Zachary Slepian

COSMOLOGIST

Chamberlain Fellow & Einstein Fellow

Lawrence Berkeley National Laboratory & Berkeley Center for Cosmological Physics

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About me

Triangulating the Universe's laws and contents with large-scale structure.

profile

My work is in cosmology on a range of topics from theoretical to methodological and observational. At Harvard, my primary focus was assessing whether there is a systematic offset in the Baryon Acoustic Oscillation standard ruler due to an imprint of baryon-dark matter relative velocities on galaxy clustering today. I have computed the relative velocity effect on the 2 and 3-point galaxy correlation functions and developed a novel, ultra-fast algorithm for measuring the 3-point correlation function, with a Fourier version suitable for upcoming large-scale structure surveys such as Euclid, LSST, and DESI.

I applied this 3-point algorithm to the SDSS DR12 CMASS sample of ~800,000 Luminous Red Galaxies, making the largest-ever measurement of the 3-point function in both galaxy number and maximum physical scale. This measurement was reported both in a compressed basis where one triangle side is integrated out and in the full 3-D parameter space. The latter enabled the first high-significance detection of the BAO in the 3PCF (4.5sigma) and a 1.7% precision measurement of the cosmic distance scale from the 3PCF alone. We also placed a 0.5% precision constraint on the baryon-dark matter relative velocity’s coupling to the BOSS Luminous Red Galaxies, meaning any shift in the BAO scale in this survey is 0.3% or smaller.

In analytic work supporting these observational constraints, I developed a configuration-space model for the 3PCF including redshift space distortions, the analog of the Kaiser/Hamilton formulae for the 2PCF. I have also developed an algorithm to use Fourier transforms to evaluate a pairwise defined line of sight for measurements of the anisotropic 2-point correlation function. Another work involved building a simple analytic model for the linear growth of structure showing how the BAO scale emerges in the matter transfer function.

My previous work concentrated on physically-motivated parametrizations for the dark energy equation of state (with JR Gott) and on testing thermalized bosonic dark matter against observational constraints (with Jeremy Goodman).

Education

Princeton University

AB, Astrophysical Sciences, 2011

summa cum laude

Thesis title: ''Slow roll dark energy''

Minors in Applied and Computational Mathematics and Values and Public Life.

University of Oxford

MSt, Philosophy of Physics, 2012

Harvard University

PhD in Astronomy & Astrophysics, 2016

Thesis title: ''Revitalizing the 3-Point Correlation Function Of Galaxies to Sharpen the BAO Standard Ruler''

MA in Astronomy & Astrophysics, 2014

Research

An Overview.

Scroll down for publications in each category.

Theory

Where do pen and paper meet the sky?

On large scales, the Universe is governed by gravity, and its use to analytically predict structure formation goes back to the '50s. On small scales, baryon physics makes analytic prediction almost fruitless. I search for the golden middle: opportunities to use perturbation theory, where the map is not yet the size of the territory, to probe the nature of gravity, dark matter, and dark energy.

Algorithms

Lossless compression is the goal.

Without data, theory is untested belief. But data is pollen---algorithms convert it to honey. In particular, laws of physics should hold at all locations and in all directions, corresponding to the cosmological assumptions of homogeneity and isotropy on large scales. Thus absolute position and direction can often be modded out for cosmological inference. I seek novel approaches to do this at unparalleled speed and scale.

Data

Only falsifiable theories please!

Data is both the starting and ending point. My theoretical interests are driven by what current data tells us and what future data promises, and for me a theory project is incomplete until it has been tested against the Universe.

Curriculum Vitae

As a PDF

Teaching

Teaching is a path to understanding.

profile

I have taught at the University of Oxford and at Harvard University. At Oxford, I served as ''Junior Demonstrator'' supervising first-year physics undergraduate labs in electromagnetism. This involved moving around the lab asking questions to probe whether the students were really understanding the experiments, assisting with year-end assessment of lab notebooks, and trouble-shooting if students required assistance with the lab.

At Harvard, I served as a Teaching Fellow (Harvard's word for Teaching Assistant) for four undergraduate classes, teaching during all four years of my PhD. In Spring 2013, I assisted with ''From the Big Bang to the Brontosaurus,'' a general-education class for non-majors covering 1/3 biology, 1/3 Earth's geologic history, and 1/3 astronomy, and integrating history of science with these focuses. I led hour-long weekly sections, assisted as needed one on one with homework assignments, gave one lecture, and helped guide and assess student final proijects.

In Spring 2014, I assisted with one of Harvard's two introductory astronomy classes for majors, ''Stellar and Planetary Astronomy.'' This introduced topics ranging from celestial coordinates to exoplanet detection, and was taught in a highly interactive format where after a short lecture students broke into groups and solved problems at whiteboards to guide them through learning and deriving the day's material. Thus I spent a large amount of time interacting with students both in class and in weekly 3-hour long interactive problem sessions. I also conducted 3-hour long labs, generally using Harvard's 16-inch Clay telescope. The class culminated in a final challenge where students tried to detect an exoplanet using transit photometry.

In Fall 2015, I assisted with a small physics department seminar for junior majors designed to help them gain comfort reading the primary literature as well as to familiarize them with the many different research groups in Harvard's physics department. I administered a course website (through openrev) where students could read and post questions on the reading, and I also helped students prepare presentations for the weekly 1.5-hour seminar. Another responsibility was facilitating weekly dinners and presentations by a Harvard physics faculty member on whose work that week's seminar had focused.

In Spring 2016, I taught in Harvard's ''Introductory Cosmology'' course, based on Ryden's ''Introduction to Cosmology.'' Responsibilities included creating problem sets and solutions, supervising a weekly 3-hour long problem session, giving one or two lectures, conducting midterm and final review sessions, and setting midterm and final exam problems.

Service

Highlights

profile

I consider service in a variety of capacities an essential part of my responsibility as a scientist. I serve as a referee for Monthly Notices of the Royal Astronomical Society, have served on the Local Organizing Committee for a BCCP workshop, and am a member of the Dark Energy Spectroscopic Instrument-2 (DESI-2) Working Group at LBNL.

I am a strong proponent of involving students in research at every phase of their career, and have supervised three high-school students over two summers on independent research projects. I will be supervising an undergraduate at LBNL in summer 2017.

Outreach is also an essential component of my job. I have taught math in New Jersey state prisons, conducted demonstrations for the Cambridge Science Festival, done classroom visits for 7th and 8th graders in Boston public schools, and given many public lectures. I also wrote for the popular paper-summary website ''astrobites'' for 5 years, and co-chaired the effort to bring new authors onboard for 2 years. I remain involved with the site, focusing on undergraduate outreach.

For a full list of my Service & Outreach activities, please have a look at my CV.

Contact

Contact Info

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720

(510) 495-2316

zslepian(at)LBL.gov

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