Cosmologists at Perimeter Institute seek to help pin down the constituents and history of our universe, and the rules governing its origin and evolution. Many of the most interesting clues about physics beyond the standard model (e.g., dark matter, dark energy, the matter/antimatter asymmetry, and the spectrum of primordial density perturbations], come from cosmological observations, and cosmological observations are often the best way to test or constrain a proposed modification of the laws of nature, since such observations can probe length scales, time scales, and energy scales that are beyond the reach of terrestrial laboratories.
Format results


Searching for dark energy off the beaten track
Sunny Vagnozzi University of Cambridge

The Theory, Practice, and Sociology of Physical Cosmology
James Peebles Princeton University

Cosmology and astrophysics from small scales
Shivam Pandey University of Pennsylvania

Constraining Dark Matter Interactions Throughout Cosmic History
Benjamin Wallisch Institute for Advanced Study (IAS)

Bounce Scenarios in Cosmology
16 talksCollection Number C17024Talk

Discussion Session 6
Lee Smolin Perimeter Institute for Theoretical Physics

Spinor driven cosmic bounces and their (in)stability
Shane Farnsworth Max Planck Institute for Gravitational Physics  Albert Einstein Institute (AEI)

Cosmological implications of quantum gravity proposals
Mairi Sakellariadou King's College London  Department of Mathematics

Loop quantum gravity and bounces : cosmology and black holes
Aurelien Barrau Laboratoire de Physique Subatomique et de Cosmologie de Grenoble

Loop Quantum Cosmology, NonGaussianity, and CMB anomalies
Ivan Agullo Louisiana State University (LSU)

Discussion Session 4

Latham Boyle Perimeter Institute for Theoretical Physics

Job Feldbrugge Perimeter Institute for Theoretical Physics

Neil Turok Perimeter Institute for Theoretical Physics


A Bouncing Universe approach to Fine Tuning
Stephon Alexander Brown University

Space(time) structure in models of loop quantum gravity
Martin Bojowald Pennsylvania State University


Time in Cosmology
14 talksCollection Number C16016Talk

Summaries and closing remarks

Renate Loll Radboud Universiteit Nijmegen

Adam Frank University of Rochester


Preferred Global Slicing

Chopin Soo National Cheng Kung University

Sean Gryb University of Groningen


Causality and Becoming

Fay Dowker Imperial College London

Avshalom Elitzur Israeli Institute for Advanced Research



Fundamental Time

Laurent Freidel Perimeter Institute for Theoretical Physics

Lee Smolin Perimeter Institute for Theoretical Physics

Joao Magueijo Imperial College London


Emergent Time Discussion

Carlo Rovelli Centre de Physique Théorique

Jenann Ismael Columbia University

Andreas Albrecht University of California System


Time as Organization – Downward Caustation, Structure and Complexity II

Stuart Kauffman Santa Fe Institute (SFI)

George Ellis University of Cape Town


Time as Organization – Downward Caustation, Structure and Complexity I
Barbara Drossel Technische Universität Darmstadt


Feedback over 44 Orders of Magnitude: From Gammarays to the Universe
22 talksCollection Number C16004Talk


recent advances in ParticleInCell simulations of relativistic plasmas
JeanLuc Vay Lawrence Berkeley National Laboratory



Experimental plasma physics
Frederico Fiuza Stanford University  Department of Physics

Local group simulations of galaxy formation with blazar heating
Till Sawala Durham University

TeV emission from blazars: physical models meet observations
Stefan Wagner Universität Heidelberg




The scattering transform in cosmology, or, a CNN without training
Sihao Cheng Johns Hopkins University

AGN Variability and HEAN in the age of VRO
Cyril CrequeSarbinowski Johns Hopkins University

UltraLight Dark Matter Dynamics in the Language of Eigenstates
Luna Zagorac Perimeter Institute for Theoretical Physics

Simulations of cosmic structure formation with fuzzy dark matter  Simon May, Max Planck Institute of Astrophysics
Simon May Perimeter Institute
In the fuzzy dark matter model, dark matter consists of “axionlike” ultralight scalar particles of mass around 10⁻²² eV. This candidate behaves similarly to cold dark matter on large scales, but exhibits different properties on smaller (galactic) scales due to macroscopic wave effects arising from the extremely light particles’ large de Broglie wavelengths. It has both particle physics motivations and a rich astrophysical phenomenology, giving rise to notable differences in the structures on highly nonlinear scales due to the manifestation of wave effects, which can impact a number of contentious smallscale tensions in the standard cosmological model, ΛCDM. Some of the unique features include transient wave interference patterns and granules, the presence flatdensity cores (solitons) at the centers of dark matter halos, and the formation of quantized vortices. I will present large numerical simulations of cosmic structure formation with this dark matter model – including the full nonlinear wave dynamics – using a pseudospectral method to numerically solve the Schrödinger–Poisson equations, and the significant computational challenges associated with these equations. I will discuss several observables, such as the evolution of the matter power spectrum, the fuzzy dark matter halo mass function, dark matter halo density profiles, and the question of a fuzzy dark matter core–halo mass relation, using results obtained from these simulations, and contrast them with corresponding results for the cold dark matter model.

Searching for dark energy off the beaten track
Sunny Vagnozzi University of Cambridge
Most of the efforts in searching for dark energy (DE) have focused on its gravitational signatures, and in particular on constraining its equation of state. However, there is a lot to be learned about DE by getting off the beaten track. I will first focus on nongravitational interactions of DE with visible matter, leading to the possibility of "direct detection of dark energy" (analogous to direct detection of dark matter): I will argue that such interactions can and potentially may already have been detected in experiments such as XENON1T, while discussing complementary cosmological and astrophysical signatures. I will then discuss early and latetime consistency tests of LCDM, and how these may shed light on (early and late) DE in relation to the Hubble tension. I will present two such tests based on the early ISW effect and the ages of the oldest astrophysical objects in the Universe.

The Theory, Practice, and Sociology of Physical Cosmology
James Peebles Princeton University
Sociologists have interesting things to say about the practice of natural science. I will discuss the sociological phenomenon of multiples in scientific discoveries, with examples drawn from how the ΛCDM cosmology grew, and examples of possible multiple discoveries to come from issues arising in our present welltested but certainly incomplete cosmology.

Cosmology and astrophysics from small scales
Shivam Pandey University of Pennsylvania
Complex and poorly understood astrophysics impacts our ability to constrain cosmological and astrophysical models from the large scale structure. Two major sources of systematic errors are galaxy biasing (nonlinear mapping between dark matter and galaxies) and baryonic feedback (impact of supernovae or AGN on LSS). In the first part of my talk, I will describe a hybrid perturbation theory model of galaxy biasing and show its validation at subpercent accuracy. I will then describe the cosmological constraints obtained using this model on the measurements from the first three years of observations of the Dark Energy Survey (DES). In the second part of my talk, I will describe tomographic measurements and analysis of the crosscorrelations between thermal SunyaevZel'dovich (tSZ) effect and gravitational lensing. Using data from ACT, Planck, and DES, we obtain the highest significance (20 sigma) measurements todate and use them to constrain models for the pressure profiles of halos across a wide range of halo mass and redshift. We find evidence for reduced pressure in low mass halos, consistent with predictions for the effects of increased feedback from AGN. Lastly, we also comment on application of this study to the sigma8 tension and hydrostatic mass bias as inferred from the cluster count analysis.

Constraining Dark Matter Interactions Throughout Cosmic History
Benjamin Wallisch Institute for Advanced Study (IAS)
Abstract: TBD



Feedback over 44 Orders of Magnitude: From Gammarays to the Universe
22 talksCollection Number C16004Feedback over 44 Orders of Magnitude: From Gammarays to the Universe

Aspects of Rotating Black Holes in Dynamical ChernSimons Gravity
Leah Jenks Brown University
In this talk I will give an overview of recent and ongoing work regarding rotating black holes in dynamical ChernSimons (dCS) gravity. dCS gravity is a well motivated modified theory of gravity which has been extensively studied in gravitational and cosmological contexts. I will first discuss unique geometric structures, `the ChernSimons caps,' which slowly rotating black holes in dCS gravity were recently found to possess. Motivated by the dCS caps, I will then discuss superradiance in the context of slowly rotating dCS black holes and show that there are corrections to the usual solution for a Kerr black hole. Lastly, I will comment on the observable implications for these corrections and point towards avenues for future work.
Zoom Link: https://pitp.zoom.us/j/95228483630?pwd=dWk1c3p5dUU3RXJrNEhIT2M3Tk1Kdz09

The scattering transform in cosmology, or, a CNN without training
Sihao Cheng Johns Hopkins University
Patterns and complex textures are ubiquitous in astronomical data but challenging to quantify. I will present a new powerful statistic called the “scattering transform”. It borrows ideas from convolutional neural nets (CNNs) while retaining the advantages of traditional statistics. As an example, I will show its application to weak lensing cosmology, where it outperforms classic statistics and is on a par with CNNs. I will also show interesting visual interpretations of the scattering statistics and possible extensions of this “mathematical neural network” idea. I argue that the scattering transform provides a powerful new approach in cosmology and beyond.
Related papers:
https://arxiv.org/abs/2112.01288
https://arxiv.org/abs/2103.09247
https://arxiv.org/abs/2006.08561
Zoom Link: https://pitp.zoom.us/j/91612161747?pwd=bnQrVmo4ZjBjaUdQMDBNZGhFS2NPQT09 
AGN Variability and HEAN in the age of VRO
Cyril CrequeSarbinowski Johns Hopkins University
Over the next ten years, the Vera C. Rubin Observatory (VRO) will observe ∼10 million active galactic nuclei (AGN) with a regular and high cadence. During this time, the intensities of most of these AGN will vary stochastically. Moreover, these fluctuations may also be connected to the highenergy astrophysical neutrino (HEAN) flux observed by IceCube. In this talk, I explore the prospects to quantify these fluctuations with VROmeasurements of AGN light curves and also evaluate the capacity of VRO, in tandem with various current and upcoming neutrino telescopes, to establish AGN as HEAN emitters. I find that AGN variability measurements will be so precise as to allow the AGN to be separated into up to ∼ 10 different correlationtimescale bins. I also show that if the correlation time varies as some power of the luminosity, the normalization and powerlaw index of that relation will be determined to O(10^{−4}%). Finally, I find that it may be possible to detect AGN contributions at the ~ 3\sigma level to the HEAN flux even if these AGN contribute only ~10% of the HEAN flux.

UltraLight Dark Matter Dynamics in the Language of Eigenstates
Luna Zagorac Perimeter Institute for Theoretical Physics
Selfgravitating quantum matter may exist in a wide range of cosmological and astrophysical settings: from the very early universe through to presentday boson stars. Such quantum matter arises in UltraLight Dark Matter (ULDM): an exciting axionlike particle candidate which keeps the successes of CDM on large scales but alleviates tensions on small scales. This small scale behavior is due to characteristic cores in ULDM called solitons, which also correspond to the ground state of the selfgravitating quantum system governing ULDM. We calculate the full spectrum of eigenstates and decompose simulations of ULDM into these states, allowing us to precisely track the evolution of the telltale soliton cores and the surrounding halo “skirt”. Using this formalism, we investigate formation of halos through binary soliton collisions and the dependence of the final halo product on initial parameters. We further link characteristic ULDM halo behavior—such as the soliton “breathing mode” and random walk of the center of mass—to the presence of certain modes. Finally, we comment on the relationship between eigenenergies and oscillatory timescales present in the system, as well as future directions for understanding ULDM through the language of its eigenstates.