Benjamin
Joachimi





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Welcome to my web pages. I am a lecturer in the Department of Physics and Astronomy at University College London. Before that I was a researcher at the Institute for Astronomy at the University of Edinburgh, and at the Argelander-Institut für Astronomie at Bonn University.

My research focuses on understanding the large-scale distribution of matter in the cosmos, with the goal to obtain an accurate picture of how the Universe formed and evolved, and what the fundamental laws of physics are that governed this evolution. This involves shedding light onto the elusive dark matter and dark energy, testing Einstein's theory of gravity on the largest scales, but also investigating how galaxies like our Milky Way assumed their form.
B. Joachimi


I am particularly interested in

Learning about dark physics

What is the nature of dark matter which holds together galaxies? What are the properties of dark energy which causes the Universe to expand at ever increasing speeds? And is Einstein's theory of gravity, which works beautifully in the Solar System, still correct on the largest scales? I am working on addressing these questions by a careful analysis of the positions, velocities, and shapes of very large numbers of distant galaxies, thereby exploiting cosmological probes like galaxy clustering, redshift-space distortions, and gravitational lensing.

The weak gravitational lensing effect

Gravitational lensing, the deflection of light by massive objects, causes tiny distortions in the images of distant galaxies whose light is deflected by the intervening large-scale matter distribution. This weak lensing effect can be measured by averaging over millions of galaxy images. To help turn this potentially very strong probe of the matter distribution and its evolution into a powerful and reliable cosmological tool, I investigate methods that measure the minute gravitational distortions accurately, develop statistics that extract as much cosmological information as possible, and determine high-precision estimates of the statistical uncertainties involved.

The shapes and alignments of galaxies

The shapes of distant galaxies and the way they align with each other and the surrounding matter distribution tells us how the formation and evolution of galaxies depends on the galaxy's environment. Moreover, these intrinsic galaxy alignments violate a central assumption in weak gravitational lensing theory, namely that galaxies are randomly oriented. This leads to a serious obstacle in reliably extracting cosmological information from weak lensing. All this calls for a better understanding of intrinsic alignments, which I am contributing to both observationally and theoretically.

New analysis methods and statistical tools

What is the best way to extract cosmology when one has measured several probes, and how does one deal with the resulting very large datasets? How does one make sure that the final measurement is free of any unwanted systematic effects? And how does one estimate the error bars on these measurements if one has only a limited number of simulations and relatively crude analytical models? I am addressing these questions in the context of several current and forthcoming surveys in which I am involved, e.g. the Kilo Degree Survey, the Dark Energy Survey, the Large Synoptic Survey Telescope, and the ESA Euclid mission.


... you were actually only after pretty pictures? Then look here:
Astronomy Picture of the Day
Hubble Space Telescope Album
The World at Night