Judit Prat

I am a Nordita fellow in the Physics department of Stockholm University and the KTH Royal Institute of Technology.

Judit Prat

I am a Nordita fellow in the Physics department of Stockholm University and the KTH Royal Institute of Technology. Before that I was a Schmidt AI in Science postdoctoral fellow at the University of Chicago, where I was part of the Survey Science Group. I previously completed my PhD in Cosmology at the Autonomous University of Barcelona. I am interested in learning about the nature of dark matter and dark energy using galaxy surveys, particularly with weak gravitational lensing and large-scale structure measurements. I focus on developing new methodologies to extract information from small scales and applying machine learning techniques to extract higher-order information from weak lensing mass maps.

I am also excited about sharing the science I am doing with the broader community through outreach activities. Moreover, I care a lot about equity, diversity and inclusion and ultimately about improving communities. I have led several efforts to promote and facilitate the presence of women and BIPOC (Black, Indigenous and People of Color) members in Science, and in particular in Physics. During my PhD, I was part of the Gender Equality Committee of IFAE at Universitat Autònoma de Barcelona, which I reactivated with several initiatives and after that I led the Community Engagement Group at the Astronomy and Astrophysics Department at the University of Chicago, where we pursued several efforts to promote dialogue and discussions around topics of interest to support the BIPOC members of the department. Other random facts about me include that I love making plots, flowcharts and infographics — choosing the colors is the best part ;) — doing sports and being outdoors, and generally just being around and talking to people!

Research

Cosmology from galaxy surveys

Data analysis with current surveys

A major part of my research has consisted of extracting cosmological information from large-scale structure and weak gravitational lensing measurements. In particular I have used observations from the Dark Energy Survey (DES), a photometric galaxy survey that covers ~5000 sq. deg. of the southern sky, and that has measured the positions and shapes of over one hundred million galaxies. Specifically, I have been part of the core team that has performed a so-called 3x2pt analysis, which stands for the combination of three two point correlation functions: one from position-position correlations (galaxy clustering), one from shear-shear correlations (cosmic shear), and one from position-shear correlations (galaxy-galaxy lensing). I recently wrote a review that summarizes the status of current results and is accessible as a first entry read for students or researchers who are interested in weak lensing cosmology. Here you can also find some of the main results of the DES Y3 3x2pt analysis:

  • Dark Energy Survey Year 3 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing.
  • Preparation for future surveys

    Besides this, I am also a member of the Dark Energy Science Collaboration (DESC) within the Vera Rubin Observatory's Legacy Survey of Space and Time, where I am developing and validating the pipelines that will be used when the data arrives. I have helped to develop and validate TXPipe, a modular pipeline for generating robust data vectors in cosmology analyses for the LSST. In the publication below we present the software, which is public for everyone:

  • The catalog-to-cosmology framework for weak lensing and galaxy clustering for LSST.

    Expanding frontiers

    To the small-scale regime

    Learning about the galaxy-matter connection and developing mitigation schemes

    Extracting useful cosmological information from small scale two-point measurements is complicated due to non-linearities and baryonic effects that impact the matter power spectrum and the relation between the matter and galaxy power spectrum (i.e. the galaxy bias). Because of these limitations, it is common that cosmological analyses using galaxy surveys data discard scales under a given threshold, throwing out a lot of signal-to-noise (for instance, ~50% of the total S/N was discarded in the DES Y3 3x2pt analysis). Thus, the biggest gain in constraining power for future cosmological analyses could come from including smaller scales. Actually, even using the same range of scales will be challenging since reduced statistical uncertainties will require greater precision in the modeling. Therefore, it has become essential to improve the understanding of the galaxy-matter connection, including non-linearities in the galaxy and halo biases and baryonic feedback effects. During my career, I have been part of several efforts that are working towards this goal including this work we published on the galaxy-halo connection from galaxy-galaxy lensing.

    I have also published a comparison of methods to mitigate non-local contribution from small scales in galaxy-galaxy lensing. One other related approach I have deveveloped is the lensing ratios framework, which allows us to include parts of the small scales that we do know how to model reliably (see also the DES Y1 analysis, and a CMB lensing extension of the method). This method leverages geometric information that can be incorporated at any scale and increases sensitivity to nuisance parameters such as redshift uncertainties and multiplicative shear bias, while also enhancing cosmological sensitivity—particularly for evolving dark energy models.

    To the higher-order regime

    Simulation based inference applied to weak lensing mass maps
    While the Gaussian early Universe anisotropies are well captured by two-point statistics, the late-time Universe is non-Gaussian. Thus, higher order statistics are needed to access a tremendous amount of complementary information. However, current traditional methods used to analyze galaxy survey data are still focused on two-point statistics (such as the 3x2pt analysis described above) and therefore do not use a lot of information which is present in the data. This is usually because higher order statistics are computationally very expensive and hard to model using traditional methods. In my research, I am developing novel methodologies to extract this additional information from late-time Universe observations, particularly galaxy survey data, while maintaining robustness against systematics and ensuring practical applicability to current datasets. I have recently demonstrated this approach by applying persistent homology as a summary statistic to DES Y3 weak lensing mass maps, utilizing innovative AI-based techniques within a simulation-based inference framework. This methodology yields constraints that are 70% tighter than those obtained through traditional cosmic shear two-point analysis.

    To the large-scale regime

    Combining Galaxy Surveys and Gravitational Wave Observatories
    Spatial variations in survey properties due to both observational and astrophysical selection effects generate substantial systematic errors in large-scale structure measurements in optical galaxy surveys on very large scales. On such scales, the statistical sensitivity of optical surveys is also limited by their finite sky coverage. By contrast, gravitational wave (GW) sources appear to be relatively free of these issues, provided the angular sensitivity of GW experiments can be accurately characterized. In a recent analysis we quantify the expected cosmological information gain from combining the LSST 3x2pt analysis with the large-scale auto-correlation of GW sources from proposed next generation GW experiments.

    List of publications

    To follow my latest papers, check the following ADS libraries:
  • All my papers.
  • Corresponding author papers.
  • CV
    Equity, diversity and inclusion

    Equity, diversity and inclusion

    Throughout my career, I have been part of the following groups:
  • LSST-DESC Collaboration Council member (2023 - 2025) : Organization within the DESC international collboration to improve the collboration culture and take care of internal policies.
  • IDEA group within the Astronomy and Astrophysics Department (A & A) at UChicago (2021 - 2023): It is a grassroots group of early career astronomers and physicsts in A & A and the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago.
  • Community Engagement Working Group of the A & A Department at UChicago (2020 - 2023): This group is dedicated to support Black Astronomers and Physicists at the University of Chicago.
  • The Gender Equality Committee at Institut de Física d'Altes Energies (IFAE) (2018 - 2019): This committee is dedicated to assess and improve the gender balance at IFAE.

  • Talks and initiatives

  • Community Conversations (sponsored by PSD Inclusive Climate Grant): We invited several speakers to talk about the South Side of Chicago from different perspectives.
  • Oral presentation on the report of the Gender Equality Committee activities, IFAE. May 2019.

    Resources

  • Cool places to visit in the South Side of Chicago.

  • Outreach

    Outreach talks

  • Astronomy on Tap hosted by University of Illinois. Oct 2023.
  • Talk within the Life-long Learning Fermilab progam. Sep 2021.
  • Talk at High School IES Matadepera, Barcelona. Jan 2019.
  • Talk in "Mad for Science" High School program, Barcelona. Mar 2018.
  • Public talk, Astronomical Association of Sant Cugat - Valldoreix (AASCV), Barcelona. Oct 2017.

    Participation in videos

  • Participation in Outreach Youtube channel Ani the Anisotropia.
  • Recording of a video with High School students about Maps of dark matter, emitted on local Barcelona TV. May 2017.

    Other

  • I was part of the organizer team for the first KICP Outreach Symposium, at the University of Chicago. Sep 2020.
  • Organizer of an activity to measure the Hubble Constant for the "Mad for Science" High School program, Barcelona. May 2017.
  • Local radio interview by college students about the Dark Energy Survey. Oct 2017.
  • Together with Chihway Chang, we created an infographic for the Dark Energy Survey, which you can find below. Jan 2021.

  • Chicago In Color

    Chicago In Color Project

    I am very excited to announce that I have just published a photography book with Trope. The book is called Chicago In Color and it can be ordered online. It is also available in many bookstores in the Chicago region (including in the Art Institute of Chicago and the Museum Of Contemporary Art!) and in some places in the Midwest. Here's a selection of my favorite photographs:

    ChicagoColors

    Creative Portfolio

    News

    Latest News

    • Just because we can't get out and about like we normally would, doesn’t mean we have to stop taking pictures. There’s still plenty you can do, provided you're prepared to use some imagination. Here are a few ideas to keep you shooting until normal life resumes.

      Most photographers love to shoot the unusual, and you don’t get much more unusual than These Unprecedented Times. Right now everything counts as out of the ordinary. There are a number of remarkable things about these lockdown days that are worth photographing now so we can remember them when it is all over.

      Most photographers find it hard to see interesting pictures in places in which they are most familiar. A trip somewhere new seems always exactly what our photography needed, as shooting away from home consistently inspires us to new artistic heights.

      Streets empty that are usually busy are remarkable and can evoke the sense of historical pictures from before the invention of the motorcar. Other things that are different at the moment will be queues to get into stores and the lines marked out on the floor to show how far apart we should be.

      Pretend everything is new and that you haven’t seen it before, and then you will be free to notice the leading lines, the places where one edge meets another in delightful geometric harmony, and how the ordinary things in the kitchen are transformed when the light is on or off.

      The trick here is to look slowly, and then look again. Take the time to look in detail and to look at the same thing from different angles, with different light, long lenses and wide lenses. Then move to the left a bit. You may never feel the need to leave the house again.

    • Just because we can't get out and about like we normally would, doesn’t mean we have to stop taking pictures. There’s still plenty you can do, provided you're prepared to use some imagination. Here are a few ideas to keep you shooting until normal life resumes.

      Most photographers love to shoot the unusual, and you don’t get much more unusual than These Unprecedented Times. Right now everything counts as out of the ordinary. There are a number of remarkable things about these lockdown days that are worth photographing now so we can remember them when it is all over.

      Most photographers find it hard to see interesting pictures in places in which they are most familiar. A trip somewhere new seems always exactly what our photography needed, as shooting away from home consistently inspires us to new artistic heights.

      Streets empty that are usually busy are remarkable and can evoke the sense of historical pictures from before the invention of the motorcar. Other things that are different at the moment will be queues to get into stores and the lines marked out on the floor to show how far apart we should be.

      Pretend everything is new and that you haven’t seen it before, and then you will be free to notice the leading lines, the places where one edge meets another in delightful geometric harmony, and how the ordinary things in the kitchen are transformed when the light is on or off.

      The trick here is to look slowly, and then look again. Take the time to look in detail and to look at the same thing from different angles, with different light, long lenses and wide lenses. Then move to the left a bit. You may never feel the need to leave the house again.

    • Just because we can't get out and about like we normally would, doesn’t mean we have to stop taking pictures. There’s still plenty you can do, provided you're prepared to use some imagination. Here are a few ideas to keep you shooting until normal life resumes.

      Most photographers love to shoot the unusual, and you don’t get much more unusual than These Unprecedented Times. Right now everything counts as out of the ordinary. There are a number of remarkable things about these lockdown days that are worth photographing now so we can remember them when it is all over.

      Most photographers find it hard to see interesting pictures in places in which they are most familiar. A trip somewhere new seems always exactly what our photography needed, as shooting away from home consistently inspires us to new artistic heights.

      Streets empty that are usually busy are remarkable and can evoke the sense of historical pictures from before the invention of the motorcar. Other things that are different at the moment will be queues to get into stores and the lines marked out on the floor to show how far apart we should be.

      Pretend everything is new and that you haven’t seen it before, and then you will be free to notice the leading lines, the places where one edge meets another in delightful geometric harmony, and how the ordinary things in the kitchen are transformed when the light is on or off.

      The trick here is to look slowly, and then look again. Take the time to look in detail and to look at the same thing from different angles, with different light, long lenses and wide lenses. Then move to the left a bit. You may never feel the need to leave the house again.

    • Just because we can't get out and about like we normally would, doesn’t mean we have to stop taking pictures. There’s still plenty you can do, provided you're prepared to use some imagination. Here are a few ideas to keep you shooting until normal life resumes.

      Most photographers love to shoot the unusual, and you don’t get much more unusual than These Unprecedented Times. Right now everything counts as out of the ordinary. There are a number of remarkable things about these lockdown days that are worth photographing now so we can remember them when it is all over.

      Most photographers find it hard to see interesting pictures in places in which they are most familiar. A trip somewhere new seems always exactly what our photography needed, as shooting away from home consistently inspires us to new artistic heights.

      Streets empty that are usually busy are remarkable and can evoke the sense of historical pictures from before the invention of the motorcar. Other things that are different at the moment will be queues to get into stores and the lines marked out on the floor to show how far apart we should be.

      Pretend everything is new and that you haven’t seen it before, and then you will be free to notice the leading lines, the places where one edge meets another in delightful geometric harmony, and how the ordinary things in the kitchen are transformed when the light is on or off.

      The trick here is to look slowly, and then look again. Take the time to look in detail and to look at the same thing from different angles, with different light, long lenses and wide lenses. Then move to the left a bit. You may never feel the need to leave the house again.

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