From Quantum Gravity to Cosmology

28 Sept 2026, 09:15 → 2 Oct 2026, 21:20 Asia/Tokyo

Lecture Hall (Kavli IPMU)

Luca Marchetti (Kavli IPMU)

Description

This workshop will bring together researchers in quantum gravity and cosmology to discuss major open questions at the interface between the two fields. Although the $\Lambda$CDM model has been remarkably successful, increasingly precise cosmological observations have also brought a number of challenges and tensions into sharper focus. At the same time, substantial progress in quantum gravity, particularly in its applications to cosmology, has opened new possibilities for investigating the quantum origin of the universe and for seeking a more fundamental understanding of both the successes and the possible limitations of the standard cosmological model.

Addressing these questions requires closer interaction between communities that often work with different concepts, methods, and perspectives. The workshop will therefore bring together researchers from a broad range of approaches to quantum gravity and cosmology. To make the discussions accessible across these different backgrounds, speakers will be encouraged to begin their talks with a broad introduction before moving on to more specialized results and open problems.

The program will also leave substantial time for discussion, with the aim of encouraging active participation, the exchange of ideas, and sustained scientific interaction. More broadly, the workshop seeks to strengthen connections between the quantum gravity and cosmology communities, foster new collaborations, and contribute to a deeper understanding of the fundamental physics underlying our models of the universe.

Confirmed Invited Speakers

• Niayesh Afshordi — University of Waterloo; Perimeter Institute for Theoretical Physics
• Benjamin Bose — University of Edinburgh
• Suddhasattwa Brahma — Indian Statistical Institute, Kolkata
• Robert Brandenberger — McGill University
• Clare Burrage — University of Nottingham
• Salvatore Capozziello — University of Naples Federico II
• Bianca Dittrich — Perimeter Institute for Theoretical Physics
• Kristina Giesel — Friedrich-Alexander-Universität Erlangen-Nürnberg
• Olaf Hohm — Humboldt-Universität zu Berlin
• Antonino Marcianò — Fudan University
• Shinji Mukohyama — RESCEU and Yukawa Institute
• Ryo Namba — Shizuoka University
• Daniele Oriti — Universidad Complutense de Madrid
• Misao Sasaki — APCTP
• Asato Tsuchiya — Shizuoka University
• Yuko Urakawa — KEK Theory Center
• Victoria Venken — Kavli IPMU, The University of Tokyo
• Jun’ichi Yokoyama — Kavli IPMU, The University of Tokyo

 

Participation

Both in-person and remote participation will be possible.

Registration will remain open until 7 September 2026, and existing registrations may be modified until 13 September 2026, including for the purpose of submitting or updating an abstract.

Please note that abstract submissions for contributed talks and posters are open only to participants attending the workshop in person.

Contributed talks and poster session

The scientific program will include a limited number of contributed talks, as well as a poster session. Participants submitting an abstract for an oral presentation may also choose to have it considered for a poster if it is not selected for a talk.

Abstract submissions are open only to participants attending the workshop in person and must be made through the registration form.

 

Organizers

Scientific Organizing Committee

• Robert Brandenberger — McGill University
• Elisa Ferreira — Kavli IPMU, The University of Tokyo
• Oliver Friedrich — Ludwig-Maximillians-University Munich
• Luca Marchetti — Kavli IPMU, The University of Tokyo and OIST
• Daniele Oriti — Universidad Complutense de Madrid
• Mairi Sakellariadou — King’s College London
• Thomas Thiemann — Friedrich-Alexander-Universität Erlangen-Nürnberg
• Jun’ichi Yokoyama — Kavli IPMU, The University of Tokyo

Local Organizing Committee

• Luca Marchetti (chair)  — Kavli IPMU, The University of Tokyo and OIST
• Elisa Ferreira — Kavli IPMU, The University of Tokyo
• Jun’ichi Yokoyama — Kavli IPMU, The University of Tokyo

Monday 28 September

09:15 → 09:30 Initial Remarks

09:30 → 10:20 Invited Talks

09:30 TBD

Speaker: Jun'ichi Yokoyama

10:20 → 11:00 Coffee Break

11:00 → 12:40 Invited Talks

11:00 Black hole resonances and cosmological particle production: an exact WKB approach

Perturbations around a non-trivial spacetime background encode much of the physics we can extract from it --- the ringdown spectrum of a perturbed black hole, or particles produced in a cosmological background. In both cases the relevant mode equation is treated in the WKB approximation, whose solutions are used to impose boundary conditions on the asymptotic states that fix the quasinormal-mode spectrum or the vacuum/particle content. However, the WKB series is only asymptotic and generically divergent: it is not globally valid and its naive truncation carries an intrinsic ambiguity tied to the Stokes phenomenon. In this talk, using black hole quasinormal modes and cosmological particle production as two concrete examples, I discuss how the exact WKB analysis, the Borel resummation of the WKB series along its associated Stokes graph, removes this ambiguity and yields exact, globally valid quantization conditions. I will highlight the common structure behind both applications, illustrating how the global analytic structure of wave equations encodes physical information beyond a finite-order WKB expansion.

Speaker: Ryo Namba

11:50 Locality of soft dressings: inflationary perturbations and the infrared-finite S-matrix

Infrared divergences in inflationary correlators raise the question of what is observable during inflation. We formulate a locality condition on how the soft modes are attached to the hard sector — patch by patch on a super-horizon lattice — and show that it ensures the infrared regularity of inflationary perturbations, the soft theorems for the correlation functions, and the suppression of loop corrections. We close by commenting on the parallel with the infrared structure of QED, where the finiteness of the S-matrix is controlled by an analogous condition on the dressing of hard states.

Speaker: Yuko Urakawa

12:40 → 14:00 Lunch

14:00 → 14:10 Announcement

14:10 → 15:00 Invited Talks

14:10 TBD

Speaker: Misao Sasaki

15:00 → 15:45 Coffee Break

15:45 → 16:45 Contributed Talks

15:45 High Frequency Spectrum of Primordial Gravitational Waves

"Cosmological inflation predicts the existence of primordial gravitational wave background. Originating as quantum fluctuations of the metric during inflation, slow roll inflation models predict a nearly flat spectrum for modes that exit the Hubble horizon during the accelerated expansion epoch. The spectrum naturally extends to high frequency modes that never enter the superhorizon regime. These modes experience resonance boosts from the oscillating background during reheating and therefore carry information about the inflaton oscillations and thermal history.

We present the production mechanism, calculation methods and features of the resulting spectra which can be potentially used to distinguish inflation models. We show that modified models of gravity like the Gauss-Bonnet term corrected inflation leave a strong high frequency graviton footprint. If ever measured using future technology, this can distinguish the higher curvature models from pure General Relativity ones."

Speaker: Kamil Mudrunka

16:00 Rolling Downhill, Averaging Uphill: Nonlinear Inflation on the Lattice

Inflation is almost always described as small ripples on a fixed, smooth background --- but that picture is expected to break down when fluctuations grow large. We push into that regime with 3+1 lattice simulations in which the field and the local expansion evolve together. This lets us follow inflation into the strongly nonlinear regime relevant for primordial black holes, where the power spectrum becomes very large. I will show how differential expansion reweights the physical volume --- "volume selection" --- and can reverse the volume-averaged inflaton, and discuss what this means for perturbation theory.

Speaker: Pankaj Saha

16:15 Preheating in Einstein-Cartan Higgs inflation

In this talk, I will discuss the preheating dynamics in Higgs inflation in the Einstein-Cartan formalism, especially focusing on unitarity violation. It is known that Einstien-Cartan gravity provides a framework for a broad class of Higgs inflation models, including the metric and Palatini Higgs inflation. It smoothly connects these two limits by the coupling between Higgs and the Nieh-Yan term. The metric limit suffers from unitarity issue during preheating while the Palatini case is free from such problem. Therefore, it is expected that the Nieh-Yan coupling also controls the unitarity violation in the Einstein-Cartan framework. I will discuss how to calculate the particle production during preheating and how to determine the transition point that separates the unitarity-violating and unitarity-safe regimes in the parameter space of Einstein-Cartan Higgs inflation.

Speaker: Minxi He

16:30 Consistency conditions and gravitational-wave probes of spacetime symmetries

Spacetime-symmetry breaking provides a well-motivated avenue for probing departures from general relativity and possible low-energy signatures of quantum gravity. I will discuss theoretical consistency conditions that arise when diffeomorphism symmetry is broken, focusing on the distinction between explicit and spontaneous breaking and the associated constraints on the gravitational dynamics. I will then show how gravitational waves provide complementary observational tests of these scenarios. In particular, symmetry breaking can modify the polarization content and propagation of gravitational waves, as well as the primordial gravitational-wave spectrum. Together, these theoretical and observational considerations illustrate how gravitational-wave measurements can test both the consistency and phenomenology of modified spacetime symmetries.

Speaker: Nils A Nilsson

16:45 → 17:45 Discussion

16:45 Discussion session: Early Universe

Tuesday 29 September

09:30 → 10:20 Invited Talks

09:30 Hořava-Lifshitz Quantum Cosmology

Hořava-Lifshitz gravity is a candidate theory of quantum gravity characterized by anisotropic scaling between space and time and improved ultraviolet behavior. In this talk, I will first review the basic ideas of the theory, its current status, and its implications for cosmology. I will then discuss recent developments in quantum cosmology based on Hořava-Lifshitz gravity, highlighting novel features of the quantum universe that arise from the modified structure of gravity at high energies.

Speaker: Shinji Mukohyama

10:20 → 11:00 Coffee Break

11:00 → 12:40 Invited Talks

11:00 Primordial Signatures of the Quantum Bounce: The robustness of analytic approximations near the bounce in hybrid loop quantum cosmology

In loop quantum cosmology (LQC), the classical big bang singularity is replaced by a quantum bounce. This raises the possibility that pre-inflationary quantum gravity effects could leave observable imprints on primordial cosmological perturbations. However, extracting such signatures requires assumptions about the evolution of perturbations through the high curvature regime and their initial quantum state.

This talk examines the robustness of analytic predictions for the primordial power spectrum in hybrid LQC, focusing on the recently proposed non-oscillatory vacuum state with asymptotic Hamiltonian diagonalisation. Following a brief overview of different approaches to cosmological perturbations in LQC, the extent to which predicted primordial signatures depend on detailed modelling of the bounce will be investigated. In particular, the talk compares within the hybrid approach different approximations to the time dependent effective mass of the perturbations near the bounce, including the Pöschl-Teller potential commonly used in analytic treatments. The resulting power spectra are found to be largely insensitive to these details, provided that the characteristic scale associated with the bounce is correctly captured.

As an outlook, the possible implications of moving beyond the treatment of perturbations on an effective modified cosmological background are discussed, together with the first steps towards deriving their dynamics from an underlying 4D covariant theory of modified gravity with a homogeneous cosmological sector that reproduces the effective dynamics of LQC.

Speaker: Kristina Giesel

11:50 TBD

Speaker: Bianca Dittrich

12:40 → 14:10 Lunch

14:10 → 15:00 Invited Talks

14:10 Emergent spacetime and a quantum gravity explanation of the dark sector

Quantum gravity is expected to unravel the fundamental quantum structure of spacetime and geometry. In several approaches the very continuum notions of spacetime and geometry are in fact emergent, rather than fundamental, and so is their dynamics. This goes beyond the intuition of effective field theory approaches to quantum gravity, and opens the possibility of quantum gravity affecting also large-scale features of the universe. We report on a series of recent results in the context of the group field theory formalism, in which the fundamental quantum interactions among "atoms of space" produce, in a continuum, hydrodynamic-like approximation, an effective cosmological dynamics characterized by a late-time acceleration of a dynamical dark energy type, as well as a dark matter-like contribution.
The underlying mechanism and models, as well as the approximations leading to the effective cosmological dynamics, but also the generality and robustness of the result, will be discussed.

Speaker: Daniele Oriti

15:00 → 15:45 Coffee Break

15:45 → 17:00 Contributed Talks

15:45 Quantum Gravity Corrections: From Background Dynamics to Cosmological Perturbations

Effective quantum gravity models often specify corrections to homogeneous cosmological evolution, leaving open how these corrections extend to cosmological perturbations. I will present a framework for constructing scalar and tensor perturbations around a given modified FLRW background. Here, spatial covariance and a Birkhoff-type vacuum requirement constrain its extension to inhomogeneous geometries. For a specified matter realization and class of local actions, the resulting response to curvature and anisotropy determines the quadratic perturbation action and the corresponding Mukhanov–Sasaki equations. As an example, I will consider an effective model motivated by loop quantum gravity with a canonical inflaton and discuss its propagation properties, stability conditions, and general-relativistic limit.

Speaker: Hongguang Liu

16:00 Inflationary Attractors from Canonical Quantum Gravity

In this talk, I will revisit the inflationary quantum cosmology framework where the Ashtekar formulation of gravity minimally coupled to a scalar field admits a full non-perturbative quantum minisuperspace description of inflation. By extending prior analytic treatments, I will show how inflation can be a robust dynamical attractor across a wide range of scalar potentials, even with mild slow-roll violations. This indicates that, in this formulation, inflation naturally emerges from generic initial conditions, and the quantum evolution can consistently extend beyond the classical singularity, opening the possibility of cyclic or pre-Big Bang phases and suggesting potential resolutions of the Borde-Guth-Vilenkin theorem.

Speaker: Tatsuya Daniel

16:15 Volume as a Clock: Observers and Topology Change in Quantum Gravity

"Different notions of time arise from different choices of observer. In cosmological settings, a particularly natural choice is spacetime volume, which is conjugate to the cosmological constant. This gives rise to unimodular time, deparametrizing the Wheeler–DeWitt equation into a Schrödinger-like evolution equation. I will first explain how this construction appears in arbitrary dimensions, with particular emphasis on the four-dimensional case.

I will then use a two-dimensional toy model to quantise global Lorentzian de Sitter space, where quantum mechanically there is a region in which topology change can occur. More generally, I will explain how adding these types of observers and clocks classically encodes topology change and orientation change, and how this structure can be associated with a Morse function. If time permits, I will also comment on the holography of such observers."

Speaker: Farbod Sayyed Rassouli

16:30 Hearing the Shape of Spacetime Using Nonlinearities in Causal Set Theory

"A central goal at the interface of quantum gravity and cosmology is to characterize spacetime in purely relational, coordinate-independent terms. In early-universe cosmology, observational access to spacetime geometry is mediated not through coordinates, but through the spectral modes of quantum fields and their higher-order correlators. However, the spectrum of a linear wave operator alone generally cannot distinguish isospectral, non-isometric geometries; one cannot, in general, “hear the shape” of spacetime.
We demonstrate that local nonlinear field interactions supply the missing information to reconstruct the underlying geometry in causal set theory, a Lorentz-invariant approach to quantum gravity in which spacetime is fundamentally discrete. Starting from the spectrum of a Hermitianized Benincasa-Dowker wave operator and a local λφ⁴ interaction in its spectral basis, we extract the unitary map to the local position basis and reconstruct the underlying finite causal set up to relabeling. Rather than requiring the full N⁴ vertex, a two-leg contraction leaving only N² components suffices, as we demonstrate explicitly on a 3+1D causal set with N = 20.
Because causal order and volume determine the underlying geometry, these nonlinearities render spacetime curvature audible. Furthermore, the continuum limit of the Benincasa-Dowker operator, □ − R/2 (where R is the Ricci scalar), suggests a direct extension of this construction to continuum spacetimes. More broadly, this raises the question of how much information about microscopic spacetime structure may be encoded in the nonlinear and higher-point field correlations accessible to cosmology."

Speaker: Cristiano Sampaio

16:45 Emergent Schrödinger Symmetry in Matter-Coupled Black Hole Minisuperspaces

"A promising route toward understanding how spacetime may emerge from quantum gravity is to regard gravitational dynamics in minisuperspace as a hydrodynamic or collective limit of the underlying theory. One intriguing indication of such an emergent structure is the recurring appearance of Schrödinger symmetry in a variety of minisuperspace models. While recent work in quantum cosmology has extended this perspective to increasingly nontrivial settings, studies of static, spherically symmetric spacetimes have so far focused mainly on vacuum black-hole models. It is therefore important to ask whether Schrödinger symmetry persists once matter degrees of freedom are included.

In this talk, based on joint work with Yuki Yokokura (Kochi University of Technology), published in Phys. Rev. D 114 (2026) 024055, I will discuss static, spherically symmetric minisuperspace models coupled to matter fields. We identify three-dimensional Schrödinger symmetry in two physically distinct systems. The first consists of gravity coupled to a Maxwell field and a cosmological constant, whose classical solutions include charged Reissner–Nordström–(A)dS black holes. The second consists of gravity coupled to a massless scalar field. This case is particularly interesting because it gives rise classically to the Janis–Newman–Winicour (JNW) spacetime, which contains a naked singularity, while a massless scalar field can also serve as a relational clock in extracting classical spacetime dynamics from quantum gravity.

Interestingly, the two matter-coupled models require different choices of the lapse function and minisuperspace variables to make the Schrödinger symmetry manifest. Nevertheless, in the vacuum limit both constructions reduce to the Schwarzschild minisuperspace, where two-dimensional Schrödinger symmetry is realized in both descriptions. This provides partial evidence that the symmetry is not merely an artifact of a particular choice of lapse or minisuperspace coordinates. More broadly, our results show that Schrödinger symmetry survives the inclusion of nontrivial matter degrees of freedom, supporting its robustness as an emergent structure in gravitational minisuperspace and strengthening the possible connection between gravity and quantum hydrodynamics."

Speaker: Taishi Sano

17:00 → 18:00 Discussion

17:00 Discussion Session: Quantum Gravity

Wednesday 30 September

09:30 → 10:20 Invited Talks

09:30 Is Dark Energy Dynamical in the DESI Era?

"We investigate whether the recent DESI DR2 measurements provide or not evidences for dynamical dark energy by exploring the LambdaCDM model and its extensions. Using a comprehensive MCMC analysis with a wide range of cosmological datasets including DESI DR2 BAO, Lyman alpha data, CMBR, BBN, cosmic chronometers, and multiple Type Ia supernova compilations, we assess the statistical preference for departures from LambdaCDM. We find that neither LambdaCDM nor omega_0-omega_aCDM models reduces the sound horizon by the 7 % required to alleviate the Hubble tension. DESI DR2 consistently favors the quadrant omega_0 > -1 and omega_a < 0, indicating a preference for dynamical dark energy of the Quintom-B type at less 3 sigma level for most dataset combinations, rising to 3.8 sigma only when the DES-SN5Y supernova sample is included. Furthermore, we get neutrino mass constraints. The systematics diagnosis shows that the preference for dynamical dark energy is biased at redshift z<0.1. When these low-z SNe Ia are excluded, our analysis no longer requires a dynamical dark energy and fully restores the LambdaCDM model. The reconstructed evolution of omega(z) shows a transition from the phantom to the quintessence regime by crossing the phantom divide. Overall, DESI DR2 provides valuable new insights into dark energy but does not yet challenge completely the LambdaCDM paradigm. Forthcoming surveys,
including DESI DR3, Rubin Observatory, Euclid, Roman Space Telescope, and the Simons Observatory will be crucial for determining whether these hints of dynamical dark energy persist or are due to statistical fluctuations or residual systematics in low-redshift supernova samples.The talk is based on the review paper:
Phys.Dark Univ. 51 (2026) 102196."

Speaker: Salvatore Capozziello

10:20 → 11:00 Coffee Break

11:00 → 12:40 Invited Talks

11:00 Phantom crossing from the Standard Model and General Relativity

Suggestions of a late-time phantom crossing from DESI baryon acoustic oscillation measurements combined with cosmic microwave background and Type-Ia supernova observations have renewed interest in dynamical dark energy. We present a realization of low-redshift phantom crossing based on fermion condensation and general relativity. The mechanism arises from the interplay between a curvature-triggered fermion-condensate transition and geometric backreaction from nonlinear structure formation. The resulting cosmological evolution remains close to $\Lambda$CDM until low redshift, subsequently undergoing a smooth phantom crossing without fundamental ghost fields or modified gravity. Projecting the predicted evolution onto the CPL parametrization yields values compatible with DESI+CMB+SNIa reconstructions. For the benchmark backreaction density parameter $\Omega_{\rm BR0}= 0.0572$, we obtain $z_*\simeq0.35$, $w_0\simeq-0.76$, and $w_a\simeq-0.93$.

Speaker: Antonino Marcianò

11:50 Connecting Quantum Gravity, Effective Theory and Cosmological Observations

Connecting fundamental theories of gravity to cosmological observations remains a major challenge: predictions formulated at high energies must be translated into viable low-energy dynamics and ultimately into observable signatures. I will discuss two complementary approaches to this problem. First, I will present new cosmological constraints on a late-time dark-energy scenario motivated by Group Field Theory, illustrating how current large-scale structure and supernova observations can directly constrain parameters associated with an underlying quantum-gravity construction. Second, I will discuss how the Effective Field Theory of Dark Energy provides a more general route for exploring the space of viable low-energy, classical gravitational theories, combining theoretical stability conditions with increasingly precise cosmological observations. I will conclude with prospects for using next-generation large-scale structure data to establish a stronger connection between fundamental theory and observable cosmology.

Speaker: Bose Benjamin

12:40 → 14:10 Lunch

14:10 → 14:15 Conference Photo

14:15 → 15:00 Contributed Talks

14:15 3-form dark energy and cosmic birefringence

3-form dark energy is similar to but distinguishable from a scalar field in an expanding universe, and its tensorial structure allows for couplings that cannot be experienced by canonical scalars. We here explore the possibility that 3-form dark energy can explain the cosmic birefringence angle β ~ 0.3 degrees, as suggested by observations of the CMB polarization. We consider two EFT-inspired couplings between the 3-form and the photon, finding a dimension-4 operator is a candidate explanation, moreover predicting a photon mass around the Hubble scale. The profiles for β(z) from the model can mimic those from ALP dark energy or ALP dark matter, or be distinguishable, depending on the field configuration in the early universe.

Speaker: Tucker Manton

14:30 What can solve the Cosmological Constant Problem?

"What can solve the cosmological constant problem?

We present a modern interpretation of the problem of radiative corrections to vacuum energy from the point of view of generalised symmetries; in particular global (-1)-form symmetries associated with continuous couplings. It is generally argued that quantum gravity does not admit global symmetries. We point out that general relativity with a cosmological constant term posesses two global (-1)-form symmetries: topologically gauging one yields Henneaux-Teitelboim unimodular gravity, while gauging both leads to local vacuum-energy sequester. While the latter is known to solve the problem of radiative corrections to vacuum energy, we propose a general diagnostic for determining whether a gravitational theory admits a cosmological constant problem.
We also discuss parallels with the strong CP problem—where topological susceptibility diagnoses the problem—and illuminate related examples in 2d.

This talk is based on ongoing work with Altay Etkin, Farbod Rassouli, and Antonio Padilla."

Speaker: Benjamin Muntz

14:45 Phantom crossing and suppressed cosmic growth in a coupled Galileon dark-energy model with a scalar potential

"We construct a linearly stable scalar-field model that realizes both an
upward crossing of the dark-energy equation of state, from $w_{\rm DE}<-1$ to $w_{\rm DE}>-1$, and weakened gravitational clustering in the cold dark matter (CDM) sector. An exponential potential breaks shift symmetry and drives the background from a stable phantom phase toward the nonphantom regime, while a pure momentum-transfer interaction increases the dynamical inertia of CDM without altering its background dilution law. We derive the background and linear perturbation equations and establish the no-ghost and Laplacian-stability conditions. For perturbations deep inside the Hubble radius, where the quasi-static approximation applies, the effective gravitational coupling for CDM can fall below Newton's constant, suppressing late-time growth and small-scale matter power, while the baryonic coupling remains enhanced by Galileon braiding. A modified CLASS calculation, including the scalar-field perturbation and the full Boltzmann hierarchies, reveals large-scale signatures of transient braiding around radiation--matter equality. For the epresentative stable solutions studied here, these signatures include enhanced matter power at the lowest wavenumbers, reduced CMB temperature power over the angular multipole range $2\leq\ell\leq30$, and small shifts in the acoustic scale and the position of the first temperature peak. These results motivate a full likelihood analysis of the model."

Speaker: Masroor Pookkillath

15:00 → 15:30 Coffee Break

15:30 → 16:30 Invited Talks

15:30 The Dark Universe: from Cosmology to the Laboratory (IPMU colloquium)

We do not understand 95% of our Universe. 63% of this unknown is dark energy (or a cosmological constant), which drives the accelerated expansion of the universe and 27% is dark matter, an additional matter component which clumps together to form large halos around visible galaxies. These two dominating components of the universe have only been observed through their gravitational effects, and both represent the failure of our standard models of particle physics and gravity to explain cosmology from a fundamental physics standpoint.
In this talk I will focus on the introduction of new light scalar fields which have been suggested as possible explanations for dark matter and the accelerated expansion of the universe. I will show examples of the unusual phenomenology that can arise in such theories, and explain why properties of macroscopic objects, such as density and compactness, are important in understanding how to detect them. I’ll then show how this leads to new opportunities for precision laboratory measurements to shed light on this type of new physics.

Speaker: Clare Burrage

16:30 → 17:00 Contributed Talks

16:30 Cosmological Uncertainty Relation and Late-Universe Acceleration

The standard cosmological model lacks a microphysical origin for cosmic acceleration, while recent observations show hints of dynamical dark energy. Quantum gravitational corrections are conventionally assumed to be confined to the Planck scale, but quantum kinematic principles—much like the Heisenberg uncertainty principle—can influence dynamics across macroscopic scales. I will discuss a novel kinematic principle in minisuperspace: the size of the universe and its rate of expansion cannot be simultaneously specified with arbitrary precision. This is implemented by deforming the velocity-configuration commutation $[a,\dot{a}]$. This framework (similar to the holographic principle) implies that quantum gravity does not decouple at low energies but rather, the deformation parameter tracks the cosmological horizon scale. Cosmic acceleration therefore emerges as the macroscopic imprint of quantum gravity operating at the cosmological horizon.

Speaker: Savvas Koushiappas

16:45 New paths towards the first detection of primordial non-Gaussianities from large-scale structure

"Measuring primordial non-Gaussianities will open a new window to study quantum fluctuations and interactions during inflation. A detection of non-zero PNGs in either the cosmic microwave backgroun (CMB) or large-scale structure (LSS) however proves to be elusive. Current and upcoming LSS 3D maps will provide data with competitive and better statistical power than the CMB 2D map---provided that systematics can be kept under control.

In this talk, I will first present several new paths towards the first detection and measurement of PNGs using LSS data. These paths include either novel observables (https://arxiv.org/abs/2603.20196), new modeling methods (https://arxiv.org/abs/2607.01314) or novel statistics of LSS (in prep).

As the ongoing DESI, Euclid, LSST, SPHEREx, and the upcoming Roman, 4MOST, CSST will be all targeting PNGs as one of their primary (if not the main) science cases, it is now timely to discuss which strategies would be robust to apply to these data sets with (in many cases, very) different systematics. If time allows, I will discuss this question in the later part of my talk."

Speaker: Minh Nguyen

17:00 → 18:00 Discussion

17:00 Discussion session: Dark Sector

19:30 → 21:10 Workshop Dinner

Thursday 1 October

09:30 → 10:20 Invited Talks

09:30 Making Quantum Gravity Observable: From the Big Bang to Black Holes and Cosmic Horizons.

Quantum gravity is usually associated with distances near the Planck scale, seemingly far beyond direct experimental reach. But horizons—from black holes to the early Universe and the cosmological horizon—may provide enormous amplifiers of microscopic physics. I will begin with a pedagogical overview of how observations of these systems can turn questions about quantum gravity into falsifiable ones.

I will then focus on a concrete example in the early Universe. In quadratic gravity, asymptotic freedom at high energies and the quantum running of gravitational couplings can dynamically generate a period of slow-roll inflation. Remarkably, the same running connects a UV-complete theory of gravity to CMB observables and predicts a tensor-to-scalar ratio large enough to be tested by forthcoming experiments.

I will next discuss two complementary windows on horizon-scale quantum physics. Near black holes, quantum modifications may produce departures from the Kerr no-hair paradigm and delayed gravitational-wave echoes, providing targets for LISA, next-generation gravitational-wave detectors, and horizon-scale imaging. On the largest cosmological scales, current data allow—and mildly favour—a percent-level mismatch between the strength of gravity measured locally and cosmologically, a “cosmic glitch” that can mimic aspects of dynamical dark energy.

I will close by asking whether these apparently disparate phenomena could be telling us something common about quantum horizons, and what observations over the coming decade could do to distinguish such ideas from conventional extensions of general relativity.

Speaker: Niayesh Afshordi

10:20 → 11:00 Coffee Break

11:00 → 11:50 Invited Talks

11:00 Formalizing QFT and Quantum Gravity via Homotopy Algebras

"I will make the case for mathematical physics in general and for the need to formalize QFT and Quantum Gravity in particular, especially in light of AI. Such formalizations seem necessary in order to consolidate (and eventually computer-verify) results in formal high-energy theory and quantum gravity, which have weak community standards. I argue that a significant collection of results in high-energy theory can and should be formalized in terms of ‘homotopy-type’ techniques, and I present three examples that I am currently working on: 1) color-kinematics duality; 2) AdS mass spectra of SUGRA and cosmological perturbation theory;
and 3) QFT as factorization algebras."

Speaker: Olaf Hohm

11:50 → 12:50 Contributed Talks

11:50 Kaluza-Klein Perturbation Theory from Exceptional Field Theory

I will present a way to use Exceptional Field Theory to compute the spectrum of Kaluza-Klein modes for a set of backgrounds that can be uplifted from a 5-dimensional theory through consistent truncation. To this end, I will employ the formulation of field theory in terms of $L_{\infty}$ algebras and homotopy transfer, which maps the gauge redundant fields of supergravity to a theory with just physical modes. Thanks to this formulation it is possible to obtain the physical spectrum with a systematic procedure that allows one to untangle the non physical modes via the Higgs mechanism.

Speaker: Camilla Lavino

12:05 Gravitational-Wave Imprints of Primordial Black Hole Evaporation

At the interface between quantum gravity and cosmology, evaporating ultra-light primordial black holes (PBHs) provide a novel probe of Hawking radiation through distinctive stochastic gravitational-wave (GW) signatures. This talk will first review how induced GWs are generated via the so-called poltergeist mechanism and how these signals can probe small-scale primordial non-Gaussianity. The main focus will then be on the universal suppression of induced GWs for non-monochromatic PBH mass distributions, arising from the continuous depletion of the surviving PBH population during evaporation. The resulting high-frequency GW spectrum encodes the underlying evaporation dynamics, largely independently of the details of the initial mass function.

Speaker: Xin-Chen He

12:20 The Knitting Mechanism: From Quantum Gravity to the Emergence of Our Universe

We show how our Universe can emerge from symmetry breaking in a W_3 algebra whose components form a Jordan algebra. The underlying one-dimensional spaces with different flavors, or components, are knitted into a higher-dimensional spacetime through a knitting mechanism, with three-dimensional space emerging as part of this structure. Algebraically, the knitting mechanism is characterized by the emergence of Virasoro constraints, whose solutions give rise to the higher-dimensional space.

Speaker: Yoshiyuki Watabiki

12:35 Cosmological correlators and discrete kinematic flow on complex Hartle-Hawking saddles

The Lorentzian gravitational path integral over homogeneous and isotropic metrics in a closed universe with positive cosmological constant is dominated by complex saddle points, such as the Hartle–Hawking saddles. We ask how to compute cosmological correlators on such complex backgrounds, and how they change with respect to shifts of the kinematic variables — the kinematic flow — without tracking the bulk time evolution explicitly. We study a conformally coupled scalar test field with a cubic interaction around these saddles. We find that the wave function coefficients can be written as discrete spectral sums whose summand contains the corresponding coefficients in the Einstein static universe, which are obtainable in analogy with the cosmological polytope program for flat FLRW cosmologies: where the spatially flat case gives integrals over shifted kinematics, the discrete spectrum on the three-sphere gives sums. The coefficients depend on the final hypersurface through a phase fixed by its size, and we identify them at all orders with Srivastava–Daoust multi-variable hypergeometric functions. For the one- and two-site Feynman–Witten diagrams we derive difference equations in the kinematic variables which close on a finite set of sums, giving a discrete kinematic flow. We also discuss computation of three- and four-point correlators.

Speaker: Vikramaditya Mondal

12:50 → 14:20 Lunch Break

Friday 2 October

09:30 → 10:20 Invited Talks

09:30 Cosmology in the string theory landscape and swampland

If string theory provides a consistent theory of everything, it should make predictions about cosmology that can be checked against observation.
In spite of this, it has proved incredibly challenging to obtain any concrete testable predictions from string theory.
In the first part of this talk I will give a broad-level overview of the 'landscape' picture of string theory and how the first semi-realistic cosmologies from string theory were proposed in this way about 25 years ago today. I will discuss how in more recent years growing cracks in these constructions have been discovered and where this leaves us today: with seemingly no good controlled models in string theory with either inflation or even a positive cosmological constant.
In the second part of the talk I will discuss the 'swampland' picture, where instead one seeks to constrain effective theories at low energies by arguing that certain EFTs cannot be consistently completed into a theory of quantum gravity. As a specific example I will discuss the 'Festina Lente' bound which constrains both cosmology and the spectrum of charged particles in a theory. I will mention some upcoming results strengthening this bound.

Speaker: Viktoria Venken

10:20 → 11:00 Coffee Break

11:00 → 12:40 Invited Talks

11:00 Emergent Metric Spacetime and Early Universe Cosmology from the BFSS Matrix Model

The BFSS matrix model is a proposed non-perturbative definition of superstring theory. Starting with a thermal state of this model, I will show how a metric spacetime and early universe cosmology can emerge. Thermal fluctuations lead to scale-invariant spectra of cosmological fluctuations and gravitational waves.

Speaker: Robert Brandenberger

11:50 Emergence of (3+1)-dimensional expanding spacetime from the Lorentzian type IIB matrix model with deformations

The type IIB matrix model is a promising candidate for a nonperturbative formulation of superstring theory. In this model, the eigenvalue distribution of the bosonic matrices represents an emergent spacetime, which is determined by the nonperturbative dynamics of the model in the large-N limit. Numerical simulations are therefore indispensable for revealing the structure of the emergent spacetime. In this talk, I first review the model and the idea of emergent spacetime. I then explain the complex Langevin method, which we use to overcome the sign problem in our simulations of the Lorentzian version of the model, and a deformation of the model, which is introduced to avoid the singular-drift problem caused by the Pfaffian. This deformation is inspired by the supersymmetric deformation used to define the "polarized type IIB matrix model" in the Euclidean case. Finally, I present the results of our simulations, which show that the deformed model exhibits a phase in which a (3+1)-dimensional expanding spacetime emerges, with both space and time being smooth and real.

Speaker: Asato Tsuchiya

12:40 → 14:10 Lunch Break

14:10 → 15:00 Invited Talks

14:10 Emergent droplet geometry from multi-Matrix Quantum Mechanics

There has been a new line of development for deriving cosmological spacetimes from quantum mechanical Matrix Models. In particular, it has been shown how an expanding metric might be coarse-grained from abstract matrix degrees of freedom, and how one naturally gets a scale-invariant spectrum of primordial perturbations in this model without introducing arbitrary tunable parameters, by considering a thermal state. In this talk, I will describe how a "collective-field" formalism can be employed to derive, and explore, the emergent "droplet" geometry coming from gauged multi-matrix models, going beyond the classic example of the single matrix case, by integrating out the off-diagonal strings. Time-permitting, I will discuss perturbations around this nontrivial vacuum and their possible entanglement structure.

Speaker: Suddhasattwa Brahma

15:00 → 15:45 Coffee Break

15:45 → 17:00 Contributed Talks

15:45 Regularized Master-Field Approximation for Mass-Deformed Supersymmetric Yang–Mills Matrix Models

Matrix models offer a framework in which spacetime can emerge dynamically without assuming a fixed background geometry. Supersymmetric Yang–Mills matrix models are particularly interesting from a physical perspective, but their numerical study is hindered by the sign problem associated with the complex Pfaffian. A possible approach to this difficulty is the regularized master-field approximation, which systematically approximates the large-N theory using finite-dimensional matrices. The large-N theory is regularized by requiring a finite set of Schwinger–Dyson equations to be satisfied as accurately as possible, thereby avoiding the sign problem in principle. In this work, we apply the regularized master-field approximation to mass-deformed supersymmetric Yang–Mills matrix models and demonstrate that it enables efficient and high-precision numerical calculations.

Speaker: Reishi Maeta

16:00 Emergent Spacetime and the Arrow of Time from Non-Hermitian Effective Field Theory of Holographic Projections

In contemporary holographic cosmology, the bulk-boundary correspondence is understood via dynamic quantum error-correcting codes and quantum extremal surfaces, where isometric encodings typically guarantee strict unitariness and time-reversible mapping in Hermitian setups [1-3]. However, this conventional framework encounters a profound bottleneck when addressing non-equilibrium cosmological horizons, failing to dynamically capture the thermodynamic arrow of time and localized structural self-organization. In this work, we propose a novel paradigm where the observable FLRW spacetime emerges as a pseudo-non-Hermitian holographic projection of a fundamental collective qubit population, formalized within a Non-Hermitian Effective Field Theory (EFT) framework [4-5].

To demonstrate this mechanism, we investigate a non-Hermitian, nonlinear quantum dot dimer system subjected to a strict Population Balance Constraint Rule (sPBCR), where the nonlinear interaction arises naturally as a mean-field approximation of the underlying collective many-body state [6-7]. We show that immediately above the Exceptional Point (EP) [8-10], the effective field dynamics under this sPBCR lock the complex energy advance angle within an ultra-narrow corridor ($\theta \le 1^\circ$). This critical transition induces near-complete eigenvector alignment and collective coherence, driving an asymmetric information localization akin to the non-Hermitian skin effect. Crucially, quantum information geometry dictates that this spectral pinch-off enforces a bounded, positive entropy decay rate ($-\dot{S} > 0$), mathematically establishing a deterministic forward arrow of time characterized by macroscopic self-organization[11,12].

Extrapolating this micro-phenomenon to a cosmological scale, we interpret the Big Bang as a non-Hermitian phase transition triggered by passing through a higher-order EP. Within this framework, dark energy naturally corresponds to a cosmic-scale non-Hermitian gain that drives accelerated expansion via the fine-tuned $\theta \le 1^\circ$ spectral drive, while black hole event horizons act as edge boundaries that condense information. Our model offers a rigorous, publication-ready perspective on the origin of cosmic order, demonstrating that our universe can be consistently modeled as a self-organizing non-Hermitian quantum engine that processes fundamental bulk information into dynamic spacetime topology[13]. Extrapolating this micro-phenomenon to a cosmological scale, we interpret the Big Bang as a non-Hermitian phase transition triggered by passing through a higher-order EP [14].

Keywords: Non-Hermitian physics, Exceptional points, Holographic cosmology, Arrow of time, Non-Hermitian skin effect, Quantum dot dimers, strict Population Balance Constraint Rule (sPBCR)

arXiv Subjects: Primary: hep-th; Secondary: quant-ph, gr-qc

References:
[1] Almheiri, A., Hartman, T., Maldacena, J., Shaghoulian, E., & Tajdini, A. (2021). The entropy of Hawking radiation and the quantum extremal surface. Rev. Mod. Phys., 93(3), 035002.
[2] Harlow, D. (2018). TASI lectures on quantum error correction and AdS/CFT. Rev. Mod. Phys., 90(4), 045002.
[3] Bousso, R. (2002). The Holographic Principle. Rev. Mod. Phys., 74(3), 825.
[4] Polchinski, J. (1992). Effective Field Theory and the Fermi Surface. arXiv:hep-th/9210046.
[5] Georgi, H. (1993). Effective Field Theory. Ann. Rev. Nucl. Part. Sci., 43, 209.
[6] Weinberg, S. (1989). Testing Quantum Mechanics. Ann. Phys., 194, 336.
[7] Pitaevskii, L., & Stringari, S. (2016). Bose-Einstein Condensation. Oxford.
[8] Bender, C. M., & Boettcher, S. (1998). Phys. Rev. Lett., 80, 5243.
[9] Hatano, N., & Nelson, D. R. (1996). Phys. Rev. Lett., 77, 570.
[10] Ashida, Y., Gong, Z., & Ueda, M. (2020). Non-Hermitian Physics. Adv. Phys., 69, 249.
[11] Amari, S. I., & Nagaoka, H. (2000). Methods of Information Geometry. AMS.
[12] Brody, D. C. (2014). Biorthogonal Quantum Mechanics. J. Phys. A, 47, 035305.
[13] Breuer, H. P., & Petruccione, F. (2002). Open Quantum Systems. Oxford.
[14] Weinberg, S. (2008). Cosmology. Oxford."

Speaker: Yutaka Tamaura

16:15 From Cosmology to Local Gravity: Testing an Infrared-Completed f(Q) Model Across Scales

"Symmetric teleparallel gravity provides a geometrically distinct route to extending General Relativity, but a viable cosmological model must remain consistent across both cosmological and local gravitational regimes. We investigate an analytic infrared-completed f(Q) model designed as a controlled deformation of the symmetric teleparallel equivalent of General Relativity (STEGR).

The cosmological branch is calibrated by E(0)=1 and the present deceleration parameter q0, with no additional hidden background-level fitting freedom once the cosmological parameters are specified. We examine its viability through branch continuity, fQ>0, background regularity, present acceleration, and recovery of the matter-dominated regime. The cosmological evolution is further characterized using the jerk hierarchy and the Om(z) diagnostic and benchmarked against cosmic-chronometer H(z) measurements together with supernova- and BAO-style distance residuals relative to ΛCDM.

We then independently examine the local weak-field sector. The model recovers the Newtonian limit at O(v^2), while the resulting leading PPN envelope remains below the tested Cassini/Shapiro scale for the fiducial branch. This provides a reproducible framework for studying how modified gravitational dynamics can connect cosmological evolution with controlled recovery of General Relativity at local scales [1].

[1] D. Kaykı, L. Yıldız, and E. Güdekli, “Cosmographic Admissibility and Weak-Field Recovery in an Analytic Infrared-Completed f(Q) Gravity Model,” International Journal of Geometric Methods in Modern Physics, 2650316 (2026). https://doi.org/10.1142/S0219887826503160"

Speaker: Deha Kaykı

16:30 Post-Newtonian Analysis of Lorentz-Violating Kalb-Ramond Gravity

This thesis explores a Lorentz-violating modification of gravity through the inclusion of a massive Kalb-Ramond (KR) field coupled nonminimally to curvature. Motivated by the need to probe gravitational dynamics in ultra-low acceleration regimes where General Relativity and Newtonian gravity may be incomplete, we develop a post-Newtonian framework to study wide binary systems as natural testbeds for such deviations. Starting from a modified action that incorporates the KR field mass, self-interactions, and Lorentz-violating background terms, we derive the coupled Einstein-Kalb-Ramond field equations. We adopt a static, spherically symmetric configuration and implement a numerical scheme to solve for the gravitational potential Φ(r) and the KR field components b01(r) and b23(r) in the presence of a double-peaked Gaussian matter distribution mimicking a wide binary system. The resulting potential deviates from the Newtonian 1/r form and admits a modified profile with a Yukawa-like decay, characterized by an exponential suppression at large distances. This numerically derived potential reflects the impact of the KR field and Lorentz-violating interactions on gravitational dynamics in the weak-field, long-range regime. The analysis provides a self-consistent demonstration of how antisymmetric tensor fields can generate observable modifications to the gravitational potential without invoking dark matter.

Speaker: Mohammad Faiz Khan

16:45 Curvature-Induced Dynamical Spacetime Dimension in Extended General Relativity and Cosmology

"We investigate a covariant extension of General Relativity in which the local effective dimension of spacetime is promoted to a dynamical, curvature-induced degree of freedom on an underlying four-dimensional manifold. Deviations from four dimensions are encoded through a scalar field ε(x), defining D_eff(x)=4−ε(x), which enters the gravitational action through a dimension-dependent weight multiplying the Einstein–Hilbert term together with an associated scalar potential.

The resulting field equations can be expressed in terms of a curvature-sensitive effective potential V(ε,R), allowing the effective spacetime dimensionality to respond dynamically to the local Ricci curvature. In the limit ε→0 and v(ε)→1, the weak-field regime of General Relativity is continuously recovered. The theory therefore has a scalar–tensor-like structure, with the additional field governing effective dimensionality rather than representing an independent matter component.

We examine two benchmark applications: static, spherically symmetric configurations and a spatially flat FLRW cosmological background. In both cases, curvature-induced dimensional effects lead to controlled departures from standard General Relativity, producing modifications of compact-object mass–radius relations and small corrections to the cosmological expansion history.

This framework provides an effective relativistic setting for studying curvature-dependent spacetime dimensionality and its possible role in strong-gravity and cosmological regimes while retaining a controlled General-Relativistic limit [1].

[1] L. Yıldız, D. Kaykı, and E. Güdekli, “Curvature-Induced Dynamical Effective Spacetime Dimension in an Extension of General Relativity,” The European Physical Journal C 86, 267 (2026). https://doi.org/10.1140/epjc/s10052-026-15514-5"

Speaker: Lina Yildiz

17:00 → 18:00 Discussion

17:00 Discussion session: Challenges and Opportunities

18:00 → 18:10 Closing Remarks