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

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

Recap of galaxy rotation Orbital velocities rise OK but then dont fall off. Something wrong –Either gravity not 1/r 2 (!!) –or there is more mass than we can see. –This is dark matter Slide 3

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Where must the mass be? Recall M = r v 2 /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 = 3v 2 4G r 2 = v2v2 G hr Slide 5

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

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

New particles we havent yet seen: –Neutrinos –Lightest Supersymmetric particles (neutralinos) –Axions (Big bang remnants). What could WIMPs be? Slide 8

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. Slide 9

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

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

Detecting WIMPs Several current + future experiments This is a NaIAD detector from UKDMC, Boulby Mine Look for recoil from Heavy WIMP None confirmed yet. Slide 12

How are they doing? UKDM Other planned experiments Slide 13

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. Slide 14

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 Slide 15