Transcript Power Point Presentation

Astronomy 350
Cosmology
Professor Lynn Cominsky
Department of Physics and Astronomy
Offices: Darwin 329A and NASA EPO
(707) 664-2655
Best way to reach me:
[email protected]
March 4, 2003
Lynn Cominsky - Cosmology A350
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Is the Universe really
expanding?
 Take a look at the handout
 How do you interpret the 3 graphs?
 Try to draw graphs of a Universe that
has
Static galaxies (not moving)
 Turbulent galaxies (random motion)
 Rotating galaxies (like the planets around
the Sun)
 Uniformly contracting galaxies

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Cepheid variables and Nebulae
 In 1923, Hubble used new Mt. Wilson 100 inch
telescope to observe Cepheid variables in the nearby
“nebula” Andromeda. He recognized that the fuzzy
patches called nebulae were actually distant galaxies,
outside of our own Milky Way
This relation
was calibrated
by Hertzsprung
and Shapley
L =K P1.3
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Hubble Law
The Hubble constant
Ho = 558 km s -1 Mpc -1
is the slope of these graphs
Compared to modern
measurements, Hubble’s
results were off by a
factor of ten!
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Redshift and Doppler Shift
 Redshift z is a non-relativistic approximation
that relates the Doppler shift to the velocity of
the object
 Redshift is determined by comparing
laboratory wavelength lo to observed
wavelength l
z
March 4, 2003
=
Dl
lo
=
l - lo
lo
Lynn Cominsky - Cosmology A350
=
v
c
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Hubble Law
 v = Ho d = cz where
 v = velocity from spectral line measurements
 d = distance to object
 Ho = Hubble constant in km s-1 Mpc -1
 z is the redshift
Space between
the galaxies
expands while
galaxies stay the
same size
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Measuring Distance and
Time
If the Universe expands at a constant rate
v = Hod and d= vt
Solving for t, we find the age of the
Universe:
t = 1/Ho = 9.78 x 109 y h-1
where h = Ho/(100 km s-1 Mpc-1)
Current value for Ho is 68 km s-1 Mpc-1
So the age of the Universe is ~14 x 109 y
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Expanding Universe
 Expanding Universe of Students

The farther things are, the faster they are
moving away
 Expanding Universe of Dots

Why it looks as though you are at the
center of the expanding universe
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Doppler shifting spectral lines
 When a star or galaxy is moving, its spectral lines are
Doppler shifted by Dl = lobs – llab
 The redshift Z = Dl/llab
 The velocity of the star or galaxy is found from the
redshift using v = ZAVG c
H and K lines
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Hubble constant
 You will plot the velocities (km/sec) vs. the distances
(Mpc) that you obtain from a sample of galaxies
 The slope of this line is known as the Hubble
constant – it is given in km/sec/Mpc
H = v/D
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The age of the Universe
 The age of the Universe is the inverse of the
Hubble constant
 Age = 1/H = D/v
 But H must be in the correct units!
 D must be in km
 V must be in km/sec
 This will give you the age of the Universe in
seconds
 To convert to years, divide by 3.15 x 107
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Web Resources
 CLEA Astronomy Laboratories
http://www.gettysburg.edu/academics/physics/clea
/CLEAsoft.overview.html
Hubble diagram
http://physics.ucsd.edu/students/courses/fall2001/
physics5/notes/ch17/sld005.htm
 Thanks also to Dr. Gordon Spear for providing
his Hubble Law Lab spreadsheet!
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Web Resources
 Astronomy picture of the Day
http://antwrp.gsfc.nasa.gov/apod/astropix.html
 Imagine the Universe
http://imagine.gsfc.nasa.gov
 Ned Wright’s ABCs of Distance
http://www.astro.ucla.edu/~wright/distance.htm
 Prof. Pogge’s class notes at OSU
http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit4/nebulae.html
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