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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