Astro Seminar Series - VIA ZOOM

Wednesday, February 17, 2021 11:30 am - 11:30 am EST (GMT -05:00)

Alex Krolewski

Alex Krolewski is an AMTD Fellow at the University of Waterloo and Perimeter Institute for Theoretical Physics.  He received his BA in Physics and Astrophysics from Harvard in 2015, and recently finished his PhD in Astrophysics at UC Berkeley in June 2020 under the guidance of Martin White and Simone Ferraro. 

His research focuses on testing cosmology and gravity from cross-correlations with current and upcoming galaxy surveys.  Motivated by a variety of recent lensing measurements suggesting a lower amplitude of density fluctuations (σ8) in the local universe than implied by the CMB, he has been working on measuring σ8 and the universe's matter density using galaxies from the all-sky unWISE infrared imaging and Planck CMB lensing maps.  He is involved in other cross-correlation measurements with this sample, including the integrated Sachs-Wolfe effect and the kinetic Sunyaev-Zeldovich effect.  He has also worked on clustering redshifts to determine the redshift distributions of photometric surveys, allowing their use in cosmological analyses even in the absence of photometric redshifts.  Finally, he is planning for future cross-correlation efforts as a member of the CMB S4, Simons Observatory, and DESI projects.

Talk Title and Abstract:

Cosmology from CMB lensing tomography to z=2 with galaxies from the unWISE catalog

Recent measurements of the amplitude of structure from weak lensing surveys suggest that the late-time universe is slightly less clumpy than expected, with the parameter S8 ~10% lower than the best-fit Lambda-CDM model from Planck.  This tension is significant at the ~3 sigma level, and whether it represents new physics or systematics in the measurement remains an open question.  CMB lensing tomography provides a test of the robustness of the weak lensing measurement, with entirely different sources of systematic error. We cross-correlate three samples of galaxies from the unWISE photometric catalog (at median redshifts 0.6, 1.1 and 1.5) with the most recent Planck CMB lensing maps.  The resulting combined significance of ~80 at 100 < ell < 1000 is the highest-significance detection to date of CMB lensing cross-correlations, and implies percent-level measurements of the amplitude of structure.  I discuss ongoing work to infer cosmological parameters from this measurement, and to mitigate the largest source of systematic error in the measurement, the uncertain redshift distribution of the unWISE galaxies.

Would you like to join this Zoom seminar?  Please email Donna Hayes.