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A Brief Tour of the Universe Dont Panic

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The Sun in UV

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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 )

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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

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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

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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

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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!

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Life only works in three dimensions. There can be no networks in two dimensions. There can be no stable planetary orbits in higher dimensions.

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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 Space has three dimensions.

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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.

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The final word Whereof one cannot speak, one must be silent. Ludwig Wittgenstein