1. Richard E. Hughes Dark Matter & GLAST; p.1 The Lure of Dark Matter Richard E. Hughes Department of Physics “The most incomprehensible thing about the universe is that it is comprehensible” - Albert Einstein

2. Richard E. Hughes Dark Matter & GLAST; p.2 Dark Matters….. This is luminous matterThis is dark matter It is tempting to look at the universe, seeing stars and galaxies, clusters of galaxies and come to the conclusion that what you SEE is the matter, and what you don’t see is empty space. But, you would be wrong! There is general agreement that, in fact, MOST of the matter in the universe is in a form that we can’t SEE. This matter is imaginatively referred to as “Dark Matter”.

3. Richard E. Hughes Dark Matter & GLAST; p.3 Rotation Velocities in Our Solar System  The falloff in speeds as the planets get further from the sun is called “Keplerian decline”  It comes from Kepler’s 3rd Law: The squares of the times to complete one orbit are proportional to the cubes of the semi-major axis of the ellipse.

4. Richard E. Hughes Dark Matter & GLAST; p.4 Newton’s Generalization Isaac Newton generalized this with his 3 Laws  The rotational speed of ANY object is only dependent on how much mass is INSIDE its orbit  This applies to our solar system  But ALSO applies to  Rotation of stars around galactic centers  Rotations of galaxies in clusters of galaxies

5. Richard E. Hughes Dark Matter & GLAST; p.5 The Milky Way Galaxy Our sun is in the Milky way galaxy, about 28,000 light-years from its center. The speed of the solar system relative to the galactic center is approximately 220 km/s. At this speed, it takes about 200 million years to make one complete revolution. A galaxy is composed of stars and other material which are held together by gravity. The name ‘Milky Way’ comes from the band of light that can be seen during dark nights in the summer. This band is actually an edge-on view of the galaxy, and it is believed that when viewed “head on” it is a spiral galaxy. The Celestial River

6. Richard E. Hughes Dark Matter & GLAST; p.6 The Milky Way Galaxy  The COBE satellite was designed to investigate a phenomenon called the Cosmic Microwave Background.  COBE is sensitive to infra-red (IR) wavelengths of light.  The Milky Way viewed in the visible, is obscured by dust.  However, viewed in the IR, the Milky Way shows a clear central bulge overlaying a thin disk, as expected of an edge-on view of a spiral galaxy:

7. Richard E. Hughes Dark Matter & GLAST; p.7 The Milky Way Galaxy The image shown is a rendition of what we believe the Milky Way galaxy looks like if it were viewed head on:  The radius is about 50,000 light-years  The sun is about 28,000 light-years from the center  Near the Orion arm  Between the arms Perseus and Sagittarius

8. Richard E. Hughes Dark Matter & GLAST; p.8 The Rotation Curve For the Milky Way The same sort of rotation curve can be made for the Milky Way galaxy. Given that the Sun is on the outer edge of the galaxy (about 2/3 out), we expect that most of the mass is inside the galactic radius of the Sun. So we should see a decreasing rotation curve, like we do for the solar system. But instead, it is FLAT (if not increasing).

9. Richard E. Hughes Dark Matter & GLAST; p.9 What we expected, NOT! These two curves are VERY different. Why? Our solar system orbits the center of the milky way galaxy just like the earth orbits the sun… so we expect Keplerian decline in the speeds of stars as one moves from the center, but we don’t see it.

10. Richard E. Hughes Dark Matter & GLAST; p.10 How about other galaxies? NGC 6503: Galaxy in Constellation Draco

11. Richard E. Hughes Dark Matter & GLAST; p.11 Yet another galaxy…

12. Richard E. Hughes Dark Matter & GLAST; p.12 Whatever it is, its DARK!

13. Richard E. Hughes Dark Matter & GLAST; p.13  Stars and gas in the galactic disks follow circular orbits whose velocity depends on the inner mass only:  A flat rotation curve means that the total M(<r) increases linearly with r, while the total luminosity approaches a finite asymptotic limit as r increases. Clearly a large amount of invisible gravitating mass (more than 90% of the total mass in the case of the Milky Way and other examples) is needed to explain these flat rotation curves.  This invisible mass is referred to as DARK MATTER  Is there any other supporting evidence? Dark Matter! Why are the rotation curves flat?

14. Richard E. Hughes Dark Matter & GLAST; p.14 Example of Gravitational Lensing Foreground cluster of galaxies CL0024+1654 (constellation Pisces) Blue galaxy behind the cluster “lensed” copy of blue galaxy

15. Richard E. Hughes Dark Matter & GLAST; p.15 Example of Gravitational Lensing

16. Richard E. Hughes Dark Matter & GLAST; p.16 Gravitational Lensing

17. Richard E. Hughes Dark Matter & GLAST; p.17 What is causing the Lensing? The majority of the dark matter is distributed broadly and smoothly in the cluster, covering a region on the sky more than 1.6 million light-years across. The mass of the individual cluster galaxies appears as pinnacles on this mountain of dark matter mass. Overall, the dark matter in the cluster outweighs all the stars in the cluster's galaxies by 250 times! From http://www.bell-labs.com/org/physicalsciences/projects/darkmatter/darkmatter1.html

18. Richard E. Hughes Dark Matter & GLAST; p.18 What the Universe is Made Of

19. Richard E. Hughes Dark Matter & GLAST; p.19 What and where is the dark matter? The dark matter can’t be in the central disk of galaxies. Why? Interstellar clouds would be much thinner (due to gravitational forces of the dark matter. So the dark matter must be in “halos” of the galaxies. What the dark matter is NOT: 1) Stars: even faint ones would radiate some light. 2) Dust: we would not be able to see our own galaxy or others, since dust absorbs and scatters light What some the dark matter MIGHT be: 1) Black holes 2) Dim, old white dwarfs which are no longer bright 3) Proto-stars in which fusion did not start What most of the dark matter SEEMS to be: Some new form of elementary matter

20. Richard E. Hughes Dark Matter & GLAST; p.20 Super Particles? Particles making up “normal” matter: stars, planets, people, etc Shadow particles: NONE have been observed yet… but one of these predicted Particles could be the source of dark matter: it is called the NEUTRALINO.

21. Richard E. Hughes Dark Matter & GLAST; p.21 The Neutralino  Predicted to exist for reasons that have NOTHING to do with dark matter… BUT… has properties that would make it a very good candidate  There might be enough dark matter particles in the halo of galaxies that the dark matter particles will collide from time to time  Since the dark matter particle is its OWN anti-particle, when the particles collide, they will ANNIHILATE High energy photons from dark matter annihilation

22. Richard E. Hughes Dark Matter & GLAST; p.22 “Seeing” dark matter Unfortunately, the atmosphere is a shield to high energy gamma rays. To “see” them, we need to go above the atmosphere - we need a satellite!

23. Richard E. Hughes Dark Matter & GLAST; p.23 Gamma-ray Large Area Space Telescope

24. Richard E. Hughes Dark Matter & GLAST; p.24 Viewing the universe in many different wavelengths See http://www.ipac.caltech.edu/Outreach/Multiwave/gallery3.html for image citations.http://www.ipac.caltech.edu/Outreach/Multiwave/gallery3.html

25. Richard E. Hughes Dark Matter & GLAST; p.25 GLAST in Action!

26. Richard E. Hughes Dark Matter & GLAST; p.26 Launch of Satellite  GLAST will launch in 2007  First data in ~2008  Will we “see” dark matter?  We will be looking for ANOMALOUS sources of gamma rays  If they have the right properties, this could be the signature of dark matter!  Stay Tuned!

27. Richard E. Hughes Dark Matter & GLAST; p.27 GLAST Mission GLAST measures the direction, energy & arrival time of celestial gamma rays GLAST is two instruments: - Large Area Telescope(LAT) measures gamma-rays in the energy range ~20 MeV - >300 GeV - Gamma-ray Burst Monitor(GBM) provides correlative observations of transient events in the energy range ~20 keV – 20 MeV Launch: Feb 2007 Orbit: 550 km,28.5 o inclination Lifetime: 5 years (minimum)

28. Richard E. Hughes Dark Matter & GLAST; p.28 GLAST LAT Overview: Design e+e+ e–e–  Si Tracker pitch = 228 µm 8.8 10 5 channels 12 layers × 3% X 0 + 4 layers × 18% X 0 + 2 layers Data acquisition Grid (& Thermal Radiators) Flight Hardware & Spares 16 Tracker Flight Modules + 2 spares 16 Calorimeter Modules + 2 spares 1 Flight Anticoincidence Detector Data Acquisition Electronics + Flight Software 3000 kg, 650 W (allocation) 1.8 m  1.8 m  1.0 m 20 MeV – 300 GeV CsI Calorimeter Hodoscopic array 8.4 X 0 8 × 12 bars 2.0 × 2.7 × 33.6 cm  cosmic-ray rejection  shower leakage correction ACD Segmented scintillator tiles 0.9997 efficiency  minimize self-veto

29. Richard E. Hughes Dark Matter & GLAST; p.29 Gamma Ray Bursts  Gamma Ray Bursts are intense flashes of gamma rays lasting from fractions of a second to hours, some with fading afterglows visible for months. What are they?  collisions between highly dense neutron stars or black holes?  signatures of the birth of a black hole?  Example: GRB 990123 Distance: 10 billion light-years Size: emitting region is light-seconds across Power: at maximum up to 1,000,000,000,000,000,000 (quintillion) times the Sun's power or roughly equal to the energy released by 100 billion Suns in a year's time  GLAST should observe more than a 200 bursts per year BATSE map of its 2704 detected GRBs Artists conception of a GRB

30. Richard E. Hughes Dark Matter & GLAST; p.30 Active Galactic Nuclei (AGN)  AGN are a special class of glaxies that are the source of tremendous energy, shining with power equivalent to trillions of suns. It is believed that at the center of these objects there lies a supermassive black hole, which ejects jets of matter in opposite directions at nearly the speed of light.  If one of the jets is directed toward us the AGN is referred to as a Blazar  GLAST will detect thousands of blazars and will try to answer questions like:  How are the jets formed?  How is the matter in the jets accelerated to such fantastic speeds?  Is a billion-solar-mass black hole really the central power source? Hubble HeritageHubble Heritage image of M87 Schematic diagram of an AGN

31. Richard E. Hughes Dark Matter & GLAST; p.31 GLAST is an International Mission Germany FranceSwedenItaly USAJapan NASA - DoE Partnership on LAT LAT is being built by an international team Si Tracker: Stanford, UCSC, Japan, Italy CsI Calorimeter: NRL, France, Sweden Anticoincidence: GSFC Data Acquisition System: Stanford, NRL, Ohio State GBM is being built by US and Germany Detectors: MPE

32. Richard E. Hughes Dark Matter & GLAST; p.32

33. Richard E. Hughes Dark Matter & GLAST; p.33 Why study  -rays ? Gamma-rays carry a wealth of information  -rays offer a direct view into Nature’s largest accelerators  the Universe is mainly transparent to  -rays: can probe cosmological volumes.  -rays readily interact in detectors, with a clear signature.  -rays are neutral: no complications due to magnetic fields; point directly back to sources, etc.

34. Richard E. Hughes Dark Matter & GLAST; p.34 Searching for Dark Matter  If we believe that Dark Matter really does exist, how do we look for it?  Well, we need a model. And one which is pretty handy is the Standard Model!  Well, actually not the Standard Model, but a close relative, which involves something called “SuperSymmetry”  A particle predicted by the SuperSymmetry theory is called the Neutralino  This particle is predicted for reasons having NOTHING to do with dark matter, but – in a happy coincidence – it COULD BE that the neutralino is the mysterious source of Dark Matter.  Once the neutralino is made, it can’t decay into something else  UNLESS: it meets its antiparticle.