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The Big BangThe Big Bang

Where do we come from, where are we going?

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GoalsGoals

• Where did the Universe come from?• Where is it going?• How can we see the past?• How can we learn about the future

from seeing the past?

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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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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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The Big Bang• Big Bang: the event from which the

Universe began expanding.• Into what did the Universe expand?• Where was the Big Bang?• Where is the center of the Universe?

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Cosmological Principle

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

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

• This is the cosmological principle.

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

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Cosmological Redshift

• Another way to think of it:• Galaxies aren’t zooming through a fixed

Universe.• Expanding Universe is carrying

galaxies along.• Galaxy redshifts:

– Aren’t Doppler shifts.– Are cosmological shifts.

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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

“An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job.”-Steven Hawking, A Brief History of Time

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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 helped solve the problem: Cepheids.

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Hubble Space Telescope

• Use HST to find Cepheids in other galaxies.

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Hubble’s Constant

• In recent decades, Ho = 50 – 100 km/s/Mpc.

• Difference in distances by factor of 2.• Difference in age of Universe by factor of 2.

• Recent HST results: Ho = 65 km/s/Mpc

Recall: T = 1/ Ho

T = 15 billion years• Is this older than the oldest stars?• Recent HST result says the oldest white

dwarfs are 13 billion years old!

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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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The Future from the Past

• Is the Universe:– Slowing down?– Speeding up?– Staying the same velocity?

• In the past, was the Universe:– Going faster?– Going slower?– Going the same velocity?

• The Universe is a time machine.

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Lookback Time• We see everything as it once was.

Young Old

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60s

50s

Baby Boomer Universe

40sFarther away we look, further back in time we see!

90s

80s70s

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What We See

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Nearby Galaxies

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Hubble Deep Field

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V

Distance (Lookback time)

Present Past

Are We Slowing Down?• In our experience,

things slow down over time.

• Is the Universe slowing down at all?

• Plot distance versus velocity.

• Use supernovae as “standard candles.”

– Distant supernovae (large lookback time).

Accelerating

SlowingV=HoD

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Are We Slowing Down?

• Unseen mass making stars move fast:

Dark Matter• Unseen energy

accelerating galaxies:

Dark Energy

Slowing

Accelerating

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Homework #18

• For Friday read Bennett Ch23:• Do Ch23:

– Problem 4, 8