Measuring Cosmic Shear Sarah Bridle Dept of Physics

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PASCAL Challenge
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Contents of our Universe
70 Dark Energy
5 Baryonic Matter
25 Dark Matter
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Gravitational Lensing
Light from a distant galaxy is bent around a mass
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Gravitational Lensing
Galaxies seen through dark matter distribution
analogous to Streetlamps seen through your
bathroom window
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Cosmic Lensing
gi0.2
Real data gi0.03
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Atmosphere and Telescope
Convolution with kernel
Real data Kernel size Galaxy size
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Pixelisation
Sum light in each square
Real data Pixel size Kernel size /2
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Noise
Mostly Poisson. Some Gaussian and bad
pixels. Uncertainty on total light 5 per cent
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Typical galaxy used for cosmic shear analysis
Typical star Used for finding Convolution kernel
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GREAT08 Data

• 10 000 images
• divided into 10 sets

One galaxy per image Kernel is given One shear
per set Noise is Poisson
100 000 000 images Divided into 1000 sets
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GREAT08 Results
You submit g1, g2 for each set of images
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GREAT09 and Beyond

• Kernel is not given (just kernel images)
• Galaxies with more complicated shapes
• Kernel is more realistic
• Many objects in each image
• Correlated noise
• Poisson Gaussian noise bad pixels
• Shear is different for each galaxy
• Kernel is not the same for every galaxy
• Cosmic rays, satellite tracks, saturated stars
• Multiple exposures
• Pixels not exactly square or on a grid

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GREAT08 Timeline

• 8 Feb 2008 GREAT08 Handbook public
• Feb 2008 Produce simulations
• Mar 2008 Internal analysis of simulations
• May 2008 Release simulations
• Leaderboard starts containing best internal
  results
• Nov 2008 Competition deadline
• Dec 2008 Workshop Release final report
• Input shears public

www.great08challenge.info
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GREAT08 Summary

• 100 million images
• 1 galaxy per image
• De-noise, de-convolve, average ? shear
• gi 0.03 to accuracy 0.0003 ? Q1000 ? Win!

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cosmocoffee.info
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GREAT08
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Understanding Cosmology
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GREAT08 Summary

• 100 million images
• 1 galaxy per image
• De-noise, de-convolve, average ? shear
• gi 0.03 to accuracy 0.0003 ? Q1000 ? Win!

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Questions for you

• Is there too much astronomy to learn?
• Yes
• No
• Is timeline (6 months) right?
• Too short
• Fine
• Too long
• Is this potentially on your research agenda?
• No
• Maybe
• Yes