Aristotle (384-322 BC) showed Earth is round, way before Columbus!

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Aristotle (384-322 BC) showed Earth is round,way
before Columbus!

• Shadow of earth on moon always part of a circle.
• Sinking ship as it is seen going to sea.
• Walk north, NCP is higher in the sky.

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Eratosthenes 200 BC calculation of size of earth
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• Aristotles geocentric theory.

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• Going beyond the Earth to get distance of the
  Moon
• Must use parallax (similar to binocular vision).

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Two eyes on opposite parts of Earth
MOON
p
EARTH

• Ptolemy(200 AD) discusses previous work.
• 4000 miles radius of Earth
• Earth radius/(2p orbit radius) Angle p / 360o.
• 60 Earth radii distance from p 1o.
• 240,000 miles or about 1.3 light second travel
  time.
• Stilted conversation between astronauts and
  President Nixon.

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Aristarchus (310--230BC) proposed that Earth
went around Sun, way before Copernicus.

• Shadow of round Earth on Moon.
• Deduced Moon is about 1/3 size of Earth (Modern
  1/4)

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• Aristarchus studied times 3rd Q, new, 1stQ of
  the Moon versus 1st, full, 3rd.
• Found Sun was at least 21x Moons distance

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• As revealed by solar eclipse, the sun and moon
  are about the same ANGULAR SIZE.
• But the sun is a lot bigger by a factor equal to
  ratio of distances.

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• Aristarchus results
• Sun is 21xMoons distance or 21xsize of Moon
• Moon is 1/3 the size of the earth.
• Sun is 21x(1/3)7 x size of the earth. Modern
  value 100gtgtEarth!
• Concluded such a big sun couldnt circle earth.

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Aristarchus idea was rejected. Parallax from
one side of Earths orbit to other was expected.
Stars too far and parallaxes to small for
ancient Greeks to accept or measure. Largest p lt
1/3600 degree.
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Ptolemys 125 AD book Almagast

• Elaborated geocentric theory to quantitatively
  account for and predict observed motions of
  planets, wanderers in the sky.
• Seven wanderers Sun, Moon, Mercury, Venus, Mars,
  Jupiter, Saturn. 7 objects.

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RenaissanceNicolaus Copernicus (14731543)

• Revived sun-centered idea ignoring failure to
  observe parallax.
• Simpler model than earth-centered
• Simpler calculations and could calculate relative
  sizes of planet orbits.

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• New!
• Copernicus could calculate relative sizes of
  planet orbits.
• Know maximum elongation angle at Earth
• SunPlanetEarth angle is 90o
• Triangle gives dist. PlanetSun in AU
• Also did for Mars, Jupiter, Saturn

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• Copernicus calculated the sizes of the planets
  orbits RELATIVE to the Earths orbit size (1 AU).
• Death bed publication, why?

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Believing the Earth circled the Sun was
dangerous. Giordano Bruno burned at stake in 1600

• There is a single general space, a single vast
  immensity which we may freely call void in it
  are innumerable globes like this on which we live
  and grow.
• I await your sentence with less fear than you
  pass it. The time will come when all will see as
  I see.

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The Distance of the Nearest Star

• Recall Copernicus found relative distances of
  planets in solar system.
• Copernicus calculated the sizes of the planets
  orbits RELATIVE to the Earths orbit size (1 AU).
• But exactly how big is the Earths orbit and the
  solar system in miles or km?
• To 1700s AU very poorly known.

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Basilica of San Petronio, a solar observatory
1576 by Egnatio Danti, a mathematician and
Dominican friar

• Aristarchus 1 AU 1520 Earth radii
• 1650 Giovanni Cassini (France) found that Sun was
  much farther and Solar System much bigger than
  previously thought gt17,000 Earth radii.
• Expanded Solar System (universe) over 10x

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Cassinis San Petronio method
SUN
EARTH SUMMER SOLSTICE
EARTH WINTER SOLSTICE

• Noon Sun (black solid line) is farther south or
  lower from overhead (red) at both summer and
  winter solstices than it would be if infinitely
  far away (dashed lines)
• Differences permit calculation of Suns distance
  in Earth radii.

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Halley is famous for calculating the 75 yr
elliptical orbit of Halleys comet and
predicting its 1758 return. Haley had an idea to
more precisely measure the AU in Earth radii.
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• Halleys parallax transit method. Venus crosses
  the Sun along different lines depending on the
  latitude of the observer on the Earth.
• The lines differ by no more than 44 seconds of
  arc on disk.
• Exaggerated in the drawing. The Sun is half a
  degree, 40 times the max difference).

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1761,1769 transits observed from all over Earth,
even Tahiti!

• First accurate results, 1
• 93 million mi, 150x106 km.
• Space probes, radar 23,500 Earth radii within
  meters.
• 8 minutes blissful ignorance if Sun vanishes!

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The long quest for stellar parallax

• German astronomer Karl Bessel.
• Visual observation, special telescope.
• First accurate measuremnt of parallax of star 61
  Cygni in 1838.
• Tiny, about 0.3 seconds of arc. One arc sec
  1/3600 o

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Tiny parallaxes simplify calculating distance.

• Nearest star (besides the Sun) has a parallax of
  0.75 sec of arc, less than 1/3600 of a degree 1
  arc sec.
• Distance in parsec 1/parallax in arc sec.
• Distance 1/0.75 1.33 parsecs
• 1 parsec 3.26 light years or 206,265 AU
• One light year 6 trillion miles.

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Nearest star amazingly far away.

• The double star in the figure, Alpha,Proxima
  Centaurus.
• 1.3 pc away, four light years travel time.
• Our info about it is over 4 years out of date!

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Next Expansion GalileosStarry
Messengerhttp//www.rarebookroom.org/Control/gals
id/index.html
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The Milky Way and the Pleiades as seen with the
telescope.
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• All sky photo
• Meteor shower.
• MW circles whole sky.
• Galileo found the MW was a multitude of dim
  stars.
• Bright stars uniform over the sky.

Milky Way
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Thomas Wright in 1750 clarified Galileos
discovery.

• Bright stars scattered uniformly over sky.
• Milky Way divides the sky into two equal halves.
• We are in mid-plane of a somewhat thick disk of
  stars.
• Nearby stars above, below, and to sides make up
  bright stars on sky.
• Stars so distant they appear as haze, mark out
  the disk plane.

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• Bright only.
• No dim stars
• in this direction.
• 
  
• Bright
  
• 
  
• dim
  
• 
  
• Bright only.
• No dim stars
• in this
  direction.

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1785 Herschel surveyed and found our place in MW
Galaxy

• Looked in different directions w. telescope
• Counted of stars of diff apparent magnitudes
• Assumed all were like the sun
• Plotted number versus distance in different
  directions.
• Disk a few thousand light years in size. Wrong!!
• Sun in center. Wrong!!

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• This is a mosaic of visual photos of the entire
  sky.
• Dust lanes that block view of galaxy center
  misled Herschel.
• Also more H gas than dust.

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True Size Place

• Early 20th century, Harlow Shapley
• Undergrad journalism at U. of Missouri, Columbia
• Wanted to take an archeology course one semester.
  It wasnt offered (but astronomy was).
• Became director of Harvard College Observatory.

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In 1920s, Shapleys method

• Estimated distances of globular star clusters
  above and below the absorbing dust of the Milky
  Way disk. (eg 47 Tuc gt106 stars, 50 pc).
• Used RR Lyrae variable stars as distance
  indicator.
• Periods of lt1day and 100x Suns luminosity.
• Got distances from RR Lyrae intensity and
  luminosity.

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• Most globular clusters are seen on one side of
  the sky
• Shapley plotted their directions and distances
• A spherical swarm whose center is in the
  direction of the nearby stars of the
  constellation Sagittarius.
• Center 30,000 light years away.

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Modern MW Map

• Disk has mostly stars, galactic clusters of stars
  (like Pleiades) some H, He gas, and dust.
• Halo has swarm of globular clusters, scattered
  old stars, and other unknown objects.
• Nuclear bulge is mostly stars some gas, dust.

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Galaxies A big step beyond.

• Initially were nebulae, fuzzy patches of light
  beyond the solar system.
• Messier cataloged 100 in 1700s for comet
  hunters.
• M31 is 31 in the list.
• Herschel in early 1800s cataloged 1000s in New
  General Catalog e.g. NGC205

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Study of nebulas via spectroscope.

• Some showed emission line spectra
• These were clouds of thin gas in our galaxy.
• Mostly hydrogen
• Supernova ejection or gas which may form stars
  later.
• Others had continuous spectra with absorption
  lines.
• Like a stars spectrum. Figure below
• Astronomers were uncertain about these.
• Collection of stars or one star and reflecting
  dust?

M87
Absorption line spec.
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Cepheid Variables

• Shapleys RR Lyrae stars (100xSun)
• not luminous enough to
• use outside MW Galaxy
• Herietta Leavitt(Harvard)1915
• Cataloged 1000s of variable stars!
• Found a new variable 10,000 x Sun, Cepheids
• Useful outside MW gal.

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An Example of a Cepheid Variable in MW Galaxy.

• Are pulsating giant stars
• Very distinctive because of their brightness
  variation gt 1 day up to 50 days.

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• Leavitt found Cepheid variable stars, in a
  satellite galaxy of MW.
• All at same distance.
• Found period-luminosity relation,
• Observe P, know luminosity, compare to intensity.
• Can thus estimate distance of the galaxy

10,000 Sun
100 Sun
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Getting M31s Distance

• 1920s Edwin Hubble observed what was called the
  Great Nebula in Andromeda Top photo.
• IDed Cepheid variables.
• Hubbles negative photogt
• Var! marks his exciting discovery of a Cepheid.
• Compared M, m to get distance

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M31s huge distance from Cepheids

• The graph shows magnitudes of 20 day Classical
  Cepheids if they were at 10 pc (33 lyr), -5.
• Hubble found M31s 20 day Classical Cepheid
  apparent magnitudes are 20.
• 1010 times less intense.
• 3x106 lyr
• Math details 33 ly x v(1010 )
  
• reversing the inverse square
  law.

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Expanding the Andromeda Nebula

• The farther away, the bigger the physical size
• Modern result Andromeda Nebula, M31, is 780,000
  pc, 780 kpc,
• 2.4 million LY away, far outside MW Galaxy
• Radio observations gt disk 3o in angular
  diameter.
• 3o/360o Size/(2px2.4 million LY)
• Size about 126,000 LY in diameter.
• Andromeda Galaxy, bigger than Milky Way Galaxy.
• But this info is two million yr old

Earth
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Systems of galaxies The Local Group

• Several million light years acrossOne million pc

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Beyond Local Group

• Virgo Cluster
• 50,000,000 LY
• 17,000 kpc away
• Over 1000 galaxies
• 7 million LY size
• 12x size moon.
• Giant E, M87, center

M87
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.
Humason

• 1920s Edwin Hubble and Milton Humason worked
  together at Mt Wilson observatory. Hubble
  getting distances of galaxies and Humason getting
  spectra.
• Expected to find some coming toward us, some
  away.
• Thats what they and others found for nearby
  galaxies.
• But when they got distances and spectra of more
  distant galaxies, Hubble noticed a pattern.

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Redshift-Distance Correlation

• Hubble used distance indicators (variable stars,
  novae, supernovae etc) to estimate distance.
• Humason got spectra. Identified characteristic
  element lines (eg Calcium)
• Measured wavelength compared to sample on earth.
• They found larger redshifts the larger a galaxys
  distance.

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Measuring Redshifts

• Distances are in Millions of pc 3.26 million LY
  on x axis.
• Pair of dark Calcium absorption lines in spectra.
• V0 wavelengths indicated by blue lines.
• Red arrowsred shifts.
• Calculated velocities in km/s are plotted on y
  axis.

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First graph data Hubble made pointing out the
pattern.

• More distant galaxies have larger redshifts
  (velocities away).
• Distance is most uncertain quantity.

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The velocity-distance relation Real expansion
We're the center
US
x
x
x
x
x
x
x
x
x
x
x
x
x
x
NO, WE ARE!
THEM

• Are we at rest and all galaxies flying away from
  us?
• No! Doppler shifts are relative. Dont know
  whats moving.

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Implications of recession

• Galaxies jammed together in past.
• When ? E.g. car 120 miles away at 60 mi/hr left
  about 120/602 hr ago.
• Hubble law, a galaxy D kpc away, recession V
  km/s.
• Left us, D/V billion years ago.
• The universe had a beginning.
• Einstein thought this was not the case.
• Math note 1 kpc in km / 1 km/s 1 billion yrs
  

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Early Evidence for Origin of Universe

• In 1826, Olber pointed out a problem with a
  perpetual unchanging universe
• It would incinerate the earth!
• Why? The number of stars inside progressively
  larger imaginary spheres or cubes increases with
  
• Intensity of the light from each star on earth
  decreases like
• The product of these two, the total intensity
  increases as . Light from a large volume
  would be enough to burn up the earth.

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• Less mathematical description.
• In an unchanging, perpetual universe, all lines
  of sight from the earth would eventually
  intersect the surface of a star.
• The entire sky should be as bright as the
  photosphere of the sun!
• No dark night sky!

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Why arent we incinerated?

• In 1848, Edgar Allen Poe (of all people)
  suggested a solution.
• The universe had to have a finite age.
• The limited speed of light would prevent the
  light of stars more distant than the age of the
  universe (in light years) from reaching us.

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Age problem from Hubbles initial plot

• Hubble law, a galaxy 1.9 Mpc1,900 kpc away,
  recession VHxD1000km/s.
• Left us, 1,900 kpc/1000km/s2 billion years ago.
• Problem Radioactive dating of Earth Solar
  System 4.5 billion yr
• Math note
• 1 kpc in km / 1 km/s
• 1 billion yrs

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An under appreciated astronomer

• Walter Baade born, educated in Germany.
• Came to US in 1931 to Mount Wilson Observatory,
  home of the world's largest telescope (100).
• During WWII, he, an enemy alien, was confined to
  Los Angeles County with almost unlimited use of
  the most powerful telescope in the world.
• Lights of LA were briefly darkened.
• Early 50s, corrected and enlarged Hubbles
  distances so Universe older than Earth.

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Modern velocity vs distance relation.

• VHD
• HHubble constant 72 km/s per Mpc.
• Much less steep.
• Age D/V 1/H
• Age 14 billion yr. gt Earth or Solar System

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Critical density of the universe?

• Analogy with ball thrown upward from surface of
  earth.
• Go up and return
• Fly away forever (escape)
• Return or escape depends on speed of ball, radius
  of earth, and mass of earth.
• Since density is mass/volume, whether returns or
  escapes depends on speed of ball and DENSITY of
  earth.

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Future?

• Galaxy receding with speed, v, from center of an
  expanding sphere of galaxies of radius, r, is
  attracted to the sphere center determined by r,
  sphere density, ?, and gravitational constant,
  G.
• Critical density depends only on current local
  Hubble constant Hv/r
• ?critical 3H2/ 8Gp

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Bottom line numbers

• If the density is 9x10-27 kg/m3 then the galaxies
  will just barely recede forever despite
  gravitational action of matter and dark matter.
• This is called the critical density.
• This universe is called the flat universe.
• Present day density (luminous and dark matter) is
  only 1/4 the critical density.

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Time increases downward

• Some galaxies ??/? zgt1! Why?
  COSMIC red shift for
• 
  expansion. Not Doppler.
• 
  Rubber sheetstretches ?
• 
  while wave travels
• 
  
  
• Time 1 Milky Way
  
  Quasar emitting
• 2
• I see it
• 3

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White dwarf supernovas permit estimate of still
larger distances of galaxies
Measure Doppler shiftgt speed of
recession Compare apparent magnitude and known
absolute magnitudes to estimate distances like we
discussed for Cepheids. Get recession velocities
of galaxies at larger distances and back in time
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Redshift vs. Distance Type I SN (dots)

• Line is uniformly
• expanding universe
• (no gravity deceleration,
• no acceleration)
• Horizontal axis also past time.
• Accelerating universe points would be below curve
• Gravitationally decelerating points would be
  above the curve.
• Observed SN are below the linegt
• Acceleration due to new component dark
  energy.
• which began to dominate 5 billion years
  ago.

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Separation/todays separation between
galaxiesvs time

• Red curve best fits SNI data

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What was it like at origin?

• Extrapolate back whats happening in MW galaxy
  today.
• Today uniform H, He gas is forming into
  concentrated masses (stars).
• In past, more H, He gas, fewer stars.
• Galaxy collapsed from H, He gas cloud.

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What was it like back then? Part II

• Today supernova heavy element enrichment of H,
  He. Galaxies receeding.
• Long ago, uniform gas H, He of our galaxy was all
  jammed together (compressed) with gas of others.
• Initially, gas expanded as part of universe
  expansion.
• Compressed gas hotter than expanded gas later.
• Initially hot, compressed H, He gas.

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The universe then and now

• Then Just hot H, He gas.
• Now Galaxies made of stars, planets, you, me.

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The Big Bang origin of the universe

• Initially, hot compressed H, He gas
• This expanded rapidly in what astronomers call
  the Big Bang
• To really prove this, you would need to see the
  universe back then.
• You would need a time machine!
• In 1948 George Gamow, pointed out that we did
  have such a time machine.

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Use of the time machine

• Finite speed of light creates a time machine.
• The sun is 8 light minutes away, we see the sun
  as it was 8 minutes ago.
• The nearest star, 4LY away, as it was 4 years
  ago.
• Andromeda galaxy as it was about 2 million yr
  ago.
• If we look 14 billion LY away, expect to see
  universe in its early, hot, compressed, uniform
  gas state.

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Universe at different distances times

• Imagine a sphere about 14 billion LY in radius
• Milky Way Galaxy in center.
• We see this part as the universe is now.
• We see edge as it was right after Big Bang 14
  billion years ago.

THEN
NOW
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The universe becomes transparent

• The early universe was composed of lots of
  energetic photons which keep protons (H nuclei)
  from combining with e- . Not transparent.
• After 300,000 yr expansion, photons and gas cool
  to 3000 K.
• H e- combine to make H atoms. Transparent.

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3000Kgt3K. 14 billion ly to here

• We do not see the hot glow of 3000K gas with our
  time machine.
• Instead, expansion of the universe would cause a
  red shift.
• Visual ? of the hot gas will become much longer
  ? microwave radiation.

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• 1960s Penzias (right) Wilson (left) worked at
  Bell Labs studying the skys radio brightness.
• They accidently discovered the red-shifted Big
  Bang radiation, a weak uniform microwave glow.
• Tedious, careful work.

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Intensity versus wave length plot of what they
found.
COSMIC BACKGROUND EXPLORER SATELLITE (COBE)

• The familiar continuous spectrum of hot, thick
  gas.
• Originally at visual wavelengths from 3000K gas.
• Red-shifted to 0.1cm APPEARS to be 3K gas. 3
  degree Kelvin radiation.

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Uniform 3K radiation

• Remove local MW dust thermal emission and motion
  of Sun in Galaxy, in Local Group, Local Group
  motion toward Virgo Cluster.
• Result is uniform through 4 digits.

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Tiny 3K variations

• COBE also found tiny variations in 3K, ?0.0003 K
  temperature fluctuations.
• Represent slightly denser regions of the gas, the
  creation of first structure in the universe.
• George Smoot, the principal investigator for COBE
  in a famous quote called this plot the face of
  God.

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Improvement on COBE,WMAP Observationscan be
used to check past density of universe
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• WMAP observations indicate a critical density of
  matter in universe at time of 3K emission.
• Gravitation of dark matter, matter, photons,
  neutrinos just equal to critical density to
  expand forever.
• Later dark energy dominates.

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Expanding discovery of universe

• Origin
• Inflation stretches to
• critical density.
• H, He light elements made in Big Bang.
• 3 K radiation now (3000 K then)
• Electrons and H nuclei combinegt space
  transparent
• Gravity (mostly dark matters) dominates
  deceleration gt formation of galaxies
• Dark energy dominates recent future expansion.

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The future

• No big crunch in future.
• Nice not to have a fiery death
• Having all the stars eventually stop shining is
  depressing.
• But that wont happen for trillions of years
• and who knows what we (or somebody else)
  might be like (or do) in a trillion years.
• We have seen a history of expansion. What
  does it mean? Thats up to you!