PhD Part A Exam by Per Andersen – University of Copenhagen

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PhD Part A Exam by Per Andersen

Doing Cosmology with Bulk Flow Magnitudes: Observational Effects

Supervisor: Jens Hjorth

Censor: Steen Hannestad (Aarhus University)

In this thesis we investigate the viability of comparing bulk flow magnitudes to predictions from linear theory, in order to test ΛCDM cosmology. This investigation is motivated by a current disagreement with ΛCDM by some bulk flow measurements in the literature, claiming observations of bulk flows larger than predicted in ΛCDM. We focus on observational effects that might bias our measurements of bulk flows, such as the effects of survey geometry, and the asymmetry of the bulk flow magnitude distribution. We find that all techniques to measure a value for the bulk flow overestimate its magnitude, and this is made worse both by sparse sampling and by incomplete sky coverage. Further work is needed before these effects are fully understood, and until that work has been carried out we conclude that the standard χ2 analysis of the bulk flow vector components is still the most viable method to compare bulk flow measurements with theory. Finally, future work should perform the same analysis but calculate cosmological parameters rather than bulk flow velocity, and see if the same biases exist. Since our study shows observational effects will cause false detections of large bulk flows, it is critical to confirm that the same observational effects do not bias cosmological parameters.

Read the thesis here>>