Lecture 7

Debate 2: What is Dark Matter?

Discovery of Dark Matter

• dark matter neither emits nor absorbs light, our primary means of observation

• BUT dark matter can be observed by its gravitational effect on 1) orbits of stars and galaxies and 2) light, whose path it can bend

• Fritz Zwicky notes in 1933 that outlying galaxies in Coma cluster moving much faster than mass calculated for the visible galaxies would indicate

  Hubble Space Telescope Image of the Center of the Coma Cluster (movie)

  Fritz Zwicky

  • finds large orbital speeds (about 1000 km/s)

  • cluster should have flown apart long ago because visible stars in galaxies cannot account for all gravity necessary to keep cluster together

  • calculates cluster must be 9/10's dark matter or ``missing mass''

    

• Zwicky predicts in 1937 that dark matter could be investigated by observing galaxies that act as gravitational lenses, something that came to pass only in the 1990's

  Schematic of Foreground Galaxy Acting as Gravitational Lens on Background Quasar (movie of light bent by mass)

  Cluster of Galaxies Acting as Gravitational Lens on Background Galaxies

  

  To see a movie of a model of a cluster (yellow galaxies) moving in front of background galaxies (faint blue galaxies) and gravitationally lensing them, click here.

  To read more about gravitational lensing, click here or here.

• Vera Rubin discovers dark matter in galaxies using rotation curves

  

  • rotation curve shows how average velocity of stars changes with distance from center of galaxy

    

  • before Rubin's measurements, it was thought that velocities of stars in outer parts of galaxies would decrease relative to center
    • if mass ran out at large distances from galaxy's center in same manner as light, then stars far from center should feel same mass as those closer in

    • therefore, fast moving stars far from center should escape eventually, leaving only slower moving stars

    • thus, average velocity of stars far from center should be smaller that that of stars closer in

    Edge-on Galaxy

    

    At a given distance from the center, stars can move faster on average if there is more mass inside that radius to hold onto them. Therefore, rotation curve rises from center as stars feel more and more mass. In this model, the mass runs out at some distance from center, and fastest stars at large distances can escape, lowering average velocity of stars and decreasing curve at large distances.

    Velocities of stars actually remain constant out to large distances from center. This ``flattening'' of rotation curve is important evidence that galaxies contain large amounts of unseen material, more widely spread than the stars and gas, whose effects we see only through its gravitational impact on material that we can observe. This dark matter causes mass felt by stars to increase even at large distances from center, trapping fast moving stars and keeping average velocity of stars up. In many galaxies, estimates of the relative amounts of luminous and invisible matter show that ``dark matter'' outweighs what we can see - sometimes by as much as ten times.

    Observed Rotation Curve - Surprise!

    

Check out this rotation curve applet.

But is dark matter the only explanation for these results?

• two examples from our Solar System
  • discovery of Neptune by its gravitational effect on Uranus -- unseen matter

    

    • wobbling of Uranus's orbit led to search for unseen body

    • Neptune discovered as a result, the only planet detected first by its gravitational effect on another planet

    • early unseen mass (or ``dark matter'') problem that lead to actual detection

  • precession of perihelion of Mercury -- incomplete physics

    

    • change in orbit originally thought to be due to unseen body called Vulcan

    • precession later found to result from bending of space by Sun, a consequence of Einstein's General Theory of Relativity (movie)

    • so, unseen mass not required in this case, but a change in known physics

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

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

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

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