agenda read unit 8 distance to galaxies (review) redshift hubble’s law the big bang
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Agenda
Read Unit 8 Distance to galaxies (review) Redshift Hubble’s Law The Big Bang
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Calculating distances to other galaxies
Two standard candles:
Cepheid variable stars Type I supernovae
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NASA
10 billion ly away!
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Cepheid variables
Gene Smith, UCSD/CASS
Cepheids are a standard candle.
Discovered by Henrietta Leavitt. (photo: AAVSO)
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Harvard
Supernovasin 4 galaxies
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How do we determine whether a galaxy is approaching or receding?
The Doppler Effect.
Next step: velocities
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Demonstration: doppler buzzer
How does the buzzer sound change as it moves away?
A. Louder
B. Softer
C. Higher pitch
D. Lower pitch
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Source moving away from you
The light is shifted toward the red: longer wavelength, lower pitch (frequency)
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Source moving toward you
The light is shifted toward the blue:shorter wavelength, higher pitch (frequency)
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What about galaxies?
Galaxies have specific spectral lines that can be identified.
Measure wavelength. Compare to the “rest” wavelength. The difference is the doppler shift. From doppler shift we get speed.
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Hubble’s distance velocity relation
(Distance from us)(Spe
ed a
t w
hich
gal
axy
rece
des
from
us)
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Two years later (note scale)
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Hubble’s law
The more distant a galaxy is, the faster it’s receding
This means it has a larger redshift!! The universe is expanding!
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The universe:
Is expanding (Hubble’s law) Has galaxies evenly distributed
throughout (homogeneous) Contents evolve with time (H fuses to
He, etc)
Any theory must be consistent with these facts!
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Space is getting bigger!
Wikipedia
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The universe has no center
Consider only the surface of the balloon
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The big bang
An explosion of space, not in space A consequence of relativity
Some heat is left over from this Cosmic background radiation Seen everywhere in the universe
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How old is the universe?
How can we estimate? Precisely map the expansion seen now from
Earth Run the physics “backwards”
Many ways to check. Examples: Age of oldest stars (globular clusters) WMAP measurements => 13.7 Billion years
(plus or minus 1%)
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Boomerang team
Tiny differences in temperatureseen in every direction!
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The universe is 2.73 K +/- 0.0001
WMAP
The whole sky is filled with radiation left over from the Big Bang
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What if this heat comes from galaxies and clusters?
How would the heat be distributed in the night sky?
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What is the ultimate fate of the universe?
Possibilities to consider:
Big Crunch Growth will slow down Growth will remain constant Growth will continue to speed up
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NASAEvidence suggests this is happening
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Summary
The universe is expanding Actually, it’s speeding up! Heat from the Big Bang is seen The matter in the universe in changing
with time (because of stars)
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What is the universe made of?
Dark matter 25%
Free H and He 4%
Stars 0.5%
Neutrinos 0.3%
Heavy elements 0.03%
Dark energy 70% Pie chartFrom Wikipedia
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Group activity #10
Hubble’s law