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 don’t fall off.

• Something wrong– Either gravity not 1/r2 (!!)– or there is more mass than we can see.– This is “dark matter”

Slide 3

Slide 4

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

= 3v2

4Gr2

= v2

Ghr

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 haven’t 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 Mo 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