UK Cosmology Meeting

Wednesday 25 Mar 2026, 00:00 → 17:00 Europe/London

Lecture Theatre 664 City&Guilds Building (Imperial College London, Abdus Salam Centre for Theoretical Physics)

Sebastian Cespedes (Imperial College London), Claudia de Rham, Sadra Jazayeri (Imperial College London)

The next UK Cosmology Meeting will take place at Imperial College London on March 25th 2026. [Registration is closed]. 

This one-day event aims to bring together the whole cosmology community in the UK to discuss recent trends in theoretical and observational cosmology. We will have several talks by young researchers and PhD students in the field, a plenary talk by Danièle A. Steer (LPENS and Université Paris Cité), and a poster session. 

Please feel free to email us if you have any questions. 

 

 

UKcosmo.pdf

09:00 → 09:45 Arrival Coffee

09:45 → 10:00 Welcome to the Abdus Salam Centre

10:00 → 11:00 Morning Session 1: Contributed Talks

10:00 New tests of parity violation in the early universe: CMB polarisation experiment and calibration

Precise polarization measurements of the Cosmic Microwave Background (CMB) provide an opportunity to rigorously test the standard cosmological model. The correlation between E and B modes, indicative of cosmic birefringence, is expected to arise from parity-violating physics in the Universe. However, detecting these EB correlations remains extremely challenging as it demands exceptionally precise polarization calibration of observational instruments. In this talk, I will present a novel calibration method for CMB polarimeters that circumvents the need for external polarized sources or self-calibration techniques. By cross-correlating data from two instruments observing the same region of the sky, we demonstrate that the difference between their misalignment angles can be resolved with data only, in a model independent-way. Using the flagship of calibration implemented in the Small Aperture Telescope (SAT) of Simons, we can precisely calibrate a wide range of experiment. As a proof of concept, we show that using this new calibration methods allows us to calibrate the Large Aperture Telescope (LAT) and Planck. This can be then be used to constrain parity-violation, whether it be cosmic birefringence or primordial EB power spectrum.

Speaker: Claire Rigouzzo (King's College)

10:20 Phantom Crossing with Quintom Models

We develop a two-scalar field quintom model, which utilises both a quintessence-like and a phantom-like scalar field, enabling a smooth and stable transition across the w = −1 phantom divide as hinted by recent measurements of Baryonic Acoustic Oscillations (BAO) by the Dark Energy Spectroscopic Instrument (DESI) Data Release 2. We explore a range of initial conditions and potential configurations that facilitate such a phantom-to-quintessence-like crossing, and find that this can be naturally realised with hill-top or cliff-face potentials bound from above. We study how varying these conditions affects the dynamics of the system, calculate the background observables and compare them with DESI, CMB, and Type Ia supernova data, identifying a viable parameter space for our model. In particular, we find that a potential featuring a hyperbolic tangent form can successfully reproduce the desired phantom crossing, although such models can suffer from fine-tuning effects. Finally, we discuss prospects for distinguishing such models with upcoming state-of-the-art cosmological observations.

Speaker: Lisa Goh (University of Edinburgh/ Institute for Astronomy)

10:40 Scalar-Induced Gravitational Waves from Alternative Thermal Histories

Primordial curvature perturbations enhanced at small scales can generate a stochastic background of gravitational waves at second order. The shape and amplitude of this signal depend not only on the primordial power spectrum but also on the Universe’s thermal history. While many studies assume radiation domination, an early matter-dominated era followed by reheating leaves characteristic imprints on the scalar-induced gravitational wave spectrum. The speed and nature of the transition between these eras can significantly modify the gravitational wave spectrum. In this talk, I introduce the topic, current literature, and show how we compute the kernel beyond sharp analytical approximations, and implications for LISA.

Speaker: Aya Ghaleb (Swansea University)

11:00 → 11:30 Coffee Break

11:30 → 12:20 Morning Session 2: Remembering Tom Kibble & Plenary Talk by Danièle Steer

11:30 Plenary Talk: Repeated gravitational wave bursts from cosmic strings

50 years after Tom Kibble’s seminal paper on cosmic strings, research on strings remains active and their GW signatures are currently being searched for by, for instance, the LVK and PTA collaborations. While much focus is on the stochastic GW background from the ensemble of cosmic strings, cosmic string loops also generate short duration GW bursts with a characteristic waveform. In this talk I will explain some of the features of these burst signals, and also point out that they are repeated, since cosmic string loops evolve quasi-periodically in time. Furthermore they will always appear from essentially the same position in the sky. These different points may help with parameter estimation (sky position, string tension…). We will estimate the number of GW repeaters for LVK and LISA, and show that the cosmic string tension that can be probed scales as detector sensitivity to the sixth power, which raises hope for detection in future GW detectors.

If time permits, I will also recall an old model for the evolution of the number density of cosmic string loops, developed by Tom, Ed Copeland and myself. We attempted to include the effects of possible loop fragmentation. This model has recently been revisited, in a different context, and I will outline some of the salient aspects of the results. After all, after 50 years, there is still a lot of disagreement over the cosmic string loop number density!

Speaker: Danièle Steer

12:20 → 12:40 Gong Show

12:20 Poster Pitch

Speaker: Gerald Barnert

12:22 Poster Pitch

Speaker: Alexander Roskill

12:24 Poster Pitch

Speaker: Lawrence Berry

12:26 Poster Pitch

Speaker: Enrico Specogna

12:28 Poster Pitch

Speaker: Aditya Verma

12:30 Poster Pitch

Speaker: Zizang Qiu (Edinburgh University)

12:32 Poster Pitch

Speaker: Laurens Smulders

12:34 Poster Pitch

Speaker: Lorenzo Tranchedone

12:36 Poster Pitch

Speaker: Haoyang Jin

12:38 Poster Pitch

Speaker: Arthur Whyley

12:40 → 13:45 Lunch+ Poster Session I

13:45 → 14:45 Afternoon Session 1: Contributed Talks

13:45 The Sartorial Universe: Dressing and Cutting from Flat Space to dS

While the cosmological wavefunction has received much attention over the years, being the object dual to CFT correlators in attempts at constructing a dS/CFT correspondence, the in-in correlator has several nice properties which render it in some sense "simpler" — despite being constructed from the wavefunction. In this talk, I will discuss how in-in correlators can be directly obtained from flat-space amplitudes for theories of conformally coupled and massless scalars. The so-called "dressing rules" which permit this uplifting involve attaching auxiliary propagators to flat-space diagrams to effectively soak up the energy which is no longer conserved in de Sitter. This representation also provides a way to study the analytical properties of the in-in correlators by endowing cuts with respect to external variables in momentum space with a natural diagrammatic interpretation, leading to some relations which may be useful for bootstrapping correlators.

Speaker: Joe Marshall (Durham University)

14:05 Cosmic Lockdown: When Decoherence Saves the Universe from Tunnelling.

I will discuss vacuum selection for a spectator scalar field in an asymmetric double-well potential during inflation, including environmental decoherence. Using numerically solved master equations, we show that the ratio between the inflationary Hubble scale and the spectator mass controls which vacuum is selected: heavy fields relax to the true vacuum, while light fields display greatly enhanced false vacuum occupation. Decoherence suppresses interference and tunnelling, effectively locking the field into its stochastically selected minimum and stabilizing light false vacua on inflationary timescales. I am presenting the results of a joint work with Jaewoo Joo, Greg Kaplenek, Vincent Vennin and David Wands.: arXiv:2512.14204 [hep-th]

Speaker: Robson Christie (University of Portsmouth)

14:25 CMB Cosmology with Simulation-Based Inference

Simulation-based inference (SBI) provides a likelihood-free framework for cosmological parameter inference directly from forward simulations. In this work, we apply SBI methods to CMB cosmology and implement a full Planck-like analysis pipeline, including realistic simulations and data processing.

Speaker: Xing Li (Imperial College London)

14:45 → 15:10 Gong Show

14:45 Poster Pitch

Speaker: Callum Bell

14:47 Poster Pitch

Speaker: Trevor Cheung

14:49 Poster Pitch

Speaker: Marisol Jimenez Cruz

14:51 Poster Pitch

Speaker: Olga Garcia Gallego

14:53 Poster Pitch

Speaker: Wei-Ning Deng

14:55 Poster Pitch

Speaker: Neel Shah

14:57 Poster Pitch

Speaker: Serena Giardino

14:59 Poster Pitch

Speaker: Angus Macdonald

15:01 Poster Pitch

Speaker: Antonio Najera

15:03 Poster Pitch

Speaker: Saba Rahimy

15:05 Poster Pitch

Speaker: Giulia Borghetto

15:08 Poster Pitch

Speaker: Sinah Legner

15:10 → 15:40 Coffee Break+ Poster Session II

15:40 → 17:00 Afternoon Session 2: Contributed Talks

15:40 Redshifts and physical properties of KiDS-1000 galaxies using the pop-cosmos galaxy population model

Accurate redshift estimation and physically motivated source sample selection are central challenges for precision weak lensing cosmology. In this talk, I will present our recent work on inferring physical properties of weak lensing galaxies in the Kilo-Degree Survey (KiDS-1000) using a generative model for the galaxy population. The framework enabling this work is pop-cosmos, a calibrated galaxy population model that allows principled Bayesian inference of individual galaxy redshifts and physical properties for millions of KiDS-1000 sources. Validation against spectroscopic samples demonstrates low bias and scatter in the inferred photometric redshifts, while physical property inference enables the construction of weak lensing catalogues that mitigate intrinsic alignment systematics. Overall, our results demonstrate how the pop-cosmos galaxy population model can deliver accurate redshifts and galaxy properties, crucial for connecting galaxy evolution physics and weak lensing cosmology with Rubin LSST and Euclid.

Speaker: Anik Halder (University of Cambridge)

16:00 Constraining Momentum Exchange in the Dark Sector

Interactions between dark energy and dark matter offer a potential solution to cosmological tensions. Specifically, models characterised by pure momentum exchange have been suggested to alleviate the S8 tension, as they lead to a suppression in structure growth rate at late times. In this talk, I will discuss how the choice of dark energy model impacts the ability of future data releases to constrain the coupling strength of these interactions. I show that to obtain robust constraints, it is crucial to account for dark energy perturbations when computing the linear matter power spectrum, particularly in the limit where the equation of state w approaches -1.

Speaker: Nathan Cruickshank (University of Portsmouth)

16:20 Empirical dark matter halo profiles from weak lensing

The structure of dark matter haloes is often described by radial density profiles motivated by cosmological simulations. These are typically assumed to have a fixed functional form (e.g. NFW), with some free parameters that can be constrained with observations. However, relying on simulations has the disadvantage that the resulting profiles depend on the dark matter model and the baryonic physics implementation, which are highly uncertain. Instead, we present a method to constrain halo density profiles directly from observations. This is done using a symbolic regression algorithm called Exhaustive Symbolic Regression (ESR). ESR searches for the optimal analytic expression to fit data, combining both accuracy and simplicity. We apply ESR to a sample of 149 galaxy clusters from the HSC-XXL survey to identify which functional forms perform best across the entire sample of clusters. We identify density profiles that statistically outperform NFW under a minimum-description-length criterion. The preferred ESR profiles have shallow inner slopes and a maximum within the observed radial range. As a practical application, we show how the best-fitting ESR models can be used to obtain enclosed mass estimates. We find masses that are, on average, higher than those derived using NFW, highlighting a source of potential bias when assuming the wrong density profile. These results have important knock-on effects for analyses that utilise clusters, for example cosmological constraints on 𝜎8 and Ωm from cluster abundance and clustering. Beyond the HSC dataset, the method is readily applicable to any data constraining the dark matter distribution in galaxies and galaxy clusters, such as other weak lensing surveys, galactic rotation curves, or complementary probes.

Speaker: Alicia Martin (Univerity of Oxford)

16:40 Searching for Cosmic (Super)strings in the CMB and the Stochastic Gravitational-Wave Background

Cosmic strings were once considered a leading candidate for the origin of cosmic structure, but high-resolution measurements of the cosmic microwave background (CMB) have ruled them out as the primary source of density fluctuations. Nevertheless, there remains ample room within the ΛCDM framework for the existence of cosmic strings or superstrings at subdominant levels. As these networks evolve, they generate loops that may decay via gravitational-wave emission, producing a stochastic gravitational-wave background (SGWB). In this talk, I will review the state-of-the-art modelling of cosmic and superstring network evolution and their predicted CMB and GW signatures. I will then present recent constraints and search results for cosmic (super)strings from NANOGrav pulsar-timing data and Atacama Cosmology Telescope (ACT) CMB observations.

Speaker: Juhan Raidal (University of Nottingham)