2024 Simons Collaboration on Learning the Universe Annual Meeting

Greg Bryan
Columbia University

Learning the Universe

The Simons Collaboration on Learning the Universe (LtU) has, over the past three years, made enormous progress towards our goal of creating an effective and robust framework to use observations of the universe to simultaneously infer fundamental cosmological properties and to develop new insights into the physics of galaxy formation. To do this, we have assembled and integrated scientists from four very different fields (and a dozen institutions) into a coherent and interactive collaboration that is accomplishing science none of us could have done alone. Having built most of the connections, theoretical foundations and software tools, we are now moving to apply them to our overarching goal. I will provide an overview of our project plan, briefly highlighting the achievements we have made toward this goal and the places where more work is required. I will describe some of the technical, organizational and sociological challenges that we have faced in this project and some of the solutions that we have discovered (as well as some of the mistakes we have made). Finally, I will describe the enormous potential that continued work in this area promises to deliver and describe the collaboration’s vision for the years ahead.
 

Lars Hernquist
Harvard University

Black Hole Seeding, Accretion and Feedback

Cosmological simulations have shown that including supermassive black holes is essential to replicate observed galaxy populations. This is especially true for massive galaxies, whose star formation is halted by black hole feedback. However, simulations have approximated these black holes due to incomplete theories on their origin, growth, and feedback processes. In Learning the Universe, we have undertaken an ambitious program to develop more physical treatments of the dynamics of supermassive black holes, how they accrete gas from their surroundings and grow, and the detailed way energy produced by accretion affects gas in and around galaxies. In this talk, I review progress made to date within the LtU collaboration on our three principal goals related to these aspects of modeling supermassive black holes in cosmological simulations and describe a fourth avenue of investigation that grew out of an external collaboration concerning the “seeding” of black holes in simulations. Going forward, we will exploit the progress to date to formulate a new subgrid model for feedback that will be used in the next generation of cosmological simulations of galaxy formation. We will use the novel approach for black hole seeding to test our models for black hole dynamics and fueling by comparing the outcomes of simulations to data from the James Webb Space Telescope. Given the degeneracy between feedback from stellar evolution and supermassive black holes, this will require close coordination with the star formation group and, ultimately, the cosmological modeling group.
 

Guilhem Lavaux
Institut d’Astrophysique de Paris / Centre National de la Recherche Scientifique

Inferring Initial Conditions

Our present observations of the universe are a direct consequence of the interaction of two ingredients: the physical laws of nature and the initial configuration of gravitational fluctuations in the primordial universe. The Learning the Universe (LtU) collaboration aims to provide insights into these two ingredients by leveraging large data and a high degree of phenomenological understanding at the small scale of these same data. Constructing physically plausible digital twins of our universe from data is the driving motivation behind the BORG working group of the LtU collaboration. At the heart of this endeavour is the Manticore project, an ambitious effort to reconstruct the initial conditions of our universe within a (6 Gpc)^3 volume at 6 Mpc resolution, utilizing gold-standard galaxy catalogues (e.g., 2M++, SDSS main and LRG, SDSS-BOSS) spanning 0 < z < 0.8. This talk will highlight significant modelling enhancements, stringent new tests and compelling early results. Beyond Manticore, we will also showcase new advances in the modelling of the initial conditions of the Local Group. The resulting data products promise to catalyze advancements in supernovae and gravitational wave cosmology, galactic physics, shear lensing and multi-wavelength cross-correlations. We expect that it will help notably at understanding and reducing cosmological tensions between data sets. Finally, the Manticore framework is designed to integrate forthcoming datasets from missions like DESI, Euclid and Roman, ensuring its relevance and impact throughout the next decade.   Eve Ostriker
Princeton University

Star Formation and Feedback

I will review the accomplishments and future plans for the LtU collaboration’s work on modeling star formation, the interstellar medium and galactic winds. We have developed, tested and deployed new subgrid models that are crucial for galaxy formation modeling in both numerical simulations and semi-analytic models. The stellar content of galaxies—the most direct observable on cosmic scales—depends on (1) the rate of star formation in galactic gas, (2) the rate of gas ejection from galaxies due to radiative, supernova and cosmic ray feedback and (3) the rate of gas accretion from the circumgalactic medium. These processes occur on scales far below the resolution of cosmological simulations, making subgrid models necessary. We use high-resolution numerical radiation magnetohydrodynamic algorithms to directly simulate the star-forming interstellar medium, which also naturally produces galactic outflows. We consider a wide range of conditions (varying gas, star and dark matter content, as well as metallicity) and use these simulations to calibrate new subgrid models for the dependence of star formation and galactic outflow rates on galaxy properties measurable at the much coarser resolution of cosmological simulations. The accuracy and robustness of our subgrid models are tested using different types of numerical simulations and through comparisons with observations where possible. We also design methods to implement multiphase winds in cosmological simulations, requiring novel treatments to follow crucial interactions between thermal phases and maintain energy conservation. We collaborate with the cosmological simulation and synthetic observation groups to deploy the models we have developed. In the past year, we have rolled out several key deliverables, with papers published or submitted. This talk will summarize our results in star formation and wind modeling, leading into work to be described by later speakers. Additionally, I will discuss our plans for the coming year, as we ramp up a new simulation framework to include additional physics features, test new subgrid model approaches for larger cosmological simulations and further explore parameter spaces revealed by recent high-redshift observations.
 

Laurence Perreault-Levasseur
Université de Montréal

Accelerated Forward Models and Robustness

Large-scale upcoming survey data will allow us to shed light on fundamental questions such as the nature of dark energy and dark matter, and the properties of neutrinos, gravity and the early universe. However, the full power of these surveys will only be reached if theoretical modelling achieves the same level of quality and accuracy as their data. Cosmological numerical simulations are key to developing such theoretical models and thus are crucial for the interpretation of these datasets. As we attempt to probe the universe with increasing precision and accuracy on increasingly larger scales, however, the computational complexity of simulations quickly becomes intractable. In this talk, I will review our collaboration’s work on developing the next generation of machine-learning empowered simulation methods to accelerate the simulation of large-scale cosmological volumes. To emulate dark-matter only evolution, we have developed a number of gravity solvers, learning from numerical training sets and enabling the advanced inference techniques employed by the collaboration. This includes an exceptionally precise learned symbolic emulator for the linear power spectrum and for the non-linear power spectrum. It also includes physics-informed machine-learning methods to learn corrected evolution equations that improve accuracy without additional computational expense. Moreover, we have developed halo and galaxy emulators, such as the Charmemulator and PineTree, as well as diffusion-based models to learn to imprint probabilistic galaxies on dark matter density fields. Finally, we have developed a suite of statistical methods to ensure the accuracy of our trained emulators that can assess the quality of the distribution of the produced simulations compared to traditional methods, and even to real data, in extremely high-dimensional spaces.
 

Rachel Somerville
Flatiron Institute

Synthetic Observations and LtU Connections

One of the paramount challenges that we face in extracting scientific insights from observational data is in making robust connections between first-principles simulations and observables. During this first phase of the Learning the Universe collaboration, we have focused on creating realistic mock galaxy surveys similar to the SDSS survey of nearby galaxies, and mock CMB skies similar to observations from the ACT survey. In this talk, I will describe the end-to-end mock galaxy survey pipeline that has been developed by the LtU team, which allows us to produce realistic SDSS-like “lightcones” including predictions for galaxy photometry. A critical part of this effort has been developing a simple but realistic model for how interstellar dust impacts the light that emanates from galaxies. I will describe radiative transfer simulations that have been used to motivate and ground our dust modeling efforts, as well as the new dust model. This will lead to recent work exploring the interplay between the dust parameters and cosmological and astrophysical parameters using our LtU Simulation Based Inference machinery. I will describe the “LtU Connections” flagship project, in which we are laying the groundwork to perform SBI with physics-grounded simulations in the space of observables. On the CMB side, we have developed multiple techniques to turn the output of fast dark-matter only simulations into thermal and kinematic Sunyaev-Zel’dovich CMB lensing signals, which will also be presented. Finally, I will end with our plans for the next phase in connecting simple theory to the observed world.
 

Volker Springel
Max Planck Institute for Astrophysics

The Promise of Next-Generation Hydrodynamic Cosmological Simulations

Cosmological hydrodynamical simulations are an indispensable and uniquely powerful tool to link fundamental parameters of cosmological theories with small-scale astrophysics, thereby allowing predictions of numerous observables far into the non-linear regime. Within the cosmological modelling group of LtU, we seek to build upon the recent successful calculation of IllustrisTNG by expanding the physical faithfulness of the numerical treatments of star formation and black hole growth, as well as their associated energetic feedback processes, and in addition, enlarging the size and statistical power of the leading cosmological models, as this is required to take full advantage of upcoming new survey data. In my talk, I will present results we recently obtained in this research program, for example in the form of the MillenniumTNG simulations, a suite of new, very large volume simulations that make substantial inroads towards interfacing galaxy formation with precision cosmology. I will also review the methodologies we currently pursue to obtain future multi-physics, multi-scale simulations that realize more reliable and thus more predictive simulations. The road towards such next generation of galaxy formation simulations is rich with challenges and opportunities and is profoundly intertwined with current technological trends, many of them we actively pursue within LtU.
 

Ben Wandelt
Institut d’Astrophysique de Paris

Implicit Likelihood Inference and Go Big

To achieve the LtU goals of transforming cosmological data analysis requires scaling simulation and analysis techniques beyond the capability of traditional methods. The implicit likelihood inference group has pioneered cutting-edge, bespoke machine learning methods for high-dimensional inference with nonlinear simulations. I will showcase our flagship “Go Big” project, where we are applying these techniques to the SDSS CMASS North Galactic Cap dataset — the largest dataset ever analyzed using simulation-based, implicit likelihood techniques. This analysis exceeds previous proofs of principle by an order of magnitude in data volume. I will detail the key innovations that enable this scaling, including our approaches that rethink the modeling of large-scale structure surveys from the ground up. These developments are priming us for the next phase of scaling our methods to present-day and next-generation surveys while including parameterizations of galaxy-scale astrophysics and survey systematics. Already, the public release of our LtU-ILI inference framework is spurring community-wide adoption of the simulation-based inference techniques pioneered by our collaborators. This is evidenced by applications ranging from the recent weak lensing analyses of KiDS-1000 data to investigations of high-redshift galaxies with JWST. Looking ahead, I will outline our recent advances extending this modeling framework to enable joint analyses of large-scale structure and CMB data, specifically targeting datasets from ACT and the upcoming Simons Observatory. This work is paving the way for comprehensive, multi-probe cosmological inference at a scale previously thought impossible.

Participation in the meeting falls into the following four categories. An individual’s participation category is communicated via their letter of invitation.

Group A – Organizers and Speakers

• Economy Class: For flights that are three hours or less to your destination, the maximum allowable class of service is Economy class.
• Premium Economy Class: For flights where the total air travel time (excluding connection time) is more than three hours and less than seven hours per segment to your destination, the maximum allowable class of service is premium economy.
• Business Class: When traveling internationally (or to Hawaii/Alaska) travelers are permitted to travel in Business Class on those segments that are seven hours or more. If the routing is over budget, a premium economy or mixed-class ticket will be booked.

Group B – Funded Participants
The foundation will arrange and pay for round-trip air or train travel to the conference as well as hotel accommodations and reimbursement of local expenses. Economy-class airfare will be booked for all flights.

Group C – Unfunded Participants
Individuals in Group C will not receive financial support, but are encouraged to enjoy all conference-hosted meals.

Group D – Remote Participants
Individuals in Group D will participate in the meeting remotely.

Air & Rail
For funded individuals, the foundation will arrange and pay for round-trip travel from their home city to the conference.

All travel and hotel arrangements must be booked through the Simons Foundation’s preferred travel agency.

Travel specifications, including preferred airline, will be accommodated provided that these specifications are reasonable and within budget.

Travel arrangements not booked through the preferred agency, including triangle trips and routing/preferred airlines outside budget, must be pre-approved by the Simons Foundation and a reimbursement quote must be obtained through the foundation’s travel agency.

All costs related to changes made to ticketed travel are to be paid for by the participant and are not reimbursable. Please contact the foundation’s travel agency for further assistance.

Personal & Rental Cars
Personal car and rental trips over 250 miles each way require prior approval from the Simons Foundation via email.

Rental cars must be pre-approved by the Simons Foundation.

The James NoMad Hotel offers valet parking. Please note there are no in-and-out privileges when using the hotel’s garage, therefore it is encouraged that participants walk or take public transportation to the Simons Foundation.

Hotel
Funded individuals who require hotel accommodations are hosted by the foundation for a maximum of three nights at Hotel AKA NoMad, arriving one day before the meeting and departing one day after the meeting.

Any additional nights are at the attendee’s own expense. To arrange accommodations, please register at the link included in your invitation.

Hotel AKA NoMad
131 Madison Avenue
New York, NY 10016
https://www.stayaka.com/hotel-aka-nomad

Overview:
Funded individuals will be reimbursed for meals and local expenses including ground transportation. Expenses should be submitted through the foundation’s online expense reimbursement platform after the meeting’s conclusion.

Expenses accrued as a result of meetings not directly related to the Simons Foundation-hosted meeting (a satellite collaboration meeting held at another institution, for example) will not be reimbursed by the Simons Foundation and should be paid by other sources.

Below are key reimbursement takeaways; a full policy will be provided with the final logistics email circulated approximately 2 weeks prior to the meeting’s start.

Meals:
The daily meal limit is $125; itemized receipts are required for expenses over $24 USD. The foundation DOES NOT provide a meal per diem and only reimburses actual meal expenses up the following amounts.

• Breakfast $20
• Lunch $30
• Dinner $75

Allowable Meal Expenses

• Meals taken on travel days (when you traveled by air or train).
• Meals not provided on a meeting day, dinner for example.
• Group dinners consisting of fellow meeting participants paid by a single person will be reimbursed up to $75 per person and the amount will count towards each individual’s $125 daily meal limit.

Unallowable Meal Expenses

• Meals taken outside those provided by the foundation (breakfast, lunch, breaks and/or dinner).
• Meals taken on days not associated with Simons Foundation-coordinated events.
• Minibar expenses.
• Meal expenses for a non-foundation guest.
• Ubers, Lyfts, taxis, etc., taken to and from restaurants in Manhattan.
• Accommodations will be made for those with mobility restrictions.

Ground Transportation:
Expenses for ground transportation will be reimbursed for travel days (i.e. traveling to/from the airport or train station) as well as subway and bus fares while in Manhattan are reimbursable.

Transportation to/from satellite meetings are not reimbursable.

Attendance
In-person participants and speakers are expected to attend all meeting days. Participants receiving hotel and travel support wishing to arrive on meeting days which conclude at 2:00 PM will be asked to attend remotely.

COVID-19 Vaccination
Individuals accessing Simons Foundation and Flatiron Institute buildings must be fully vaccinated against COVID-19.

Entry & Building Access
Upon arrival, guests will be required to show their photo ID to enter the Simons Foundation and Flatiron Institute buildings. After checking-in at the meeting reception desk, guests will be able to show their meeting name badge to re-enter the building. If you forget your name badge, you will need to provide your photo ID.

The Simons Foundation and Flatiron Institute buildings are not considered “open campuses” and meeting participants will only have access to the spaces in which the meeting will take place. All other areas are off limits without prior approval.

If you require a private space to conduct a phone call or remote meeting, please contact your meeting manager at least 48-hours ahead of time so that they may book a space for you within the foundation’s room reservation system.

Guests & Children
Meeting participants are required to give 24 hour advance notice of any guests meeting them at the Simons Foundation either before or after the meeting. Outside guests are discouraged from joining meeting activities, including meals.

With the exception of Simons Foundation and Flatiron Institute staff, ad hoc meeting participants who did not receive a meeting invitation directly from the Simons Foundation are not permitted.

Children under the age of 18 are not permitted to attend meetings at the Simons Foundation. Furthermore, the Simons Foundation does not provide childcare facilities or support of any kind. Special accommodations will be made for nursing parents.

Meeting & Policy Questions
Christina Darras
Event Manager
[email protected]

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