Hubble and the Distance Ladder

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Hubble and the Distance Ladder

• Dr Bryce

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Homework

• Worth 25 of final grade
• 289 homework points available
• A score of 250 or higher will receive full credit
  i.e. 25
• 240 or higher will receive 24
• 230 or higher 23 etc
• i.e. scores are rounded down
• BUT there are 39 points that you dont need to
  score
• NO EXTRA CREDIT FOR SCORES gt 250

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Extra credit exercise

• All groups have been hampered by bad weather
• So far
• 0,1 1/25
• 2,3 2/1, 3/5, 3/8
• 4,5 3/19
• 6,7 3/26
• 8,9 2/20, 2/21, 2/22, 4/5

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• Week beginning April 9th, Groups 0-1, 2-3, 8-9
• April 16th 4-5, 6-7
• April 23rd 0-1, 4-5
• April 30th 6-7
• Remember it is extra credit and not compulsory

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White-dwarf supernovae can also be used as
standard candles
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Step 5 Apparent brightness of white-dwarf
supernova tells us the distance to its
galaxy (up to 10 billion light-years)
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Tully-Fisher Relation Entire galaxies can also
be used as standard candles because galaxy
luminosity is related to rotation speed
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Edwin Hubble
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The Great Debate

• Before Hubble, some scientists argued that
  spiral nebulae were entire galaxies like our
  Milky Way, while others maintained they were
  smaller collections of stars within the Milky Way
• The debate remained unsettled until someone
  finally measured their distances

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Hubble settled the debate by measuring the
distance to the Andromeda Galaxy using Cepheid
variables as standard candles
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The spectral features of virtually all galaxies
are redshifted ? Theyre all moving away from us
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• By measuring distances to galaxies, Hubble found
  that redshift and distance are related in a
  special way

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Hubbles Law velocity H0 x distance
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Redshift of a galaxy tells us its distance
through Hubbles Law distance
velocity H0
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Distances of farthest galaxies are measured from
redshifts
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Thought Question

• Your friend leaves your house. She later calls
  you on her cell phone, saying that shes been
  driving at 60 miles an hour directly away from
  you the whole time and is now 60 miles away. How
  long has she been gone?
• A. 1 minute
• B. 30 minutes
• C. 60 minutes
• D. 120 minutes

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Thought Question

• Your friend leaves your house. She later calls
  you on her cell phone, saying that shes been
  driving at 60 miles an hour directly away from
  you the whole time and is now 60 miles away. How
  long has she been gone?
• A. 1 minute
• B. 30 minutes
• C. 60 minutes
• D. 120 minutes

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The expansion rate appears to be the same
everywhere in space The universe has no center
and no edge (as far as we can tell)
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Cosmological Principle

• The universe looks about the same no matter
  where you are within it
• Matter is evenly distributed on very large scales
  in the universe
• No center no edges
• Not proved but consistent with all observations
  to date

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Thought Question

• Your observe a galaxy moving away from you at 0.1
  light-years per year, and it is now 1.4 billion
  light-years away from you. How long has it taken
  to get there?
• A. 1 million years
• B. 14 million years
• C. 10 billion years
• D. 14 billion years

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Thought Question

• Your observe a galaxy moving away from you at 0.1
  light-years per year, and it is now 1.4 billion
  light-years away from you. How long has it taken
  to get there?
• A. 1 million years
• B. 14 million years
• C. 10 billion years
• D. 14 billion years

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Hubbles constant tells us age of universe
because it relates velocities and distances of
all galaxies Age 1 / H0
Distance Velocity
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Distances between faraway galaxies change while
light travels Astronomers think in terms of
lookback time rather than distance
distance?