Astrophysics

Current and near-future cosmological surveys can provide powerful tests of gravity on the largest scales. However, in order to extract this information, it is essential to understand the host of parameter degeneracies which may arise. I present two approaches towards this goal, with a focus on the use of weak gravitational lensing measurements in combination with other probes. First, I discuss a novel expression for a weak lensing power spectrum under alternative theories of gravity. Obtained by considering small deviations from GR, this expression separates various physical effects of modifications to GR, thus allowing a quantitative understanding of degeneracies between gravitational parameters. Second, I present an investigation into the theoretical uncertainty inherent to the popular gravity-testing statistic E_G. By improving our understanding of the theoretical value of E_G, we can use it to make more robust statements about the viability of alternative theories of gravity on cosmological scales.

On small scales, our understanding of dark matter in galaxies is incomplete. One example is that high resolution simulations predict a large number of subhalos in dark matter halos, which should be seen in our Milky Way galaxy as a host of satellite galaxies. Many plausible astrophysical mechanisms have been proposed to explain why we don't clearly see large numbers of such satellite galaxies, but this remains a test of the cold dark matter scenario that has yet to be passed. Gravitational lensing provides a direct probe of subhalos in distant galaxies, but requires high sensitivity and angular resolution. I will talk about a search that is now started using strong lensing of the mm and submm-wave emission from dust grains in high-redshift star-forming galaxies. The properties of the dust emission are well-matched to the typical expected scales of the dark matter subhalos, and ALMA (the Atacama Large Millimeter Array) now has the sensitivity and angular resolution to start to see the lensing signature of these subhalos. We haven't found any yet in early data, but we should see some soon if the cold dark matter paradigm is correct.