Thursday, July 24, 2014

The 200 Million Dollar Assumption

There used to be the Six Million Dollar Man, but now with inflation and all, we have the 200 Million Dollar Assumption. In some of the Slides from the workshop on Science on the Sphere, you may have noticed this slide from Tom Kitching on Weak lensing on the sphere

I went ahead and asked Tom what he meant by mentioning a 200M$ assumption, here is what he responded:

Dear Igor,

Thank you for your email.

The the plot on the slide is from this paper where we investigated the impact of the "Limber approximation" on weak lensing statistics and forecasted cosmological parameter error. The approximation replaces Bessel functions with delta functions to make calculations easier, and should be ok for small scales (where the functional approximation is asymptotically ok). What we found was that the predicted marginalised 1-sigma error bars using the Limber approximation are about 20% larger than when the approximation is not used.

Future experiments such as Euclid, LSST and SKA are to cost about 1 billion euros/dollars so a 20% increase in error bars on the key parameters is the equivalent cost of nearly 200M.

This was a slightly provocative statement aimed to stimulate discussion on how we can avoid this and other approximations to fully exploit these new experiments. In fact it reminds me of a sporting analogy where it is often said that the "aggregated effect of marginal gains" (e.g. can result in wins; where small differences can all add up to a significant difference.

I hope that helps, let me know if you have any further questions.

Best Regards
Thanks Tom !

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