Download Chapter 34: Cosmology FYI 1. Radar Ranging 2. Triangulation idea

Chapter 34: Cosmology
Cosmology:
How The Universe Works
Cosmology is the study of the structure and
evolution of the universe. To understand the
universe we need to be able to tell
– Where things are
– Where things are moving
How does the universe work?
What do we know of “how thing
are, were, and are to be”
be”?
Don’
Don’t have all the answers but
some evidence is accumulating
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How do we measure distances,
to stars and galaxies, etc.?
FYI
1. Radar ranging
2. Triangulation
3. Brightness - distance
4. Doppler shift
•There are about 100-400
billion stars in our galaxy!
•There are about 100-200
billion galaxies in our
universe!
R.R.
T.
We see about 10,000 “stars”
with the naked eye in the desert
--some are galaxies. Milky
Way is about 100,000 light
years across
B.-D.
D.S.
relatively short
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2. Triangulation idea
B
Set up similar triangles –angles are
the same.
θ
Applicability? Only to earth’s nearest neighbors -- moon,
sun, solar system
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relatively long
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1. Radar Ranging
Distance = (speed of light)
x (time) x 1/2
scale of distances
x
b
3 θ
5
a 4 meters c
120 meters C
A
P1: If you have similarity and know one side.
x=?
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Triangulation of Star Positions
But we want to see even farther!
θ1
θ2
3. Brightness
Three quantities are related:
1. Absolute brightness
Know θ1, θ2, length
Applicability
1. Use on ~10,000 stars in the near neighborhood (100 light
years) of the sun (just part of our galaxy)
2. Requires a telescope for such small angle differences.
(First done in the mid 1800s).
2. Apparent brightness
3. Distance to the star (unknown)
If you knew first two,
two, you could
calculate the third.
third. (100 watt bulb)
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Herzsprung and Russell
knew that
a.
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Harlow Shapley was able to point out that we
are not at the center of the galaxy
Stars vary in temperature and that means in “color”
He found the position by
measuring distances to the
globular star clusters that
orbited about the center of the
Milky Way. (early 1900’s)
blue-hot and bright
white - not quite so hot and bright
yellow- so-so
red- cool
b. The color can be measured from the earth.
H. &. R. assumed that from color of the star they could
get the absolute brightness. Then with apparent
brightness calculate the distance
They check their method for stars close enough to get
distances b triangulation. It worked
Globular Cluster
Harlow Shapley
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Position of the Sun in the Milky Way
The sun is 2/3 the way
from the galaxy center
to the edge.
Its position was found by
measuring distances to
the globular star
The Sun
clusters that orbited
about the center of the
Milky Way.
Variable stars were used
to find the distances.
x
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Variable Stars as Distance
Indicators
A class of very bright stars called Cepheids, have a tight
relation between their real brightness and their pulsation
periods.
Pulsation period Æ real brightness Æ measured brightness Æ distance
The primary mission of the Hubble Space Telescope was to
find distances to nearby galaxies using Cepheid variable
stars.
45 days
30 days
*
*
*
15 days
x
10 days
Globular Clusters
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External Galaxies
Soon it was realized that
all spiral-shaped “nebula”
were galaxies of stars
similar to the Milky Way.
It wasn’t until 1923 that
we fully realized the
universe consisted of
more than the Milky Way.
Using Cepheid variable
stars, Edwin Hubble
determined that the
Andromeda galaxy was a
separate system of stars.
Andromeda galaxy is 200 million light
years away
Our Galaxy is not unique
Edwin Hubble with his 200 inch telescope
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On average over 100
billion stars per galaxy!
Huge! The universe
itself is only 105 time
larger than a typical
galaxy.
At least 50 billion of
them.
Sketch of a galaxy: William Parsons, 1845
HST photo of the galaxy
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The Hubble Deep Field
Looks back 13 billion years,
at 10,000 galaxies.
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Elliptical Galaxies
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Spiral Galaxies
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Barred Spirals
Irregular/Peculiar
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4. Cosmological Redshift
and The Doppler Effect
Galaxy Clusters
Galaxies are held together in small to large
groups by gravity.
A profound discovery of the early 1900s was that
all distant galaxies have their absorption and
emission spectral lines shifted to the red.
the amount of the shift is related to the speed of the
galaxy.
Demo: bell tower recording
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Measuring Motion
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Doppler Spectroscopy
The Doppler effect causes waves to shift up in frequency as
they move towards you and down in frequency as they
move away
Light examples
Doppler broadened laser
cosmological redshift
Video: train pitch
Demo: swinging sound
source, web animation,
ripple tank
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shifted toward the red
Discrete Spectrum
from hydrogen atoms
in the laboratory
lower frequency
discrete spectrum from hydrogen atoms moving
with a star or galaxy away from the viewer
“Red Shift” – faster moving stars are farther away,
more shift
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Cosmological Redshift
Spectral emission (absorption) lines have a characteristic
pattern
The pattern is shifted to the red for distant stars
The amount of shift related to the speed of the star
Galaxies far away
P2: A galaxy that is six million light years away
1) is six million years old right now
2) appears to us as it looked six million years
ago
3) was six million years old when the light left
it
4) is within our own galaxy
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Distances
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Hubble Law (~1925)
The Hubble Law is used to find distances to the
farthest galaxies
The speed a galaxy 55 million ly
moves away from us
is proportional to its
distance.
The farther away the
galaxy is, the faster
it recedes.
This could mean we
are at the center of the
universe – but not
likely.
200 million ly
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Distance Measurements
Summary
Expansion
If the universe is expanding,
the farther away two galaxies
are, the faster they move
apart.
Also, all galaxies would
appear to be moving away
from all other galaxies so our
view is not unique.
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Video: 4 galaxies on balloon
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radar ranging
within solar system
triangulation
nearest 10,000 stars in
galaxy
distance-brightness
within our galaxy and
nearby galaxies
cosmological redshift
beyond nearby galaxies
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Types of Cosmologies
☼
☼
Why is Night Sky Dark?
☼
☼
Early ideas of the universe (steady state) held that it
was:
☼
☼
☼
☼ ☼
☼
☼
Infinitely old
Infinitely large
Unchanging in time
☼
☼
☼
☼
☼
☼
Olber
But this cannot be true because
The universe is seen to be expanding and thus it is changing
with time.
An infinitely large and infinitely old universe would never
know darkness. This is Olber’s paradox
Earth
All lines of sight end on a star! The sky should be ablaze with star light!
The solution is that the universe has a beginning
Light has not had time to reach us from the greatest distances
Glowing stars and galaxies have not always existed.
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Big-Bang Theory
Basic Ideas
•Each galaxy started by condensation of
huge clouds of hydrogen gas.
•Particularly dense regions compressed by
gravity to for stars.
•Currently our universe is dominated by
fusion of hydrogen to form helium.
•Structure is stable now, but what happens
when hydrogen is used up
the universe began with an incredibly dense
concentration of mass energy
in the process of rapid expansion and cooling matter was
formed
point-like particles (electrons, quarks, photons, etc.)
nucleons (protons and neutrons)
simple nuclei (hydrogen and helium)
more complex nuclei, atoms, and molecules
Age of the universe is ~ 14 billion years old
The fate of the universe depends on its mass and energy
Open, it keeps expanding forever
Closed, it collapses back in on itself
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What observations must a good
model of the universe explain?
*1. The “expanding” universe.
(the red shift)
2. The “age” of the universe.
*3. The abundance of hydrogen (75%), helium
(24%), and other elements. (process of particle
formation)
*4. The microwave radiation (3K). (even in black
space, started with the big bang)
5. The structure of observed matter.
The model called “the Big Bang” does quite well
and is now widely accepted by scientists – the
modern science “creation view”. PS 110A-Hatch-Ch. 34 40
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Steps in Big-Bang Theory
Time
Temp
10 -43 sec
huge
???
10 -35 sec
10 26 deg
quarks, gluons, electrons,
neutrinos, photons
protons and neutrons
.001 sec
10
11
deg
9
Structure
3 m inutes
10 deg
3
H , 3 H e, 4 H e nuclei
1/2 hour
10 8 deg
2
H nucleus
500,000 yrs 30000 deg atom s
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Experimental Evidence
Experimental Evidence
cosmological red shift
Everything is moving away from us
universal microwave radiation (3K)
helium/hydrogen ratio (25%/75%)
Expect low frequency leftover energy from an
explosion of that magnitude
Fusion early after the big bang would produce
that ratio of H to He (~ 3 min.)
Big Bang
Stellar Fusion
Supernova
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Wavelength (mm)
Evidence Supporting the Big
Bang
What does the big bang predict for
the future of the universe?
We see the expansion
We see a faint glow left from the
original fireball.
It depends:
IF the universe is “open”...the galaxies will separate forever.
Space
time will expand forever. Fewer suns will be formed and finally the
stars will all burn out.
Called the Cosmic Microwave
Background or 3° K Background
Radiation.
Arno Penzias and Robert Wilson
Nobel Prize, 1978
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IF the universe is “closed”...the universe will cool and the galaxies
will slow down then collapse.
a “Big Collapse” or “Big Crunch”
What makes the difference?
Mass – don’
don’t forget dark matter (not emitting light).
Are there bounds to the heavens?
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Expansion reverses closed
Expansion Stops - flat
Now
Closed
“Big Bang”
Time
“Big Crunch?”
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Size of Universe
Size of Universe
Now
Flat
Closed
“Big Bang”
Time
“Big Crunch?”
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Expansion Continues - open
Size of Universe
Now
Supernova Explosion 1987 IA
Open
Flat
Closed
Before
During
“Big Bang”
Time
“Big Crunch?”
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Type IA Supernova
The Ultimate Fate of the Universe
Gravity always pulls, never pushes.
So the expansion should slow down.
Spectrum and brightness are perfectly correlated
(days to brightest portion, months to die out)
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Video: spectrum as function of (fast) time
Which model (flat, open, etc.)
is correct?
By looking in the past we can compare the shape of
the universe expansion curves. We use supernovae
explosions to check brightness, distance, and
velocities. At most one star per galaxy per decade.
During the last ten years we find that none of these is
correct. The universe expansion is speeding up
faster than all these predict.
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----unless Einstein is correct and there
is a “cosmological constant force”
expanding space. Gravity fights at
first but loses out in the end.
Associated with dark energy (matter)
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Accelerating expansion
• Video: Type IA supernova explosion.
• 25% dimmer than the open growth
predicts.
• Video: expansion for runaway growth.
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The Ultimate Fate of the Universe
Question
P3: Of the methods described for
Not enough mass - open
universe
Just right - flat universe
Too much mass - closed
universe (bang-bangbang?)
determining distances, the one which is
limited to measuring distances within our
solar system is:
a. brightness-distance relationships
b. radar ranging
c. triangulation
d. Doppler spectroscopy
Another force - runaway
universe.
This is what we think is
happening!
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P5:Forces slowing the expansion of
the universe are due to
Question
P4: Consider light observed on earth that was emitted
by atoms moving with stars or galaxies. Which
statements are true?
a. You can actually see little red atoms that have been
shifted to the left. They have small beady red eyes
and cannot be trusted.
b. The spectral colors emitted by the atoms moving with
most of those objects are shifted toward higher
frequencies. The shift is greater for objects that
are farther away. This “red shift” is the basic
evidence for the belief that the universe is expanding.
c. none of the above
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a) The cosmological
interaction
b) The nuclear
interaction
c) The electromagnetic
interaction
d) The gravitational
interaction
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