Some Distances Distance to moon 10 times around the earth 1.3
light seconds Sun 400 times the distance to the moon 8.3 light
minutes Nearest star 4.2 light years Andromeda galaxy 2.5 million
light years
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A Fruity Comparison If the sun were a grapefruit and all other
distances were scaled accordingly The earth would be a pinhead 10
meters away. The nearest star would be 2000 miles away.
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The Milky Way Galaxy 100,000 light years in diameter 225
million years rotation period 100 billion stars as many sand grains
as there are in one meter cube. It would take you 3000 years to
count them.
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A More Comparisons If the earths orbit were the size of a
pinhead and all other distances were scaled accordingly The nearest
star would be 300 yards away. Our galaxy would span the entire
United States.
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Galaxy Cluster
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Final Comparisons If the milky way were 20 yards across --
Andromeda would be 600 yards away. The farthest distance we could
see would be the size of the United States and contain 100 billion
galaxies.
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Hubble Plot Recession Speed vs. Distance
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The Hubble Constant The Hubble Time ~ the age of the
universe
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The Cosmological Principle Everything is at the center!
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18 Geometry on a sphere
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19 R(t 2 ) * * * * time = t 1 time = t 2 Space is expanding:
The distance from here to there keeps getting farther. Space is
expanding: The distance from here to there keeps getting farther.
R(t 1 )
Slide 20
20 Simple solutions to Friedmans equations R(t) t scale factor
time k= +1 k=0 k= -1 Big BangBig Crunch now
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Critical density The critical density is equivalent to six
hydrogen atoms per cubic meter. If the density of the universe is
less than this it will expand forever. If the density is greater,
it will re-collapse.
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22 Shape #1: spherical, closed geometry finite, unbound k = +1
l General Relativity: the Universe can have only one of three
possible shapes
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23 Shape #2: saddle-shaped, open geometry infinite, unbound k =
-1 l
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24 Shape #3: flat or Euclidean geometry infinite, unbound
(unbound = no edge) k = 0 l But the Universe has shape #3
Slide 25
The three Scenarios: k=+1 The density of the universe is
greater than the critical density. The universe will expand for a
while and then re-collapse in the big crunch. The curvature is
positive like the surface of a sphere. All straight lines
eventually come back to where they started.
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The three scenarios: k=-1 The density of the universe is less
than the critical density. The universe will go on expanding
forever. Space has negative curvature, e.g.
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The three scenarios: k=0 The universe has exactly the critical
density. The universe will go on expanding, but the expansion will
eventually coast to a stop. Euclid was right. As a matter of fact,
space is flat. And yet the density of matter that we can see is
much less than the critical density.
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29 Energy Content of the Universe + baryons + dark matter +
dark energy = crit = 1 k = 0
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Summary So Far The universe is infinite (so far as we know). It
has flat, Euclidean geometry. Its expanding. If it has always
expanded at the same rate, there would have been a time 13.5
billion years ago when everything we can now see was compressed to
a point.
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When matter (or radiation) is compressed, it heats up. When it
expands it cools down. So what is the temperature of the universe
now?
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2.73 degrees K Now run the clock backwards, starting now.
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t=350,000 years a.b.b. T=3000 degrees Neutral hydrogen breaks
up to form electrons and protons. So the universe is no longer
transparent.
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The Universe As Seen from the Outside But heres what we see the
cosmic microwave background
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t=3 minutes a.b.b. T= one billion degrees The only nuclei
around are isotopes of hydrogen, helium, and a smidgeon of lithium.
These now break up to make protons and neutrons.
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t=one second a.b.b. Protons and neutrons break up to make
quarks.
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This is the realm of quantum gravity. We have no idea what
happened before this time.
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Five Things That Had To Be Exactly Right Critical Density
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How Light Nuclei Are Formed
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How Stars Make Energy
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What Could Go Wrong If the strong force were slightly stronger
the universe would only consist of protons and di- protons. If it
were slightly weaker there would be no deuterons and stars couldnt
shine.
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Five Things That Had To Be Exactly Right Critical Density The
strong force
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How Stars Make Heavy Elements etc.
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Until It Makes Iron
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And Then
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Ashes to ashes, dust to dust
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And Then New Stars
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Five Things That Had To Be Exactly Right Critical Density The
strong force The structure of
Slide 52
The cosmic microwave background Galaxy clusters are seeded by
small variations in density.
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Five Things That Had To Be Exactly Right Critical Density The
strong force The structure of The strength and scale of the
fluctuations
Slide 54
Antimatter??? Presumably the universe started out with an equal
number of particles and antiparticles. All the particles and
antiparticles should then annihilate leaving nothing but photons.
This almost happened. There are a billion photons for every proton.
This doesnt make sense!
Slide 55
Life only works in three dimensions. There can be no networks
in two dimensions. There can be no stable planetary orbits in
higher dimensions.
Slide 56
Five Things That Had To Be Exactly Right Critical Density The
strong force The structure of The strength and scale of the
fluctuations Space has three dimensions.
Slide 57
Anthropic principles The weak anthropic principle: since we
exist the universe must have the properties necessary for our
existence. The strong anthropic principle: the universe must have
the properties necessary for our existence.
Slide 58
The final word Whereof one cannot speak, one must be silent.
Ludwig Wittgenstein