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
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.
Dark Matter and Dark Energy
4 December 2006
Today:
• Dark matter: evidence and possible
forms
• Dark energy: evidence and
possible forms
Weighing Galaxies
NGC 7541
Vera Rubin
“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!
Gravitational Lensing
Again, galaxies are found to weigh about 10x more than
the total weight of their stars.
Hubble Deep Field
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…
Searching for WIMPs
LHC (“Large Hadron Collider”),
Geneva, under construction
Recent result from latest supernova data:
The expansion is accelerating!!!
Velocity -->
Alex Filippencko,
U.C. Berkeley
•
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.
“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.
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.
Details of CBR tell us…
Space is “flat” on cosmic scales, and filled with
mass/energy in the following mixture: