Experimental Cosmology at UCR

Steve Choi's Picture

The primary focus of our group is captured precisely by this phrase from Stephen Hawking's A Brief History of Time: "our goal is nothing less than a complete description of the universe we live in." To this end, we develop innovative methods to enable a newer and deeper view of our cosmos, aiming to reveal its origins, composition, and evolution. Our research spans from developing sensitive astronomical instrumentation with superconducting detectors to analyzing and interpreting microwave sky data using computer clusters.

Cosmology has advanced tremendously in recent years as a result of a wealth of data from sensitive instruments. Notably, observations of the cosmic microwave background (CMB) have played a crucial role in solidifying the current cosmological paradigm. The standard Lambda-cold-dark-matter (LCDM) model, in particular, has been remarkably successful in describing the observations of our universe with only six free parameters. Despite the exciting progress, several fundamental questions remain unanswered: How did the universe begin? What are the properties of dark energy and dark matter? Is new physics required to resolve the ongoing cosmological tensions?

Improved mapping of the microwave sky will illuminate new directions to tackle the open questions in fundamental cosmology. We work with the CCAT Observatory, Atacama Cosmology Telescope, Simons Observatory, and CMB-S4 to pursue a range of cosmological and astrophysical sciences. Prospective undergraduate and graduate students, as well as postdocs, are encouraged to reach out and explore research opportunities available within these projects.

I am currently an Assistant Professor in the Department of Physics and Astronomy at the University of California, Riverside (2024–present). My postdoctoral research was done at Cornell University as a Presidential Fellow then an NSF Fellow (2018–2023). Before that, I earned my PhD in Physics at Princeton University (2012–2018). In my free time, I like to play tennis and chess (bullet).

Research highlight:

CCAT FYST — **CCAT Observatory:** picture of FYST, a new six-meter telescope being built in Germany (with me in front of it for scale; July 2024). FYST will be located at a 5,600-m altitude site in the Atacama Desert. The telescope is currently under construction for commissioning and first light in 2025!

CCAT 280 GHz array — **Detector arrays for CCAT:** large arrays of kinetic inductance detectors (KIDs) are being developed to provide the best 200–1000 GHz measurements to date with CCAT. This emerging superconducting detector technology offers an unmatched scalability to field tens of thousands detectors on CCAT for an unprecedented sensitivity. (more here and here)

DR4 PS — **ACT Data Release 4 (DR4):** power spectrum analysis of the ACT data independently confirmed the Planck measurement of the Hubble constant. This strengthened the ongoing “Hubble tension” that indicates the discrepancy of the Hubble constant as measured with the CMB versus the so-called distance ladder approach using Cepheid variables and Type Ia supernovae. (more here)

AdvACT HF array — **Advanced ACTPol:** transition edge sensor arrays for ACT that produced some of the most sensitive high-resolution CMB data yet, leading to the recent CMB gravitational lensing results from ACT (DR6). The upcoming DR6 power spectrum results will provide opportunities to look for cracks in LCDM to potentially alleviate some of the cosmological tensions. (more here)

dust synchrotron PS — **Polarized Galactic dust and synchrotron emissions:** analysis of the Planck and WMAP data revealing to a high significance the spatial correlation of the polarized dust and synchrotron emissions from the Milky Way. Understanding these interstellar components is essential in both cosmological and Galactic sciences. (more here)

Steve K. Choi