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What do acoustic scale observations tell us about dark energy?

I explore how cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) measurements constrain cosmological models. The CMB angular scale provides robust constraints on the ratio of sound horizon to angular diameter distance, limiting possible deviations from the standard ΛCDM model. The null energy condition for a separate dark energy component imposes strict inequalities on BAO observables relative to ΛCDM predictions, restricting the freedom to fit new data within standard cosmological frameworks. I’ll discuss what this means for latest BAO results and other possible interpretations.

• Speaker: Antony Lewis (University of Sussex)
• Monday 20 October 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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Dynamics of nearly isolated, lenticular vortices: From waves to bulk instabilities?

In geophysical and astrophysical settings, rotating stratified flows often exhibit large-scale, nearly isolated vortices. Examples include the Mediterranean eddies in the Atlantic ocean, or the Great Red Spot (GRS) in Jupiter. These vortices have been widely studied using shallow-water or quasi-geostrophic models for decades. In particular, these models have successfully explained why these vortices maintain nearly lenticular shapes through time. However, prior reduced models have a blind spot when it comes to predicting the internal dynamics of such vortices, despite the fact that they are far from being motionless in their bulk (e.g. as observed for the GRS ). Various instabilities may sustain small-scale turbulence and accelerate the decay of large-scale vortices on long time scales.

Here, I will present a reduced model accounting for the bulk dynamics of large-scale pancake-like vortices. This model, which is developed in the framework of an interdisciplinary collaboration between pure and applied mathematics, is largely inspired by some ideas and methods pioneered by astrophysicists (e.g. S. Chandrasekhar or N. Lebovitz). First, I will describe the properties of the normal modes, because wave motions are often key to understanding the transition to turbulence in geophysical flows. As in the rotating non-stratified case, it will be shown that the wave spectrum solely consists of eigenvalues, and that the eigenvectors are all smooth. Moreover, it will be explained why some low-frequency waves/modes, which are governed by a mixed hyperbolic-elliptic problem for the velocity, can exist below the usual cutoff frequency of inertia-gravity waves. Next, by combining local and global stability methods, I will discuss whether some bulk instabilities could sustain small-scale bulk turbulence in strongly deformed stratified vortices.

• Speaker: Jérémie Vidal (ENS Lyon)
• Monday 20 October 2025, 16:00-17:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Mattias Brynjell-Rahkola.

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Hydrogen-deficient binary stars with magnetic braking

Hydrogen-deficient binary stars comprise one star which has been stripped of its hydrogen through mass transfer to a binary companion. Observations show that the companion is able to accrete several solar masses without spinning up to critical rotation, and so there must be a mechanism to drain spin angular momentum from the accretor. We test magnetically coupled winds and magnetic star-disc coupling as possible mechanisms and find that, while the disc coupling is negligible, the winds are sufficient to allow the accretor to gain mass without spinning up to critical rotation. However, in order to fully replicate observations, time-dependent scalings of the dynamo-generated magnetic field are needed.

• Speaker: David Bour / IoA
• Wednesday 22 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Miscibility in sub-Neptunes

Sub-Neptunes are found around 50% of Sun-like stars in our galaxy. Despite their ubiquity, we lack a comprehensive understanding of their interior structure. I will present the first evolving interior structure model for sub-Neptunes that accounts for the expected miscibility between silicate magma and hydrogen. I will discuss the concept of “binodal surfaces”, which represent phase transitions within sub-Neptunes and provide a physically/chemically informed boundary between a planet’s “interior”’ and “envelope”. I will discuss the many implications of miscibility, and a potential observational route to testing its prevalence.

• Speaker: James Rogers / IoA
• Wednesday 15 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Strong cosmic censorship for de Sitter black holes

We discuss modern formulations of the strong cosmic censorship conjecture (SCCC) and possible resolutions supported by rigorous non-linear results for the spherically symmetric Einstein-Maxwell-scalar field system. We show that the presence of a positive cosmological constant suggests a violation of the SCCC at a fundamental level of regularity. Indeed, the blueshift mechanism occurring at the Cauchy horizon can be counterbalanced by the dispersive effects encoded in the exponential Price law along (cosmological) black hole event horizons. On the other hand, we show that, if non-smooth black hole solutions are allowed, then the aforementioned violations are non-generic in a positive co-dimension sense.

• Speaker: Flavio Rossetti (Gran Sasso Science Institute)
• Friday 24 October 2025, 13:00-14:00
• Venue: Potter Room / Zoom .
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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arXiv:2504.18637v2 Announce Type: replace Abstract: The Sunyaev-Zeldovich (SZ) effect is a window into the astrophysical processes of galaxy clusters, and relativistic corrections (the "rSZ") promise to provide a global census of the gas feedback within clusters. Upcoming wide-field millimeter-wave surveys such as the Simons Observatory (SO), Fred Young Submillimeter Telescope, and CMB-S4 will make increasingly precise measurements of the SZ effect and its relativistic corrections. We present simulated full-sky maps of the rSZ effect and a fast code to generate it, for use in the development of analysis techniques and pipelines. As part of the websky simulation suite, our mock observations have semi-realistic cross-correlations with other large-scale structure tracers, offering insights into the formation and evolution of galaxy clusters and large-scale structure. As a demonstration of this, we examine what an SO-like experiment can learn from the rSZ effect. We find that high significance detections will be possible, provided that the instrumental systematics are under control, and that the evolution of cluster temperatures with mass and redshift can be probed in a manner complementary to X-ray measurements.

arXiv:2510.09569v1 Announce Type: new Abstract: Context. Current large-scale, high-cadence surveys, such as the Zwicky Transient Facility (ZTF), provide detections of new and rare types of transients and supernovae whose physical origins are not well understood. Aims. We investigate the nature of SN 2021lwz at a redshift z=0.065, an overluminous supernova (SN) of absolute magnitude, $M_{g} \sim -20.1$ AB, falling in the lower range of superluminous supernovae (SLSNe) luminosities, and discovered in a faint dwarf galaxy with an absolute magnitude of $M_{g} \simeq -14.5$ AB. Methods. SN 2021lwz is studied using optical spectroscopy, photometry and imaging linear polarimetry obtained during several follow-up campaigns. All the data are used to analyse and model the evolution of the explosion. Comparisons with other SNe of well known or rarer types are investigated. Results. SN 2021lwz belongs to the rare class of rapidly evolving transients. The bolometric light curve rises in about 7 days to a peak luminosity of about 5 x10^{43} erg/s, at a rate of 0.2 mag/day close to the peak. Spectroscopy modeling reveals more similarities with a normal Type Ic-like SN than with a SLSN before peak, showing broadened lines after peak. Light curve modeling shows that the Arnett model of the bolometric light curve using a radioactive source ($^{56}$ Ni) is not able to reasonably explain the light curve evolution. A magnetar model seems more appropriate, suggesting that the explosion of low ejecta mass ($M_{\rm ej} \sim 0.24 ~M_\odot$) took place in a low mass ($M \sim 10^{6.66}~M_\odot$) dwarf galaxy of specific star-formation rate about ten times larger than typical star-forming galaxies. Conclusions. Given its spectroscopic properties and the low ejecta mass needed to model its light-curve, SN 2021lwz does not match with many core-collapse H-poor SNe Types. It shares similarities with rarer transients like SN 2014ft, iPTF 16asu and SN 2018gep.

Tracking subcritical dynamo transitions and minimal dynamo seeds

In some astrophysical flows known to be linearly stable, finite-amplitude perturbations with favourable spatial structure can nonlinearly trigger a transition from a non-magnetic, non-turbulent state to self-sustained dynamo action and turbulence. Such transitions are suspected to significantly impact spin-down in radiative stellar layers or accretion rates in stellar discs. I will first present numerical examples of nonlinearly-triggered Tayler-Spruit dynamos in a spherical shell and zero-net-flux MRI dynamos in a quasi-Keplerian plane flow. I will then discuss how optimal control can identify stable, nontrivial (M)HD equilibria without requiring prior knowledge of the transition mechanisms.

• Speaker: Florence Marcotte (Université Côte d'Azur, INRIA, CNRS)
• Monday 13 October 2025, 16:00-17:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Mattias Brynjell-Rahkola.

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arXiv:2510.07637v1 Announce Type: new Abstract: Supernova H0pe is a multiply-imaged Type Ia supernova (SN Ia) and the second lensed SN to yield a measurement of the Hubble constant by the time-delay cosmography method, finding $H_0 = 75.4^{+8.1}_{-5.5} \text{km s}^{-1} \text{Mpc}^{-1}$. We investigate the seven lens modeling approaches used to derive $H_0$, assessing their agreement with $\Lambda \text{CDM}$ constraints from SN Ia surveys through a purely observational comparison. While photometrically derived magnifications yield distance moduli in line with $\Lambda \text{CDM}$ expectations, our comparison reveals that lens model predictions, even the most precise ones, consistently overestimate the magnification, with a offset of $ \Delta \mu > 1$ mag. This known bias, already appreciated by modeling teams, is independently confirmed through our analysis and highlights the value of lensed SNe as a tool to test model accuracy. If unaccounted for, such magnification biases can propagate into uncertainties in derived cosmological parameters, including $H_0$, and affect the interpretation of future precision measurements. These findings highlight a critical challenge for precision cosmology using strongly lensed transients. With next-generation surveys such as LSST, Roman, and Euclid poised to discover many more gravitationally lensed supernovae, the development and validation of robust, accurate lens models will be essential for using these rare events to probe cosmology.

arXiv:2502.20437v3 Announce Type: replace Abstract: We introduce the THESAN-ZOOM project, a comprehensive suite of high-resolution zoom-in simulations of $14$ high-redshift ($z>3$) galaxies selected from the THESAN simulation volume. This sample encompasses a diverse range of halo masses, with $M_\mathrm{halo} \approx 10^8 - 10^{13}~\mathrm{M}_\odot$ at $z=3$. At the highest-resolution, the simulations achieve a baryonic mass of $142~\mathrm{M}_\odot$ and a gravitational softening length of $17~\mathrm{cpc}$. We employ a state-of-the-art multi-phase interstellar medium (ISM) model that self-consistently includes stellar feedback, radiation fields, dust physics, and low-temperature cooling through a non-equilibrium thermochemical network. Our unique framework incorporates the impact of patchy reionization by adopting the large-scale radiation field topology from the parent THESAN simulation box rather than assuming a spatially uniform UV background. In total, THESAN-ZOOM comprises $60$ simulations, including both fiducial runs and complementary variations designed to investigate the impact of numerical and physical parameters on galaxy properties. The fiducial simulation set reproduces a wealth of high-redshift observational data such as the stellar-to-halo-mass relation, the star-forming main sequence, the Kennicutt-Schmidt relation, and the mass-metallicity relation. While our simulations slightly overestimate the abundance of low-mass and low-luminosity galaxies they agree well with observed stellar and UV luminosity functions at the higher mass end. Moreover, the star-formation rate density closely matches the observational estimates from $z=3-14$. These results indicate that the simulations effectively reproduce many of the essential characteristics of high-redshift galaxies, providing a realistic framework to interpret the exciting new observations from JWST.

arXiv:2510.07534v1 Announce Type: new Abstract: We review the state of the evidence for the existence and observational appearance of supermassive black hole binaries. Such objects are expected from standard hierarchical galaxy evolution to form after two galaxies, each containing a supermassive black hole, have merged, in the centre of the merger remnant. A complex interaction is predicted to take place with stars and gas in the host galaxy, leading to observable signatures in weakly as well as actively accreting phases. Direct observational evidence is available and shows examples of dual active galactic nuclei from kpc scales down to parsec scales. Signatures of possibly closer supermassive black hole binaries may be seen in jetted black holes. The interaction with stars and gas in a galaxy significantly affects the hardening of the binary and hence contributes to uncertainties of the expected gravitational wave signal. The Laser Interferometer Space Antenna (LISA) should in the future detect actual mergers. Before the launch of LISA, pulsar timing arrays may have the best chance to detect a gravitational wave signal from supermassive black hole binaries. The first signs of the combined background of inspiralling objects might have been seen already.

arXiv:2510.07369v1 Announce Type: new Abstract: Wide-field slitless spectroscopy (WFSS) is a powerful tool for studying large samples of galaxies across cosmic times. With the arrival of JWST, and its NIRCAM grism mode, slitless spectroscopy can reach a medium spectral resolution of $(R\sim 1600)$, allowing it to spatially resolve the ionised-gas kinematics out to $z\sim 9$. However, the kinematic information is convolved with morphology along the dispersion axis, a degeneracy that must be modelled to recover intrinsic properties. We present the Grism Emission-line Kinematics tOol ($\texttt{geko}$), a Python package that forward-models NIRCam grism observations and infers emission-line morphologies and kinematics within a Bayesian framework. $\texttt{geko}$ combines S\'ersic surface-brightness models with arctangent rotation curves, includes full point-spread function (PSF) and line-spread function (LSF) convolution, and leverages gradient-based sampling via $\texttt{jax}$/$\texttt{numpyro}$ for efficient inference. It recovers parameters such as effective radius, velocity dispersion, rotational velocity, rotational support, and dynamical mass, with typical run times of $\sim$20 minutes per galaxy on GPUs. We validate performance using extensive mock data spanning position angle, S/N, and morphology, quantifying where degeneracies limit recovery. Finally, we demonstrate applications to real FRESCO H$\alpha$ emitters at $z\approx 4-6$, recovering both rotation- and dispersion-dominated systems. $\texttt{geko}$ opens the way to statistical studies of galaxy dynamics in the early Universe and is publicly available at https://github.com/angelicalola-danhaive/geko.

Title to be confirmed

Abstract not available

• Speaker: Antony Lewis (University of Sussex)
• Monday 20 October 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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The Nature of Rocky Exoplanets Via Study of White Dwarf Stars

Venerable Exoplanetary Systems, a brief history: Revelations from 100 years ago to some new discoveries via an unusual, entirely unanticipated, observational technique. The discussion will include the first ever observational evidence for the existence of an extrasolar planetary system, the first observational evidence for the existence of a differentiated extrasolar planet (i.e., one with a crust and mantle), and the remarkable discovery of objects with 100s of times larger abundances of the light elements Be and B than previously seen in any astronomical object.

• Speaker: Prof. Ben Zuckerman
• Tuesday 14 October 2025, 13:00-14:00
• Venue: HOYLE LECTURE THEATRE + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr. Mariona Badenas Agusti.

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arXiv:2505.02233v2 Announce Type: replace Abstract: We investigate the degeneracy between the effects of ultra-light axion dark matter and baryonic feedback in suppressing the matter power spectrum. We forecast that galaxy shear data from the Rubin Observatory's Legacy Survey of Space and Time (LSST) could limit an axion of mass $m = 10^{-25}\,\mathrm{eV}$ to be $\lesssim 5\%$ of the dark matter, stronger than any current bound, if the interplay between axions and feedback is accurately modelled. Using a halo model emulator to construct power spectra for mixed cold and axion dark matter cosmologies, including baryonic effects, we find that galaxy shear is sensitive to axions from $10^{-27}\,\mathrm{eV}$ to $10^{-21}\,\mathrm{eV}$, with the capacity to set competitive bounds across much of this range. For axions with $m \sim 10^{-25}\,\mathrm{eV}$, the scales at which axions and feedback impact structure formation are similar, introducing a parameter degeneracy. We find that, with an external feedback constraint, we can break the degeneracy and constrain the axion transfer function, such that LSST could detect a $10^{-25}\,\mathrm{eV}$ axion comprising 10\% of the dark matter at $\sim 3 \sigma$ significance. Direct reconstruction of the non-linear matter power spectrum provides an alternative way of analysing weak lensing surveys, with the advantage of identifying the scale-dependent features in the data that the dark matter model imposes. We advocate for dedicated cosmological hydrodynamical simulations with an axion dark matter component so that upcoming galaxy and cosmic microwave background lensing surveys can disentangle the dark matter-baryon transfer function.

arXiv:2510.06558v1 Announce Type: new Abstract: The CosmoCube project plans to deploy a global 21-cm spectrometer with 10-100 MHz observation band in a lunar orbit. The farside part of such an orbit, i.e. the part of orbit behind the Moon, offers an ideal site for accurately measuring the 21-cm signal from the Dark Ages, Cosmic Dawn and Epoch of Reionization, as the effects of the Earth's ionosphere, artificial radio frequency interference (RFI), and complex terrain and soil are all avoided. Given the limitations of a satellite platform, we propose a receiver calibration system design based on a Radio Frequency system-on-chip, consisting of a Vector Network Analyzer (VNA) sub-system, and a source switching sub-system. We introduce the measurement principle of the VNA, and discuss the effect of quantization error. The accuracy, stability and trajectory noise of the VNA are tested in laboratory experiments. We also present the design of the source-switching sub-system, generating mock datasets, showing that the imperfect return loss, insertion loss, and isolation of surface-mounted microwave switches have a minimal effect on the sky foreground fitting residuals, which are within $\pm10$ mK under optimal fitting condition. When all possible measurement errors in reflection coefficients and physical temperatures are taken into account, the foreground fitting residuals for the 50-90 MHz part of the spectrum remain around $\pm20$ mK.

arXiv:2510.06315v1 Announce Type: new Abstract: Understanding how galaxies assemble their mass during the first billion years of cosmic time is a central goal of extragalactic astrophysics, yet joint constraints on their sizes and kinematics remain scarce. We present one of the first statistical studies of the $\mathrm{H}\alpha$ size-mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of $z\approx 4-6$ from the FRESCO and CONGRESS NIRCam grism surveys. We measure the $\mathrm{H}\alpha$ morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool $\texttt{geko}$, and complement them with stellar continuum sizes in the rest-frame FUV, NUV, and optical, obtained from modelling of imaging data from the JADES survey with $\texttt{Pysersic}$. At $z\approx5$, we find that the average H$\alpha$ sizes are larger than the stellar continuum (FUV, NUV and optical), with $r_{\rm e, H\alpha}= 1.17 \pm 0.05$ kpc and $r_{\rm e,cont} \approx 0.9$ kpc for galaxies with $\log(M_{\star} ~\rm [M_{\odot}])= 9.5$. However, we find no significant differences between the stellar continuum sizes at different wavelengths, suggesting that galaxies are not yet steadily growing inside-out at these epochs. Instead, we find that the ratio $r_{\rm e, H\alpha}/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($\Delta \rm MS$), consistent with an expansion of H$\alpha$ sizes during episodes of enhanced star formation caused by an increase in ionising photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support $v/\sigma$, which could be tracing accreting gas illuminated by the \Ha\ emission. Finally, we find that about half of the elongated systems ($b/a < 0.5$) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.

Einstein Metrics, Interacting QFT’s and Confinement in Four and Five Dimensions

M-theory provides a geometric framework to describe a variety of interesting quantum field theories in which the QFT ’s arise from Einstein metrics. We motivate a precise definition of this framework which (partly) takes the form of the space of complete, asymptotically conical Ricci flat manifolds in various dimensions. We show how this provides insights into various strongly coupled systems such as non-Abelian gauge theories in four and more dimensions and leads to confining string theories in four and five dimensions. The four dimensional strings can be compared to flux tubes in Yang-Mills theories.

• Speaker: Bobby Acharya (ICTP Trieste)
• Thursday 09 October 2025, 12:00-13:00
• Venue: MR9/Zoom https://cam-ac-uk.zoom.us/j/81827907718.
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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Dust cosmological collapse

Cosmological collapse, where gravitational pull overcomes the universe’s expansion, can create primordial black holes or other bound structures. The ultimate fate of such a collapse depends on the initial distribution of matter and its equation of state, as pressure counteracts the gravitational infall. Then, in the dust case, where there is no pressure, shouldn’t a black hole always form? In this talk, I will attempt to answer this question using numerical relativity simulations, discussing the challenges of this problem and the progress we have made so far. Additionally, as this is in full GR, it presents an opportunity to characterise the cosmological spacetime; therefore, I will also describe this using gravito-electromagnetism and Weyl invariants.

• Speaker: Robyn Munoz (Sussex University)
• Friday 17 October 2025, 13:00-14:00
• Venue: Potter Room / Zoom .
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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An Ever-Evolving Story of the Chemical Composition and Accretion History of the HR 8799 Planets

As the poster child of directly-imaged exoplanets, the HR 8799 system consists of four young gas giant planets that are likely to be in mean motion resonance. Their formation origin and evolutionary history remain uncertain but can be constrained by a rich set of archival and newly-obtained proprietary data. I will present the detection of key molecular species such as H2O and CO in the planetary atmosphere using the combination of high-contrast imaging and high-resolution spectroscopy. Despite solid detections, interpreting the data through spectral retrieval analysis has been controversial, largely due to systematics introduced by inhomogeneous data sets and differing underlying model assumptions. I will present a calibration procedure using benchmark brown dwarfs to understand the systematic errors. This leads to an accurate constraint of atmospheric composition, which in turn allows us to robustly trace the formation history. Moreover, using a sample of directly-imaged exoplanets including the HR 8799 planets, we convert their atmospheric metallicities to the accreted mass of solid during formation through a Bayesian framework that marginalizes the probabilities of disk conditions, formation locations, planetary interior structures, and accretion physics. We show that more than 50 M⊕ solid mass has been accreted, pointing to an extremely early formation time scale (

• Speaker: Ji Wang
• Thursday 06 November 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Jan Scholtz.

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