10B The Big Bang Where do we come from, where are we going?

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The Big BangThe Big Bang

Where do we come from, where are we going?

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GoalsGoals

• What’s the cosmological principle?• Where did things begin?• Where will things end?• What’s the temperature of space?

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Hubble’s Law

• Recall: All galaxies are moving away from us.

• The farther away the faster they go.

• V = Ho x D

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You Are Here

Large Scale StructureFarther away we look, further back in time we see!

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Cosmological Principle

• Isotropy – The view from here is the same in all directions.

• Homogeneity – We live in an average place the same as any other.

• This is the cosmological principle.

• Implies: – Universe has no edge!– Universe has no center!

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Olber’s Paradox

• Why is it dark at night?

• In an infinite and unchanging universe:Every line of sight should end at the surface of a star.

• Universe is neither.

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Expanding Universe• If galaxies are all moving away, then at

some point they were all much closer.• Hubble’s Law implies the Universe is

expanding.

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Age of the Universe• Since all galaxies are moving away

from us, how long has it been since all galaxies were together?time = distance / velocityvelocity = Ho x distance

time = distance / (Ho x distance)

time = 1/Ho

• Measured Ho ~ 65 km/s/MpcT ~ 15 billion years

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Age Disagreements

• Until recently, much disagreement on the value of Ho and therefore, the age of the Universe.

• Need to know the distance to some galaxies in order to know the slope of velocity versus distance.

• Different methods yielded different distances.

• Some values of Ho yielded an Universe younger than some of its stars.

• HST has helped solve the problem: Cepheids.

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The Big Bang• Big Bang: the explosion out of which

the Universe began expanding.• Into what did the Universe expand?

Nothing.• Where was the Big Bang? Everywhere.• Where is the center of the Universe?

Nowhere.

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The End of the Universe

• Will the universe expand forever?

• Depends on the density of the Universe.

• Too big: Big Crunch– Closed Universe– Bound Universe

• Too small: Big Freeze– Open Universe– Unbound Universe

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Critical Density

• Dividing line is the critical density.

o is the ratio of measured density to the critical density.

• If o > 1 then closed, o < 1 then open.

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Density of the Universe

• Add up all the mass we see and o = 0.01

• But we know there is some dark matter in galaxies and clusters.

• How much?• Think ~10 x more dark matter than

“light” matter.

• Cosmologists think o < 0.3

• Result: Open Universe Big Freeze!

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Are we Slowing down?• In our experience,

things slow down over time.

• Is the Universe slowing down at all?

• Use supernovae as “standard candles.”– Get distance to

distant supernovae.

• Plot distance versus velocity.

Slowing

Accelerating

Dark Energy

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Observational Clues• Instant of the Big Bang:

– Very Dense– Very hot

• Universe one giant blackbody.– Temperatures gamma rays

• As space expands, wavelengths of light expand with it Cosmological Redshift

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3 Kelvin

• Recall: p 1/T

• Since longer wavelengths mean cooler blackbodies then space should be cooling.

• Today, it should be ~3 K

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COBE

• Satellite measured the intensity of energy from space at a range of frequencies.

2.7 K

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Isotropy

• Amazingly uniform temperature in all directions.

• Deviations < 0.0002 Kelvin

10 x more energy in background than everywhere else.