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3rd Nordic Cosmology Meeting
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Europe/Tallinn
Tallinn, Estonia
Tallinn, Estonia
Kohtu 6/1, 10130 Tallinn
Hardi Veermäe
(National Institute of Chemical Physics and Biophysics (EE)),
Ville Vaskonen
Description
The Third Nordic Cosmology Meeting will take place from the 23rd to the 25th of September in Tallinn.
The meeting will bring together researchers from the Nordic and Baltic regions to discuss recent progress in cosmology. Following the previous events held in Stockholm (Oct 2023) and Helsinki (Mar 2025), the meeting features invited and contributed talks across a broad range of topics and aims to bring people together, encourage collaboration, and strengthen connections within the community.
Invited Speakers
- Mark Hindmarsh (University of Helsinki, Finland)
- Oksana Iarygina (Nordita, Sweden)
- Carlo Marzo (KBFI, Estonia)
- Edvard Mörtsell (Stockholm University, Sweden)
- Kai Schmidt-Hoberg (DESY and University of Southern Denmark)
- Elmo Tempel (Tartu Observatory, Estonia)
- Thomas Tram (Aarhus University, Denmark)
Special guest speaker: Eleonora Di Valentino (University of Sheffield, UK)
Topics
The topics concern theoretical and observational cosmology, including:
- Dark matter
- Dark energy
- Inflation
- Baryogenesis
- Cosmological tensions
- Gravitational waves
- Multimessenger cosmology
- Structure formation
- Cosmic dawn
Abstract Submission & Registration
The deadline for abstract submission is 26th of August 2026 (23:59 UTC+2).
The deadline for registration is 9th of September 2026 (23:59 UTC+2).
Funding
Centre of Excellence TK202 Foundations of the Universe
Participants
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Coffee break
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Registration
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Opening remarks
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Invited talks
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1
Cosmological Inference in the Age of Emulators
Precision cosmology relies on accurate theoretical predictions from Einstein–Boltzmann solvers combined with increasingly sophisticated statistical inference. As cosmological observations become increasingly precise and the need to explore a broad landscape of cosmological models grows, computational efficiency has become a central challenge for modern cosmological inference.
In this talk I will discuss recent developments in accelerating cosmological inference with a particular focus on emulation techniques. I will present the CONNECT framework for fast and accurate emulation of cosmological observables and highlight applications including parameter estimation, profile likelihood analyses, and Bayesian evidence calculations. I will also introduce CLiENT, a framework for direct emulation of cosmological likelihoods, enabling substantial further gains in computational efficiency. These developments, together with related work in the community, are making it possible to explore more complex cosmological models and fully exploit the scientific potential of next-generation cosmological observations.
Speaker: Thomas Tram -
2
Pandemic sterile neutrino dark matter
We propose a novel mechanism to generate sterile neutrinos νs in the early Universe, by converting ordinary neutrinos να in scattering processes νsνα → νsνs. After initial production by oscillations, this leads to an exponential growth in the νs abundance. We show that such a production regime naturally occurs for self-interacting νs, and that this opens up significant new parameter space where νs make up all of the observed dark matter. Our results provide strong motivation to further push the sensitivity of X-ray line searches, and to improve on constraints from structure formation.
Speaker: Kai Schmidt-Hoberg
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1
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Lunch
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Invited talks
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3
From curiosity to manifesto: The particle interpretation of gravity
Recent understanding in quantum field theory has shown that gravity can be consistently described as the propagation of a massless spin-2 particle. This perspective remains somewhat overlooked or simply seen as a mere curiosity. In this talk, we revisit the main steps behind this interpretation and argue that the particle picture is more than an equivalent reformulation: it provides a richer framework for exploring the gravitational phenomena and fundamental cosmological questions that remain unexplained.
Speaker: Carlo Marzo (NICPB)
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3
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Contributed talks
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4
Cosmic variance and ergodicity in finite systems with correlations
I will discuss the difference between ensemble averaging and volume averaging in cosmological systems with long-range correlations. I will present quantitative results for the difference between the two for cosmological systems, where the averaging volume is necessarily finite so that the ergodic limit is not reached.
Speaker: Syksy Räsänen -
5
Entanglement in an expanding universe
In the first part I discuss the evolution of entanglement entropy for a massless field within a spherical region in an expanding background. The formalism is applied to the inflationary period and the subsequent era of radiation domination, starting from the Bunch-Davies vacuum. Each field mode evolves towards a squeezed state upon horizon exit during inflation, with additional squeezing when radiation domination sets in. This results in the enhancement of the entanglement entropy. A volume term develops in the radiation-dominated era, and becomes the leading contribution to the entropy at late times.
In the second part I discuss the form of the entanglement entropy in various
gravitational backgrounds (de Sitter and anti-de Sitter space, the Einstein
universe) focusing on the structure of the divergences. Universal
coefficients are determined for ultraviolet and infrared divergent terms.
In the third part I discuss the use of the finite part of the entropy for the
calculation of c- and a-functions.Speaker: Prof. Nikolaos Tetradis (National and Kapodistrian University of Athens (GR)) -
6
Peak Theory Revisited: asphericity and non-Gaussianity
The study of peaks in the primordial perturbations is a fundamental cornerstone of cosmology, with important implications for the formation of galaxies, primordial black holes (PBHs), and gravitational waves (GWs). In this talk, I will revisit the theory of peaks, first discussing their conditioning in coordinate space. I will then introduce a spherical decomposition of peaks and discuss its advantages for their generation and characterization. Finally, I will show how this formalism can be extended to non-Gaussian fields.
Speaker: Gabriele Perna
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4
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Coffee break
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Lightning talks
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7
Simulation-Based Inference for Cosmological Field Reconstruction and Parameter Estimation
Simulation-based inference (SBI) enables Bayesian analysis of complex cosmological data when only a forward model is available, while field-level inference (FLI) aims to perform inference in a maximally efficient way and retain more information than summary-statistic pipelines. In this talk, I will highlight recent advances and applications of SBI and FLI in cosmology. First, I will show how field-level SBI can be used to reconstruct cosmological fields from incomplete and noisy data. Using Gaussian neural posterior estimation with a trainable mean and covariance, and combining classical conjugate-gradient solvers with neural networks, our method captures complex spatial correlations, denoises observations, and probabilistically reconstructs missing regions. We demonstrate this approach on the challenging task of inferring the 3D dark matter field and its initial conditions. I will then describe how this method can be combined with graph neural networks to reconstruct fields at small scales from point cloud galaxy data, and how it can be embedded in an active-learning framework for dynamic SBI, enabling joint inference of fields and cosmological parameters by steering simulations toward the most relevant regions of the parameter space.
Speaker: Oleg Savchenko -
8
Galaxy Superclusters in SDSS DR12 BOSS: Identification and Properties
The cosmic web is a complex network of matter spanning a wide range of scales and densities, with galaxy superclusters representing the largest coherent overdense structures that are generally gravitationally unbound. Their formation mechanism remains an open question, and large galaxy surveys are needed to characterise their properties with sufficient statistical significance. We present a catalogue of galaxy superclusters identified from the SDSS-BOSS DR12 spectroscopic galaxy catalogue over the redshift range $0.15 < z < 0.70$. Superclusters are identified using a modified Friends-of-Friends (mFoF) algorithm that accounts for survey selection effects. Structure identification is performed using an overdensity threshold of D = 5.0 based on both the number-density (12,784 superclusters) and luminosity-density (10,243 superclusters) fields, with negligible differences between the two approaches. We estimate the confidence level of each identified structure and select those with at least 5-sigma confidence that they are inconsistent with a random distribution for further analysis. The resulting catalogue has a mean supercluster size of 55 Mpc, a mean richness of 25 galaxies and a mean luminosity density contrast of 12.95. The largest supercluster is of size 378 Mpc at a redshift of 0.53, and the richest supercluster has 503 galaxies at a redshift of 0.34. The properties of these superclusters may be used as probes to constrain or test different cosmological models.
Speaker: Sumedh Ghamande -
9
Cosmological gravitational particle production: Scalars and fermions
In this talk, I will discuss the gravitational production of scalars and fermions during inflation. For scalars, I will compare Bogolyubov coefficient and Starobinsky stochastic approaches, showing that they only agree in the limit of infinitely long inflation. High-scale inflation is very efficient in particle production, leading to constraints on the existence of free, light, and stable scalars: such particles are viable only if their masses are below the eV scale or if the reheating temperature is in the GeV range. This motivates freeze-in at stronger coupling, which I will explore in the framework of warm dark matter production. For fermions, I will show how the production efficiency depends on the particle mass, which is generated via the Yukawa coupling and sensitive to the corresponding scalar field value. Scalar fields can experience large quantum fluctuations during inflation, driving the average field to the Hubble scale and above. Thus, fermions can be very heavy during inflation, enhancing particle production. Based on 2503.14652, 2509.01673, and 2602.20242.
Speaker: Duarte Feiteira -
10
Axion Misalignment Across First-Order Phase Transitions
When the axion mass is generated during a first-order phase transition and becomes non-vanishing only inside expanding true-vacuum bubbles, the standard picture of misalignment production is qualitatively modified. Using lattice simulations in an expanding universe, we study dark matter production within such a framework and identify two distinct regimes. For rapid transitions, the onset of oscillations is delayed until bubble percolation, enhancing the relic abundance. For slower transitions, spatial gradients generated by expanding bubbles suppress the effective misalignment angle through the bubble misalignment mechanism. We derive a semi-analytical expression for the relic density that provides a unified description of both regimes and accurately reproduces the simulation results. Finally, we show how this mechanism also modifies isocurvature perturbations and the small-scale matter power spectrum, with important implications for axion minicluster formation.
Speaker: Galymzhan Baltabay (NICPB, Taltech) -
11
Landau–Zener Formula and Resonant Axion–Photon Conversion in Neutron Star Magnetospheres
Resonant axion–photon conversion in neutron star magnetospheres provides a promising avenue for probing axions and axion-like particles through polarization and spectral observations. Existing studies commonly employ the Landau-Zener (LZ) approximation to estimate the conversion probability at resonance. In this work, we examine the validity of this approximation across the parameter space relevant for millimeter-to-optical observations.
We demonstrate that, in the non-adiabatic regime, the LZ result is equivalent to the stationary phase solution of the axion-photon propagation equations. Building on this correspondence, we derive simple analytical criteria that determine when the resonance remains sufficiently localized for the LZ approximation to be applicable. We find that, for resonances producing optical and higher frequency photons, the characteristic resonance width often becomes comparable to or larger than the scale over which the neutron star magnetosphere varies. In this regime, the assumptions underlying the LZ approximation break down, causing the predicted axion-photon conversion probability to deviate significantly from the numerical result.
Our analytical predictions are validated through direct numerical solutions of the coupled propagation equations for realistic dipole neutron star magnetospheres. Applying these results to optical polarization measurements of the pulsar B0656+14 and the magnetar 4U 0142+61, we revisit existing constraints on the axion-photon coupling and find that they are generally weaker than previously reported when the limitations of the LZ approximation are properly taken into account.
Speaker: Topi Sirkiä (University of Helsinki) -
12
Non-perturbative aspects of curvaton primordial non-Gaussianities
In the curvaton scenario, strongly non-Gaussian curvature perturbations arise naturally from the post-inflationary dynamics of the spectator field.
In this talk, we present a simple procedure to directly construct the non-linear mapping between the full non Gaussian curvature perturbation field and its Gaussian counterpart, for an arbitrary curvaton potential.
Additionally, we investigate the non-perturbative effects of strongly non-Gaussian curvaton regimes on the primordial power spectrum, showing that primordial supermassive black hole seeds can be produced while simultaneously evading the $\mu$-distortion constraints.Speaker: Mr Sasha Allegrini (KBFI, Tallinn and Tallinn U. Tech.) -
13
A Disformal Affair: How Dark Matter Mimics Lorentz Violation
We study the effects of general disformal metric transformations on fermionic fields, showing that the Dirac action acquires additional kinetic and axial-current couplings. When the sourcing field takes a nontrivial background value, apparent Lorentz-violating effects arise, which we classify within an effective field theory. For scalar and vector ultralight dark matter, precision tests of rotation and boost invariance place stringent constraints on the couplings. We further derive consistency conditions requiring the absence of ghost degrees of freedom, which for multi-scalar transformations demands a degenerate field-space metric, yielding consistent two-field modified gravity models.
Speaker: Apostolos Tsampodimos -
14
Emergent dark matter effect from Cartan Khronon gravity
In the gauge-theoretic formulation of gravity within the Cartan Khronon framework, time is described with a symmetry-breaking clock field, the khronon. Combined with the chiral structure of the theory, general relativity is recovered in the right-handed limit, supplemented by an additional degree of freedom. We show that this extra component gives rise to phenomena typically attributed to dark matter and the $\Lambda$CDM model emerges within a self-contained theoretical framework. This talk discusses the properties of this emergent dark matter and its implications in different phases of cosmology.
Speaker: Lucy Zheng (University of Tartu) -
15
Dark matter in the Cartan geometry?
Despite its immense observational significance, the nature of dark matter remains elusive, and possible interpretations proliferate. Rather than a separate particle sector, it might be tempting to instead consider a modification of geometry: in particular, attention is drawn to the recent Cartan Khronon theory of gravity, which aims to solve a multitude of problems simultaneously. Indeed, through a reformulation of the coframe geometry viz. the Cartan radius vector in self-dual gravity, a scalar ideal dust degree of freedom appears, alongside an interpretation of cosmological time, the presence of Lorentz symmetry breaking, and forwarding to an entire proposal of "khronogenic" cosmology. But past a brief review, the interest is on whether, how and when the interpretation of such sectors as a geometric effect is justified, as it touches upon questions of observation and measurement, parameter count, duality (Proca-Kalb-Ramond), and physical and mathematical equivalence.
Speaker: Priidik Gallagher (University of Tartu) -
16
Dynamical systems analysis of Newer GR cosmology
We explore the cosmological dynamics of a non-metricity-based theory, the so-called Newer General Relativity for a flat FLRW metric. We introduce the fundamental concepts of symmetric teleparallel gravity and Newer GR cosmology. We focus on two physically motivated one-parameter subclasses for which the post-Newtonian parameters are the same as in GR and no ghost modes emerge. We study two models: one involving a single fluid and one with dust and radiation. For each case, we determine the (angular) fixed points, bounces and turnarounds, potential singularities, the existence of effective dark energy and we draw conclusions about the possible cosmological scenarios.
Speaker: Vasiliki Karanasou (University of Tartu) -
17
Bianchi cosmologies with four-dimensional groups of motion in New General Relativity
Our current understanding of the Universe is that it is isotropic, which means that spherically symmetric models of spacetime are the preferred choice for any geometrical considerations about the Universe. However, some observations point to an asymmetry in the CMB, the "axis of evil," that could be explained by a non-isotropic Universe. This can potentially be leveraged to help solve another issue, namely the problem of strong coupling preventing an analysis of cosmological perturbations in teleparallel theories. In this work, we make use of three geometries that have a weaker symmetry than SO(3) and belong to the so-called Bianchi classes/types II, III and IX, to calculate analytical and/or numerical solutions to the 1-parameter NGR field equations. We also engage in an initial analysis of the precise behaviour of these solutions.
Speaker: Paul Martin Kull (University of Tartu) -
18
Spacetime energy in general parallel relativity
Because of the mixing of gravity and inertia in GR, no coordinate-independent local energy density of the gravitational field can be defined. Many years of theoretical explorations have revealed that the same GR field equations can also be derived from a more general teleparallel connection which carries torsion and nonmetricity while the curvature vanishes. After further development of the theories, the proposed “general parallel relativity” (G||R) allows gravity and inertia to be theoretically disentangled while maintaining the experimentally well tested GR field equations. The aim of the PhD project is to test the G||R proposal for more complicated spacetimes and establish a comprehensive picture of gravitational/spacetime thermodynamics.
In my talk, I'm going to give an overview of general teleparallel equivalent of general relativity (GTEGR) and G||R. After that I will show the derivation of analytical expression for Schwarzschild spacetime energy. Additionally I'll briefly go over the results of the work on Kerr-Newman spacetime in G||R framework which has yielded us an analytical result for the spacetime energy.
Speaker: Roald Heinrich Ivask -
19
Fifth forces in cuscuton gravity: implications of non-dynamical scalars for structure growth
The cuscuton is a scalar field that can modify gravity without introducing an additional propagating degree of freedom. In this talk, I will discuss what happens when matter is minimally coupled in the Jordan frame, resulting in a conformal coupling to the cuscuton in the Einstein frame, and, in particular, whether a non-propagating field can nevertheless mediate a fifth force. I will show that the cuscuton perturbation, although fixed by a constraint, can respond to matter perturbations and modify their gravitational interaction. I will then illustrate the cosmological consequences in two simple classes of dark-energy solutions, including tracking and sign-switching scenarios. Besides the direct fifth force, the same coupling modifies the expansion history, particle masses, recombination, and the growth of structure, leading to modifications of the CMB and matter power spectrum.
Speaker: Ilaria Andrei
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7
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Reception
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Invited talks
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20
Gravitationally Lensed Supernovae: Lessons from the First Decade
Gravitationally lensed supernovae enable independent measurements of the highly disputed value of the Hubble constant. Over the past decade, the sample of known lensed supernovae has grown into a small but diverse population. I will review our efforts to discover, follow-up and model these systems, highlighting both their promise for precision cosmology and the observational and modelling challenges that remain.
Speaker: Edvard Mörtsell
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20
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Coffee break
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Invited talks
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21
Cosmology with the next-generation 4MOST survey
I will present the 4MOST survey, focusing on the 4MOST surveys that will help constrain current cosmological models.
Speaker: Prof. Elmo Tempel (University of Tartu)
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21
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Contributed talks
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22
The challenge of the largest structures in the Universe
Understandng the properties, formation and evolution of the cosmic web is one of the main tasks in cosmology.
In my talk I introduce the largest structures in the cosmic web, and the compatibility of these structures with the LCDM cosmolocigal model.
In the Local Universe the richest structures in the cosmic web are very rich galaxy superclusters and their complexes - the Sloan Great Wall and the BOSS Great Wall. Moreover, rich galaxy clusters and superclusters form a quasiregular pattern with two huge perpendicular planes with extent of several hundreds of megaparsecs, the Local Supercluster plane and the Dominant supercluster plane. The characteristic distance between superclusters in this pattern is 120 - 140 h(-1) Mpc. The origin of these patterns in the supercluster distribution is not yet clear. I discuss whether the presence of such structures can be explained within the standard LCDM cosmological model.Speaker: Maret Einasto -
23
Is there evidence for a violation of the cosmological principle?
The cosmological principle, which states that the Universe is statistically homogeneous and isotropic on sufficiently large scales, is a foundational assumption of the standard cosmological model. Reported discoveries of clustering on gigaparsec scales appear to contradict this assumption and motivate inhomogeneous cosmologies. I examine two of the recently claimed discoveries, and conclude that they do not constitute evidence for a departure from the cosmological principle.
Speaker: Till Sawala
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22
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Lunch
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Contributed talks
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24
Constraints on the primordial tensor perturbation power spectrum from CMB and gravitational wave observations
Inflation predicts scalar and tensor perturbations. The former give rise to spatial curvature (or isocurvature) perturbations, and the latter lead to a stochastic background of primordial gravitational waves. The simplest inflationary models lead to a nearly scale-invariant power-law spectra for both perturbations and to a consistency condition, $n_t = -r/8$, between the tensor spectral index $n_t$ and the tensor-to-scalar ratio $r$. In these models, in therms of the slow-roll parameter, we have $r=16\epsilon$ and $n_t=-2\epsilon$. However, in many inflationary scenarios this relation can be broken, so that $n_t$ can be independent of $r$, and $n_t$ can significantly differ from zero. Assuming the standard adiabatic $\Lambda$CDM model plus a power-law tensor perturbation power spectrum, we derive observational constraints for the tensor-to-scalar ratio and the tilt of the tensor spectrum. Even if one was to believe in the simplest inflationary models, it is important to test whether the data favor the nearly scale invariant spectrum dictated by the consistency condition or whether a significantly negative or positive tilt of the tensor spectrum is favored.
Currently the Planck CMB temperature and polarization E-mode data used together with the BICEP/Keck Array B-mode data allow for relatively large positive tilts.
Parametrizing the tensor contribution by two independent primordial tensor-to-scalar ratios $(r_1,\,r_2)$ at $k_1 = 0.005\,$Mpc$^{-1}$ and $k_2 = 0.02\,$Mpc$^{-1}$, we obtain constraints $r_{0.005} < 0.030$ and $r_{0.02} < 0.098$ at 95% confidence level. We show that the future LiteBIRD CMB B-mode data will be able to shrink the allowed parameter space area in the $(r_{0.005}, r_{0.02})$ plane to less than one hundredth of the currently allowed area. Distinguishing $n_t = -r/8$ from exact scale invariance will stay impossible, but radical, theoretically motivated, departures from $n_t = -r/8$, which are consistent with the current data, could be distinguished with LiteBIRD.Another avenue is the use of an upper bound for the energy density of the stochastic gravitational wave background, $\Omega_{\rm GW} (25\, {\rm Hz}) < 2.8 \times 10^{-9}$, provided yesterday by the LIGO, Virgo, and KAGRA data, which lies at 22 orders larger $k$ than the scales probed by the CMB B mode. If the tensor power spectrum followed the strict power law over this whole $k$ range (note the huge extrapolation), then a large region of positive $n_t$ values would lead to a direct detection of stochastic primordial gravitational wave background that LIGO, Virgo, KAGRA has not seen. However, I will explain why improving the constraints on $\Omega_{\rm GW}$ even by several orders of magnitude from the current ones would not restrict $n_t$ significantly more.
Speaker: Jussi Väliviita -
25
Beyond the basics: exploring gravitational waves from strong phase transitions
A leading candidate for a stochastic source of primordial gravitational waves would be a first order phase transition. Strong electroweak-scale phase transitions in particular stand a good chance of being constrained by gravitational wave data from future missions such as LISA and Taiji, and thus it is important to understand the signal from strong transitions. Unfortunately strong transitions also lead to highly nonlinear behaviour over many length- and timescales, which makes for a challenging topic of study both for analytical and simulation approaches. In arXiv:2505.17824 we presented results of large-scale simulations for a strong detonation and deflagration, demonstrating features including the saturation of the signal. Here we extend this work to a scan across wall velocities, bubble separations and phase transition strengths. Employing the field-fluid simulation model (and our SCOTTS code) we study gravitational waves from strong hybrids, explore the role of droplets, and investigate the 'low-frequency' shallow slope. We believe that studies like this will be crucial to solving the 'inverse problem' of constraining (or detecting) a phase transition, given gravitational wave data.
Speaker: David Weir (University of Helsinki) -
26
Sphaleron rate for first-order electroweak phase transitions
The origin of the observed baryon asymmetry of our universe remains a mystery. Electroweak baryogenesis (EWBG) scenarios remain viable candidates which can be constrained by ongoing particle physics and upcoming gravitational wave experiments. The Standard Model has baryon number violating processes, the sphalerons. However, EWBG models have to introduce beyond the Standard Model physics at the electroweak scale to address the lack of a first-order phase transition (and notable CP violation) in the electroweak sector. The viability of these models hinges on the sphaleron rate freezing out fast enough as one transitions from the confinement-like phase to the Higgs phase. The computation of this rate in the increasingly complex BSM landscape can become untractable. Luckily, most of BSM models at high temperatures can be mapped perturbatively into an effective dimensionally reduced 3D Higgs+SU(2) theory. We can thus constrain many BSM models by studying the sphaleron rate in the parameter space of the simpler EFT. In this talk I will present a non-perturbative lattice determination of the sphaleron rate in the full parameter space relevant for first-order phase transitions. With these results we can obtain a robust model independent constraint for the strength of the transition that lead to viable baryogenesis scenario.
Speaker: Jaakko Annala -
27
Wave-optics lensing of gravitational waves
Gravitational waves (GWs) are barely absorbed, scattered or dispersed as they travel through the cosmos. They are therefore an appropriate messenger to probe regions opaque to light and very distant GW sources. When GWs encounter a massive object, however, they are deflected and distorted by the object's gravity through the gravitational lensing effect. This distortion by gravitational lensing on GW signals provides a way to reveal faint and invisible massive objects (lenses) they may cross. In this talk, I will describe the wave-optics lensing signatures such as diffraction and interference, which are imprinted by low mass lenses and when the source, the lens and the observer are closely aligned. I will show how the detectability of the wave-optics lensing signatures changes with the system parameters, and how we can use lensing to learn about the environment of GW sources.
Speaker: Helena Ubach -
28
Signatures of $10-10^4\,{\rm M}_{\odot}$ Dark Matter halos in LISA via Stochastic Diffraction
Cold Dark Matter predicts a population of low-mass halos which are sensitive to its fundamental nature and the primordial power spectrum, yet remain undetected. Although elusive, their discovery may be possible thanks to wave-optics lensing of gravitational waves (GWs) by the superposition of many halos along the line of sight. We study the statistical properties of \textit{stochastic diffractive lensing}, which imprints correlated fluctuations on the amplitude and phase of the original waveform. The stochastic distortions can be described by an orthogonal basis that captures the dominant ``tones'' associated with the dark matter properties, or \textit{dark timbre}, which is not degenerate with binary source parameters. LISA is most sensitive to halos of $\mathcal{O}(10\text{--}10^4\,M_\odot)$, and because the imprint recurs in every source, stacking $\sim(50,500)$ loud binaries could confirm them at the $(2,5)\sigma$ level ($\sim0.2$ to $\gtrsim4\sigma$ for realistic merger rates and different concentration estimations). The per-event signal is only $\mathcal{O}(10^{-3})$ in cold dark matter, demanding major advances in waveform accuracy and data analysis. Even short of that reach, stochastic diffraction places stringent bounds on models that enhance small-scale structure, such as axion miniclusters and primordial black holes.
Speaker: Juan Urrutia (KBFI)
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24
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Coffee break
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Contributed talks
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29
Tachyonic production of dark relics: quantum 2PI formalism with momentum exchanging collisions
Oscillating spacetime curvature can drive the production of dark matter during reheating, and accurately quantifying this requires the use of both non-perturbative and non-equilibrium methods. This tachyonic instability has previously been studied using 2-particle irreducible -formalism in the Hartree approximation. However, modelling the non-thermal dark matter distribution and its subsequent evolution requires accounting also for momentum exchanging collisions. I will detail a self-consistent approximation scheme for reducing the beyond-Hartree 2PI equations of motion to a generalized quantum Boltzmann equation, which can then be solved with standard methods. The approach can be used on both stable and unstable field excitations. This presentation is based on arXiV:2606.02039 and its preceeding article arXiv:2406.17468.
Speaker: Olli Väisänen -
30
Scalar field with nonminimal couplings to metric-affine gravity and inflationary model building
We study a scalar field nonminimally coupled to metric-affine gravity within a wide class of actions that are linear in the affine curvature and contain all independent parity-even and parity-odd terms quadratic in torsion and nonmetricity as well as Nieh-Yan-like derivative couplings between the scalar-field derivative and the four independent torsion and nonmetricity vectors. Integrating out the non-Riemannian connection gives an equivalent metric scalar-tensor theory in which the non-Riemannian interactions are encoded in the effective kinetic function. We illustrate how these modifications affect the potential of the Einstein frame canonical scalar field and discuss the implications for inflationary model building.
Speaker: Laur Järv (University of Tartu) -
31
Universal mapping of observables for Palatini inflationary attractors
We study single-field slow-roll inflation in the context of Palatini gravity for the class of non-minimally coupled $\xi$-attractors, i.e.,\ models where the same function $f(\phi)$ fixes both the non-minimal coupling, $1+\xi f(\phi)$, and the inflationary potential, $V(\phi) = V_0 f(\phi)^2$. As is well-known, for an arbitrary $f(\phi)$, the number of $e$-folds is, at leading order, independent of $\xi$. Thanks to this, we provide an immediate mapping between the observables of the non-minimally coupled setup and those of the minimally coupled one. In the strong coupling limit, the tensor-to-scalar ratio is, as usual, suppressed, while the scalar spectral index is shifted towards larger values, with the magnitude of the shift depending only on the tensor-to-scalar ratio associated with the original potential $V(\phi)$.
Speaker: Christian Dioguardi (KBFI)
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29
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Break
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Colloquium
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32
Cracks in the Standard Cosmological Model: Anomalies, Tensions, and Hints of New Physics
The ΛCDM model has long served as the standard paradigm in cosmology, offering a remarkably successful description of the Universe’s evolution. Yet, as observational precision continues to improve, persistent tensions have emerged across a range of probes, including the well-known Hubble constant discrepancy. While individual datasets may each align with ΛCDM, their collective interpretation reveals significant discordances that challenge the model’s internal consistency. In this talk, I will review the most prominent tensions in modern cosmology and assess their implications. I will present recent results pointing to hints of dynamical dark energy and interactions within the dark sector. I will also reflect on the growing influence of methodological choices, such as dataset selection and model assumptions, in shaping our cosmological conclusions.
Speaker: Eleonora Di Valentino (University of Sheffield)
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32
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Dinner
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Invited talks
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33
The post-inflationary axion mass in a minimal scaling model
We calculate the dark matter axion mass in the post-inflationary scenario in a minimal model with a set of numerical simulations on $12288^3$ grids. The minimal model contains only one extra scalar field which breaks the Peccei-Quinn U(1) symmetry at a temperature $f_a \sim 10^{11}$ GeV, creating a string network. The strings become the boundaries of domain walls at the QCD transition, which draw the strings together and annihilate them. The decay of the strings and domain walls leaves behind axions, which can constitute part or all of the dark matter. I report on the measurements of the comoving axion number density following the collapse of the string-wall system, and give an estimate of the dark matter axion mass in the scaling scenario, the most accurate to date. Prospects for detection at haloscopes are briefly reviewed.
Speaker: Prof. Mark Hindmarsh (University of Helsinki, University of Sussex) -
34
Gravitational Waves and Magnetic Fields from Axion Inflation
In the coming years, the search for primordial gravitational waves will combine direct detection with space-based observatories and measurements of Cosmic Microwave Background B-mode polarization. Accurately interpreting these measurements depends heavily on understanding their production mechanisms. A particularly compelling scenario involves gravitational wave generation through the interaction of an axion with gauge fields during inflation. I will present recent advances in axion inflation with non-Abelian gauge fields, focusing on the signatures in the primordial gravitational wave background and their correlation with primordial magnetic fields. I will also discuss how the emergence of a primordial plasma via the Schwinger effect alters the early universe dynamics, modifying both the gravitational wave signal and primordial magnetic field predictions.
Speaker: Oksana Iarygina (Nordita)
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33
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Coffee break
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Contributed talks
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35
The theory of Axion Relic Pockets
Axion Relic Pockets (ARPs) can form in cosmic phase transitions and comprise all of dark matter. The pocket interiors are filled by the false vacuum phase, stabilised against collapse by a hot, kinematically trapped axion gas: ARPs are balloons filled with relativistic axions. I will review the theory and phenomenology of ARPs, and report on recent progress in understanding their formation and stability when the axion couples to matter, including prospects for observable signatures at terrestrial experiments and astronomical observatories.
Speaker: David Marsh -
36
SU(N) Baryon formation in the large N limit and Dark Matter
I discuss baryon formation in the early Universe in a confining SU(N) gauge theory containing quarks with masses well below the confinement scale and transforming in the fundamental representation of the gauge group. Assuming Casimir scaling of the confining potential between static SU(N) charges, I show that baryon formation is hindered by a Casimir bottleneck: for small quark clusters, which constitute the initial stages of N-quark baryon assembly, destruction processes greatly outweigh formation processes. Due to the large suppression in their relic number density, cosmologically stable SU(N) baryons can provide dark matter candidates with masses substantially larger than conventionally assumed.
Speaker: Enrico Nardi (National Institute of Chemical Physics and Biophysics (EE)) -
37
Gravitational Waves from Dimension-6 Assisted Peccei - Quinn Phase Transitions
We discuss a simple model-independent extension of the minimal KSVZ axion model, augmenting the Peccei - Quinn effective potential with a dimension-6 operator invariant under the Peccei - Quinn symmetry. We show that this operator can trigger a first-order phase transition and describe the following cosmological consequences. We map out the parameter space, consistent with present observational constraints, in which the phase transition produces a gravitational wave signal observable at current and future interferometers, and we elaborate on the resulting dark matter phenomenology. Finally, we present specific ultraviolet completions that generate the dimension-6 operator, each leading to distinct phenomenological signatures, which motivates our model-independent approach.
Speaker: Kristjan Müürsepp (INFN-LNF, Frascati and NICPB, Tallinn, Estonia) -
38
Are PTA measurements sensitive to Non Gaussianities?
The non-Gaussianity of PTA timing residuals has been proposed as a way to tell a finite population of supermassive black hole binaries apart from a cosmological background. I will argue that for tests making no assumptions about the population or the GW spectrum, there is a fundamental obstruction to this approach. Such a test must first whiten the data in the response eigenbasis, since the Hellings–Downs covariance alone produces spurious rejections of Gaussianity. What remains is accessible only through the overall scale — which, absent a model, must be estimated from the same realization. That estimation destroys the signal: for a single dominant source the cancellation is exact and analytic, leaving a distribution set by the array geometry rather than the source population. Impose a spectral model or consistency across frequency bins and the scale is no longer a free nuisance, but the conclusion is then only as good as the model assumed. Two consequences: a model-agnostic detection of non-Gaussianity would point to systematics rather than the GWB, and anisotropy remains the discriminator worth pursuing.
Speaker: Jonas el Gammal
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35
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Closing remarks
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