Cosmology and extragalactic astronomy

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

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

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Recap of galaxy rotation

• Orbital velocities rise OK but then dont fall
  off.
• Something wrong
• Either gravity not a 1/r2 (!!)
• or there is more mass than we can see.
• This is dark matter

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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

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What could dark matter be?

• Two classes of candidates
• MACHOs
• Massive Compact Halo Obects
• WIMPs
• Weakly Interacting Massive Particles

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What could MACHOs be?

• Discrete objects that are very difficult to
  detect
• Brown dwarfs / large Jupiters
• White dwarfs
• Small black holes.

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What could WIMPs be?

• New particles we havent yet seen
• Neutrinos
• Lightest Supersymmetric particles (neutralinos)
• Axions (Big bang remnants).

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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.

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MACHO events

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

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..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.

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Detecting WIMPs

• Several current future experiments
• This is a NaIAD detector from UKDMC, Boulby Mine
• Look for recoil from Heavy WIMP
• None confirmed yet.

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How are they doing?
UKDM
Other planned experiments
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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.

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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