Lecture 12

Debate 2: What is Dark Matter? (cont.)

Evidence for baryonic dark matter

• gravitational microlensing detects previously undiscovered objects (MACHOS) in Milky Way's halo

  • halo white dwarfs and other objects with masses less than 100 solar masses may constitute up to 2-4 times as much matter as in all visible stars in Milky Way

    

  • still apparently not enough to make up all of dark matter (at most 25% of total mass of our Galaxy)

  • but perhaps microlensing experiments underestimate the fraction of Galaxy's total mass in baryons

    • not sensitive to dark objects with masses larger than 100 solar masses

    • if distribution of mass in our Galaxy's halo is clumpy, then microlensing measurements in particular direction (towards Large Magellenic Cloud) may not be fair estimate of all baryons

• hot X-ray gas in clusters was a surprise, as was cold gas clouds that make up Lyman-alpha forest, so maybe there is other hot or cold gas in wavelengths and/or places that we have not probed

• nucleosynthesis and cosmic microwave background measurements predict baryon density to be a few percent (~0.04-0.06) of critical density (note that, by convention, astronomers typically express baryon density, total mass density, and luminous matter density as a fraction of the density needed to close the Universe)

• baryon density already larger than for luminous matter in galaxies (~0.04-0.06 of critical density vs. ~0.005 of critical density) --> some if not all of dark matter in galaxies is baryonic (Jupiter-sized planets, long-dead stars, etc)

• density of baryons is within factor of 5 of total density of dark + light matter in Universe (as much as ~0.04-0.06 of critical density for baryons vs. ~0.3 of critical density for all matter), so is there really need for non-baryonic matter?

• only one non-baryonic candidate, the neutrino, has been actually detected, and neutrinos are not enough to account for all dark matter

Summary of contributions to matter-energy density of Universe

Component	Mass Fraction (relative to critical density)	Comment
luminous matter	~0.005	light emitting baryons only
all baryons	~0.04-0.06	from nucleosynthesis predictions and CMB measurements, factor of ~10x luminous baryons
all light and dark matter	~0.3	from adding observed light from mass derived from bulk flows, factor of ~5x all baryons
dark energy	~0.7	from supernovae and CMB measurements

To read some more about dark matter, go to the Hand-Outs and Reference Materials page.

To read some questions and answers about dark matter, click here.

Try playing around with the links on this site or this site, or clicking here.

To read a somewhat more advanced review of dark matter problems, click here.

To read what an on-line textbook has to say about dark matter, click here.

If you want to read about detecting "WIMP"s, click here.

If you want to read about detecting "MACHO"s, click here.

If you want to read about detecting dark matter in general, click here.

For a cosmology primer, check out the this page or this page.