Modern Cosmology

History of the Universe. Original figure by NASA WMAP team, edited by Zhilei Xu
I am interested in studying the universe mainly via observing the Cosmic Microwave Background (CMB) and the 21-cm radiation. Since both of them were emitted in the early universe, they carry critical information of the majority of the universe’s history.

1. Cosmic Microwave Background (CMB)

1.1 Cosmology Large Angular Scale Surveyor (CLASS)

CLASS at the Atacama Dessert
CLASS telescopes Observing at the Atacama Desert. Photo credits: CLASS Collaboration
I am a core member of the team that designed and built the Cosmology Large Angular Scale Surveyor (CLASS). My contribution covers both instrumentation and data analysis, with part of them presented in the following publications:

Z. Xu. Cosmology Large Angular Scale Surveyor (CLASS): Integration and Testing of CLASS at 40 GHz. PhD thesis, Johns Hopkins University, Aug 2017,

J. W. Appel, Z. Xu, I. L. Padilla, K. Harrington, B. P. Marquez, et al. On-sky performance of the CLASS q-band telescope. ApJ, 876(2):126, May 2019,

Z. Xu, M. K. Brewer, P. F. Rojas, Y. Li, K. Osumi, B. Pradenas, et al. Two-year cosmology large angular scale surveyor (CLASS) observations: 40 GHz telescope pointing, beam profile, window function, and polarization performance. The Astrophysical Journal, 891(2):134, Mar 2020.

 

1.2 Simons Observatory (SO)

Working in the LATR
Working in the LATR. Photo credits: Derick Crucius
My main focus in the Simons Observatory (SO) is working with the University of Pennsylvania team to design, manufacture, integrate, and test the Large Aperture Telescope Receiver (LATR). The LATR is the largest cryogenic receiver with a < 100mK  cryogenic stage. The design was presented in the following publications:

N. Zhu, J. L. Orlowski-Scherer, Z. Xu, et al. Simons Observatory large aperture telescope receiver design overview. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, volume 10708, page 1070829, July 2018,

J. L. Orlowski-Scherer, N. Zhu, Z. Xu, et al. Simons Observatory large aperture receiver simulation overview. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, volume 10708, page 107083X, July 2018,

G. Coppi, Z. Xu, et al. Cooldown strategies and transient thermal simulations for the Simons Observatory. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, volume 10708, page 1070827, July 2018,

Z. Xu, G. Chesmore, et al. The Simons Observatory: Metamaterial Microwave Absorbers (MMA) and the Cryogenic Applications, in preparation, 2020.

In addition to the LATR development, I am also one of the leads in the SO pipeline working group (PWG) focusing on the beam, calibration, and polarization calibration of the instrument.
I also co-chair the Talks Panel within the Simons Observatory.

2. 21-cm Observations

Hydrogen Epoch of Reionization Array (HERA)

Hera 19 complete, Feb. 2016. Photo credits: HERA Collaboration
I am joining the HERA collaboration (link) to study the Epoch of Reionization by observing the radiation from high-redshift neutral hydrogen atoms.

3. Cross-correlation

I am interested in cross-correlating different data sets, including galaxy surveys, CMB data sets, 21 cm data etc., to understand the history of the universe and the evolution of the galaxies. One initial work we did by cross-correlating the Planck and Dark Energy Survey (DES) data sets was published in:

S. Pandey, E. J. Baxter, Z. Xu, et al. Constraints on the redshift evolution of astrophysical feedback with sunyaev-zel’dovich effect cross-correlations. Phys. Rev. D, 100:063519, Sep 2019.