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Orbits and Galaxy Shape
• elliptical orbits• random orientations• different speeds in
different directions
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Orbits and Galaxy Shape
Halo and Bulge:• elliptical orbits• random orientations
Disk:• nearly circular orbits• same direction• a little “bobbing” up and down
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Thought Question:Which of the following mergers of spiral and
elliptical galaxies is most unlikely?
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orderly orbits in spiral galaxies are disrupted during collisions:
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• collisions between galaxies affect star orbits
• many examples – especially in galaxy clusters
Environment
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Stars versus Gas
“Collisions” of star groups don’t result in star collisions…
stars are small with lots of space between
BEFORE AFTER
Collisions of gas clouds result in “shocks” … kinetic energy converted to heat and radiated away
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Galaxy Properties
Spiral Elliptical
Shapes flat disk + round bulge
elliptical (football)
Overall color bluish (young, high-mass stars)
reddish (giant stars)
Gas, Dust yes – in disk little or none
Where? mostly in emptier regions of space
often in clusters of galaxies
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Zoom in on a seemingly blank piece of sky…
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Observe galaxies to try to learn…
• what does the universe look like?
• what was its past?
• what is its future?
Cosmology
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How Does the Universe Look?
Isotropic?: Does it look almost the same in all directions to us?
count galaxies in different directions
Homogeneous?: Would it look almost the same to observers anywhere?
measure distances to many galaxies
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How Does the Universe Look?I feel left
out…Isotropic?: Does it look almost the same in all directions to us?
count galaxies in different directions
Homogeneous?: Would it look almost the same to observers anywhere?
measure distances to many galaxies
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Thought Question
Which of these 4 universes is homogeneous?
(Enter the letters for all correct answers.)
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Thought QuestionWhich of these 4 universes is isotropic according to the stick figure?
(Enter the letters for all correct answers.)
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Is the Universe Isotropic?isotropic: looks the same in all directions to us
All-sky surveys of millions of galaxies show there are no special directions
blocking by Milky Way
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Is the Universe Homogeneous? harder to measure: need to measure galaxy distances and
survey faint, distant galaxies
some “clumping” of galaxies on small scales, but there seem to be no special places
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Is the Universe Homogeneous?
galaxies line up in filaments, with clumps where they meet
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EVIDENCE: Expansion of the Universe (1928)
Almost all galaxies have redshifts
appear to be moving away from us
more distant galaxies have larger redshifts…
BLUE RED
REDBLUE
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COINCIDENCE? HMMM…
WHEW! DID SOMETHING DIE
IN HERE?
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Hubble’s Law
vr : recession velocity
d: distance
H0: Hubble constant
22 km/s / Mly
(1 Mly = 1 Mega light-year = 106 light-years)
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Hubble’s Law(fast forward)
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Consequence #1Consequence #1
what we see:
alien galaxy feels stationary to them
they observe the same Hubble Law!
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Consequence #2Consequence #2All distances change by
the same percentage in the same time:
implies space is stretching uniformly to make Hubble’s Law
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Imagine running time backwards:
galaxies would meet in same place at same time t in past (BIG BANG)
Consequence #3Consequence #3
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THEORY: A “Big Bang”
Idea:
• universe had a beginning
• early universe was hot, dense gas
Tests:
• Did “Big Bang” leave evidence?
THEORY: a scientific idea that has survived repeated tests of its predictions
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Evidence for a “Big Bang”
What clues do we have that there was a BEGINNING to the Universe?
• Dark Sky at Night
• Expansion of the Universe (Hubble’s Law)
• Cosmic Microwave Background Radiation
• Abundances of Chemical Elements
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The Observable Universe
If universe is NOT infinitely old, then…
only see galaxies whose light had time to reach us
universe may be MUCH larger, but there’s an edge (“HORIZON”) to what we can see
more distant galaxies look younger to us: “LOOKBACK TIME”
OBSERVABLE UNIVERSE
UNOBSERVABLE UNIVERSE
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Thought Question:Imagine you are observing a high-mass star located in a
distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years.
• How long before we receive light from this star’s supernova?
100 MILLION LIGHT-YEARS
?
“I see a blue star right now.”
(Enter your answers in millions of yrs.)
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Thought Question:Imagine you are observing a high-mass star located in a
distant galaxy 100 million light-years away. By analyzing the starlight, you are able to tell that the star appears to be 10 million years old. From its properties, you predict that the star has a total lifetime of 50 million years.
• When will or when did the supernova actually occur?
100 MILLION LIGHT-YEARS
?
“I see a blue star right now.”
(Enter your answers in millions of yrs. If it already happened, make your answer negative.)
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Lookback
100 MILLION LIGHT-YEARS
40 MILLION LIGHT-YEARS
“The star just blew up!!!”
“The star has been
dead for 60 million years.”
“The star looks like it
has 40 million years
to live.”
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“DARK AGES”
UNOBSERVABLE UNIVERSE
(early in history)
How far back in time can we possibly look?
“DARK AGES”: before galaxies and stars were born
Universe was once OPAQUE gas too dense to let light through
Universe History
US
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Stretching and RedshiftsRedshift tells us about total amount of stretching
universe has done while the light has been traveling
EXPANDING UNIVERSE:
PRESENT
DISTANT PAST
MEDIUM PAST
LARGER REDSHIFT FOR MORE DISTANT GALAXY
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Universe ScaleWe can’t measure size of universe (especially if infinite), so
compare distances at different times in history:
Distances between non-moving galaxies stretch in same way light wavelengths do:
PRESENT:
PAST:
density of the universe was larger in the past:
RU =1 z=0: present
RU<1, z>1: past
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Thought Question:The most distant object that has been seen has a
redshift of about z = 8. Roughly how far was it from us when it released the light we see today?
A. 9 times its present distanceB. 8 times its present distanceC. 7 times its present distanceD. Its present distanceE. 1/7th its present distanceF. 1/8th its present distanceG. 1/9th its present distance
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EVIDENCE: Cosmic Microwave Background Radiation (1965)
Microwave and radio from all directions in sky with nearly equal brightness:
MAP OF ENTIRE SKY:
thermal radiation with very low temperature
T = 2.728 K
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EVIDENCE: Cosmic Microwave Background Radiation (1965)
If Big Bang was an explosion at one place in space, we should see a glow only from that place…
BIG BANG seems to have happened EVERYWHERE
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Cosmic Microwave Background Radiation
thermal radiation with temperature T = 2.728 K
Expansion of universe stretches light, made temperature lower today:
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The Horizon ProblemOpposite sides of observable
universe somehow “knew” to have almost the same temperature today
takes longer than age of universe for light to travel from one side to the other…
HORIZON
Choose a number between 1 and 1 million.
538,681
538,681
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INFLATION: age 10-35 s to 10-32 s space expands faster than light
can travel by huge amount (1050 times)
parts of then-observable universe move so far away we never see them again
needed to explain:
isotropic microwave background
homogeneous universe
History of the Universe
OUR OBSERVABLE UNIVERSE TODAY
THEN-OBSERVABLE UNIVERSE (BEFORE INFLATION)
OBSERVABLE UNIVERSE AFTER INFLATION
BEFORE:
AFTER: