Gravity2026: New Frontiers in Cosmology

27 Apr 2026, 00:30 → 1 May 2026, 18:15 Asia/Seoul

IBS Science Culture Center (CTPU, IBS HQ)

Overview:

The aim of the workshop is to foster comprehensive and in-depth discussions on the rapidly expanding frontiers of gravitational physics. The program will cover a wide spectrum of topics central to cosmology, including gravitational waves, early- and late-time cosmology, and theories of modified gravity. 

This workshop is mainly organized by the IBS Center CTPU-CGA and is held on-site at the Institute for Basic Science in Daejeon, Korea. There is no registration fee. f you wish to attend, please register by 29 March 2026, 23:59 KST. We will also accept a number of contributed talks: if you wish to give a contributed talk, please register for the workshop by the above date and submit a title and abstract by 5 April 2026, 23:59 KST. The selection will be made by the organizing committee.  Please note that we are not able to support VISA applications from non-invited speakers except in special cases. 

For information about previous events, please see:

Gravity2022: Current challenges in black hole physics and cosmology

Gravity2023: Dawn of field theoretic approach

Gravity2025: New horizon of black hole physics

 

Important deadlines:

Registration: 29 March 2026, 23:59 KST

Abstract submission: 5 April 2026, 23:59 KST

 

Confirmed Invited Speakers:

Ana Achucarro (Leiden U, Netherlands)

Antonio De Felice (YITP, Japan)

Jaume Garriga (ICCUB, Spain)

Fawad Hassan (Stockholm U, Sweden)

Donghui Jeong (Penn State U, USA)

Tsutomu Kobayashi (Rikkyo U.)

Matteo Magi (IBS, Korea)

Takahiro Matsubara (KEK, Japan)

Viatcheslav Mukhanov (Munich U/KIAS, Germany)

Shinji Mukohyama (YITP/RESCEU, Japan)

Nils Albin Nilsson (IBS, Korea)

Changbom Park (KIAS, Korea)

Seong Chan Park (Yonsei U, Korea)

Sabir Ramazanov (MSU, Russia)

Mairi Sakellariadou (King’s Coll. London, UK)

Misao Sasaki (APCTP/IPMU, Korea and Japan)

Teruaki Suyama (Institute of Science Tokyo, Japan)

Gianmassimo Tasinato (Swansea U, UK)

Alexander Vikman (CEICO, Czech Republic)

 

Local Organizing Committee:

Sebastian Bahamonde, Mohammad Ali Gorji, Dong-Won Jung, Nils Albin Nilsson, Hyunbae Park, Masahide Yamaguchi

 

Scientific Advisory Committee:

Katsuki Aoki (YITP), Antonio De Felice (YITP), Francesco Di Filippo (Goethe U.), Mohammad Ali Gorji (IBS), Shinji Mukohyama (YITP/RESCEU), Naritaka Oshita (YITP), Masroor C. Pookkillath (Sogang U.), Kazufumi Takahashi (Nihon U.), Masahide Yamaguchi (IBS)

 

Contact:

For general inquiries, please use ctpu-cga-admin@ibs.re.kr

 

Please note that the organizers of Gravity2026 will not arrange accommodations for attendees except invited speakers.

Please do not respond to any messages from suspicious agencies or persons offering to arrange accommodation.

 

 

Hosted and organized by

Monday 27 April

08:00 → 09:00 Registration

09:00 → 09:20 Social: Opening remarks

09:20 → 10:20 Invited: Changbom Park - "No Evidence for a Phantom-Divide-Crossing Dark Energy"

Convener: Chair: Mohammad Ali Gorji

09:20 Changbom Park - "No Evidence for a Phantom-Divide-Crossing Dark Energy"

We apply an extended Alcock–Paczyński (AP) test to the SDSS data to constrain the dark energy equation of state with the Chevallier–Polarski–Linder (CPL) parametrization. The extended AP test method uses the full shape of redshift-space two-point correlation funcion(CF) as the standard shape in order to measure the expansion history of the universe. We calibrate the standard shape by using the cosmology-dependent nonlinear evolution of the CF shape in the Multiverse simulations. Further validation of the method and calibration of possible systematics are performed based on mock samples from the Horizon Run 4 simulation. Using the AP test alone, we constrain the flat CDM plus CPL-type dark energy model (flat wCPLCDM) to have Wm=0.289_-0.029^+0.031, w_0=-0.798_-0.102^0.192 and w_a=-0.165_-0.945^+0.610. The result does not show evidence for a dynamically evolving dark energy model. When combined with other results from the low-redshift universe, such as the PantheonPlus supernova compilation and DESI BAO data, the constraint on w_a becomes w_a=-0.124_-0.368^+0.334, which is again consistent with zero.

10:20 → 10:50 Coffee break

10:50 → 11:50 Invited: Takahiro Matsubara - "Tensor Fields and Integrated Perturbation Theory"

Convener: Chair: Swagat Mishra

10:50 Takahiko Matsubara "Tensor Fields and Integrated Perturbation Theory"

I will discuss a general framework for correlation analyses of cosmic structures based on nonlinear perturbation theory. In particular, I will outline a new methodology for studying the spatial correlations of astronomical objects described by general tensor quantities, including scalars, vectors and higher-order tensors. The method is based on irreducible representations of the two- and three-dimensional rotation groups, and shows how rotationally invariant observables can be constructed and predicted using perturbation theory.

11:50 → 12:30 Invited: Nils A. Nilsson - "Testing spacetime symmetries with current and future gravitational-wave observations"

Convener: Chair: Swagat Mishra

11:50 Nils A. Nilsson - "Testing spacetime symmetries with current and future gravitational-wave observations"

Gravitational waves are now standard tools in modern cosmology, including as probes of fundamental physics. In this talk, I will describe recent results in the efforts to test the symmetries of general relativity using gravitational-waves. Motivated by quantum-gravity phenomenology, I will describe the effective-field theory approach to breaking symmetries and how they can be tested, and I will show state-of-the-art constraints from gravitational-wave data as well as forecasts for future observations. Finally, I will comment on constraints on specific quantum-gravity candidate theories.

12:30 → 14:00 Lunch break

14:00 → 15:00 Invited: Teruaki Suyama - "Graviton Floor"

Convener: Chair: Hyunbae Park

14:00 Graviton Floor

It has been observed that the Universe is permeated by the cosmic photon background, ranging from radio waves to gamma rays. We investigate the conversion of the photon background into gravitons in the presence of background magnetic fields in blazar jets. This graviton background constitutes a graviton floor for high-frequency gravitational wave detectors searching for new physics, analogous to the neutrino floor.

15:00 → 16:00 Invited: Donghui Jeong - "PowerFull: Modeling wide-angle relativistic power spectrum with the TAM formalism"

Convener: Chair: Hyunbae Park

15:00 Donghui Jeong - "PowerFull: Modeling wide-angle relativistic power spectrum with the TAM formalism"

The upcoming generation of wide and deep galaxy surveys, such as SPHEREx and Euclid, will provide a unique opportunity to probe the ultra-large scales of structure formation, where primordial non-Gaussianity, often parameterized by fNL, is expected to leave its most detectable imprint. At the same time, these surveys inevitably enter regimes where relativistic and wide-angle effects become significant, requiring careful modeling to extract unbiased cosmological information. We address these challenges by applying the total-angular-momentum (TAM) formalism to describe redshift-space clustering beyond the flat-sky approximation. The standard Fourier-mode description, which characterizes distortions by the angle between a mode and a line of sight, becomes ill-defined over wide fields, whereas the TAM basis naturally separates radial and angular contributions and incorporates the relevant relativistic effects. Within this framework, we provide a new parameterization of wide-angle predictions that can be compared directly with survey data. We then introduce PowerFull, a modification of the Julia-based 2-F AST package, which enables efficient computation of the full relativistic angular power spectrum. Finally, using Fisher information matrixbased analyses, we show that neglecting these effects biases parameter estimation, and that achieving O(1) precision on fNL with surveys like SPHEREx requires a consistent relativistic and wide-angle treatment of galaxy clustering.

16:00 → 16:30 Coffee break

16:30 → 17:10 Contributed: Leonid Gurvits - "Multi-messaging prelude: precursors of gravitational wave emitters at the milliarcsecond scale"

Convener: Chair: Hyunbae Park

16:30 Leonid Gurvits - "Multi-messaging prelude: precursors of gravitational wave emitters at the milliarcsecond scale"

Formation of super-massive black hole binaries (SMBHB) is deemed to be inevitable in various cosmological models. Their search poses one of the most challenging problems of modern observational astrophysics. Dissipation of kinetic energy in SMBHB controls the evolution of these objects and leads to coalescing into a single black hole. This process (often called “inspiralling”) is accompanied by increasingly intensive emission of gravitational waves (GW) and ends with the final GW burst (a “chirp”). While the first direct detection of GW made by the LIGO and Virgo collaboration in 2015 dealt with coalescence of stellar-mass black holes, recent results by multiple Pulsar Timing Arrays (PTA) increased attention to the SMBHB population as a likely source of the GW background.

SMBHB objects remain rather elusive: at present, there are only several dozens of candidates of which just a handful can be treated as certain cases. Direct detections of the components of SMBHB at the sub-parsec scales remain beyond reach for today’s observing techniques at all domains of the electromagnetic spectrum.

Recently several AGNs distinguished by oscillating astrometric positions at the milliarcsecond angular scale with periods of several years attracted our attention as SMBHB candidates. Our study does not allow us to “see” directly the components of possible SMBHBs. But we see a “smoking gun” of orbital motion in these potential SMBHBs. Several examples of such the oscillating behaviour are detected with VLBI astrometry. We analyse the evolution of these binary systems leading to coalescence and associated with this GW outburst.

The estimates presented in this work provide inputs into design studies of future mm/sub-mm VLBI systems with spaceborne radio telescopes. Such the systems will allow us to resolve images of binary SMBHBs at the microarcsecond angular scales, principally unachievable with the Earth-based observational facilities.

17:30 → 19:30 Social: Conference reception

Tuesday 28 April

09:00 → 10:00 Invited: Mairi Sakellariadou - "Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA's O1-O4a runs"

Convener: Chair: Shinji Mukohyama

09:00 Mairi Sakellariadou - "Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA's O1-O4a runs"

I will summarise the most recent LVK results searching for a gravitational-wave background of cosmological origin.

10:00 → 10:30 Coffee break

10:30 → 11:30 Invited: Ana Achucarro - "Ultra-slow-turn inflation"

Convener: Chair: Yusuke Yamada

10:30 Ana Achucarro - "Ultra-slow-turn inflation"

Multifield inflation models involving ultra-light fields such as moduli and axions are a generic outcome of string and supergravity cosmology. In this talk we consider under what conditions their primordial perturbation spectra may be consistent with observations on the largest (CMB) scales. The usual route is to stabilise the light fields so as to recover single-field inflation. Another possibility is if massless, frozen isocurvature perturbations can source adiabatic perturbations efficiently over many e-folds. I will discuss a third option, that has emerged more recently, in which the isocurvature perturbations are tachyonic (unstable) but the instability is shut off by an exponentially decaying turning rate. We call this regime “ultra-slow-turn” (by analogy with the ultra-slow-roll regime where the first slow roll parameter decays exponentially).

[Based on arxiv 2603.03204 with P. Christodoulidis, J-O. Gong and O. Iarygina.]

11:30 → 12:30 Invited: Jaume Garriga - "Primordial black holes and baby universes from ultra-slow roll"

Convener: Chair: Yusuke Yamada

11:30 Primordial black holes and baby universes from ultra-slow roll

A number of inflationary scenarios seeding PBH from adiabatic perturbations also lead to the production of localized eternally inflating relics, which in turn also lead to the formation of PBH once the ambient universe thermalizes. Here, we compare the relative contribution from both channels, focusing on the case of a transient ultra-slow roll (USR) phase followed by a sharp transition to slow roll. Special attention is given to the distribution of initial conditions for high peaks, including the relevant mean profiles and shape dispersion around them. The profiles are numerically evolved to determine the thresholds for both channels. Our analysis essentially validates the logarithmic template for non-Gaussianity, whose divergence signals the onset of eternal inflation. Observational consequences are briefly discussed.

12:30 → 14:00 Lunch break

14:00 → 14:40 Contributed: Bum-Hoon Lee - "Cosmic Gravitational Microwave Backgrounds and the Einstein-dilaton-Gauss-Bonnet Cosmology"

Convener: Chair: Ivan Rybak

14:00 Bum-Hoon Lee - "Cosmic Gravitational Microwave Backgrounds and the Einstein-dilaton-Gauss-Bonnet Cosmology"

After the motivation for frameworks beyond Einstein's gravity and the ΛCDM paradigm, we explain the Cosmology of the Einstein-dilaton-Gauss-Bonnet (EdGB) gravity, one of the simplest extensions through the inclusion of higher-curvature terms. Particular attention is given to the emergence of a novel phase within dEGB cosmology at high temperatures, supplementing the established radiation, matter, and dark energy-dominated phases characteristic of the ΛCDM model. Out of the many implication of EdGB cosmology, the main focus is on the Cosmic Gravitational Microwave Background (CGMB) from the hot plasmas consisting of the standard model particles in the early universe whose peak frequency is close to 100GHz. We point out that CGMB can be enhanced up to the visible range for the detectors near future with the present technology. This should lead either to the new discovery of the Ultra High Frequency gravitational waves of CGMB or the constraints to the EdGB theory if not.

14:00 → 14:40 Contributed: Vicharit Yingcharoenrat - "Large n-point functions in resonant inflation"

Convener: Chair: Yuhang Zhu

14:00 Vicharit Yingcharoenrat - "Large n-point functions in resonant inflation"

In this talk, I will first review the derivation of the unitarity bound in a single-field inflationary model with small and rapid oscillatory potential. I will show that this revised unitarity cutoff is somewhat higher than the naïve cutoff, 4 \pi f. Using this new cutoff, in contrast to the standard scenario where most of the observable information is encoded in the power spectrum, in this regime the oscillatory signal predominantly appears in higher-order correlation functions with large n. Finally, I will identify a phenomenologically relevant window in which n-point functions with 3 < n < 9 are potentially observable.

14:40 → 15:20 Contributed: Daiki Saito, Yi-Zen Chu

Convener: Chair: Yuhang Zhu

14:40 Daiki Saito - "Long-wavelength perturbations around an anisotropic universe"

We extend the gradient-expansion (separate-universe) approach to anisotropic inflationary backgrounds driven by a scalar field coupled to a U(1) gauge field. By choosing an appropriate gauge, we establish a correspondence between linear perturbations and background variations, where the background geometry is given by Bianchi type I spacetimes. We identify a conserved quantity W that generalizes the Wronskian for adiabatic modes. Using this, we show that the comoving curvature perturbation does not freeze on superhorizon scales — it retains memory of the anisotropic phase even after the universe becomes isotropic. Furthermore, we derive a direct relation between the spatial curvature and a gravitational-wave mode, revealing how scalar perturbations can source tensor modes in the presence of anisotropy.

15:00 Yi-Zen Chu - "What Constitutes A Gravitational Wave In An Expanding Universe?"

Our understanding of gravitational waves produced by isolated astrophysical systems is primarily based on gravitational perturbation theory off a flat spacetime background. This leads to the common identification of gravitational radiation with massless spin-2 waves. In this talk, I will argue that gravitational waves may no longer be solely "spin-2" in character once the background spacetime is our expanding universe instead. As a result of the mixing between gravitational and other degrees of freedom, scalar "spin-0" gravitational waves may exist during the radiation-dominated epoch of our universe; as well as during its current accelerated expansion phase -- provided the main driver is not the cosmological constant, but some extra "Dark Energy" field. Moreover, during the radiation-dominated era, spin-0 Cherenkov gravitational waves may even be generated if its material source were traveling faster than 1/\sqrt{3}.

14:40 → 15:20 Contributed: Swagat Saurav Mishra, Toshiki Takadera

Convener: Chair: Ivan Rybak

14:40 Swagat Saurav Mishra - "Dynamical Dark Energy from the Braneworld"

Recent high-precision observations from the DESI collaboration, combined with CMB and supernova data, suggest that DE might be dynamical in nature. Remarkably, the data indicate that the DE equation of state (EoS) may have transitioned from a phantom-like regime (w < −1) in the past to a quintessence-like one (w > −1) today, implying a recent crossing of the phantom divide at w = −1. In this work, we examine a broad class of thawing/decaying scalar-field models -- including quadratic, quartic, exponential, symmetry-breaking, and axion potentials -- evolving within a ghost-free braneworld, where our (3+1)-dimensional Universe is embedded in a (4+1)-dimensional bulk spacetime. Such models arise naturally in higher-dimensional frameworks, including string theory. We show that they can successfully reproduce the observed phantom-divide crossing, yielding an evolution of the Hubble parameter and an effective DE equation of state that closely matches the recent DESI DR2 observations. Our MCMC analysis further demonstrates that these models provide a statistical fit comparable to the widely used CPL parametrization, underscoring their excellent agreement with current data.

15:00 Toshiki Takadera - "Halos and voids in the EFT of dark energy"

The large-scale structure of the Universe is one of the most promising probes of the nature of gravity. In this talk, we focus on halos and voids, and study their evolution and abundance in Horndeski and DHOST theories. Using the Effective Field Theory (EFT) of dark energy framework, we investigate the impact of individual EFT parameters on the formation and abundance of these structures. We further derive constraints on the EFT parameters from halo formation, based on the requirement that real solutions for scalar field fluctuations exist at all times.

15:20 → 15:50 Coffee break

15:50 → 16:30 Contributed: Perseas Christodoulidis, S Mahesh Chandran

Convener: Chair: Rinku Maji

15:50 Perseas Christodoulidis - "Dissipative EFT of inflation"

We construct a gravitational open extension of the effective field theory of inflation in the Schwinger-Keldysh framework. While physical symmetries allow many open operators in the Schwinger-Keldysh action, most of them overconstrain the equations of motion, yielding inconsistent dynamics. We identify the minimal open operators compatible with propagating scalar and tensor modes and build the gravitational action, recovering dissipative models of inflation.

16:10 Mahesh Chandran - "Inflation-collapse duality breaking in a Planck-scale sensitive cosmological quantum simulator"

We propose the experimental simulation of cosmological perturbations governed by a Planck-scale induced Lorentz violating dispersion, aimed at distinguishing between early-universe models that generate a scale-invariant power spectrum. Employing a novel variant of the scaling approach for the evolution of a Bose-Einstein condensate with both contact and dipolar interactions, we capture the hitherto unobserved phenomenon of trans-Planckian damping. We show that Wands’ duality is subsequently broken via a spectral tilt at small scales for inflation, and at large scales for the initial collapse phase of bounce. With this, we demonstrate how cold-atom experiments can help isolate potential low-energy signatures of Planck-scale physics in the power spectrum, and in turn, help reinterpret current observations of the Cosmic Microwave Background. [Refs: https://doi.org/10.1140/epjc/s10052-025-15187-6]

15:50 → 16:10 Contributed: Zahra Davari

Convener: Chair: Boris Kamenetskaia

15:50 Zahra Davari - "From Tomographic Alcock–Paczyński Measurements to Physical Quintessence Models"

We present a likelihood-guided framework to interpret tomographic Alcock--Paczy\'{n}ski (AP) measurements in physically motivated quintessence dark-energy models. Rather than constraining scalar-field potentials directly from data, we map external constraints on the Chevallier--Polarski--Linder (CPL) parametrization—derived from tomographic AP measurements, Type~Ia supernovae, and DESI baryon acoustic oscillation data—onto quintessence parameter spaces. For each scalar-field realization, we solve the background dynamics and fit the equation-of-state history with an effective CPL form, $(w_0^{\rm eff}, w_a^{\rm eff})$. Statistical weights are assigned using an externally reconstructed likelihood in $(\Omega_m, w_0, w_a)$, enabling efficient identification of viable models. Applying this method to several representative potentials, we find that statistically favored models cluster around $w_0 \simeq -0.9$ with mild redshift evolution, consistent with tomographic AP constraints.

16:10 → 17:10 Discussion/Networking

16:30 → 17:10 Discussion/Networking

Wednesday 29 April

09:00 → 10:00 Invited: Misao Sasaki - "Primordial Black Holes and Induced Gravitational Waves"

Convener: Chair: Yusuke Yamada

09:00 Primordial Black Holes and Induced Gravitational Waves

Primordial Black Holes (PBHs) have become one of the most exciting
topics in cosmology.
They are presumably formed from rare, large curvature perturbations on
scales that can't be accessed by CMB and large scale structure
observations.
If PBHs exist, they will lead to many interesting observational phenomena.
Among them are the induced gravitational waves (IGWs) that may be
detectable in various frequencies.
In this talk, I will first review the IGW phenomenology in the PBH scenario.
Then I will focus on the IGWs from the PBH isocurvature perturbation,
and discuss their predicted features.

10:00 → 10:30 Coffee break

10:30 → 11:30 Invited: Sabir Ramazanov - "Cosmic domain walls: theory and lattice observations"

Convener: Chair: Antonio De Felice

10:30 Sabir Ramazanov - "Cosmic domain walls: theory and lattice observations"

I will report results of lattice simulations for cosmic domain walls. The main focus will be on the spectra of gravitational waves produced by these topological defects. There are various types of spectra to be discussed, which originate from different types of domain walls: long-living domain walls, annihilating ones, as well as melting walls characterized by a time-dependent tension. On the theoretical side, I will focus on the scaling of domain walls, i.e., the fact that they evolve in a self-similar manner, and their curvature radius remains of the order of the horizon size. This property is likely to be related to the intense particle emission by domain walls, and I will discuss the possible mechanism underlying such emission.

11:30 → 12:30 Invited: Gianmassimo Tasinato - " Gravitational Waves Probes of Fundamental Physics: Theory Perspective"

Convener: Chair: Antonio De Felice

11:30 Gianmassimo Tasinato - "Gravitational Waves Probes of Fundamental Physics: Theory Perspective"

12:30 → 12:40 Group photo (Location: Main Hall)

12:40 → 14:00 Lunch break

14:00 → 14:40 Contributed: Ido Ben-Dayan - "Suppressed Intrinsic Curvature Gravity"

Convener: Chair: Georg Trenkler

14:00 Ido Ben-Dayan - "Suppressed Intrinsic Curvature Gravity"

We consider various mechanisms of modifying the effect of intrinsic curvature in gravity with respect to general relativity. Two primary approaches are studied. First, by considering a Lagrange multiplier or an auxiliary field. Second, by non-minimal coupling between a scalar field and the intrinsic curvature scalar. We reproduce the basic solutions of FLRW cosmology, black hole solutions, Lense-Thirring effect, and gravitational waves. The speed of gravitational waves is modified in comparison to the speed of light. A certain limit of our theory corresponds to the lowest-order Carroll gravity. Hence, our theory is a different UV completion of Carroll gravity, compared to the usual expansion of a small speed of light. Carroll gravity limit also has an enhanced symmetry, making the reduced or vanishing extrinsic curvature technically natural. Although the intrinsic curvature and its modification may depend on the foliation, we use the covariant formalism and show that such a foliation always exists. Perhaps the most important outcome is that in the non-minimal coupling approach, the intrinsic curvature appearing in Einstein's field equations can defer from the one appearing in the geodesic equation potentially leading to important observational consequences.

14:00 → 14:40 Contributed: Konstantinos F. Dialektopoulos - "Primordial black holes as cosmic expansion accelerators"

Convener: Chair: Fumio Uchida

14:00 Konstantinos F. Dialektopoulos - "Primordial black holes as cosmic expansion accelerators"

I will present a novel mechanism for cosmic acceleration driven by primordial black holes with effective repulsive behavior. Using a new Swiss Cheese cosmological framework, I will discuss four black hole spacetimes—Hayward, Bardeen, Dymnikova, and de Sitter-Schwarzschild—to show that this acceleration emerges naturally from the geometry itself. The results suggest that ultra-light PBHs could drive inflation without requiring an inflaton, while PBHs with larger masses and moderate abundances, slightly before matter-radiation equality, can produce a substantial amount of early dark energy, helping to alleviate the Hubble tension.

14:40 → 15:20 Contributed: Javier Moreno, Eve Dones

Convener: Chair: Georg Trenkler

14:40 Javier Moreno - "Regular black holes in pure AdS"

It has been recently established in Phys. Lett. B861 (2025) 139260 that asymptotically flat black holes exist in pure gravity theories in dimension greater than four. We extend this result to asymptotically five dimensional Anti-de Sitter spacetimes and perform bottom-up holographic explorations: Hawking-Page phase transitions, quasinormal modes of black holes providing decay times of excitations in the finite temperature conformal field theory and U-shaped string probes leading to the quark-antiquark potential in the dual field theory. The gravitational action contains a bared, negative cosmological constant and an Einstein-Hilbert term, supplemented by an infinite series of higher-curvature terms of the family known as Quasi-topological (QT) Lagrangians, leading to a lapse function in terms of infinite series which can be resumed to an analytic function for different coupling choices. The QT terms are defined as those satisfying a Birkhoff’s theorem and possessing a holographic c-function, and exist at all orders in the curvature, for arbitrary dimension greater than four. Besides presenting the general formulae, we explore the physics of three specific cases leading to Hayward, Dymnikova and Bardeen-like regular black holes. We also found the field redefinition that permits recasting the α′3′3W44 correction of Type IIB supergravity on A5A5​ × S55 as a series of QT terms. As anticipated in JHEP11 (2019) 062, due to the presence of a bared AdS radius L, the perturbative field redefinition renormalizes Newton’s constant by an additive term of the form α′3′3L−6−6R and induces lower order terms of the form α′3L−4R2α′3L−4R2 and α′3L−2R3α′3L−2R3, both of which can be simultaneously written as QT terms, without spoiling the structure of the highest curvature term of the form α′3R4α′3R4. Before finishing, beyond holography, we also provide an extended thermodynamic setup, that via variations of the dimensionful α and L, allow obtaining the Euler relation between finite thermodynamical quantities for the black hole of the theory.

15:00 Eve Dones - "Radiation-reaction effects at 3PN order in scalar-tensor theories"

With the advent of third-generation gravitational-wave detectors, it has become crucial to extend analytical frameworks used to model waveforms from compact binary systems to alternative theories of gravity. In this talk, I will present recent results on radiation-reaction effects in a class of scalar–tensor theories. Using the post-Newtonian multipolar post-Minkowskian (PN–MPM) formalism, we compute the dissipative contributions to the ten Noetherian conserved quantities, which can be classified into two types: Schott and pseudo-Schott terms. In contrast to general relativity, where only pseudo-Schott terms contribute to the waveform phasing at 4.5PN order for quasi-circular orbits, we find that Schott terms begin to contribute already at 3PN order, even in the quasi-circular case. These contributions must therefore be consistently included in the future when extending the energy flux and waveform, currently known up to 2.5PN order, to 3PN order.

14:40 → 15:20 Contributed: Pritha Bari, Tomotaka Kuroda

Convener: Chair: Fumio Uchida

14:40 Pritha Bari - "Signatures of Primordial Gravitational Waves on the Large-Scale Structure of the Universe"

Primordial gravitational waves (GWs), beyond their direct detection prospects, can induce second-order scalar perturbations. These tensor-induced scalar modes evolve similarly to standard matter perturbations and leave distinct imprints on the large-scale structure (LSS). In this talk, I will present a detailed study of these effects, including analytical results for the evolution of induced density contrasts across radiation- and matter-dominated eras, and the role of GWs energy density fluctuations as a source. I will also discuss how the resulting non-Gaussianity reflects the nature of the primordial GWs spectrum, ranging from scale-invariant to sharply peaked models. These findings suggest a novel observational avenue to probe primordial GWs using galaxy surveys and LSS data, opening a complementary window to the early Universe beyond traditional GWs detectors.

15:00 Tomotaka Kuroda - "Insights for non-Markovianity in the stochastic formalism"

The stochastic formalism can be understood as an effective field theory for IR modes, which incorporates quantum corrections from UV modes at the coarse-graining scale. As the name suggests, the dynamics are described by Langevin equations. In general, the system is non-Markovian, meaning it depends on past history by construction in stochastic inflation. In this talk, I will present and discuss several approaches to analyzing the system, including the Markovian approximation and full numerical computations that account for non-Markovian effects.

15:20 → 15:50 Coffee Break

15:50 → 16:30 Contributed: Rinku Maji, Michiru Uwabo-Niibo

Convener: Chair: Hao Jiao

15:50 Rinku Maji - "Cosmological Implications of Unification: Topological Structures and Gravitational Waves"

Grand unified theory (GUT) provides a rationale for the arbitrariness of the Standard Model (SM) and explains many enigmas of nature at the outset of a single gauge group. The GUTs predict the proton decay and the spontaneous symmetry breaking (SSB) of the higher symmetry group may lead to the formation of topological defects, which are indispensable in the context of cosmological observations. Topological defects such as magnetic monopoles, cosmic strings, and their composite structures arise naturally in various GUTs through symmetry-breaking phase transitions in the early Universe. I will discuss the formation of string and its various composite structures in SO(10) GUTs. We will discuss the realizations of 'metastable' strings (MSS) and 'quasistable' strings (QSS) in SO(10) symmetry breakings. We will explore the stochastic gravitational wave background emitted from such networks of composite structures. The gravitational waves emitted from MSS and QSS with superheavy (GUT scale) strings can explain the recent exciting evidence of the stochastic gravitational waves in the NANOGrav and other pulsar timing array data.

16:10 Michiru Uwabo-Niibo - "Cosmic Neutrino Background Detection with Tritium"

The Cosmic Neutrino Background (CNB) is a robust thermal relic of the Big Bang and a potential probe of neutrino mass properties and of the Universe at O(1) second. A leading direct-detection strategy is neutrino capture on tritium, where observing a distinct capture peak requires excellent effective energy resolution. In practice, binding and solid-state effects can broaden the endpoint spectrum, potentially pushing experiments into a background-dominated regime with significant modeling uncertainties. In this ongoing work, we perform a unified sensitivity study for (i) an energy-only endpoint analysis and (ii) a joint energy–angle analysis that exploits the CNB dipole anisotropy. Using profile-likelihood methods with nuisance parameters, we quantify the exposure and systematic-control requirements for discovery. The energy-only approach can hit a systematic “floor,” where increasing exposure no longer improves significance, while an angular analysis can cancel leading normalization systematics and provide a complementary handle on backgrounds.

15:50 → 16:30 Contributed: Valentina Danieli, Dong Ha Lee

Convener: Chair: Pavel Petrov

15:50 Valentina Danieli - "Dark graviton sensing with magnetically levitated superconductors"

Magnetically levitated superconductors provide a promising platform for detecting ultra-light dark matter through precision force measurements. In this talk, I will discuss our study of spin-2 dark matter—the dark graviton—and its effects on such systems via both matter and electromagnetic couplings. The former induces a tidal, strain-like force similar to a continuous gravitational wave, while the latter generates an effective current that produces an oscillating magnetic field. We model the system as a driven harmonic oscillator and derive the expected signals across the dHz–kHz frequency range. While the sensitivity to matter coupling is not competitive with existing bounds, the electromagnetic coupling offers a compelling opportunity. In particular, levitated superconductors could become leading probes of dark graviton interactions with photons at low frequencies, provided noise sources are well controlled.

16:10 Dong Ha Lee - "A QCD vacuum motivated model of dynamical dark energy"

The nature of dark energy has been a growing point of debate in recent years, particularly after the DESI measurements of the Baryon Acoustic Oscillations. While frequentist metrics appear to indicate a growing preference for a dynamical dark energy, some bayesian approaches indicate otherwise. Beyond this, there also lies the question of whether there exists a physical motivation behind any phenomenological parametrisation of dynamical dark energy. The model described in this talk is motivated from computations of the non-perturbative QCD vacuum, which we parametrise into a dynamic, non-local contribution to the energy density, described by two additional cosmological parameters. I will describe how this model compares against both CPL and ΛCDM on the latest cosmological datasets, employing both frequentist χ² statistics and Bayesian model comparison. For the latter, the evidence is estimated from MCMC chains via the learnt harmonic mean estimator method which bypasses the need for the computationally expensive, nested sampling to obtain estimates of the evidence. This marks one of the first use cases of this method in cosmological bayesian model comparison.

16:30 → 17:10 Free discussion

17:30 → 19:00 Social: Banquet (on the third floor of the workshop venue)

Thursday 30 April

09:00 → 10:00 Invited: Viatcheslav Mukhanov - "Regularized Quantum Instantons"

Convener: Chair: Sebastian Bahamonde

10:00 → 10:30 Coffee break

10:30 → 11:30 Invited: Seong Chan Park - "Superheavy Dark Matter: theory and experiment"

Convener: Chair: Dong-Won Jung

10:30 Seong Chan Park - "Superheavy Dark Matter: theory and experiment"

I will discuss the current status of dark matter physics and suggest new territory of superheavy regime.

11:30 → 12:30 Invited: Alexander Vikman - "Stable Quantum Ghost"

Convener: Chair: Dong-Won Jung

11:30 Alexander Vikman - "Stable Quantum Ghost"

I will discuss stable systems with ghosts and how they can even become more stable upon quantization. This stability is due to the existence of a positive definite integral of motion. I will show how to canonically quantize a classically stable system of a harmonic oscillator polynomially coupled to a ghost with an unbounded-below kinetic energy. I will prove that i) the integral of motion has a positive discrete spectrum, ii) the Hamiltonian has a point spectrum unbounded in both directions, iii) the evolution is manifestly unitary, iv) the vacuum is well-defined, and v) the squared canonical variable expectation values remain bounded. Numerical solutions of the Schrödinger equation confirm these results. I will argue that the discrete spectrum of the integral of motion enforces stability for extended interactions.

12:30 → 14:00 Lunch break

14:00 → 14:40 Contributed: Jibril Ben Achour - "Jet launching and acceleration of charged particles in the Kerr magnetosphere: An electrogeodesic approach"

Convener: Chair: Hui-Yu Zhu

14:00 Jibril Ben Achour - "Jet launching and acceleration of charged particles in the Kerr magnetosphere: An electrogeodesic approach"

The launch of relativistic jets of plasma on astrophysical to cosmological scales is observed in a variety of astrophysical sources. While these jets can be reproduced by general relativistic magneto-hydrodynamics (GRMHD) and particle-in-cells (GRPIC) simulations of the dynamical Kerr magnetosphere, the development of analytic models to describe the physics of the jets has remained limited.

In this talk, I will review a first attempt to analytically study the motion of charged particles in the magnetosphere of the Kerr black hole. I will first review several key notions for the extraction of energy from a rotating black hole: (i) the amount of accessible spinning energy one can extract from a Kerr black hole, (ii) the variety of models to extract energy, and (iii) the basic of the Blandford-Znajek model and its limitations. Then, I will review under which conditions one can solve the motion of charged particles in a magnetized Kerr black hole which is the ket target of an analytic kinetic approach. I will use this opportunity to review (i) the integrability of the geodesic motion (for neutral particles) in the Kerr geometry and (ii) under which condition this result can be extended to the motion of charged particles for specific magnetosphere models. Based on this review, I will present a model where the motion of charged particles and their ejection can be studied fully analytically. This will allows us to show that (i) the radial acceleration is triggered by both the rotation of the black hole and its magnetization, (ii) the existence of a new magnetic frame-dragging effect dominating the standard Lense-Thirring effect, and (iii) that at a given spin, there is a critical magnetization threshold allowing charged particles to be accelerated (i.e. observed as blueshifted for an asymptotic observer) and finally (iv), that these particles can be accelerated only in a specific region whose maximal radius depends on the spin and magnetization of the black hole. Possible generalizations to more complex magnetosphere black hole models, and to neutrons stars will be discussed.

14:00 → 14:40 Contributed: Pavel Petrov, Che-Yu Chen

Convener: Chair: Theodoros Nakas

14:00 Pavel Petrov - "Nonsingular Cosmologies in Horndeski Gravity"

We investigate a range of non-singular cosmological scenarios within the framework of Horndeski gravity. In particular, we construct a bouncing Universe model and propose a minimal setup that realizes a non-pathological Genesis scenario. Both constructions allow for a fully stable transition to the kination epoch, during which General Relativity (GR) is restored. These scenarios successfully evade the no-go theorem, almost at the expense of potential strong-coupling issues in the early phase. The requirement of strong-coupling avoidance imposes strict constraints on the model parameters and, in certain cases, forbids the generation of a red-tilted scalar spectrum. Furthermore, we explore an alternative scenario in which the Genesis phase precedes a period of Starobinsky inflation. We show that the corrections from the Genesis phase can naturally account for the recent ACT observations. Based on: 2207.04071, 2503.02626, 2509.04832, 2601.14008

14:20 Che-Yu Chen - "Light in the darkness: Inner shadow to probe quantum gravity"

Understanding the possible observational features of black holes in quantum theory is a promising avenue to bridge the theoretical formulation of quantum gravity and astrophysical observations. In this talk, I will consider models of compact objects in the presence of horizon-scale quantum modifications, in which the objects have no horizon in the usual sense. Focusing on the shadow images of these objects, I will discuss possible observational signatures of these horizon-scale modifications. In particular, it turns out that the absence of a horizon, under certain assumptions on the surrounding emission profiles, universally results in an excess intensity within the inner shadow of the images in a model-dependent manner. The excess intensity within the inner shadow could be a model-independent image feature to probe horizon-scale quantum effects.

14:40 → 15:20 Contributed: Mang Hei Gordon Lee, Calvin Chen

Convener: Chair: Theodoros Nakas

14:40 Mang Hei Gordon Lee - "Propagator positivity for cosmological correlators"

I will discuss how to construct positivity bounds for cosmological correlators

15:00 Calvin Chen - "Holomorphic structure of massive scalar fields in (A)dS_2"

It is well-known that conformal transformations and closely related to holomorphicity in two dimensions. In particular, holomorphic operators such as the stress tensor in a conformal field theory can be used to show holomorphic splitting of the classical phase space of solutions. In this talk, I will argue this is also a feature of certain massive scalar fields in (A)dS_2 with integer scaling dimensions \Delta = k+1, marked by the existence of a pair of (anti-)holomorphic higher-spin currents. We explore the consequences of this to describe their quantisation and subsets of their linear and non-linear symmetries. In particular, we point out that the theories admit mode expansions reminiscent of standard 2d CFTs in complex coordinates, with which we are able to construct operators implementing global conformal transformations and Virasoro symmetry. We further leverage holomorphicity of the currents to show that the full set of symmetries of theories with k>0 is captured by a chiral algebra, contained in the one of the massless theory. This allows us to identify integrable deformations for k \leq 2.

14:40 → 15:20 Contributed: Stephen Angus, Masroor C. Pookkillath

Convener: Chair: Hui-Yu Zhu

14:40 Stephen Angus - "Clustering dynamics in O(D,D) string cosmology from double field theory"

The low-energy limit of string theory contains additional gravitational degrees of freedom, which together with the metric are naturally embedded in the O(D,D)-symmetric framework of double field theory (DFT). Extending this O(D,D) symmetry to additional matter fixes a modified minimal coupling in string frame, while the resulting DFT energy-momentum tensor is enhanced to include dilaton pressure and skew-symmetric contributions. First I will review DFT cosmology and present some analytic solutions, including candidate bouncing cosmologies. Following this I will discuss perturbations, and I will explore the possibilities for structure formation and consistency with clustering observations.

15:00 Masroor C. Pookkillath - "Effective field theory of coupled dark energy and dark matter"

We formulate an effective field theory (EFT) of coupled dark energy (DE) and dark matter (DM) interacting through energy and momentum transfers. In the DE sector, we exploit the EFT of vector-tensor theories with the presence of a preferred time direction on the cosmological background. This prescription allows one to accommodate shift-symmetric and non-shift-symmetric scalar-tensor theories by taking a particular weak coupling limit, with and without consistency conditions respectively. We deal with the DM sector as a non-relativistic perfect fluid, which can be described by a system of three scalar fields. By choosing a unitary gauge in which the perturbations in the DE and DM sectors are eaten by the metric, we incorporate the leading-order operators that characterize the energy and momentum transfers besides those present in the conventional EFT of vector-tensor and scalar-tensor theories and the non-relativistic perfect fluid. We express the second-order action of scalar perturbations in real space in terms of time- and scale-dependent dimensionless EFT parameters and derive the linear perturbation equations of motion by taking into account additional matter (baryons, radiation). In the small-scale limit, we obtain conditions for the absence of both ghosts and Laplacian instabilities and discuss how they are affected by the DE-DM interactions. We also compute the effective DM gravitational coupling Geff​ by using a quasi-static approximation for perturbations deep inside the DE sound horizon and show that the existence of momentum and energy transfers allow a possibility to realize Geff​ smaller than in the uncoupled case at low redshift.

15:20 → 15:50 Coffee break

15:50 → 16:30 Contributed: Hugo Roussille - "A new integrable parametrization for deformations to the Kerr photon ring"

Convener: Chair: Eve Dones

15:50 Hugo Roussille - "A new integrable parametrization for deformations to the Kerr photon ring"

The first direct electromagnetic observation of a black hole (BH) was done by the EHT collaboration in 2019, triggering the possibility to test General Relativity (GR) in its strong field regime by probing the curvature of spacetime close to BHs. The observed image contains in particular the "critical curve", which is the imprint of the photons emitted by the accretion disk and trapped around the BH for several orbits. Measuring this critical curve and comparing it to the GR prediction constitutes a new kind of test of this theory: it is therefore crucial to compute this observable for beyond-GR BHs. In this work, we compute analytically such a critical curve for the "Kerr off-shell" family of spacetimes which describes a broad class of rotating BHs beyond the Kerr solution. This family of solutions is the most general extension of Kerr that preserves its "Killing tower" of symmetries, making the geodesic motion integrable. We study several concrete examples, studying in particular for the first time solutions with polar deformations with respect to Kerr. We conclude on the feasibility of a test of GR using future critical curve observations.

15:50 → 17:10 Contributed: Minxi He, Joakim Flinckman, Zi-Yu Tang, Yusuke Manita

Convener: Chair: Masroor Pookkillath

15:50 Minxi He - "Unitarity of single scalar field with non-minimal coupling"

A non-minimally coupled scalar field with quartic self-coupling is widely studied in the literature but the estimation of the unitarity violation scale has not been consistently discussed, especially in the Jordan frame. In this talk, I will show how to consistently calculate the cutoff scale in both the Jordan frame and the Einstein frame, emphasizing the importance of potential of the scalar field. In particular, I will discuss the six-point scattering amplitudes and explicitly show the consistency of the results in both frames as expected.

16:10 Joakim Flinckman - "Gravity beyond a single metric: Ghost-free interactions of spin-2 fields"

The fundamental interactions of nature are formulated in terms of fields classified by mass and spin. Two of the most successful theories, General Relativity (GR) and the Standard Model (SM), can be derived from fields of fixed mass and spin by imposing theoretical consistency conditions. The SM contains well-understood interactions for fields with spin s<2, whereas local interacting theories with s>2 face fundamental obstacles, placing s=2 in a special position. Spin-2 fields are intrinsically linked to gravity, yet theories of interacting spin-2 fields remain comparatively unexplored. Lovelock’s theorem essentially establishes GR as the unique theory for a single massless spin-2 field, so it is natural to ask whether GR is part of a larger structure, as electromagnetism was later understood to sit within electroweak theory. This may be relevant to open questions in gravitational physics, such as dark matter, dark energy, the Hubble tension, and, ultimately, quantum gravity. However, theories of interacting spin-2 fields are notoriously plagued by ghosts—pathological fields with negative kinetic energy—and requiring their absence severely restricts the allowed interactions. In this talk, I will discuss multi-gravity theories, containing multiple interacting spin-2 fields, focusing on their theoretical consistency via the absence of ghosts.

16:30 Zi-Yu Tang - "A photon cloud induced from an axion cloud"

It is known that the axion-photon coupling can lead to quantum stimulated emission of photons and classic exponential amplification of electromagnetic (EM) fields at half the axion mass frequency, when the axion density or the coupling constant is sufficiently large. In this work, we studied the EM photon cloud induced from an axion cloud around a Kerr black hole in the first order of the coupling constant classically. In the presence of a static EM background (such as the extended Wald solution motivated by astrophysical environments), we found that an EM photon cloud emerges, oscillating at the same frequency as the axion cloud and growing exponentially in accordance with the axion cloud when the superradiant condition for the axion field is satisfied. The evolution of the EM photon cloud with time and azimuthal angle is obtained analytically while the cross-sectional distribution is solved numerically. The induced EM field exhibits symmetries that are markedly different from those of the background EM field. Consequently, the induced photon cloud forms an unstable bound configuration that emits EM waves to spatial infinity while being replenished by the axion cloud, providing a potential observational signature of both the presence of an axion cloud and axion-photon coupling.

16:50 Yusuke Manita - "Agnostic mode decomposition and nonlinear resonance in black hole ringdowns"

Quasi-normal modes (QNMs) govern the ringdown of perturbed black holes and encode key information about the remnant's mass, spin, and strong-field geometry. However, their non-orthogonality makes unambiguous mode decomposition a persistent challenge. In the first part of this talk, I present two agnostic, data-driven methods — the Matrix Pencil Method (MPM) and AAA rational approximation — that extract QNM frequencies and amplitudes directly from time-domain waveforms without prior assumptions about which modes are present. Visualized in the complex frequency plane, these tools give an immediate overview of the mode content, achieve high resolution capable of resolving closely-spaced overtones, and are sensitive enough to detect Price-law tails. Crucially, the extracted amplitudes remain stable under variation of the analysis time window, making them reliable for precision spectroscopy. In the second part, I discuss nonlinear QNM resonances in Kerr black holes. While linear perturbation theory already reveals a near-resonance between distinct overtone branches, the nonlinear sector is richer: at third perturbative order, quasi-normal modes incident on the black hole source new modes through Absorption-Induced Mode Excitation (AIME). I show that the first overtone of the dominant AIME mode satisfies a near-resonance condition with the quadratic source frequency, leading to a significant amplification of its amplitude. Although the resonant contributions partially cancel when summed, the residual imprint on the waveform morphology is observable — with potential implications for ringdown modeling and gravitational-wave data analysis.

Friday 1 May

09:00 → 10:00 Invited: Fawad Hassan - "Ghost-free theories of multiple spin 2 fields: bimetric and multimetric interactions"

Convener: Chair: Nils Albin Nilsson

10:00 → 10:30 Coffee break

10:30 → 11:30 Invited: Antonio De Felice - "Dark Energy in Light of Current Observations and Modified Gravity"

Convener: Chair: Sebastian Bahamonde

10:30 Antonio De Felice - "Dark Energy in Light of Current Observations and Modified Gravity"

Current cosmological observations have established the accelerated expansion of the Universe, yet the physical nature of dark energy remains elusive. We summarize the current status of cosmology as inferred from observational data and discuss the implications for the nature of dark energy. We review the main observational probes, highlight existing tensions between datasets within the standard ΛCDM framework, and present theoretical and experimental efforts aimed at addressing these challenges.

11:30 → 12:30 Invited: Tsutomu Kobayashi - "Spinoptics in the presence of axion-like particles in curved spacetime"

Convener: Chair: Sebastian Bahamonde

11:30 Tsutomu Kobayashi - "Spinoptics in the presence of axion-like particles in curved spacetime"

We study the propagation of high-frequency electromagnetic waves in curved spacetime in the presence of axion-like scalar fields. We employ the spinoptics approximation and extend the covariant effective action approach recently developed by Frolov to incorporate axion–photon coupling. The resulting spinoptics equations are solved in the background of a Schwarzschild black hole dressed with an axion field, with particular emphasis on the appropriate construction of a complex null tetrad associated with null rays in the presence of the axion. We show that axions affect photon trajectories differently from gravitational fields.

12:30 → 14:00 Lunch break

14:00 → 14:40 Invited: Matteo Magi - "Poincaré Gauge Gravity with Extra Massive Spin-2 in Cosmology"

Convener: Chair: Nils Albin Nilsson

14:40 → 15:10 Coffee break

15:10 → 16:10 Invited: Shinji Mukohyama - "Emergent time and big-bang"

Convener: Chair: Mohammad Ali Gorji

15:10 Shinji Mukohyama - "Emergent time and big-bang"

We proposes a new avenue for understanding the cosmological singularity. The standard cosmological model contains a generic initial singularity usually referred to as the big bang. Herein, we present a novel idea to extend the description of our Universe beyond this limit. The proposal relies on rewriting physics in a purely Riemannian, i.e. locally Euclidean, 4-dimensional space and the emergence of Lorentzian patches owing to the interaction of all matter fields to a clock field that is responsible for a signature change. If our universe is contained within one of these patches, the initial singularity is replaced by a smooth boundary on which the signature of the physical metric flips. In this talk, we first define the model and draw the necessary conditions on its arbitrary functions for solutions to exist. Next, we prove the existence of solutions that lead to an emergent universe with a primordial (almost) de Sitter phase. To finish, we discuss the consequences of this construction for the universe on scales much larger than our observable universe: a large ``Euclidean sea'' in which Lorentzian islands locally emerge and host an expanding universe potentially similar to ours. While speculative, this scenario has specific features that can be tested, and this talk sets the basis for further phenomenological investigations.

16:10 → 17:00 Social: Closing discussion/farewell