Exploring the Universe with cosmological surveys

The physics of our Universe, its history in the past and its future evolution, has fascinated scientists across all fields and the general public for centuries. Today, the cosmology community faces the fortunate situation of being showered with an unprecedented amount and quality of observational data. The ongoing wide-field imaging, spectroscopic, and cosmic microwave background surveys (Dark Energy Survey, Hyper Suprime Cam Survey, Kilo-Degree Survey, Baryon Oscillation Spectroscopic Survey, Planck, South-Pole Telescope, Atacama Cosmology Telescope) are in the final stages of data collection and provide fantastic data sets to explore. 

These endeavors will be superseded in the next decade with the advent of the Large Synoptic Survey Telescope, the Dark Energy Spectroscopic Instrument, and the SPHEREx, Euclid, and WFIRST satellite missions, and the CMB-Stage 4 experiment.

Our lab is interested in asking the right physics questions that can be answered with the individual data sets and to combine them in joint analyses. We strive to answer fundamental questions about the physics of our Universe, dark energy, dark matter, gravity, and inflation:

1) What is the underlying physical mechanism driving the accelerated expansion of the Universe?
2) What is the mass and number of neutrino species?
3) What is the nature of dark matter and the connection of dark matter overdensities and galaxy formation?
4) Is General Relativity a complete description of our theory of gravity or does it need to be modified as a function of the environment or the distances involved?
5) Can we measure the physics of inflation, i.e. of the very first second when the Universe began? 

We explore these questions through a combination of theoretical modeling and statistical data analysis. Come join us on this exciting journey!


Encoding Theory - CosmoLike

CosmoLike is a software framework for complex likelihood analyses of cosmological surveys. It is unique in its integrated ansatz of jointly analyzing large-scale structure probes of the Universe and it possesses sophisticated covariance computation capabilities that include the higher-order terms of the matter density field.
The code has been used in several forecasts exploring joint analyses of cosmological probes (Eifler et al., 2014; Krause & Eifler, 2017; Schaan et al., 2017) and systematics mitigation strategies, such as the impact of baryons and intrinsic alignment (Eifler et al., 2015; Krause et al., 2016; Huang et al., 2019). On the observational side the code was used in the weak lensing analysis of Sloan Digital Sky Data (Huff et al., 2014) and the recent DES Year 1 analysis (Krause et al., 2017; Abbott et al., 2018b). The code is also used in the survey optimization and forecasting effort of the NASA Wide Field Infrared Survey Telescope (WFIRST) Science Investigation Team on “Cosmology with the High Latitude Survey” Dore et al. (2018), and most recently it has been used to optimize the cosmology analysis of LSST data (The LSST Dark Energy Science Collaboration et al., 2018; Lochner et al., 2018).

CosmoLike Projects

CosmoLike is a vehicle for many theory and data analysis projects, some associated with specific surveys, others very generally applicable. We constantly try to extend the code to model more complex analysis with higher precision, accuracy, and speed. Learn more about CosmoLike


Kinematic Lensing (KL)

The main noise contribution in traditional weak lensing (WL) measurements originates from the large dispersion of the intrinsic ellipticity distribution of galaxies. This so-called shape noise dilutes the desired measurement of the shear effect, which is a factor of 10-50 lower. As a consequence, traditional WL methods require a large ensemble of galaxies to measure the shear effect, which leads to the inclusion of a substantial fraction of faint, small galaxies in these ensembles. Unsurprisingly, faint galaxies are strongly affected by uncertainties in shape and redshift measurement algorithms, which are hard to control at the level of WFIRST and LSST.

The basic idea for the kinematic lensing is depicted on the right. In an image, an inclined rotating circular disk has elliptical isophotes. When the image of this galaxy is sheared, the isophotes remain elliptical (in the weak shear limit, |g| << 1) with a new axis ratio and position angle. New photometric axes are inferred from this ellipse in the sheared image, and information about the original axes is lost. The case is different however, with kinematic measurements. The unsheared circular disk has kinematic axes that are perpendicular to one another and are aligned with the unsheared photometric axes. This cross shape becomes skewed when the velocity map is sheared; the kinematic axes are no longer perpendicular and they are misaligned with the photometric axes inferred from the sheared isophotal ellipse in the imaging data.

Kinematic Lensing (KL) reduces shape noise by more than an order of magnitude and either removes or strongly mitigates the most important sources of theoretical and observational systematic errors inherent in traditional weak lensing techniques (most prominently, intrinsic alignment, shear and photo-z calibration).

Kinematic Lensing Projects


KL Intrinsic Alignment and Magnification modeling

Although KL is less likely affected by systematics than traditional weak lensing, magnification and alignment effects can still impact the measurement and have to be parameterized. Goals of the project: Develop the theoretical background for KL systematics, suggest and efficient parameterization, and quantify the impact of systematics on KL measurements.


First KL measurement - Pilot Study with Keck+HST data

We obtained spectroscopic data from Keck DEIMOS and high-resolution HST archival data and are building a pipeline to conduct the first KL measurement using Abell 2218 as a foreground lens. 


Explore KL applications for WFIRST

The WFIRST grism with resolution of R~600 might be a viable instrument for a KL measurement. We are exploring corresponding instrument limitations, define a galaxy sample with a sufficient S/N, and quantify the science return of such a KL mission.  

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