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Cosmology and extragalactic astronomy

This lecture delves into the mysteries of dark matter, exploring its potential existence and implications for cosmology. We will discuss what dark matter is, where it might be found, and the two main classes of candidates: MACHOs (Massive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). Key experiments aimed at detecting these elusive particles will be examined, including past searches for MACHOs and current and future efforts to identify WIMPs. Understand the significance of dark matter, which constitutes 90% of the universe's mass, and its vital role in shaping cosmic structures.

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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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