Announcements

1. Announcements • Homework 14 due Wednesday (5 questions) • Monty Python Challenge offer good through this Wednesday! • Final exam in SL 228 next Monday or Tuesday. Worth 100 points (20% of grade). Comprehensive, but will emphasize galaxies and cosmology

2. Since this course began . . . • Earth has spun on its axis about 100 times, causing the sun, stars, etc. to appear to rise and set each time; • The moon has orbited the earth 3.5 times, going through its phases; • Earth has completed more than 1/4 of its orbit around the sun, allowing us to see different stars and planets at night and causing changes in the sun’s position in our sky and the seasons; • Mercury has completed about one orbit around the sun; Venus about half an orbit; Mars only 1/6; outer planets much less; • Nearby stars have typically moved about 1 AU with respect to our sun, causing their positions in the sky to shift by a fraction of an arc-second; • Our solar system has completed a little over one billionth of its orbit around the galactic center; • The observable universe has expanded (and grown older) by one part in 40 billion.

3. Dark Matter and Dark Energy 4 December 2006

4. Today: • Dark matter: evidence and possible forms • Dark energy: evidence and possible forms

5. Weighing Galaxies NGC 7541 Vera Rubin

6. “Dark Matter” • In the solar system (all mass at the center), outer planets move slower than inner planets. • In a spiral galaxy, outer stars move slightly faster than inner stars. • Conclusion: The mass of a galaxy is not as centrally concentrated as the stars are; most mass is much farther out, and doesn’t shine!

7. The Andromeda Galaxy

8. Gravitational Lensing Again, galaxies are found to weigh about 10x more than the total weight of their stars.

9. Hubble Deep Field

10. What is the dark matter? • Gas clouds? These would absorb starlight; we know that their total mass (within a galaxy) is much less than that of the stars. • Dust particles? These would block our view of distant stars and galaxies; total mass is quite small. • “Jupiters” or “brown dwarfs” (“MACHOs”)? These can be detected by gravitational lensing when they pass in front of a star. Searches indicate that there aren’t enough of them to account for much of the dark matter. • Black holes? Again, gravitational lensing would have been detected. • Neutrinos? These are known to exist in large numbers, but their masses are probably negligible. • Some undiscovered species of weakly interacting massive particles (“WIMPs”)? This is actually the most favored hypothesis at present. Best guess for mass of each WIMP is 30 to 5000 x proton’s mass. • No such thing? Maybe we’re wrong about the law of gravity…

11. Searching for WIMPs LHC (“Large Hadron Collider”), Geneva, under construction

12. Recent result from latest supernova data:The expansion is accelerating!!! • Very distant galaxies are moving slower than expected according to Hubble’s law. • We see these galaxies as they were, billions of years ago. • Conclusion: The rate of expansion was slower in the distant past. Velocity --> Alex Filippencko, U.C. Berkeley

13. “Dark Energy” • A property of space itself (probably), in the sense that it fills otherwise empty space with a uniform density • Creates a repulsive force, pushing everything apart • Originally hypothesized by Einstein to balance the gravitational pull of the stars on each other • Quantum physics seems to predict HUGE amounts of dark energy • Until recently, most physicists thought that somehow these quantum effects cancel out to leave no dark energy at all • Given that the expansion of the universe is accelerating, the density of dark energy must be very small but nonzero. Nobody knows why.

14. Our best picture of the early univese Details of the cosmic background radiation tell us how matter was originally distributed, and whether space is curved on large scales.

15. Details of CBR tell us… Space is “flat” on cosmic scales, and filled with mass/energy in the following mixture: