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The Euclid Satellite and Gravitational Lensing

By Peter L Taylor, on 30 January 2018

In 1999 the physics community was stunned when it was discovered that far from slowing down under the pull of its own gravity, the universe is actually expanding at an accelerating rate. This is the cosmic acceleration problem and it poses one of the largest challenges in physics today. Here, at UCL’s Mullard Space Science Laboratory (MSSL) we are leading the effort to find a solution.

Parts for the Euclid Satellite, which will conduct a massive astronomical survey, are being built on site. Meanwhile we prepare for the arrival of the groundbreaking Euclid data by developing and testing techniques to distinguish between theories of cosmic acceleration.

We take advantage of an effect of Einstein’s theory of general relativity called gravitational lensing. As light from distant sources travels towards Earth, it is deflected by the gravitational attraction of intervening mass slightly distorting the shape of the observed image. This effect is normally very small, but by measuring the shape of billions of galaxies, amazingly we can statistically differentiate between theories.  This type of analysis has been conducted before, but to get the most out of next generation experiments we must hone our methods. When comparing the two leading statistical techniques, we found both have serious drawbacks. Nevertheless we have shown that the two methods are just special cases of a more general theory: narrowing our search for a better approach.

Irrespective of the statistical technique that is ultimately used, we must always accurately predict the lensing signal to compare to the real universe. An intermediate step involves determining the power spectrum that describes the ‘clumpiness’ of matter on different length scales, at different times. We are working on a publicly available code that will help researches who generate these spectra determine whether their output is accurate enough to meet the needs of upcoming lensing experiments like Euclid.

Gravitational lensing is very a delicate business. Large collaborations must identify, understand and mitigate annoying instrumental and astrophysical effects that contaminate the lensing signal. UCL is leading the charge in preparation for Euclid’s launch in the early 2020s.

 

Image credit to ESA

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