Testing gravity with cosmic shear

Tonight a lecture at UCL: Dr Donnacha Kirk –  “Testing Gravity with Cosmic Shear”

Donnacha’s lecture was about his research using weak gravitational lensing, to help decide between  theories of modified gravity and the existence of dark matter. Not an easy analysis as you’re looking at the shapes of galaxies after they’ve been modified by gravitational lensing, but how do you know they’ve been lensed and are not just that shape to start with!

I sat and thought about light crossing the universe. In my reference frame, shouldn’t I see it take no time at all? Shouldn’t the lifetime of a photon be forever? If lengths contract in the direction of motion, then for a photon travelling at the speed of light, isn’t the universe squished right down to nothing? Shouldn’t its journey be instantaneous?  Travelling at the speed of light, wouldn’t my clock stop, and time cease to exist? And why is the speed of light so important? Isn’t it just dictated by the ‘tension’ of the electromagnetic field? Why is that so important? Why is Gravity so weak in comparison? Where is all the ‘negative mass’? How did structure manage to form in an expanding universe? The ‘weak’ gravity managed to overcome the outward expansion!? Do observations of the bullet cluster show that dark matter doesn’t interact easily? Do they reveal that the individual components of dark matter are widely dispersed  (just like stars in a Galactic collision) ? The dark matter doesn’t have time to interact? Is dark matter simply slowly moving electrically neutral particles? Neutrinos? Some strange form of neutron molecule? Some more ‘exotic’, slow moving WIMP? Why did the acceleration of the expansion kick in at z≈0.5? Why does particle physics disagree so badly with observation of the vacuum energy?

So many questions, we will never stop. Maybe the fun isn’t in the goal, but in the pursuit 🙂


~ by jamesdow2013 on March 21, 2013.

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