Download Class 28, 27 July

SMDEP Physics
Today: Astronomy, Extrasolar Planets, Cosmology,
Dark Matter, Dark Energy, Black Holes and more
http://www.astro.yale.edu/krines/smdep
New address:
http://www.cfa.harvard.edu/~krines/smdep/
Test Results
• Mean = 20.3/37 (max:33)
• Median = 20
• Stddev = 5.5
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CLASS PARTICIPATION AND HW
Homework: 36 possible, mean: 26, max:34
Bonus HW: 22 possible, mean: 16, max:22
Group revotes: 36 possible, mean: 20, max: 30
Total of above: 94 possible, mean: 62, max: 81
Next Year
• KR moving to Boston
– New email: [email protected]
– SMDEP Physics webpage to end (move?)
• Full-time research, so please email for
help with physics courses
• Open invite:
– If you’re in Boston, I’ll take you out to
lunch
– Email ahead of time
Blackbody Radiation
• Photons emitted by object at temperature T
– Characteristic shape: more photons as frequency
increases until peak, then fewer with highfrequency tail
– Peak wavelength: lT = 3 x 10-3 m K
• Examples:
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Sun: T=6000 K, peak at l = 450 nm -> optical
Light bulb: T=2200 K, peak at l = 1.4 x 10-6 m (IR)
People: T=300 K, peak at l = 10 x 10-6 m (IR)
Echo of Big Bang: T=3K, peak at l = 1 mm
(microwave)
Blackbody Radiation
The Sun
• Why does the Sun shine?
– Late 19th century: as ball of gas contracts, gas
releases potential energy as radiation
• Timescale of 10 million years
– Must be other source of energy
• Nuclear fusion: ‘Burn’ hydrogen into helium
• E=mc2
• Timescale of: (total energy available)/(Power)
– Total energy available is 10% of Sun’s mass, converted at
0.008% efficiency
– Power is light output by Sun
– Timescale of 10 billion years
Pioneer 10 and Pioneer 11 were launched in
the 1970s to explore planets in the outer
solar system. They transmit radio signals
back to Earth at a specific frequency. To
hear these signals, scientists on Earth
tune their radio dishes to:
1.
2.
3.
The same frequency
transmitted by Pioneer
10
A higher frequency
A lower frequency
Aliens on a planet orbiting a nearby star
send a laser signal out to space at a
specific wavelength. If astronomers
could detect this laser over an entire
orbit and plotted wavelength versus time,
they would see:
1.
2.
3.
4.
A constant wavelength
A continuously
increasing wavelength
A continuously
decreasing wavelength
A sine wave
Extrasolar Planets
• Key concepts: Gravity, Orbits, Center of
Mass, Circular Motion as Simple Harmonic
Motion, Light as Wave, Doppler effect
– Star much much brighter than planet
• Also, diffraction limit
– Both planet and star orbit center of mass of
system
– Monitor velocity of star by watching spectral lines
from atoms
• Lines show Doppler shifts
• Circular motion looks like oscillation -> sine wave!
Extrasolar Planets
Extrasolar Planets
A Quick History of the Universe
• Big Bang: space is infinitely dense, infinitely hot
– Known laws of physics break down
• After Big Bang, space expands, Universe cools
– At some temperature, protons and neutrons form
– A little later, the protons and neutrons make Helium (most
Helium in the universe was formed in the first 3 minutes
after the Big Bang)
– Nuclei, electrons, and photons in big “soup”
• Nuclei try to collapse (gravity), photons push back (pressure)
• This leads to OSCILLATIONS!
• Size of oscillations measures geometry of universe (know
physical size, angle, so can measure geometry)
– “Soup” cools, atoms form, and photons escape with nearly
uniform temperature
• These photons get stretched by space, become microwaves
Expansion of Universe: Hubble’s Law
• Is universe static or dynamic?
– If static (and eternal), very small fluctuations would grow via
gravity -> not static
– Einstein suggests “cosmological constant” keeps this from
happening -> acts like “anti-gravity”
• Key concepts: Kinematics (relation between velocity,
time, and distance), Electron orbits, Doppler effect
– Nearly all galaxies have Doppler effect towards longer
wavelengths -> “redshift”
– Size of Doppler effect measures velocity
– Velocity increases with distance: v = H d
• Imagine snapshot of middle of marathon
Expansion of Universe: Hubble’s Law
Hubble’s data
Expansion of Universe: Hubble’s Law
• Key concepts: Kinematics, Electron orbits, Doppler
effect, Gravity
– All matter in universe attracts all other matter in universe
via gravity -> expect expansion to decelerate (negative
acceleration)
– Measure velocity versus distance with supernovae
• Supernovae are “standard candles”: constant power
• Brightness tells you distance
• Doppler effect (redshift) tells you velocity
– Observations show acceleration!!!
• Observations wrong?
• General relativity wrong?
• Dark energy? Produces “anti-gravity” similar to Einstein’s
cosmological constant
• Imagine snapshot of middle of marathon
Expansion of Universe: Hubble’s Law
My research: Galaxy Clusters
• Key concepts: Gravity, Orbits, Light as Wave,
Doppler effect, Temperature is Kinetic
Energy
– Massive ‘ball’ of galaxies, Doppler shift measures
velocity, velocity measures mass
• Apply v = sqrt(GM/R)
– Equate kinetic energy to temperature
• KE = (1/2)mv2 = (3/2)kT
• Blackbody radiation -> m -> X-ray!
– Hydrogen gas orbiting cluster emits X-rays!
• Measured temperature provides check of mass estimate
My research: Galaxy Clusters
My research: Galaxy Clusters
• Key concepts: Gravity, Orbits, Light as Wave,
Doppler effect, Temperature is Kinetic Energy
• Galaxy clusters contain “fair sample” of stuff in the
universe
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Measure total mass with galaxy velocities, gas temperatures
Total starlight measures mass in stars
X-ray emission measures mass in diffuse gas
Total of (stars+gas) is only 15% of total mass!
• Difference is “dark matter”
• Evidence also from velocities of stars orbiting
galaxies
My research: Galaxy Clusters
What do we know about dark
matter?
• Behaves like “ordinary matter” in some
ways
– Clumps via gravitational collapse
– Doesn’t provide pressure support
• (If you push normal matter together, it resists
as the particles interact. Dark matter particles
pass straight through each other without
interacting)
• Similar to neutrinos, but neutrinos don’t
have enough mass to be dark matter
My research: Galaxy Clusters
• Key concepts: Gravity, Orbits, Light as Wave, Doppler effect,
Temperature is Kinetic Energy
• Largest structures in universe still forming today and in recent
history
• Amount and nature of dark energy affects growth of structure
– More dark energy (“anti-gravity”) means less growth of
structure
• Measure amount of structure (number of clusters) in nearby
universe
– More clusters mean more structure
• Measure amount of structure in distant universe
– Light travels at finite speed, so looking far away is looking
into past
• Difference measures amount of growth, shows effects of dark
energy
Signs of a bad observing run
My Goals for this Course
• Strengthen back and arm muscles with heavy
textbook
• Cover several topics from first-semester
physics courses
• Encourage independent thinking and group
work, stimulate discussion
• Improve conceptual understanding as
measured by pre and post-quiz
• Lay foundations for fall courses, MCAT
• Convince you that physics is comprehensible,
interesting, and (sometimes) fun!
My Goals for this Course
• All met and far exceeded expectations
• Thank you for all your hard work and
dedication!