About

Jonathan Blazek is an Assistant Professor in the Department of Physics at Northeastern University. His research focuses on observational and theoretical cosmology, with a primary emphasis on understanding the Universe through astronomical surveys that cover large areas of the sky. He develops modeling and statistical methods to connect these observations to the underlying cosmological model, with particular attention to applications of galaxy clustering and weak gravitational lensing to map out the structure of the Universe. Blazek utilizes analytic techniques, cosmological simulations, and novel applications of artificial intelligence in his work. He is actively involved in major research collaborations, including the Dark Energy Survey, the Rubin Observatory Dark Energy Science Collaboration, and the Roman Space Telescope. His efforts also include combining data sets from multiple experiments observing at different wavelengths and employing different techniques to produce the most informative tests of cosmological models. Prior to his current position, Blazek was a postdoctoral fellow at EPFL in Switzerland and at Ohio State University. He earned his Ph.D. from the University of California, Berkeley.

Research topics

• Astronomy
• Physics
• Astrophysics
• Chemistry
• Psychology

Selected publications

• Euclid preparation. Calibrated intrinsic galaxy alignments in the Euclid Flagship simulation

  arXiv (Cornell University) · 2026-01-12

  preprintOpen access

  Intrinsic alignments of galaxies are potentially a major contaminant of cosmological analyses of weak gravitational lensing. We construct a semi-analytic model of galaxy ellipticities and alignments in the \Euclid Flagship simulation to predict this contamination in Euclid's weak lensing observations. Galaxy shapes and orientations are determined by the corresponding properties of the host haloes in the underlying $N$-body simulation, as well as the relative positions of galaxies within their halo. Alignment strengths are moderated via stochastic misalignments, separately for central and satellite galaxies and conditional on the galaxy's redshift, luminosity, and rest-frame colour. The resulting model is calibrated against galaxy ellipticity statistics from the COSMOS Survey, selected alignment measurements based on Sloan Digital Sky Survey samples, and galaxy orientations extracted from the Horizon-AGN hydrodynamic simulation at redshift $z=1$. The best-fit model has a total of 12 alignment parameters and generally reproduces the calibration data sets well within the $1σ$ statistical uncertainties of the observations and the \flagship simulation, with notable exceptions for the most luminous sub-samples on small physical scales. The statistical power of the calibration data and the volume of the single \flagship realisation are still too small to provide informative prior ranges for intrinsic alignment amplitudes in relevant galaxy samples. As a first application, we predict that \Euclid end-of-mission tomographic weak gravitational lensing two-point statistics are modified by up to order $10\,\%$ due to intrinsic alignments.

  PublisherDOI

• Euclid preparation. Testing analytic models of galaxy intrinsic alignments in the Euclid Flagship simulation

  arXiv (Cornell University) · 2026-01-12

  preprintOpen access

  We model intrinsic alignments (IA) in Euclid's Flagship simulation to investigate its impact on Euclid's weak lensing signal. Our IA implementation in the Flagship simulation takes into account photometric properties of galaxies as well as their dark matter host halos. We compare simulations against theory predictions, determining the parameters of two of the most widely used IA models: the Non Linear Alignment (NLA) and the Tidal Alignment and Tidal Torquing (TATT) models. We measure the amplitude of the simulated IA signal as a function of galaxy magnitude and colour in the redshift range $0.1

  PublisherDOI

• Differentiable Stochastic Halo Occupation Distribution with Galaxy Intrinsic Alignments

  Open MIND · 2026-02-04

  preprintSenior author

  We present diffHOD-IA, a fully differentiable implementation of a halo occupation distribution (HOD) model that incorporates galaxy intrinsic alignments (IA). Motivated by the diffHOD framework, we create a new implementation that extends differentiable galaxy population modeling to include orientation-dependent statistics crucial for weak gravitational lensing analyses. Our implementation combines this HOD formulation with an IA model, enabling end-to-end automatic differentiation from HOD and IA parameters through to the galaxy field. We additionally extend this framework to differentiably model two-point correlation functions, including galaxy clustering and IA statistics. We validate diffHOD-IA against the reference halotools-IA implementation using the Bolshoi-Planck simulation, demonstrating excellent agreement across both one-point and two-point statistics. We verify the accuracy of gradients computed via automatic differentiation by comparison with finite-difference estimates for both HOD and IA parameters. We present science use cases leveraging gradients in the simulations to recover the IA parameters of a galaxy field representative of the TNG300 simulation. Finally, we apply diffHOD-IA in a Hamiltonian Monte Carlo analysis and compare its performance with halotools-IA and a neural-network-based emulator, IAEmu. Unlike emulator-based approaches for statistics, diffHOD-IA provides differentiability at the galaxy catalog level, enabling integration into field-level inference pipelines and extension to arbitrary summary statistics for next-generation weak-lensing analyses. Our code is publicly available.

  DOI

• Constraining the Stellar-to-Halo Mass Relation with Galaxy Clustering and Weak Lensing from DES Year 3 Data

  The Open Journal of Astrophysics · 2026-01-08

  articleOpen access

  We develop a framework to study the relation between the stellar mass of a galaxy and the total mass of its host dark matter halo using galaxy clustering and galaxy-galaxy lensing measurements. We model a wide range of scales, roughly from to , using a theoretical framework based on the Halo Occupation Distribution and data from Year 3 of the Dark Energy Survey (DES) dataset. The new advances of this work include: 1) the generation and validation of a new stellar mass-selected galaxy sample in the range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⋆</mml:mo> </mml:msub> <mml:mi>/</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>9.6</mml:mn> </mml:mrow> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>11.5</mml:mn> </mml:mrow> </mml:math> ; 2) the joint-modeling framework of galaxy clustering and galaxy-galaxy lensing that is able to describe our stellar mass-selected sample deep into the 1-halo regime; and 3) stellar-to-halo mass relation (SHMR) constraints from this dataset. In general, our SHMR constraints agree well with existing literature with various weak lensing measurements. We constrain the free parameters in the SHMR functional form <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⋆</mml:mo> </mml:msub> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>=</mml:mo> <mml:mo>log</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mi>ϵ</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>+</mml:mo> <mml:mi>f</mml:mi> <mml:mrow> <mml:mo stretchy="true" form="prefix">[</mml:mo> <mml:mo>log</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:mi>/</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo stretchy="true" form="postfix">]</mml:mo> </mml:mrow> <mml:mo>−</mml:mo> <mml:mi>f</mml:mi> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>0</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>f</mml:mi> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mi>x</mml:mi> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>≡</mml:mo> <mml:mo>−</mml:mo> <mml:mo>log</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mi>α</mml:mi> <mml:mi>x</mml:mi> </mml:mrow> </mml:msup> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo>+</mml:mo> <mml:mi>δ</mml:mi> <mml:msup> <mml:mrow> <mml:mo stretchy="true" form="prefix">[</mml:mo> <mml:mo>log</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:mo>exp</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:mi>x</mml:mi> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo stretchy="true" form="postfix">]</mml:mo> </mml:mrow> <mml:mi>γ</mml:mi> </mml:msup> <mml:mi>/</mml:mi> <mml:mrow> <mml:mo stretchy="true" form="prefix">[</mml:mo> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:mo>exp</mml:mo> <mml:mrow> <mml:mo stretchy="true" form="prefix">(</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msup> <mml:mo stretchy="true" form="postfix">)</mml:mo> </mml:mrow> <mml:mo stretchy="true" form="postfix">]</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>log</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mn>11.506</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.404</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.325</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>log</mml:mo> <mml:mi>ϵ</mml:mi> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mn>1.632</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.181</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.306</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>α

  PublisherDOI

• Dark Energy Survey: Implications for cosmological expansion models from the final DES baryon acoustic oscillation and supernova data

  Physical review. D/Physical review. D. · 2026-01-30 · 6 citations

  article

  International audience

  PublisherDOI

• Smokescreen: A Python package for data vector blinding and encryption in cosmological analyses

  arXiv (Cornell University) · 2026-04-20

  preprintOpen access

  Smokescreen is an open-source Python library for data-vector concealment (blinding) in cosmological analyses. Data-vector blinding works by applying cosmology-dependent shifts to the observed data vector, moving it away from the true cosmological signal without affecting its statistical properties, so that analysts cannot infer the true result until the analysis is frozen and the blinding is lifted. The package computes these shifts using Firecrown likelihoods applied to data vectors stored in the SACC format, ensuring that the theoretical model used for blinding is identical to that used for inference whilst remaining agnostic to the specific observable being blinded. To prevent accidental unblinding, the original SACC file, containing the true cosmology, is encrypted. Although developed for the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), Smokescreen is applicable to any experiment using Firecrown likelihoods and the SACC data format.

  PublisherDOI

• Tidal alignment and tidal torquing modeling for the cosmic shear three-point correlation function and mass aperture skewness

  arXiv (Cornell University) · 2026-01-14

  preprintOpen access

  We present a model for the intrinsic alignment contamination of the shear three-point correlation function and skewness of the mass aperture statistic using the tidal alignment and tidal torquing (TATT) formalism. We compute the intrinsic alignment bispectra components in terms of the TATT model parameters. We consider two effective field theory approaches in the literature, relate them to the TATT model parameters and an extension to TATT that includes the velocity-shear (VS) parameter. We compare the impact of changing between NLA, TATT, and TATT+VS on the theoretical computation of the 3PCF using the best fit parameters and tomographic redshift distributions from Dark Energy Survey Year 3. We find that the TATT model significantly impacts the skewed triangle configurations of the 3PCF. Additionally, including the higher-order effects from TATT can introduce opposite effects on the two-point function and on the mass aperture skewness, damping the signal of the former while boosting the signal of the latter. We argue that a joint 2PCF+3PCF analysis with the TATT model can help break the degeneracy between its model parameters and provide more robust constraints on both cosmology and intrinsic alignment amplitude parameters. We show that typical values of order unity for the intrinsic alignment parameters introduce differences of around $10\%$ between NLA and TATT predictions.

  PublisherDOI

• Brightest Cluster Galaxy ellipticity as proxy for halo shape: Orientation bias, assembly bias, and potential selection effects in SZ-selected clusters

  ArXiv.org · 2026-03-24

  articleOpen access

  The orientation of triaxial galaxy clusters with respect to the line-of-sight is expected to be one of the prime sources of scatter and potential bias in optical observables (e.g., richness and weak-lensing signal) of galaxy clusters. In this work, we use the observed shape of the central Brightest Cluster Galaxy (BCG) as proxy for the orientation along the line-of-sight for clusters selected via the Sunyaev-Zel'dovich (SZ) effect from the South Pole Telescope (SPT) and Atacama Cosmology Telescope (ACT) surveys, matched to optically selected clusters from the Dark Energy Survey Year 3 (DES). We construct two samples of clusters that are designed to be identical in SZ mass estimate and redshift but with the roundest vs. the most elliptical BCGs, which we expect to correspond to BCGs (and clusters) with major axes aligned along the line-of-sight vs. in the plane of the sky, respectively. We find that the optical richness of round-BCG clusters is $\sim 10$\% larger than that of elliptical-BCG clusters, in agreement with the expectation from projection effects and presenting the first such detection in data. The density profiles, however, are not in agreement with the expectation from projection effects: the 1-halo term (below $6~h^{-1}\rm{Mpc}$) of both the weak-lensing and galaxy density profiles are the same for the subsamples, contrary to previous studies based on X-ray selected clusters. In the 2-halo regime (above $6~h^{-1}\rm{Mpc}$), we find a significant excess of the elliptical-BCG cluster profiles compared to the round-BCG cluster profiles, which is the opposite of the expectation from numerical simulations. We hypothesize that the intrinsic shape of the BCG reflects not just the orientation angle, but also intrinsic properties of the cluster which can affect both the SZ signal and the amplitude of the 2-halo term.

  PublisherOA PDF

• Tidal alignment and tidal torquing modeling for the cosmic shear three-point correlation function and mass aperture skewness

  ArXiv.org · 2026-01-14

  articleOpen access

  We present a model for the intrinsic alignment contamination of the shear three-point correlation function and skewness of the mass aperture statistic using the tidal alignment and tidal torquing (TATT) formalism. We compute the intrinsic alignment bispectra components in terms of the TATT model parameters. We consider two effective field theory approaches in the literature, relate them to the TATT model parameters and an extension to TATT that includes the velocity-shear (VS) parameter. We compare the impact of changing between NLA, TATT, and TATT+VS on the theoretical computation of the 3PCF using the best fit parameters and tomographic redshift distributions from Dark Energy Survey Year 3. We find that the TATT model significantly impacts the skewed triangle configurations of the 3PCF. Additionally, including the higher-order effects from TATT can introduce opposite effects on the two-point function and on the mass aperture skewness, damping the signal of the former while boosting the signal of the latter. We argue that a joint 2PCF+3PCF analysis with the TATT model can help break the degeneracy between its model parameters and provide more robust constraints on both cosmology and intrinsic alignment amplitude parameters. We show that typical values of order unity for the intrinsic alignment parameters introduce differences of around $10\%$ between NLA and TATT predictions.

  PublisherOA PDF

• Prospects for the detection of gamma rays using Cherenkov telescopes enhanced by a ground array observatory

  Open MIND · 2026-01-31

  preprint

  We consider the Single-Mirror Small-Size imaging atmospheric Cherenkov Telescopes (SST-1M) to be located inside a high-altitude array of Water-Cherenkov Detectors (WCDs) inspired by the Southern Wide-field Gamma-ray Observatory (SWGO). For such a hybrid observatory, using detailed Monte Carlo simulations, we show an improvement in the flux sensitivity of monocular and stereoscopic SST-1M observation by about 60% and 30% above 10 TeV, respectively, due to the improved gamma/hadron separation when additional parameters from the WCD array are used. We also discuss further benefits of the hybrid SWGO concept and its technical challenges.

  DOI

Recent grants

• Galaxy intrinsic alignments: A new window into structure in the Universe

  NSF · $401k · 2022–2026

Frequent coauthors

• A. Carnero Rosell

  281 shared

• L. N. da Costa

  Laboratório Interinstitucional de e-Astronomia

  271 shared

• D. Gruen

  266 shared

• E. Bertin

  Orange (France)

  250 shared

• A. Roodman

  SLAC National Accelerator Laboratory

  249 shared

• K. Kuehn

  Netherlands Institute for Radio Astronomy

  217 shared

• M. Carrasco Kind

  Urbana University

  203 shared

• J. Gschwend

  Laboratório Interinstitucional de e-Astronomia

  196 shared

Awards & honors

• NSF CAREER Award