standard cosmology. cosmological red-shift not really a doppler effect space itself is being...
TRANSCRIPT
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Standard Cosmology
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Cosmological Red-Shift
• Not really a Doppler effect
• Space itself is being stretched between galaxies
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Conclusions from our Observations
• The Universe has a finite age, so light from very distant galaxies has not had time to reach us, therefore the night sky is dark.
• The universe expands now, so looking back in
time it actually shrinks until…?
Big Bang model: The universe is born out of a hot dense medium
13.7 billion years ago.
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Big Bang
• The “start” of the universe, a primordial fireball the early universe was very hot and dense
intimate connection between cosmology and nuclear/particle physics
“To understand the very big we have to understand the very small”
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How does the expansion work?
• Like an explosion (hot, dense matter in the beginning), but space itself expands!
• Slowed down by gravitational attraction
• Attraction is the stronger, the more mass there is in the universe
• Scientifically described by Einstein’s General theory of Relativity (1915)
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More General than Special Relativity
• General Relativity is more general in the sense that we drop the restriction that an observer not be accelerated
• The claim is that you cannot decide whether you are in a gravitational field, or just an accelerated observer
• The Einstein field equations describe the geometric properties of spacetime
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The Idea behind General Relativity
– We view space and time as a whole, we call it four-dimensional space-time.
• It has an unusual geometry, as we have seen
– Space-time is warped by the presence of masses like the sun, so “Mass tells space how to bend”
– Objects (like planets) travel in “straight” lines through this curved space (we see this as orbits), so
“Space tells matter how to move”
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Planetary Orbits
Sun Planet’s orbit
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Effects of General Relativity
Bending of starlight by the Sun's gravitational field (and other gravitational lensing effects)
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Assumption: Cosmological Principle
• The Cosmological Principle: on very large scales (1000 Mpc and up) the universe is homogeneous and isotropic
• Reasonably well-supported by observation
• Means the universe has no edge and no center – the ultimate Copernican principle!
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What General Relativity tells us
• The more mass there is in the universe, the more “braking” of expansion there is
• So the game is:
Mass vs. Expansion
And we can even calculate who wins!
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The Fate of the Universe – determined by a single number!
• Critical density is the density required to just barely stop the expansion
• We’ll use 0 = actual density/critical density:
0 = 1 means it’s a tie 0 > 1 means the universe will recollapse (Big Crunch)
Mass wins! 0 < 1 means gravity not strong enough to halt the expansion
Expansion wins!
• And the number is: Ω0 = 1.02 +/- 0.02
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The “size” of the Universe – depends on time!
Expansion wins!
It’s a tie!
Mass wins!
Time
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The Shape of the Universe
• In the basic scenario there is a simple relation between the density and the shape of space-time:
Density Curvature 2-D example Universe Time & Space
0>1 positive sphere closed, bound finite
0=1 zero (flat) plane open, marginal infinite
0<1 negative saddle open, unbound infinite
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Back to: Expansion of the Universe
• Either it grows forever
• Or it comes to a standstill
• Or it falls back and collapses (“Big crunch”)
• In any case: Expansion slows down!Surprise of the year 1998(Birthday of Dark Energy):
All wrong! It accelerates!
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Enter: The Cosmological Constant
• Physical origin of 0
is unclear• Einstein’s biggest
blunder – or not !• Appears to be small
but not quite zero!• Particle Physics’
biggest failure
• Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign)
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Triple evidence for Dark Energy
• Supernova data
• Large scale structure of the cosmos
• Microwave background
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Microwave Background: Signal from the Big Bang
• Heat from the Big Bang should still be around, although red-shifted by the subsequent expansion
• Predicted to be a blackbody spectrum with a characteristic temperature of 2.725 Kelvin by George Gamow (1948)
Cosmic Microwave Background Radiation (CMB)
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Discovery of Cosmic Microwave Background Radiation (CMB)
• Penzias and Wilson (1964)
• Tried to “debug” their horn antenna
• Couldn’t get rid of “background noise”
Signal from Big Bang• Very, very isotropic (1
part in 100,000)
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CMB: Here’s how it looks like!Peak as expected from 3 Kelvin warm object
Shape as expected from black body
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CMB measurements improve
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Latest Results: PLANCK
• Measure fluctuations in microwave background• Expect typical size of fluctuation of ½ degree if universe
is flat• Result:
Universe is flat !
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Experiment and Theory
Expect “accoustic peak” at l=200
There it is!
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Supernova Data
• Type Ia Supernovae are standard candles• Can calculate distance from brightness• Can measure redshift• General relativity gives us distance as a function of redshift for a given universeSupernovae are further away than expected for any decelerating (“standard”) universe
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Pie in the Sky: Content of the Universe
We know almost everything about almost nothing!
1
2
3
23%
5%
72%
Dark EnergyDark MatterSM MatterSM Matter
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Properties of Dark Energy
• Should be able to explain acceleration of cosmic expansion acts like a negative pressure
• Must not mess up structure formation or nucleosynthesis
• Does not dilute as the universe expands will be different % of content of universe as time goes by