OKC@15: Celebrating 15 Years of the Oskar Klein Centre

17 Oct 2023, 09:00 → 19 Oct 2023, 22:00 Europe/Stockholm

FR4 (AlbaNova Main Building)

Ariel Marcelo Goobar (Stockholm University), Jens Jasche (Stockholm University), Michael Korsmeier (Stockholm University), Timothy Linden (Stockholm University)

Tuesday, 17 October

09:00 → 10:30 Invited Plenary Talks: Session 1

Convener: Pierluca Carenza (Stockholm University)

09:00 Welcome

Speaker: Ariel Marcelo Goobar (Stockholm University)

Welcome_OKC15.pdf

09:10 History of the OKC

TBA

Speaker: Lars Bergström (Stockholm University)

OKC_LarsB_2023.pdf

09:30 Bimetric and multimetric theories of gravity

The interactions of the gravitational metric with other spin-2 fields are strongly constrained by the absence of the so called ghost instabilities in the theory. Although the study of such ghost free interactions are well motivated theoretically, their construction and even the question of their existence have been long standing problems. I will motivate the study of such theories and then describe the ghost free theories of two interacting spin-2 fields (bimetric theories) and their known extensions to multiple spin-2 fields (multimetric theories). I will also comment on some implications and on the basic geometry of spacetime in such theories.

Speaker: Fawad Hassan (Stockholm University)

Hassan-OKC15-v2.pdf

10:00 Testing General Relativity with gravitational waves

One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes, leaving behind a geometry with light rings, ergoregions and horizons. These peculiarities are responsible for uniqueness properties and energy extraction mechanisms that turn black holes into ideal laboratories of strong gravity, of particle physics (yes!) and of possible quantum-gravity effects. I will review the status of black holes, in light of gravitational-wave observations in the last few years.

Speaker: Prof. Vitor Cardoso (University of Copenhagen)

Oskar_Klein_October_2023.pdf

10:30 → 11:00 Morning Fika

11:00 → 11:30 Invited Plenary Talks: Session 2

Convener: Joel Pearson Johansson (Stockholm University)

11:00 Modelling the multi-messenger signals of neutron star mergers

Neutron star mergers are related to a large number of (astro-)physical questions. For example, they had long been suspected to produce gravitational waves, gamma ray bursts and -by some- to produce heavy "rapid neutron capture" elements. While overall convincing, all these conjectures were only based on indirect, often theoretical arguments and none was proven by direct observations. The situation changed dramatically on August 17, 2017 when the first ever neutron star merger was detected in both gravitational and electromagnetic waves. It confirmed many expectations, but also raised new questions.

Scientists at the OKC played crucial roles in this exciting endeavour and I will give my personal view of the development of multi-messenger astrophysics in the thriving environment at the Oskar Klein Centre. I will also discuss recent progress in modelling neutron star mergers and show some of our latest results.

Speaker: Stephan Rosswog (Stockholm University)

11:30 → 12:10 Contributed Talks: Session 2

Convener: Joel Pearson Johansson (Stockholm University)

11:30 Discovering fast transients and multi-messenger counterparts with the Zwicky Transient Facility

We live in a golden era for time-domain astronomy. The fourth observing run of the LIGO-Virgo-KAGRA gravitational wave detectors is in full swing and observatories around the world search for rapidly-fading kilonovae associated with binary neutron star mergers. In addition to follow-up campaigns, wide-field surveys such as the Zwicky Transient Facility (ZTF) scan most of the observable sky with a high cadence, opening a discovery space historically difficult to explore in the optical. I will present results from real-time searches for elusive fast optical transients in ZTF survey data, in close collaboration with colleagues at OKC. These results include new constraints on kilonova rates, the discovery of a population of cosmological afterglows found independently of gamma-ray triggers, and the first optical identification of a relativistic tidal disruption event.

Speaker: Igor Andreoni

Stockholm_OKC_Oct2023_Andreoni.pdf

11:50 Hurdles on the road to the promised land: When reality meets expectations post GW170817.

Two neutron stars merge somewhere in the Universe approximately every 10 to 1000 seconds, creating violent explosions potentially observable in gravitational waves and across the electromagnetic spectrum. The transformative coincident gravitational-wave and electromagnetic observations of the binary neutron star merger GW170817 gave invaluable insights into these cataclysmic collisions. And perhaps for many, created a warped perception of what to expect. In this talk, I will discuss what we learned from GW170817 and other mergers involving neutron stars seen directly in gravitational waves or indirectly through electromagnetic radiation. I will highlight how reality has altered our understanding of the formation/evolution of such binaries and the transients they produce at the end of their lives and conclude by discussing what we expect to learn in the upcoming decade.

Speaker: Nikhil Sarin (Stockholm University)

12:10 → 13:45 Lunch

13:45 → 15:10 Contributed Talks: Session 3

Convener: Steve Schulze (Stockholm University)

13:45 Multi-messenger investigation on the neutron star equation-of-state-gravity degeneracy

In spite of its fragile elegance, General Relativity (GR) is subject to scrutiny due to several factors, such as observational evidence such as "dark matter" and "dark energy", along with theoretical challenges like inevitable singularities and the enigma of black-hole information loss. In alternative gravity theories, the macroscopic properties of neutron stars (NSs) such as mass, radius, tidal deformability etc. are modified. This creates a degeneracy between equation of state (EoS) and gravity, potentially the effect of EoS can be mimicked by using a different theories of gravity. We formulate a hierarchical Bayesian framework to simultaneously infer the EoS and theories of gravity by combining multiple astrophysical observations. We find current GW detectors do not have such sensitivity to break the degeneracy. But future GW detectors ET and CE with improved design sensitivity are expected to detect louder signals and might be able to break this degeneracy.

Speaker: Bhaskar Biswas (Universität Hamburg)

OKC_15.pdf

14:05 Numerical analysis of non-thermal emission signatures of relativistic jets through RMHD simulations

Relativistic magnetized jets are common features of Active Galactic Nuclei (AGNs), that feature multi-timescale variability and a non-thermal spectrum ranging from radio to gamma-raysThese highly magnetized jets are prone to undergo several Magneto-hydrodynamic (MHD) instabilities during their propagation in space and could trigger jet radiation and particle acceleration. In this work, we study the implications of relativistic kink mode instability on the observed long-term variability in the context of the twisting in-homogeneous jet model. We found that with the evolution of the instability, the plasma column gets turbulent in nature and generates localized shocks. These shocks give rise to flaring signatures in the high-energy band which is also evident from the instantaneous flattening of the synchrotron component of the SEDs. Additionally, we will discuss the impact of such instability-driven shocks on the synchrotron polarization signatures.

Speaker: Sriyasriti Acharya (Hamburg University)

Numerical analysis of non-thermal emission signatures of relativistic jets through RMHD simulations

14:25 Transient science the past 15 years

An overview of how the scientific field of transients such as supernovae has been transformed over the past 15 years, i.e. since the start of the OKC. Focus will be on the large sky surveys iPTF and ZTF, with possible detours to neutrino and gravitational wave counterparts and maybe an outlook for LSST.

Speaker: Jesper Sollerman (Stockholm University)

OKC15sollermanpdf.pdf

14:45 Spectral Features from Pulsars and Dark Matter in the Cosmic-Ray Electron and Positron Fluxes

High-energy cosmic-ray electrons and positrons cool rapidly as they propagate through the Galaxy, due to synchrotron interactions with magnetic fields and inverse-Compton scattering interactions with photons of the interstellar radiation field. Typically, these energy losses have been modelled as a continuous process. However, inverse-Compton scattering is a stochastic process, characterised by interactions that are rare and catastrophic. In this work, we take the stochasticity of inverse-Compton scattering into account and calculate the contributions to the local electron and positron fluxes from different sources. Compared to the continuous approximation, we find significant changes: For pulsars, that produce electron-positron pairs as they spin down, the spectrum becomes significantly smoother. For TeV-scale dark matter particles, that annihilate into electrons and positrons, the signal becomes strongly enhanced around the energy corresponding to the dark matter mass. Combined, these effects significantly improve our ability to use spectral signatures in the local electron and positron spectra to search for particle dark matter at TeV energies.

Speaker: Isabelle John (Stockholm University)

OKC15_2023.pdf

15:10 → 15:40 Afternoon Fika

15:40 → 17:00 Contributed Talks: Session 4

Convener: Justin Alsing (Stockholm University)

15:40 Dark Matter (H)eats Young Planets

I will present the effect of dark matter annihilation on the formation of Jovian planets. We show that dark matter heat injections can slow or halt Kelvin-Helmholtz contraction, preventing the accretion of hydrogen and helium onto the solid core. The existence of Jupiter in our solar system can therefore be used to infer constraints on dark matter with relatively strong interaction cross sections. In the case of spin-dependent dark matter, we derive novel constraints beyond the reach of current direct detection experiments. We highlight the possibility of a positive detection using future observations by JWST, which could reveal strongly varying planet morpholoiges close to our Galactic Center.

Speaker: Juri Smirnov (University of Liverpool)

DM (H)eats Exoplanets.key

16:00 Effects of wave packet profiles on neutrino oscillations

In the context of neutrino oscillations, the wave packet representation of propagating neutrinos has emerged as an indispensable tool for deriving the transition probabilities. Conventionally in these analyses, the neutrino wave packet is assumed to be a Gaussian profile. However, it remains unclear to what extent results depend on the assumption of Gaussianity, since (i) the assumption ignores higher-order effects and (ii) there exist profiles that cannot be approximated by a Gaussian wave packet.

We study the impact of the wave packet profile on neutrino oscillations; specifically, we consider Gaussian, Lorentzian and relativistic minimum uncertainty wave packets. Of these possibilities, the relativistic minimum uncertainty wave packets are of particular interest, as they belong to a novel class of wave packets that describe semiclassical trajectories in spacetime, minimising uncertainty in velocity rather than in phase space.

Speaker: Evan Gale (University of Queensland)

Effects of wave packet profiles on neutrino oscillations

neutrino-wp-presentation.pdf

16:20 Detecting single gravitons with quantum sensing

The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental evidence of the quantization of gravity.

Detecting single gravitons with quantum sensing

Speaker: Germain Tobar (Stockholm University)

Tobar_Presentation.pdf

Wednesday, 18 October

09:00 → 10:30 Invited Plenary Talks: Session 5

Convener: Benjamin Wallisch (Stockholm University)

09:00 Resolving the complex dynamical evolution of supermassive black holes in cosmological simulations using the KETJU code

Traditional numerical simulations employing gravitational softening are unable to resolve the small-scale dynamics and gravitational wave emission from supermassive black hole binaries. Instead, the parsec-scale dynamics is typically modelled by postprocessing the simulations using either semi-analytic methods based on orbit-averaged equations or by resimulating the core regions of the merged galaxies using an altogether separate N-body code.
An alternative is to use a hybrid approach, such as the KETJU code, recently developed in our group. The KETJU code includes algorithmically regularised regions around every supermassive black hole (SMBH). This allows for simultaneously following global galactic-scale dynamical and astrophysical processes, while solving accurately the dynamics of SMBHs at sub-parsec scales. In addition, the KETJU code also includes post-Newtonian terms in the equations of motions of the SMBHs, which allows us to directly calculate the expected gravitational wave signal from the motion of the resolved SMBH binary in mergers of massive galaxies. We demonstrate here how KETJU can be used to model the dynamics of SMBHs in a full cosmological setting and thus provide more accurate predictions for the upcoming LISA gravitational wave observatory. Finally, we also present a new public version of the KETJU supermassive black hole (SMBH) dynamics module, as implemented into the GADGET-4 code.

Speaker: Peter Johansson (University of Helsinki)

Ketju_GW_SMBH_OKC15.pdf

09:30 Commander -- lessons learned from two decades of global Bayesian CMB analsis

Detailed measurements of the cosmic microwave background during the last three decades have revolutionized our understanding of the Big Bang and the evolution of structure in the universe. These advances have been made possible through a close collaboration between instrumentalists, theorists and data analysts. As the signal-to-noise ratio of available data increases, it becomes more and more important to take into account all relevant effects at the same time, and in this talk I will present one specific framework that addresses this challenge head-on through global Bayesian modelling. This already played a major role in the analysis of ESA's Planck mission, and it is likely to play an even bigger role in next-generation experiments, as the importance of systematic uncertainties continue to increase. Furthermore, the fundamental lessons learned from this line of work are likely to be of interest to a wide range of other cosmological and astrophysical experiments as their sensitivity also continue to increase.

Speaker: Hans Kristian Eriksen (University of Oslo)

Commander -- lessons learned from two decades of Bayesian CMB analysis

10:00 Metamaterial absorbers and detectors: How to hide the notable and detect the invisible.

Cunning instrument design is at the core of most successful endeavors in experimental physics. In this talk, I will describe hardware efforts at the OKC motivated by our desire to address fundamental questions in cosmology and astroparticle physics. The talk will cover some of the fun (and not-so-fun) challenges associated with designing tunable plasmonic resonators and broadband absorbers for the microwave window on the Universe.

Speaker: Jon Gudmundsson (Stockholm University)

Metamaterials absorbers and detectors

10:30 → 11:00 Morning Fika

11:00 → 12:00 Contributed Talks: Session 6

Convener: Oindrila Ghosh (Stockholm University)

11:00 The Case for Renormalizable Quantum Gravity

In the context of perturbative quantum field theory (QFT), the addition of quadratic-curvature invariants to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. However, this theory exhibits unusual features due to an additional massive spin-2 ghost particle. In this talk, we are going to argue that despite the presence of the ghost and actually thanks to it, Quadratic Gravity can provide a consistent QFT description of the gravitational interaction at a fundamental level and explain new physics beyond Einstein's general relativity, e.g., it offers a natural explanation for the inflationary phase in the early Universe.

Speaker: Luca Buoninfante (Nordita)

OKC@15_Buoninfante.pdf

11:20 Free-Streaming Neutrinos in the Early Universe

Standard Model neutrinos decoupled from the primordial plasma about one second after the big bang and have been free-streaming through the cosmos ever since. This leaves a subtle imprint in the baryon acoustic oscillations as observed in the cosmic microwave background (CMB) and large-scale structure (LSS) of the universe. Through careful theoretical considerations of the underlying physics and observables, we have been able to extract and measure the neutrino-induced phase shift in current CMB and LSS observations. In this talk, I will present the theoretical background, new data analyses and forecasts for the free-streaming nature of neutrinos, and discuss some implications for physics beyond the Standard Model.

Speaker: Benjamin Wallisch (Stockholm University)

okc-at-15_wallisch.pdf

11:40 Constraining Cosmic Reionisation by Combining the kinetic Sunyaev-Zel'dovich Effect and the 21-cm Power Spectrum

The kinetic Sunyaev-Zel'dovich effect (kSZ) is induced by the scattering of Cosmic Microwave Background (CMB) photons with a medium of free electrons that possess a non-zero bulk velocity. These scatterings alter the small-scale CMB temperature anisotropies, which correlate with the growth of ionising regions during the Epoch of Reionization (EoR). Hence, the kSZ observations can be merged with measurements of the 21-cm power spectrum from the intergalactic medium during the EoR to gain insights into the process of reionisation and the characteristics of the first stars and galaxies.

In this talk, we propose a simple yet effective parametric method that establishes a connection between the 21-cm and kSZ power spectra. Through an MCMC analysis, we discover that these two observables exhibit complementary characteristics, leading to significantly improved constraints on reionisation compared to analysing each dataset separately.

I will present how the present-day upper limits from the MWA collaboration (Trott et al. 2020), future measurements of the 21-cm power spectrum with the SKA, and the kSZ power spectrum measurement are combined to constrain models of cosmic reionisation. Our findings demonstrate that a few well-informed measurements of the 21-cm power spectrum and kSZ data can precisely determine the reionisation history of the Universe.

Speaker: Ivelin Georgiev (Stockholm University)

Ivelin_OKC_2023.pdf

12:00 → 13:40 Lunch

13:40 → 14:00 Contributed Talks: Contributed Talks 7

Convener: Milena Crnogorcevic (Stockholm University)

13:40 Quantum superpositions of spacetimes and black holes

If relativistic gravitation has a quantum description, it must be meaningful to consider a spacetime metric in a quantum superposition. But how might such a superposition be described, and how could observers detect it? I will present an operational framework for studying “superpositions of spacetimes” via indirect measurements, on particles residing in such spacetime. After presenting the general approach, I show how it can be applied to a black hole placed in a superposition of masses where the dynamics of the particle exhibits signatures of quantum-gravitational effects reminiscent of Bekenstein’s seminal conjecture concerning the quantized mass spectrum of black holes in quantum gravity. I will close with remarks on the importance of distinguishing scenarios that are genuinely quantum-gravitational, notably with reference to recent proposals to test gravitationally-induced entanglement.

Speaker: Magdalena Zych (Stockholm University)

BH superpositions-OKC.pdf

14:00 → 14:30 Invited Plenary Talks: Session 7

Convener: Milena Crnogorcevic (Stockholm University)

14:00 Avatars of Doom / A Hole-y Universe?

Metastable systems with very long lifetimes can display signs of their eventual demise long before it happens. I will discuss those avatars of doom, and ways in which they can be studied. I will also discuss how the cosmological expansion modifies the picture of instability, and the possibility of surviving indefinitely long in a locally unstable universe.

Speaker: Frank Wilzcek

14:30 → 15:10 Contributed Talks: Session 7

Convener: Milena Crnogorcevic (Stockholm University)

14:30 The present and future of gravitationally lensed supernovae

Last year, the supernova cosmology team at the Oskar Klein Centre made an exciting discovery. With the Zwicky Transient Facility, we observed a supernova which was gravitationally lensed by a massive galaxy, resulting in four images of the same object. The lensed supernova, baptised ‘SN Zwicky’, points to a poorly understood population of low-mass lens galaxies. Gravitationally lensed supernovae are incredibly rare phenomena that can give valuable insights into high-redshift SN physics, substructures in massive galaxies, and the cosmic expansion rate. Currently, the lensed supernova field is at a turning point, as we will go from a handful of present discoveries to several hundreds per year with the advance of the next generation of telescopes. In this talk, I will present the discovery story of SN Zwicky and look ahead at the prospects of detecting lensed supernovae and constraining cosmology with the upcoming Vera Rubin Observatory.

Speaker: Nikki Arendse (Stockholm University)

14:50 Strongly lensed supernovae; Discovery to cosmology in the LSST era

Strongly lensed supernovae are excellent, independent probes to measure the Hubble constant and weigh in on the Hubble tension. In my talk I will discuss the time delay measurements for the first resolved strongly lensed Type Ia supernova, iPTF16geu. I will also review spectroscopic observations of SN Zwicky, the first lensed supernova discovered by Zwicky Transient Facility, including JWST observations in the nebular phase. I will summarise ongoing results on novel time-delay inference methods - e.g., using spectroscopy - for precision cosmology. In the LSST era, we expect tens of lensed SNe per year. In this talk, I will summarise ongoing work on the impact of the survey cadence and systematics on the lensed SN discovery rate. Finally, I will show unpublished results on constraints spatial curvature with lensed transients as a novel probe.

Speaker: Suhail Dhawan (University of Cambridge)

Stockholm_OKC15_SuhailDhawan.pdf

15:10 → 15:40 Afternoon Fika

15:40 → 16:10 Invited Plenary Talks: Session 8

Convener: Metin Ata (Stockholm University)

15:40 Is Dark Matter Primordial Black Holes?

Historically the most popular dark matter candidates have been new elementary particles, such as WIMPs and axions. However Primordial Black Holes (PBHs), black holes formed from over-densities in the early Universe, are another possibility. The discovery of gravitational waves from mergers of ~10 Solar mass black hole binaries by LIGO-Virgo has generated a surge in interest in PBH dark matter. I will overview the formation of PBHs, observational probes of their abundance and the key open questions in the field.

Speaker: Prof. Anne Green (University of Nottingham)

Green-PBHs.pdf

16:10 → 17:50 Contributed Talks: Session 8

Convener: Metin Ata (Stockholm University)

16:10 Cross-correlating galaxy catalogs and gravitational waves: a tomographic approach

Unveiling the origin of the coalescing binaries detected via gravitational waves (GW) is challenging, notably if no multi-wavelength counterpart is detected. One important diagnostic tool is the coalescing binary distribution with respect to the large scale structures (LSS) of the universe, which we quantify via the cross-correlation of galaxy catalogs with GW ones. By using both existing and forthcoming galaxy catalogs and using realistic Monte Carlo simulations of GW events, we find that the cross-correlation signal should be marginally detectable in a 10-year data taking of advanced LIGO-Virgo detectors at design sensitivity, at least for binary neutron star mergers. The expected addition of KAGRA and LIGO-India to the GW detector network would allow for a firmer detection of this signal and, in combination with future cosmological surveys, would also permit the detection of cross-correlation for coalescing black holes. Such a measurement may unveil, for instance, a primordial origin of coalescing black holes. To attain this goal, we find that it is crucial to adopt a tomographic approach and to reach a sufficiently accurate localization of GW events. The depth of forthcoming surveys will be fully exploited by third generation GW detectors such as the Einstein Telescope or the Cosmic Explorer, which will allow one to perform precision studies of the coalescing black hole LSS distribution and attain rather advanced model discrimination capabilities.

Speaker: Alessandro Cuoco (University of Turin)

OKC15_Stockholm_Ott2023.pdf

16:30 Primordial Black Holes and Gravitational Waves in Higgs-$R^2$ Inflation

Primordial black holes (PBHs), produced in the early universe, can be responsible for many phenomena, ranging from dark matter to gravitational waves. In this work, we introduce a scenario where the Standard Model Higgs running induces features in the inflationary potential motivated by Higgs inflation. We identify that a near-inflection point or a tachyonic instability can develop in the potential, depending on the specific parameter ranges. We demonstrate how enhanced curvature perturbations can be produced in both scenarios and provide specific mass ranges for the produced PBHs and the corresponding second-order gravitational wave spectra. We show that each scenario can be distinguished with the gravitational wave spectrum and highlight its implication with current and future gravitational wave observatories.

Speaker: Dhong Yeon Cheong (Yonsei University & CERN)

OKC15_DY_Cheong_final_2.pdf

16:50 Non-Gaussianity in rapid-turn multi-field inflation

Primordial non-Gaussianity is a powerful tool to discriminate between models of inflation by probing the dynamics and field content of the very early Universe. In this talk I will show that theories of inflation with multiple, rapidly turning fields can generate large, potentially observable amounts of non-Gaussianity. I will discuss a novel, analytical formula for bispectrum generated from multi-field mixing on super-horizon scales for a general theory with two fields, an arbitrary field-space metric and potential. I will explain why detection of local non-Gaussianity with an amplitude of order one would rule out all attractor models of single-field inflation and discuss what such detection would tell us about multiple-field inflation.

Speaker: Oksana Iarygina (Stockholm University)

OKC_15_Oksana.pdf

17:10 Turbulent magnetic field amplification in binary neutron star mergers

The detection of a binary neutron star merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. During the merger, several mechanisms like the Kelvin-Helmholtz instability, the winding up effect and the MRI, can amplify the initial magnetic field in the remnant to be powerful enoguh for launching a jet, with an associated short GRB. When performing simulations, simplified assumptions arise for the initial magnetic field strength and topology of the merging neutron stars. Here I will show convergent results by using high-resolution, large-eddy simulations of binary neutron star mergers, following the newly formed remnant for up to 30 milliseconds. I will specifically compare simulations with different initial magnetic field strenghts and configurations, going beyond the widespread-used aligned dipole confined within each star. I will show that the magnetic field is always amplified up to ~10^16 G in the bulk region of the remnant, while the initial topology is quickly forgotten in a timescale of few miliseconds after the merger due to the Kelvin-Helmholtz instability. In the end I will show the magnetic field evolution of a long-lived remnant (100 ms after the merger).

Speaker: Ricard Aguilera Miret (Hamburg Universität)

Presentation.pdf

17:30 Is the JWST Hinting Towards a Cosmological Model Beyond the Standard?

The epoch of reionisation marks the emergence of the first galaxies that emit light into the intergalactic medium, leading to the ionisation and heating of the Universe. This phase is a forefront area in astrophysics. While our current understanding relies on theoretical models of early galaxy formation within the standard cold dark matter (CDM) cosmology, recent observations, including early galaxy data from the James Webb Space Telescope (JWST) and the thermal evolution of intergalactic gas from the Experiment to Detect the Global EoR Signature (EDGES), have raised questions about this conventional view. I will present our initial testing of models using JWST observations of early galaxies.

Speaker: Dr Sambit Kumar Giri (Stockholm University, Nordita)

Sambit_Giri.pdf

Thursday, 19 October

09:00 → 10:30 Informal Meetings and Laboratory Tours: Informal Meetings and Laboratory Tours - Session A

10:30 → 11:00 Morning Fika

11:00 → 12:00 Informal Meetings and Laboratory Tours: Informal Meetings and Laboratory Tours - Session B

12:00 → 13:40 Lunch

13:40 → 14:10 Invited Plenary Talks: Session 9

Convener: Joshua Eby (Stockholm University)

13:40 The gamma-ray sky: insights and surprises

The talk will cover some of the lessons we learned from 15 years of the observations of the Fermi LAT satellite and what we hope to learn with the upcoming Cherenkov Telescope array.

Speaker: Gabrijela Zaharijas (Center for Astrophysics and Cosmology, University of Nova Gorica)

2023-10-OKC-Zaharijas.pdf

14:10 → 15:10 Contributed Talks: Session 9

Convener: Joshua Eby (Stockholm University)

14:10 unimpeded: Universal model comparison and parameter estimation distributed over every dataset

Cosmological tensions have recieved much attention in recent years, with the community debating the significance (or lack thereof) of the difference in parameter inferences of $H_0$, $S_8$ or $\Omega_K$ between supernovae, CMB, weak lensing and BAO datasets. Bayesian methods for quantifying tensions across high-dimensional datasets have been developed to robustly determine hidden levels of tension, although these can be expensive to implement in practice due to requirements of nested sampling and MCMC runs across multiple combinations of datasets and models.

This talk will present the current status and products of a DiRAC allocation to construct a next-generation legacy archive: a once-and-for-all coverage and public distribution of nested sampling and MCMC runs across a broad variety of models and dataset combinations, packaged in a zenodo-backed downloadable system unimpeded, as well as preliminary cosmological results. I will also highlight the state-of-the art anesthetic post-processing code, suitable for post-processing chains from MultiNest, PolyChord, dynesty and ultranest, as well as MCMC samplers.

https://github.com/handley-lab/anesthetic
https://github.com/handley-lab/unimpeded

Speaker: Will Handley (University of Cambridge)

will_handley_stockholm_2023.pdf

14:30 Self-interacting gravitational atoms in the strong gravity regime

Ultralight bosons are a prominent type of axion-like dark matter. These particles may form clouds around black holes through the superradiance process. The remaining system with a boson cloud around a black hole is often called a gravitational atom due to the similarities to the electron cloud of a hydrogen atom. In this talk I will discuss one of the first non-perturbative explorations of gravitational atoms with numerical relativity. To this end, we numerically investigate free and self-interacting ultralight scalar fields around black holes in General Relativity. We focus on complex scalar fields Φ whose self-interactions are described by the quartic potential V ∝ λ|Φ|4, and ignore the black hole spin in order to disentangle the effects of self-interactions on the boson cloud. I will present our findings for self-interacting bosonic clouds around black holes and discuss technical challenges. Our results provide useful inputs for fully dynamical numerical relativity simulations and I will end the talk with an outlook for future explorations of spinning black holes and real scalar fields.

Speaker: Alexandra Wernersson (University of Amsterdam)

Gravitational_atoms-3.pdf

14:50 AMPEL: Scientific exploration in the era of high throughput astronomical observatories

The rapid development of detector technology, including these sensitive to gravitational waves and neutrinos, has brought us to the gate of an era where we will be able to observe transient events as they unfold throughout a large fraction of the Universe. The availability of these data floods requires new systems for data processing and the consistent application of modern statistical methods ("machine learning").

I will here describe AMPEL, an open source development platform for real-time data analysis. Users develop and tune complex workflows in a local development environment which can be uploaded to a computer center for large-scale live processing or shared for reproducibility, effectively introducing the "code-to-data" paradigm in astronomy. AMPEL was developed during the ZTF survey and has been a critical component in the multi-messenger programs, including in the discovery of Tidal Disruption Events associated with extragalactic neutrinos.

AMPEL will be one of the brokers for the LSST real-time alert stream, and results from the ELAsTiCC simulations show that the technology and photometric classification methods are now mature for these data rates. My final question will instead be whether "we" are ready - the largest challenge might be in how to combine individual scientific creativity with computational requirements.

Speaker: Jakob Nordin (Humboldt-Universität zu Berlin)

OKC15.pdf

15:10 → 15:40 Afternoon Fika

15:40 → 16:30 Invited Plenary Talks: Session 10

Convener: Axel Widmark (Stockholm University)

15:40 A brief history and future perspectives of ML and Data Science activities at OKC

Advanced data analysis and machine learning techniques are increasingly required to optimally exploit large experimental or observational data sets to answer fundamental physics, astrophysics, and cosmology research questions. In recent years, OKC members' coherent efforts have created a thriving community of computational physicists and data analysts, culminating in the recent establishment of Machine Learning as an interdisciplinary working group of the OKC. I will give a brief overview of the current state and successes of this endeavor, such as the construction of a high-performance computer cluster. I will further outline planned developments of data science activities at the center. Particular emphasis will be given to strengthening technology development, training, and education, and a public outreach and knowledge transfer program. A core aspect of our knowledge and technology transfer program aims to build new University-Industry links through our OKC Alumni network.

Speaker: Jens Jasche (Stockholm University)

16:00 Has JWST Discovered Dark Stars?

The James Webb Space Telescope has found more than 700 high redshift objects (z>10), far too many to be compatible with the number of galaxies predicted in LCDM. We investigate the possibility that some could instead be Dark Stars. These would have been the first stars in the Universe, made (almost entirely) of hydrogen and helium from the Big Bang, with 0.1% of the mass in the form of Dark Matter. DS would be powered by the heat from Dark Matter annihilation rather than by fusion. The relevant types of dark matter include Weakly Interacting Massive Particles (WIMPs), and Self Interacting Dark Matter (SIDM). Although dark matter constitutes only ≲0.1% of the stellar mass, this amount is sufficient to power the star for millions to billions of years. They are very bright diffuse puffy objects (10 AU in radius) and grow to be very massive. In fact, they can grow up to ten million solar masses with up to ten billion solar luminosities. We show that three of four hi-z JADES objects in JWST data could in fact be Supermassive Dark Stars: the spectra are a good match, and in the future with better spectra the detection of a HeII 1640 absorption line would be a smoking gun for a Dark Star vs a galaxy. Once the dark matter fuel runs out and the dark star dies, it may collapse to a black hole. Thus dark stars may provide seeds for as yet unexplained supermassive black holes observed at early times as well as in galaxies today.

Speaker: Katherine Freese

16:30 → 17:10 Contributed Talks: Session 10

Convener: Axel Widmark (Stockholm University)

16:30 Comprehensive constraints on the galaxy population enabled by machine-learning

This is a story about how breakthroughs in machine learning over the past 15 years have borne a paradigm shift in how to do data analysis in Astronomy, enabling impactful science that was previously out-of-reach.
The dynamics of galaxy evolution leaves its mark on the distribution of galaxy properties in the Universe. Studying the joint distribution of galaxy characteristics (mass, metallciity, dust, star-formaiton history, etc) - how they relate to eachother and how that evolves over cosmic history - therefore remains our main observational tool for constraining galaxy evolution. However, robustly measuring the joint distribution of galaxy properties in the Universe has stood as a formidable Astronomical data analysis problem for decades, for four key reasons: (1) the stellar population synthesis (SPS) models relating galaxy charateristics to their observable SEDs are too expensive to run at the scale required to analyse large samples, (2) strong degeneracies (eg dust-age-metallicity-redshift) make sampling even individual galaxy posteriors from photometric data challenging and expensive, (3) survey selection effects complicate population-level inference, making robust hierarchical inference computationally intractable, and (4) comprehensive parametric models describing the whole galaxy population have been out-or-reach. In this talk I'll show how advances in machine-learning allow us to smash through all of these barriers and derive comprehensive, robust constraints on the galaxy population from large, deep galaxy surveys.

Speaker: Justin Alsing (Stockholm University)

16:50 Spacetime Emergence in the Real World

In this talk we tackle two open and pressing questions for the academic community:

1) Is spacetime emergent and if so can this be tested in a laboratory setting?

2) Is it possible to be an active and engaged researcher after leaving Academia?

We present initial hints that both questions have a positive answer!

Speaker: Matthew Lawson (Savantic AB)

04_talk.pdf

17:30 → 21:00 Alumni Meet and Greet: Dinner Event