2. 2. April 2007J.Wicht : Dark Matter2 Outline ● Three lecturers spoke about Dark Matter : – John Ellis, CMB and the Early Universe – Felix Mirabel, High-Energy Astrophysics – Carlos Wagner, Beyond the Standard Model (Supersymmetry) ● Why does Dark Matter (DM) matter? What is DM? ● Observational evidence for Dark Matter – the Galaxy rotation problem – Gravitational lensing and high temperature gas: ● The Abell Clusters ● The Bullet Cluster – Cosmic Background Radiation, structure formation

3. 2. April 2007J.Wicht : Dark Matter3 Why does Dark Matter matter? ● A definition of Dark Matter (DM) is hard to give : – DM is matter, not directly observed and of unknown composition. – Gravitational interaction and maybe weak interaction. ● Cosmology and cosmological evidences : a large part of the Universe is made of DM. ● Physics beyond the Standard Model (SM) : probably new particles.

4. 2. April 2007J.Wicht : Dark Matter4 DM lexicon ● Baryonic DM – MACHOs (MAssive Compact Halo Object) : Black holes, brown dwarfs (small stars), neutron stars,... ● Non-baryonic DM – Hot DM : move ultrarelativistically ● Neutrino, ??? – Warm DM : move relativistically ● Sterile neutrino (massive neutrino-like particle), ??? – Cold DM : move non-relativistically ● Neutralino (supersymmetric particle), WIMP (Weakly Interacting Massive Particle), ???

5. 2. April 2007J.Wicht : Dark Matter5 Evidence of Dark Matter : the Galaxy rotation problem ● This plots shows the orbital velocity of stars or gas as a function of the distance to the center of the galaxy. ● Discrepancy between observation and theory (Newton gravitational potential evaluated from the visible mass) of stars in the disk of spiral galaxies. – Invisible matter around the galaxy? – Newton/Einstein is wrong? ● MOND (MOdified Newtonian Dynamics) ● MOG (MOdified Gravity) – tensor-vector-scalar gravity (TeVeS) – scalar-tensor-vector gravity (STVG) A = theory B = observation

6. 2. April 2007J.Wicht : Dark Matter6 Evidence of Dark Matter : the Galaxy rotation problem ● This is not a « one galaxy » effect. This phenomena is observed in many galaxies! ● Red points with error bars are observation, the blue dashed line is Newtonian gravity. The others are modified gravity theories. astro-ph/0506370

7. 2. April 2007J.Wicht : Dark Matter7 Evidence of Dark Matter : Abell 1689 ● DM observed through gravitational lensing : predict mass without relying on dynamics. ● In this cluster of Galaxies, it is believed that the lensing is done by cold DM (hot DM would escape)

8. 2. April 2007J.Wicht : Dark Matter8 Evidence of Dark Matter : Abell 2029 ● The galaxy cluster composed of thousands of galaxies (optical image, right) enveloped in a gigantic cloud of hot gas (X-ray image, left), and an amount of DM equivalent to more than a hundred trillion (10 14 ) Suns. ● Gas is confined by the gravity of the DM. ● Agrees with the predictions of cold DM models. ● If the Universe is « like » Abell 2029 : 70 to 90 % of the mass of the universe consists of cold DM. Density

9. 2. April 2007J.Wicht : Dark Matter9 Best evidence of Dark Matter : the Bullet Cluster ● Collision of two clusters of galaxies 150 millions years ago. ● Normal matter (gas, stars) and DM behave differently during the collision : – Stars : gravitational interaction  slowed – Gas (observed in X-rays) : EM interaction  much more slowed than the stars, diffused. – DM : mass « observed » through gravitational lensing  DM is mostly collisionless. ● Really hard to explain with MOND or MOG theories (but of course theorists are trying). Matter density contour

10. 2. April 2007J.Wicht : Dark Matter10 Best evidence of Dark Matter : the Bullet Cluster Animation of the collision

11. 2. April 2007J.Wicht : Dark Matter11 Composition of the Universe : Cosmic Microwave Background Relic radiation filling the Universe. « Picture » of the Big Bang (c) John Ellis

12. 2. April 2007J.Wicht : Dark Matter12 Composition of the Universe : Cosmic Microwave Background Like a Fourrier analysis of the CMB

13. 2. April 2007J.Wicht : Dark Matter13 The CMB Power Spectrum From this spectrum, you can get the composition of the Universe :  tot = spacial curvature  b = baryonic density  CDM theory Concordance model of big-bang cosmology

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16. 2. April 2007J.Wicht : Dark Matter16 Indirect evidence : Structure formation Almost homogeneous Not homogeneous

17. 2. April 2007J.Wicht : Dark Matter17 Evidence of DM : structure formation ● DM is a necessary component to understand the large-scale structure in the Universe (last slide). ● The DM acts as a compactor of structure : – Baryonic matter was too hot and its pressure was too high to collapse to form small structures (stars,...) ● The Dark Energy is necessary to explain the observation that the expansion of the Universe is accelerating.

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