvature of the Universe.

The ratio of the odd peaks to the even peaks gives us the baryon density.

The third peak tells us about the density of dark matter.

We can alter the cosmological parameters so that the curve fits the observed data.

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Dark Matter – Then and Now

Gravitational Lensing

Abell 1689

LRG-3757

The Bullet Cluster

More evidence for dark matter

Velocity dispersion of galaxiesMeasurements of the velocities of galaxies within clusters.

Acoustic oscillationsPeriodic fluctuations in baryonic matter caused by acoustic waves in the early Universe.

Type Ia supernovae measurementsPut a limit on the amount of dark energy hence constrain dark matter

Structure formationIf the Big Bang model is correct, dark matter is required to allow structures to form.

Rotation curvesA solid body (i.e. a disk) will have a rotational velocity that is proportional to distance – as R increases, v increases.

Since , for objects that are gravitationally bound (i.e. not a solid disk), we expect v to be proportional to

Galaxies do not follow Keplerian rotation, outside of the core of the galaxy, v is approximately constant. This provides evidence for dark matter.

Galactic rotation curves

𝑣 ∝𝑅𝑣 ∝ 1

√𝑅

𝑣 ≈ constant

Cryogenic detector experimentsCryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium.

CDMSII, Soudan Mine, Minnesota

CRESST, Gran Sasso, Italy

Noble liquid experimentsNoble liquid detectors detect the flash of scintillation light produced by a particle collision in liquid xenon or argon.

XENON, Gran Sasso, Italy DEAP, Ontario, Canada

Indirect detection methodsIndirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles (the particle and antiparticle are the same) then two WIMPs colliding could annihilate to produce gamma rays or particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. The detection of such a signal is not conclusive evidence for dark matter, as the production of gamma rays from other sources are not fully understood.

EGRET - Energetic Gamma Ray Experiment Telescope

PAMELA - Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics

GLAST - Gamma-ray Large Area Space Telescope (also known as Fermi)