My Research

I am an observational Cosmologist and a professor in the department of Astronomy and Astrophysics at the University of California Santa Cruz. I lead a research group that tackles a broad range of outstanding questions in Cosmology and Galaxy Formation. I also work on various sustainability projects. Together with UCSC undergraduates, we maintain an urban food systems demonstration center on campus. We are working on composting and rain water harvesting. I am also a founding member of the UCSC faculty and staff Climate Action Now group. Finally, I am organizing a high level effort among US + Canadian institutes to tackle academic institutional barriers that limit Climate Action.  We are working on a white paper  - stay tuned!

The core of my Astrophysics team's research focuses on understanding the interplay between the dark and bright universe. My goal is to shed light on questions such as: 1) What is the co-evolution between dark matter and galaxy formation?  How does dark matter influence galaxy formation processes and in turn, do baryonic physics affect the dark matter distribution? 2) What is the distribution of dark matter on both small scales and large scales?  How do these distributions compare with predictions from the LCDM cosmological model? 3) What is the evolution of the equation of state of dark energy and is dark energy the result of a modification to the laws of gravity?

My primary expertise is in weak gravitational lensing: the deflection of light from distant galaxies by intervening gravitational potentials. This is a purely geometrical effect, free from astrophysical biases and sensitive to all mass - regardless of its baryonic or dark form. Gravitational lensing techniques have a uniquely dual ability to probe both the growth of structure (which is dominated by the distribution of dark matter) as well as the geometrical distance-redshift relation (which traces the expansion history of the universe and the equation of state of dark energy). Gravitational lensing is also powerful tool with which to probe the connection between galaxies and dark matter. Recordings from the 2021 Growth of Structure Webinar series can be found here.

Together with Will Percival, I currently serve as co-spokesperson for the DESI survey. Our 2025 results hint at dynamical dark energy and made international headline news!

I am the co-PI, together with my close collaborator Jenny Greene, of the Merian survey. Starting in 2021, Merian will use 62 nights on the Blanco telescope to build a sample of 100,000 dwarf galaxies. Here is our survey overview paper.

I run a multifaceted program called Seed Spoon Science which aims to promote underrepresented Spanish speaking STEM undergraduates, connect with local families, educate the next generation of scientists, and promote urban gardening and sustainable mindsets. We are working on our website, stay tuned!

I am gravely concerned about the Climate and Biodiversity crisis. I am leading guided readings of the IPCC Synthesis report for scientists who wish to learn more. I have also posted links and materials on this website in the Climate Crisis page.Here is a simplified glossary that you can use to help you read the Synthesis report: IPCC_vocabulary

The DESI Survey

I am a member of the Dark Energy Spectroscopic Instrument (DESI), an ambitious program to map out the properties of dark energy on the Mayall telescope.   Our team has released DR1 data which includes more than 18 million spectra.

COSMOS HST Survey

The COSMOS team has mapped out the largest contiguous area on the sky using the Hubble Space Telescope. I am a member of the COSMOS weak lensing team and created the COSMOS ACS weak lensing catalog. Our team produced more than 20 lensing related publications including a map of the distribution of dark matter that was the cover page of the journal Nature. The COSMOS ACS catalog can be found on the data products page.

Want to join our team?

Are you an undergraduate, a graduate student, or a new postdoc who would like to join our team? Check out our team members, our publications, and what we are currently working on. Prospective graduate students: we have a standardized admission process for our graduate program. Please apply at this link. If you are admitted into our program, I'd be happy to meet with you and discuss you thesis plans during our prospective visit.