1. Dark Matter Inquiring Minds Want to Know Prof. David Toback Texas A&M University

2. Inquiring Minds Want to Know I keep hearing about “Dark Matter” in newspapers, on TV, etc What are people talking about? Why is it called Dark Matter? What do we know about it? What’s the evidence for it? So what?

3. Quick Answer Stuff out in space we can’t “see” directly

4. What is Dark Matter and why do we call it Dark Matter? What do we see when we look at the Heavens? Easy to see light from things like stars Can see the light from stars AFTER it has left the star and interacted with whatever is between us and the star Anything else out there?

5. What is Matter? Lots of ways to understand the word “matter” Examples: –Rocks –You –The Earth –The Sun –Atoms Our Sun and Earth are attracted to each other by gravity They are attracted to each other because they are made of “matter” –They have “mass”

6. 6 Dark Matter There is evidence that there is “stuff” in space that we can’t “see” directly Call this stuff “Dark Matter” –Stuff that is “like atoms” in that “has gravity”

7. 7 Evidence for Dark Matter 1.The Rotation of Galaxies 2.Gravitational “Lensing” 3.Colliding Galaxies

8. 8 Evidence 1 Watch the way stars move around the center of a Galaxy

9. 9 The Planets and the Sun Gravity does a good job of predicting the planets path around the Sun

10. How Quickly do the Planets go Around the Sun? Inner planets like Mercury, Venus and Earth go quickly around the Sun Planets move MUCH slower

11. As the Solar System Turns

12. 12 What about Stars and Galaxies? Can again use Gravity to predict the orbits of stars as they move around the center of the galaxy Should look “like” the planets as they go around the Sun

13. Is that what our galaxy, the Milky Way, looks like? Does it spin like our solar system? Closest stars go around quickest?

14. 14 Data doesn’t look anything like expectations! The Data Difference due to Dark Matter?

15. How the Galaxy Turns

16. 16 Dark Matter? Data well explained by lots of “Dark Matter” we can’t see About five times as much Dark Matter as stars  Forms a “halo” around the entire galaxy

17. 17 Evidence 2 Look at the effect gravity has on light as it travels through the Universe towards us

18. 18 What happens to light as it goes past the Sun? Path doesn’t curve Path does curve Newton: only things with “real” mass “feel” the force of gravity. Light has no mass! Einstein: Objects with “Energy” move according to the curve of space-time, regardless of whether they have mass or not

19. More Detail Star looks like it’s here Can see the star directly Now the Sun curves the path of the starlight

20. The Great Experiment of 1915 Look at a star’s position “behind” the sun as it “passes” in between us and the star Result: The apparent position of a star is different when the sun is close to the path In other words, we observe that the gravity of the sun “bent” the path of the light coming from the star

21. Dark Matter and Lensing Dark Matter “Lenses” the galaxies behind them like a prism Evidence that the light coming to us is passing through lots of matter we can’t see directly

22. 22 Looking at Light from the Universe Galaxy Light from a Galaxy Looks like a Galaxy Hydrogen Cloud Looks like a Galaxy behind a cloud of hydrogen Looks like a Galaxy behind a cloud of heavy “stuff” that isn’t just stars and hydrogen Another Galaxy

23. 23 More on Lensing Sometimes we can even see more than one image of the same galaxy!

24. 24 Evidence 3 Colliding Clusters of Galaxies

25. 25 Look at Colliding Clusters of Galaxies Atoms Dark Matter Atoms Dark Matter Atoms in the Galaxies interact and slow down as they pass through each other Dark Matter doesn’t interact much so it isn’t slowed down much

26. 26 Colliding Galaxy Clusters Atoms and Dark Matter Atoms and Dark Matter The atom part and the Dark Matter part of Galaxies interact differently as they pass through each other Atoms Dark Matter

27. 27 Colliding Galaxy Clusters Atoms and Dark Matter Atoms and Dark Matter Atoms Dark Matter Light from a Galaxy Galaxy

28. 28 Evidence for This in Nature? Colliding Clusters of Galaxies Blue is the part from lensing only “Fast  Dark Matter” Red part from observing the light “Slow  Atoms”

29. 29 What IS Dark Matter? We don’t know… Still working on it…

30. Is that the whole story? What’s the Matter in the Universe? No real understanding of what this stuff is…

31. Concluding Remarks The case that there is lots of Dark Matter in the Universe is very compelling The fate of our Universe depends on Dark Matter Might be able to discover it using the Large Hadron Collider (LHC) If scientists are right, might be able to tie together Astronomy, Particle Physics and Cosmology