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Announcements• Cosmos Assignment 5, due Monday 4/26,

Angel Quiz• Monday, April 26

Quiz 3 & Review, chapters 16-23• Wednesday, April 28,

Midterm 3: chapters 16-23• Links to power point slides are now on the

course homepage• Answers to assignments are in process of

being linked to the course homepage• Up to date Grades will be posted Monday

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What have we learned?

Universe is ExpandingSpace between clusters of galaxies is getting larger

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What did we learn last class?

• Galaxies (and stars) form by Gravitational InstabilityRegions with slightly more concentrated mass

have more gravityPull in more matter, which further strengthens

gravityMass concentration grows

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What did we learn last class?• Dark Matter is needed to produce large enough

gravity to make very small initial density fluctuations grow into galaxies by now

• Other evidence for Dark Matter– Orbits in galaxies (rotation curve)– Motions in clusters of galaxies– Hot gas in clusters of galaxies

(balance of pressure vs. gravity)

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Question 1:

Because the Universe is expanding its temperature is

A. Increasing

B. Staying constant

C. Decreasing

Why?

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Question 1:

Because the Universe is expanding its temperature is

A. Increasing

B. Staying constant

C. Decreasing

Why?

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Why is the temperature of the Universe decreasing as it expands?

• Expanding against pull of gravity requires work and uses energy

• Converts kinetic energy to gravitational potential energy

Experiment: Blow on your hand with open mouth and with pursed lips.

Why is work required in this case?

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Early Universe was HOT & DENSE

• Expansion -> cooling

• Expansion -> things farther apart (lower density)Going back in time, Universe was smaller, denser

and hotter than today

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Big Bang - history of the Universe

• In the beginning, universe was so hot, photons and particles had so much energy, collisions destroyed nuclei as fast as they formed

QuickTime™ and aVideo decompressor

are needed to see this picture.

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Big Bang - history of the Universe

• As the universe expanded it cooled

• At a temperature ~ 50X center of the Sun nuclear forces could hold protons and neutrons together (time about 1 second)

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Big Bang - history of the Universe

• Primordial Nucleosynthesis (T ~ 109 K, t ~ 3 min)– Proton + Neutron -> Deuterium (2H)– Deuterium + Proton + Neutron -> Helium

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Big Bang - history of the Universe

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Big Bang - history of the Universe

• The larger the density of matter at the time of primordial nucleosynthesis

The more collisionsThe faster the fusion reactionsThe more Helium made and

the less Deuterium left

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Big Bang - history of the

Universe

(See Fig 23.11)

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Big Bang - history of the Universe

• As the Universe expanded more, its density become too small for nuclear fusion reactions (time ~ 10 min)

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Big Bang - history of the Universe

• Universe continued to expand and cool

• Era of Radiation(book calls it era of nuclei)

• Photons, Nuclei and Electrons (too hot for atoms)

• Opaque photons scatter off electrons(random walk as in Sun)

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Big Bang - history of the Universe

• Universe becomes TransparentElectrons combine with nuclei of H & He to make atomsNo more electrons to scatter photonsT ~ 3000 K, time ~ 400,000 years

• Pressure of photons no longer prevents gravity from pulling matter togetherProto-galactic clumps of matter begin to grow

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Big Bang - history of the Universe

• Era of Atoms and Galaxiesgalaxies and stars formtime ~ 1 billion yearsredshift ~ 10

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Big Bang - history of the Universe

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Big Bang -

history of the

Universe

Fig. 23.2

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Big Bang - history of

the Universe

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Alternative Theory: Steady State

• Universe is, on average, unchanging

• Universe is expanding, but new hydrogen is being created to keep average density constant(Rate too small to notice)

• All other elements are made in starsImpetus for development of theory of heavy element production in stars

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Tests

1. Relic Elements

2. Relic Radiation

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Relic Elements

TheoryObservations

Universe is 75% H 25% He

Deuterium abundanceconstrains density ofordinary matter

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Relic Radiation

• If universe was once hot and opaque, should see radiation from that time– Should come at us from all directions– Should have a thermal spectrum– Should be cold now because of expansion

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Relic Radiation: Cosmic Microwave Background Radiation

Accidental first detection

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Spectrum is Thermal, T=2.7 K

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CMB Radiation

Radiation is nearly the same from all directions,

Doppler Shift due to motion of the Milky WayT/T ~ 10-3

After subtracting emission from MW, seePrimoridial fluctuations from when universe became transparent,T/T ~ 10-5

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Expansion speed & Fate of

universe

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Fate of Universe & Mass Density

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Fig 22.18

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Type Ia Supernovae

Type Ia SN are standard candles (i.e. have identical luminosities)Observed flux decreases with increasing distanceSo apparent brightness measures distance.

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Supernovae appear fainter in an accelerating universe

• Accelerating universe was expanding slower in the past than non-accelerating universe

• Universe takes longer to get from early scale to today in accelerating universe

• Light has more time to travel from SN at early scale to us in accelerating universe

• SN appears fainter

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Fig 22.18

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Einstein’s Equations govern evolution of Scale of Universe

Mass x Acceleration = Gravitational Force

The extent to which expansion is speeding up

Due to the contents of the universe: Usually it is attractive, leading to deceleration

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Density + Pressure + Pressure

Einstein’s Theory of General Relativity:Gravitational force is proportional to

Accelerating universe implies contents have negative Pressure,to produce negative Gravity,called DARK ENERGY

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Energy density due to matter decreases as universe expands

• Number of particles in a fixed volume goes down as the universe expands (think of number of raisins in a fixed volume as the bread rises)

Decrease in energy density leads to decrease in Expansion Rate

After a while, Acceleration dominates over Deceleration

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What do we know about Dark Energy?

• Constitutes 2/3 of energy in universe

• Is smoothly distributed and invisible

Doesn’t clump into galaxies likeMatter, visible or dark

• Has negative pressure

produces Acceleration

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Fate of Universe

and Dark

Energy

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Fate of Universe

• 109 - 1014 years - Era of Stars & Galaxiesends when no new stars

• 1015 - 1037 years - Degenerate EraOnly BH, NS, WD, planetsends when protons decay

• 1038 - 10100 years - Black Hole Eraends when black holes evaporate

• Dark Eralow energy photons, electrons, neutrinos

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Problems with the Big Bang Model

1. How can two pieces on opposites sides of the universe have the same temperature at the time the universe became transparent?

They are too far apart to have communicated with each other within the age of the universe, since light from them has just now reached us half way between.

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Problems with the Big Bang Model

2. Why is the space-time geometry of the universe so nearly flat, equivalent to the sum of the Ordinary Matter, Dark Matter and Dark Energy = Critical Density?

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Inflation

At very beginning of Big Bang, the Universe underwent a tremendous expansion (inflation).

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Inflation

Before Inflation the two parts of the universe were close enough together to communicate with each other

Fig 23.14

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Inflation

• Expansion smoothes out fluctuations and makes things appear flatter (e.g. blowing up a balloon).