Different stuff dumped and/or the WD itself is different in the past => the resulting SNe could be different

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• Different stuff dumped and/or the WD itself is
  different in the past gt the resulting SNe could
  be different

• So distant SNeIa could be different from nearby
  ones in terms of brightness!

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What about dust?

• Normal dust scatters blue light more than red
  why sunsets are red

• Look at color of the SNe Ia
• They have the same color gt
• no ordinary dust (what we see in our galaxy)

• What if weird dust that only dims SNe Ia?
• Further away, more dust in the way gt more
  distant SNe would appear more dimmer but not
  redder

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• So we could be fooled, but

• There is no direct evidence for this dust

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Summary

• The two worries (evolution and dust) regarding
  SNe Ia
• gtWe need to make lots more observations and
  cross check with other methods (CMB and Clusters)
  gt

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How often does a SN go off?
About 1/100 years per galaxy gt need to look at
lots of galaxies
Took 10 years to get 40 measurements Want 10
times more to nail things downgt you need to be
very young or get clever! (look up
http//snap.lbl.gov/ )
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Next Up Clusters of Galaxies
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Core of Coma Cluster, Enlarged
7 million lt yrs
Coma Cluster
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A cluster of galaxies is the largest
self-gravitating system known in the Universe
A cluster contains about 1000 galaxies that
swarm around a gravitational center at 1000
km/sec!
The diameters are 3 million light years
This background is part of the Coma cluster
These are galaxies
This is a background cluster at 1z 1.5!
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How to we measure mass? But why?

• We know how many clusters there are per unit
  volume
• Measure of the mass of clusters
• lower limit to Wm (galaxies outside clusters are
  negligible)

• Also use evolution of clusters with time to
  derive Wm

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Now, how do we measure the mass?

• Galaxy motions

• Hot gas atmosphere

• Gravitational lens

• First method like measuring the mass of the earth
  by the motion of the Moon
• Second method like using the temperature and
  mass of the gas atoms in the Earths atmosphere
  to derive mass of Earth

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First Method
Or, centrifugal force gravitational force

• Assume we have circular motion, then mv2/R
  GMm/R2
• If M gtgt m this works
• Little ms cancel
• measure v and R, then derive M

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• Replace the M by the mass of the entire Cluster
  
• Measure the velocity of galaxies
• Assume orbits are circular

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• 3 billion years for an orbit gt
• Look at many galaxies
• To derive average circular motion
• Derive and average orbit gt
• Derive about 1015 solar masses for cluster!

• But in galaxies total see about 3 x 1013
• gt missing mass, dark matter