New Vistas in Weak Lensing

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Light rays are bent by gravity as they move past massive objects, in a way that was first predicted by Einstein. When astronomical objects are massive enough, such as whole galaxies, then the light bends enough that it can be brought to a focus, creating what is known as a "gravitational lens". Since Einstein's prediction, the study of gravitational lensing has grown enormously, and indeed gravitational lensing is one of the main reasons that we know that "dark matter" exists. Even though all light is affected in this way, so far only two sources of light have ever been used to study gravitational lensing: light from individual galaxies, and the left over light from the Big Bang, known as the Cosmic Microwave Background (CMB). This project will greatly increase the domain over which gravitational lensing is studied. In particular it will add three different new lensed fields to the two currently observed. These are (1) clouds of gas in intergalactic space (which we can view in three dimensions), (2) groups and clusters of galaxies, and (3) the field of time delays measured from CMB light seen in different directions on the sky. The last one, CMB time delay, is caused by the fact that light slows down as it passes through a gravitational lens, so that light arrives at an observer later when a lens is between them and the edge of the observable universe (where the CMB originates). The project aims to lead to an understanding of the theory behind these new fields, how best to measure the lensing effects and how to use them to make accurate maps of the dark matter in the Universe. These concepts, related to the length scales and contents of the cosmos will also be spread by the project to elementary through high school students. This will be done with educational video games, including development of "Minecraft" astronomy lesson plans, and explorable computer models of quasars and intergalactic space and the chemical constituents of the Universe. Weak gravitational lensing is arguably the best way to map out the dark side of the Universe. Billion dollar class surveys and satellites will use distortions of galaxy shapes (galaxy shear) caused by lensing to do this. The more recent adoption of lensing of the Cosmic Microwave Background (CMB) as a cosmological probe has shown the advantages of a different tracer of lensing distortions, with different strengths and systematic errors. These observations of weak lensing are only the beginning, however - any background light sources will undergo the same distortions and there is a rich phenomenology yet to be discovered. The work will expand the use of lensing anisotropies of clustering to three new areas, the Lyman-alpha forest, the galaxy correlation function, and CMB time delay. None relies on measurement of galaxy shapes which can make precise galaxy shear determinations difficult, but two can be measured to high significance with current data and one has potential implications for future surveys. The PI and Co-I have developed some of the first ideas on these topics, and the time is ripe for a concerted effort to unlock their full potential. The project will formulate estimators to derive lens parameters from raw data, and make use of realistic simulated datasets to test and develop them in the presence of non-linearities and non-Gaussianity. These estimators will be applied to data from surveys of the intergalactic medium and the distribution of galaxies. Statistical errors on the matter fluctuation amplitude at the few percent level are expected, but the true gains will be in the differing systematic errors to shear and the new ways the data can probe cosmology. These techniques will offer a new route to measuring the neutrino mass and the evolution of structure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.