Review: How does a star

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Hubbles galaxy classes
Spheroid Dominates
Disk Dominates
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Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
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Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
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Maps of galaxy positions reveal extremely large
structures superclusters and voids
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Time in billions of years
2.2
5.9
8.6
13.7
0.5
35
70
93
140
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Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls
mass into denser regions universe grows lumpier
with time
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Structures in galaxy maps look very similar to
the ones found in models in which dark matter is
WIMPs
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Cepheid variable stars are very luminous
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Cepheid variable stars with longer periods have
greater luminosities
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White-dwarf supernovae can also be used as
standard candles
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Step 5 Apparent brightness of white-dwarf
supernova tells us the distance to its
galaxy (up to 10 billion light-years)
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We measure galaxy distances using a chain of
interdependent techniques
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Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using Cepheid
variables as standard candles
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The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
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Hubbles Law velocity H0 x distance
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Distances of farthest galaxies are measured from
redshifts
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One example of something that expands but has no
center or edge is the surface of a balloon
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Cosmological Principle

• The universe looks about the same no matter
  where you are within it
• Matter is evenly distributed on very large scales
  in the universe
• No center no edges
• Not proved but consistent with all observations
  to date

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Expansion stretches photon wavelengths causing a
cosmological redshift directly related to
lookback time
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Fate of universe depends on the amount of dark
matter
Critical density of matter
Lots of dark matter
Not enough dark matter
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Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded
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Accelerating universe is best fit to supernova
data
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old
older
oldest
Estimated age depends on both dark matter and
dark energy
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The early universe must have been extremely hot
and dense
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Four known forces in universe Strong Force
Electromagnetism Weak Force Gravity
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Primary Evidence

• We have detected the leftover radiation from the
  Big Bang.
• The Big Bang theory correctly predicts the
  abundance of helium and other light elements.

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The cosmic microwave background the radiation
left over from the Big Bang was detected by
Penzias Wilson in 1965
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Background radiation from Big Bang has been
freely streaming across universe since atoms
formed at temperature 3,000 K visible/IR
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Background has perfect thermal radiation spectrum
at temperature 2.73 K
Expansion of universe has redshifted thermal
radiation from that time to 1000 times longer
wavelength microwaves
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WMAP gives us detailed baby pictures of structure
in the universe
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Abundances of other light elements agree with Big
Bang model having 4.4 normal matter more
evidence for WIMPS!
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Mysteries Needing Explanation

• Where does structure come from?
• Why is the overall distribution of matter so
  uniform?
• Why is the density of the universe so close to
  the critical density?
• An early episode of rapid inflation can solve all
  three mysteries?

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Overall geometry of the universe is closely
related to total density of matter energy
Density Critical
Density gt Critical
Density lt Critical
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Inflation of universe flattens overall geometry
like the inflation of a balloon, causing overall
density of matter plus energy to be very close to
critical density
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Patterns of structure observed by WMAP show us
the seeds of universe
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Observed patterns of structure in universe agree
(so far) with the seeds that inflation would
produce
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Sound waves red/blue high/low gas light
pressure
Many waves of different sizes, Directions
phases, all superposed
Water waves high/low level of water surface
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Seeds Inferred from CMB

• Overall geometry is flat
• Total massenergy has critical density
• Ordinary matter 4.4 of total
• Total matter is 27 of total
• Dark matter is 23 of total
• Dark energy is 73 of total
• Age of 13.7 billion years

In excellent agreement with observations of
present-day universe and models involving
inflation and WIMPs!
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Finite, But Without Edge?
2-dimensional analogy Surface of a sphere
Surface is finite, but has no edge.
An ant has no sense of the third dimension, to
him theres no center - All points are equal.
Any point on the surface can be defined as the
center of a coordinate system.
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Expanding Space
Analogy A loaf of raisin bread where the dough
is rising and expanding, taking the raisins with
it.
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The Age of the Universe
Knowing the current rate of expansion of the
universe, estimate the time for galaxies to move
to where they are today
Time distance / velocity
velocity (Hubble constant) distance
T d/v 1/H 14 billion years
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379,000 years old First light escapes Universe
already has structure (light still arriving today)
Early fluctuations become denser condensations of
matter
First stars form after 200 million years
Galaxies (groups of billions of stars) and galaxy
clusters form, according to the floorplan laid
out at 379,000 years
The Universe today lots of stars and galaxies!
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Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
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Olbers Paradox If universe were 1)
infinite 2) unchanging 3) everywhere
the same Then, stars would cover the night sky
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Night sky is dark because the universe changes
with time As we look out in space, we can look
back to a time when there were no stars