1. Evolution of Dark Matter How ideas have changed

2. Outline Timeline –Hints –People –Some Explanations right and wrong

3. What is Dark Matter? Matter that isn’t directly visible –Doesn’t have to be “weird” matter Discovered accidentally (i.e. not predicted) Still much debate –May or may not exist!

5. The first hints…1933 Fritz Zwicky –Knew galaxies in clusters move around due to gravity –Decided to “measure” this gravity

7. Zwicky & Coma Cluster 1933 #1: Calculated mass from velocity of galaxies –Mass of cluster = M gravity #2: Calculated mass from brightness & number of galaxies –Mass of cluster = M visible He performed 2 separate measurements of cluster’s mass:

8. Zwicky & C.C. 1933 Two independent measurements of mass What did he find? M gravity >>> M visible (much, much, much greater than)

9. Zwicky & C.C. 1933 In words: Galaxies appear to be moving faster than their visible matter would allow. (His result: 98% of the mass is hidden M gravity >>> M visible

10. Sinclair Smith & Virgo Supercluster 1936 Three years after Zwicky Found the same result for the Virgo Supercluster Smith Sinclair, not Sinclair Smith

11. What’s going on? Two different masses measured There can only be one correct mass for a cluster of galaxies. –What’s the deal? Easiest explanation: must be invisible matter in clusters.

12. Does this make sense? Facts: –Galaxies move faster than visible matter would allow –Gravity makes things move faster Unseen mass = more gravity = higher velocity

13. More Evidence: Rotation Curves 1939 –Completely different method for detecting dark matter –NOTHING to do with clusters

14. Rotation Curves 1939 A graph of orbital speed versus distance from center Our solar system: close means fast. Can we do the same for whole galaxies?

15. How do the orbital velocities differ as we go away from the center?

16. Babcock & Andromeda 1939 Measured rotation curve for Andromeda Galaxy. Key idea: greater mass means faster orbital speed Observed Expected* *If mass only comes from visible matter Note the difference in the shapes of curves; Rotation curve is FLAT at large radii.

17. Rotation Curves: Many Galaxies THEY’RE ALL FLAT!

18. Rotation Curve: Our Galaxy Key Idea: greater mass means faster orbit. We are orbiting faster than we should be! Easiest explanation: visible matter isn’t the only matter.

19. Some ideas 1940 Two observations that support dark matter: –Clusters of Galaxies –Galactic Rotation Curves 1940 astronomers skeptical of data –Bad observing techniques? –Dust blocking light?

20. Jumping Ahead 1970 1970s Astronomers: –Believe the data –No idea what it was Theories: –Black holes –Hot gas between galaxies –Free-Range Jupiters

21. Jumping Ahead Today Visible mass: 10%Hot gas mass: 20%

22. Today 15% Visible Matter; 85% Dark Matter (  5%) Theories: Hot Dark Matter & Cold Dark Matter –Slow-moving or Fast-moving? Cold Dark Matter seems to be the best theory today. Other theory: DM doesn’t exist!!! We don’t understand gravity on large scales.

23. DM doesn’t exist Best theory Hot gas Weird matterNon-weird matter