In this talk we'll see that : We can only see about 1% of the Universe The dark side And ask: What is the Universe made of?

The Milky Way --- our home galaxy Dirbe infrared map of the Milky Way beautifully shows the Galactic disk and bulge...

The 'edge-on' Milky Way The disk and bulge of the Galaxy are prominent at all wavelengths from gamma rays to radio but that's only a tenth of the story.

Spiral galaxies rotate strangely Emission lines from hot gaseous regions in the disk of a spiral galaxy allow its rotation to be measured. Centre This side of the galaxy is redshifted This side is blueshifted

What we'd see with dark matter spectacles Alternatives Cold dark matter distribution as found in cosmological simulations of structure formation (Moore et al 2000) I

Or maybe this.... Alternatives 'Shroud' of dim white dwarfs in a thickened disk around the Galaxy (Gates and Gyuk 2001) II

Or maybe this.... Alternatives Halo of supermassive black holes around the Galaxy (Ostriker and Lacey 1985) III

Or maybe this.... Alternatives No dark matter --- our theory of gravity is wrong IV That's all folks, there isn't any more...

DARKSTAR : constraining the dark matter Our home as we see it If the dark matter is in the form of dim stars, we should see some near the Sun If the dark matter is in the form of black holes, they could affect the Galactic disk Studies done with the NASA/ESA Hubble Space Telescope and with ESA's Hipparcos satellite Double check disk mass

Evidence: Clusters of galaxies Clusters in galaxies move at higher speed The visible mass of the galaxies themselves seems insufficient to hold them

X-rays from clusters of galaxies The same goes for the hot, X-ray emitting gas in galaxy clusters

Simulations of the whole universe Movie : density_flythru.mpg Simulations of the universe must be quite dense in order for the right numbers of galaxies and clusters to form (when compared to observations)

Simulations of clusters of galaxies Movie : xy.movie.mpg

Andromeda is headed our way! Movie : merger.mpg

It's showtime, folks! Movie : antobs.mpg Movie : mice.mpg

Through a glass darkly...

Lenses map matter...

The bullet cluster Highly significant cluster pair, because dark and visible matter are temporarily in different places (so their relative contributions can really be assessed)

Dark matter in cosmology

Spying on the neighbours... Your friend and mine, the Large Magellanic Cloud...

The DARKSTAR search for dim, low mass stars... dead stars... the 'white dwarfs'... barely alive stars... 'red dwarfs'

Results with HST HST... the Hubble Space Telescope Flynn, Gould and Bahcall searched for faint red stars in the Hubble Deep Field. Stars in the HDF marked in yellow None were found. Many were expected if they comprise the dark matter. Searching for dim, red stars

Astrophysical candidates - dim stars Fast moving white dwarf star from Ibata et al, Nature, 2000

Results with HST and ground telescopes Searching for dim, blue stars Flynn, Holopainen and Holmberg searched for faint blue stars in stellar motion catalogs Very were found. Many were expected if they comprise the dark matter.

It's probably not... red dwarfs or white dwarfs... dead stars... the 'white dwarfs'... barely alive stars... 'red dwarfs'

It's probably not... the white dwarf shroud Holopainen and Flynn 2003 Used new stellar motion surveys to search for dim white dwarfs Can rule out normal white dwarfs in a shroud

Supermassive black holes... Black holes, if they are heavy enough, cause the disk of the Galaxy to become thicker with time --- a process called disk heating. Very massive black holes Initially thin stellar disk

Supermassive black holes... Heavy black holes destroy the Milky Way Light black holes have no effect Need a fine tuned compromise... Disk has thickened

'Disk heating' observed The European Space Agency's Hipparcos satellite The European Space Agency's Hipparcos satellite has measured distances and spatial motions of 125,000 nearby stars with a huge increase in precision compared to a century of work done with telescopes on the ground

'Disk heating' observed and simulated Simulated dark halo and GMCs Required heating Time [Gyr] Kinetic energy

Can be fine tuned to fit the Hipparcos data Work well enough near the Sun and elsewhere in the disk... BUT require special pleading to work in other galaxies ? Hänninen and Flynn 2001, 2003 It's probably not... supermassive black holes

The disk mass is well understood Prediction for no disk dark matter Hipparcos Observations Holmberg and Flynn 2002, 2003 Observations match predictions very well; there is no missing matter in the disk…

Summary Baryonic Dark Matter Particle physics has cause for optimism! The evidence is building against baryons as the dark matter in galaxies...