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Cosmology and extragalactic astronomy

Cosmology and extragalactic astronomy. Mat Page. Mullard Space Science Lab, UCL. 6. Dark matter. Slide 2. 5. Dark Matter. This lecture: Dark Matter: where is it? what could it be? some experiments to find it. Slide 3. Recap of galaxy rotation.

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Cosmology and extragalactic astronomy

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  1. Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 6. Dark matter

  2. Slide 2 5. Dark Matter • This lecture: • Dark Matter: • where is it? • what could it be? • some experiments to find it.

  3. Slide 3 Recap of galaxy rotation • Orbital velocities rise OK but then don’t fall off. • Something wrong • Either gravity not a 1/r2 (!!) • or there is more mass than we can see. • This is “dark matter”

  4. Slide 4

  5. Slide 5 v2 r = Gphr 3v2 r = 4Gpr2 Where must the mass be? • Recall M = r v2/G • If M varies with radius, and v is constant • Mass proportional to radius • for a disc thickness h, density • for a sphere, density • Must extend out to visible edge of galaxy

  6. Slide 6 What could dark matter be? • Two classes of candidates: • MACHOs • Massive Compact Halo Obects • WIMPs • Weakly Interacting Massive Particles

  7. Slide 7 What could MACHOs be? • Discrete objects that are very difficult to detect: • Brown dwarfs / large Jupiters • White dwarfs • Small black holes.

  8. Slide 8 What could WIMPs be? • New particles we haven’t yet seen: • Neutrinos • Lightest Supersymmetric particles (neutralinos) • Axions (Big bang remnants).

  9. Slide 9 Detecting MACHOs • Machos are small and dense. • But they have mass, so cause curvature in spacetime. • Will focus light from a background star as they pass in front - star will change brightness.

  10. Slide 10 MACHO events • MACHO project looked for these events. • 1m telescope in Australia looked at LMC every night possible for several years. • Saw some!

  11. Slide 11 ..but not many • Not many low-mass (planet size) MACHOs. • Some 0.5 Mo events (black holes, white dwarfs?) • MACHOs could account for UP TO 40% of dark matter.

  12. Slide 12 Detecting WIMPs • Several current + future experiments • This is an old detector from UKDMC, Boulby Mine. • Current Boulby experiment is called “Drift-II” • Look for recoil from “Heavy WIMP” • None confirmed yet.

  13. Slide 13 What sort of results do they produce? UKDM (now defunct) Other experiments

  14. Slide 14 But none found so far • Big, risky science. • Would be as important for particle physics as for cosmology. • Could net the Nobel prize if found. • But you have to detect one first.

  15. Slide 15 Some key points about dark matter: • 90% of the mass in the Universe • Most of the mass in the outer parts of galaxies. • Two possible candidates for dark matter: • WIMPs • weakly interacting massive particles • searches underway, none found so far • MACHOs • massive compact halo objects • can only make up to 40% of dark matter

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