Talks

In the inner regions of protoplanetary discs, ionisation chemistry controls the fluid viscosity, and is thus key to understanding various accretion, outflow and planet formation processes. The ionisation is driven by thermal and non-thermal processes in the gas phase, as well as by dust-gas interactions that lead to grain charging and ionic and thermionic emission from grain surfaces. The latter dust–gas interactions are moreover a strong function of the grain size distribution. Previous chemical networks, including these chemical processes, did not accurately capture this dependence on the grain size distribution. In this talk, I will explain how our network – which explicitly includes a distribution of grains, at minimal extra computational cost – shows that chemical abundances (and thus resistivities) may vary by orders of magnitude for a reasonable set of dust distributions. Furthermore, I will illustrate how the charge derived on the surface of the grains is expected to severely hinder collisions between these grains in the inner disc – an important effect to be included in solving the Smoluchowski equation, governing the growth and fragmentation of grains. Finally, I will show the progress we have made towards developing 2D magnetohydrodynamic (MHD) simulations of the inner disc, including: multi-species (gas + dust distribution) hydrodynamics, radiation transport, our self-consistent chemistry, MHD resistivities and charge-dependent fragmentation and coagulation of grains.

• Speaker: Morgan Williams [Imperial College London]
• Monday 19 May 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Samuel Turner.

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In March 2025, the Atacama Cosmology Telescope (ACT) released its last cosmological analysis along with a new cosmic microwave background (CMB) dataset. The sixth data release (DR6), including data collected from 2017 to 2022, covers 40% of the sky at arcminute resolution providing the most precise maps of CMB temperature and polarization. In this talk, I will give an overview of the challenges faced during the ACT DR6 analysis and describe its constraints on fundamental assumptions of the standard cosmological model and extensions to it.

• Speaker: Adrien La Posta (University of Oxford)
• Tuesday 20 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR12, Pav. D basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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In Person

We cannot predict life. We can, instead, learn from Earth’s biodiversity and their varied molecular catalogue of markers of adaptability. Biopigments are widespread biomolecules that serve as powerful surface biomarkers of adaptability to extreme conditions on our planet. These molecules have distinct and unique spectral signatures providing a promising avenue for detecting extraterrestrial life. However, current surface models for other planets overlook Earth’s broader biodiversity. In the Solar System, current models struggle to constrain non-icy mysterious spots on the surface of the Jovian icy moon Europa for lack of matching reference spectra. In parallel, exoplanet surface models tend to overemphasize chlorophyll-based landscapes, often constrained by the assumption that photosynthesis requires visible light. This introduces unnecessary restrictions on atmospheric opacity and composition. In reality, Earth’s biosphere hosts a vast array of biopigments capable of harnessing energy across the UV to IR spectrum, driving diverse metabolisms, volatile byproducts, and environmental adaptations—many of which serve as analogues for targets to be studied with future telescopes and space missions. By integrating Earth’s biological and evolutionary diversity with astrophysical tools, I will present life-detection frameworks based on a broad spectral dataset. I will show how in situ reflectance data from Svalbard (Arctic) and Atacama Desert can help us correlate biosignatures with specific environments. These findings contribute to biologically informed planetary models, crucial for the next generation missions, including Extremely Large Telescopes (ELTs), the Habitable Worlds Observatory (HWO) and Large Interferometer For Exoplanets (LIFE), as well as NASA ’s Europa Clipper, ESA ’s Juice and Enceladus L4. These exciting new instruments will probe several planetary surfaces for a new biosphere where orange, yellow, or purple may be the new green.

• Speaker: Ligia F Coelho (Cornell Astronomy)
• Thursday 15 May 2025, 11:00-12:00
• Venue: Battcock, Room F17.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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In March 2025, the Atacama Cosmology Telescope (ACT) released its last cosmological analysis along with a new cosmic microwave background (CMB) dataset. The sixth data release (DR6), including data collected from 2017 to 2022, covers 40% of the sky at arcminute resolution providing the most precise maps of CMB temperature and polarization. In this talk, I will give an overview of the challenges faced during the ACT DR6 analysis and describe its constraints on fundamental assumptions of the standard cosmological model and extensions to it.

• Speaker: Adrien La Posta (University of Oxford)
• Tuesday 20 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR12, Pav. D basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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The Milky Way experienced a major satellite merger 10 Gyr ago which altered, but did not destroy, the early high-alpha disk and created both an accreted and an in situ inner halo. The low-alpha disk that formed subsequently became bar-unstable 8 Gyr ago, creating the b/p bulge that also contains the inner high-alpha disk stars. M31 experienced a similar major satellite merger 3 Gyr ago which greatly heated and mixed the pre-existing high-metallicity disk, and also caused a massive inflow of gas and the formation of a dynamically hot secondary inner disk. Such a merger is consistent with the wide-spread star formation event 2-4 Gyr ago seen in disk colour-magnitude diagrams, and with the major substructures and metal-rich stars in the inner halo of M31 , when comparing photometric and recent spectroscopic data with available models. The merged satellite must have had a broad metallicity distribution and would have been the third most massive galaxy in the Local Group before the merger.

• Speaker: Ortwin Gerhard, MPE (Garching)
• Thursday 15 May 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: .

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21-cm cosmology experiments have opened new frontiers in our quest to explore the early universe. However, the rapid expansion of satellite constellations in Low Earth Orbit (LEO) poses a significant threat. SpaceX’s Starlink is particularly concerning due to unintended electromagnetic radiation (UEMR) generated by its hardware and onboard electronic subsystems, as reported by observatories such as the Low-Frequency Array (LOFAR). These emissions could contaminate observations of the faint 21-cm signal, already buried beneath foreground emissions and radio frequency interference (RFI). The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a low-frequency radio telescope based in the Karoo radio reserve, South Africa, designed to detect the global 21-cm signal from Cosmic Dawn. In this talk, I will present my ongoing work assessing the extent to which Starlink impacts REACH . My approach combines orbital trajectory simulations using Two-Line Element (TLE) catalogues with geometric constraints to identify Starlink flyovers within REACH ’s field of view. These are cross-referenced with power spectral density (PSD) measurements to search for correlations indicating UEMR , including Doppler shift analysis. I conclude by outlining plans to automate this process and how this work contributes to broader efforts to safeguard radio astronomy from satellite interference.

• Speaker: Gabriella Rajpoot / Cavendish Laboratory
• Wednesday 14 May 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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Gravitational-wave observations have revealed the population of stellar remnants from a new angle. Yet their stellar progenitors remain uncertain, in particular in the case of black holes. At least a fraction of these stars is believed to form in isolated binary systems. In this talk, I will discuss how binary mass transfer affects the late evolution and final fate of massive stars. The focus will be on stars that transfer their outer layers to a companion star and become binary-stripped. Binary-stripped stars develop systematically different core structures compared to single stars. I will discuss consequences for supernovae, black hole formation, and gravitational-wave observations.

• Speaker: Eva laplace, University of Leuven, Belgium
• Thursday 22 May 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: ag2017.

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In this talk, I’ll take you on a whistle-stop tour of my journey in 21-cm cosmology – from my PhD days in Cambridge to fellowship and research scientist positions in the USA and Canada. I’ll discuss the significance of 21-cm cosmology in understanding the Universe’s first billion years and describe key projects I’ve worked on, including the SKA , HERA, EDGES , and REACH . Along the way, I’ll share some personal highlights from my time in North America, including adventures in national parks and snow sports.

• Speaker: Dr. Peter Sims (University of Cambridge)
• Tuesday 20 May 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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The matter power spectrum is one of the fundamental quantities in the study of large-scale structure in cosmology. In this talk, I will describe its small-scale asymptotic limit, and give a theoretical argument to the effect that, for cold dark matter, P(k) has a universal asymptotic scaling with the wave-number k, for k >> k_nl, viz. P(k) ~ k^(-3). I will explain how gravitational collapse drives a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and non-linear time scales are balanced, and how this balance dictates the k dependence of the power spectrum. The coldness of the dark-matter distribution function — its non-vanishing only on a 3-dimensional sub-manifold of phase-space — underpins the analysis. I will show Vlasov-Poisson simulations that support the theory, and if time permits, also describe a stationary-phase technique for deriving an equivalent result. 

• Speaker: Barry Ginat / University of Oxford
• Wednesday 14 May 2025, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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The existence of dark energy and dark matter hint that there is more to gravity than meets the eye.  A wide range of new theories, exhibiting a new scalar particle with a property called screening, indicate small-scale tests as the most promising route towards detection of new particles.  Atomic physics is especially promising.  I will discuss how pairs of atomic clocks are capable of searching for equivalence-principle violating scalar couplings to Standard Model particles, which hold the potential to detect quintessence, ultralight dark matter, and modified gravity.  Similarly, atom interferometry and atomic spectroscopy provide a window to detect new forces associated with new screened scalars as well.

• Speaker: Benjamin Elder, Imperial College London
• Friday 16 May 2025, 13:00-14:00
• Venue: MR20/Zoom: https://cam-ac-uk.zoom.us/j/89585655242?pwd=ur229qk6mRXNG2a1EQVdb7PAdUx2gU.1.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Abstract not available

• Speaker: Elena Khomenko (IAC Tenerife)
• Monday 09 June 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity is much lower than the viscosity, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm, i.e. the ratio between viscosity and resistivity) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of 1e-6 to 1e-3. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies has indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars.

Accelerating the magnetohydrodynamics solvers with graphics processing units has recently opened an avenue to numerically study low-Pm flows. With these tools we have been able to perform simulations that approach the solar Pm-values, studying both kinematic and non-linear regimes. Contrary to earlier findings, the SSD turns out not only to be possible for Pms down to 0.0031, but even to become increasingly easy to excite for Pm below approximately 0.05. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. The saturation strength of the SSD is of the order of the turbulent kinetic energy independent of the Pm, when the magnetic Reynolds number (Rm) is moderate (up to a few thousands). For higher Rm the saturation strength rapidly diminishes and reaches levels of order of magnitude lower than turbulent kinetic energy, casting a new doubt of the SSD being important in the Sun and stars. Even higher resolution studies, however, would be required to verify this robustly. For such calculations, however, extraordinary resources/quantum computers are required.

• Speaker: Maarit Korpi-Lagg [Helsinki/Espoo]
• Monday 12 May 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Mattias Brynjell-Rahkola.

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Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity $\eta$ is much lower than the viscosity $\nu$, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm=$\nu/\eta$) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of $10>6;">$$10{-3}$. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies had indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars.

Accelerating the magnetohydrodynamics solvers with graphics processing units has recently opened an avenue to numerically study low-Pm flows. With these tools we have been able to perform simulations that approach the solar Pm-values, studying both kinematic and non-linear regimes. Contrary to earlier findings, the SSD turns out not only to be possible for Pms down to 0.0031, but even to become increasingly easy to excite for Pm below $\simeq 0.05$. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. The saturation strength of the SSD is of the order of the turbulent kinetic energy independent of the Pm, when the magnetic Reynolds number (Rm) is moderate (up to a few thousands). For higher Rm the saturation strength rapidly diminishes and reaches levels of order of magnitude lower than turbulent kinetic energy, casting a new doubt of the SSD being important in the Sun and stars. Even higher resolution studies, however, would be required to verify this robustly. For such calculations, however, extraordinary resources/quantum computers are required.

• Speaker: Maarit Korpi-Lagg [Helsinki/Espoo]
• Monday 12 May 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Mattias Brynjell-Rahkola.

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The Milky Way experienced a major satellite merger 10 Gyr ago which altered, but did not destroy, the early high-alpha disk and created both an accreted and an in situ inner halo. The low-alpha disk that formed subsequently became bar-unstable 8 Gyr ago, creating the b/p bulge that also contains the inner high-alpha disk stars. M31 experienced a similar major satellite merger 3 Gyr ago which greatly heated and mixed the pre-existing high-metallicity disk, and also caused a massive inflow of gas and the formation of a dynamically hot secondary inner disk. Such a merger is consistent with the wide-spread star formation event 2-4 Gyr ago seen in disk colour-magnitude diagrams, and with the major substructures and metal-rich stars in the inner halo of M31 , when comparing photometric and recent spectroscopic data with available models. The merged satellite must have had a broad metallicity distribution and would have been the third most massive galaxy in the Local Group before the merger.

• Speaker: Ortwin Gerhard, MPE (Garching)
• Thursday 15 May 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: .

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The Dark Energy Spectroscopic Instrument (DESI) is undertaking a five-year survey spanning 14,000 square degrees of the sky, with the goal of mapping 40 million extragalactic redshifts. These observations aim to refine our understanding of the universe’s expansion history through Baryon Acoustic Oscillations (BAO) and the growth of cosmic structure via Full Shape analyses. In 2025, the DESI collaboration released BAO cosmology results from the Data Release 2 (DR2) sample, assembled from the first three years of data taking (2021 – 2024). This presentation will introduce the instrument and the survey and review the BAO measurements derived from DR2 . I will discuss the consistency of BAO constraints with other probes—-CMB (including the latest ACT DR6 CMB data) and supernovae—-and present cosmological constraints on dark energy and neutrino masses. I will conclude by providing an outlook on upcoming DESI analyses.

Zoom link: https://cam-ac-uk.zoom.us/j/86165819179?pwd=uITeMzHyCpzVlUmVufdGEJXudF0dsy.1

• Speaker: Arnaud de Mattia (IRFU, CEA, Université Paris-Saclay)
• Monday 12 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR5, Pav A basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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The Dark Energy Spectroscopic Instrument (DESI) is undertaking a five-year survey spanning 14,000 square degrees of the sky, with the goal of mapping 40 million extragalactic redshifts. These observations aim to refine our understanding of the universe’s expansion history through Baryon Acoustic Oscillations (BAO) and the growth of cosmic structure via Full Shape analyses. In 2025, the DESI collaboration released BAO cosmology results from the Data Release 2 (DR2) sample, assembled from the first three years of data taking (2021 – 2024). This presentation will introduce the instrument and the survey and review the BAO measurements derived from DR2 . I will discuss the consistency of BAO constraints with other probes—-CMB (including the latest ACT DR6 CMB data) and supernovae—-and present cosmological constraints on dark energy and neutrino masses. I will conclude by providing an outlook on upcoming DESI analyses.

• Speaker: Arnaud de Mattia (IRFU, CEA, Université Paris-Saclay)
• Monday 12 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR5, Pav A basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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I will discuss recent work where it was demonstrated that regular black holes emerge as the unique spherically symmetric solutions to certain gravitational actions that incorporate infinite towers of higher-derivative corrections. I will then illustrate what happens when one considers the collapse of spherical thin shells and dust in these theories, showing that the collapse is generically non-singular. This is based on work with Pablo Bueno, Pablo Cano and Ángel Murcia.

• Speaker: Robbie Hennigar, Durham University
• Friday 09 May 2025, 13:00-14:00
• Venue: MR9/Zoom: https://cam-ac-uk.zoom.us/j/87869493842?pwd=vGeCJJgQZa8PwZOhk1kpE0nbj6DgpJ.1.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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The High Accuracy Radial velocity Planet Searcher-3 (HARPS3) is being developed for the Terra Hunting Experiment, a 10-year observing campaign to conduct nightly observations of a carefully selected group of solar-like stars to detect long-period, low-mass exoplanets. The goal is to achieve extremely-precise radial velocity (EPRV) measurements at the level of 10 cm/s to enable the detection of an Earth-twin. Attaining this precision requires a deep understanding of all error sources: instrumental systematics, astrophysical noise, and data reduction algorithms.

To address the latter, I have developed a novel method to test the data reduction pipeline (DRP) using synthetic data. Rather than attempting to replicate the instrument’s response exactly, the method is designed to systematically probe the DRP ’s performance, identify potential biases, and validate the reduction algorithms. By injecting known inputs into the DRP and tracing their propagation, I can control all aspects of the data, test specific algorithms, and verify the accuracy of the reduction products. The aim is to use simulated data to identify systematic biases and inaccuracies that could impact EPRV measurements.

In this talk I will present my work, currently in preparation for publication, describing how I simulate the data and discussing the first results of passing the synthetic echellogram through the DRP . This approach provides a framework to assess the performance of HARPS3 during commissioning and early operations – when it comes on-sky in late 2025 – enabling us to identify issues and refine data processing techniques.

• Speaker: Alicia Anderson (Cavendish Astrophysics)
• Tuesday 13 May 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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Abstract not available

• Speaker: Adrien La Posta (University of Oxford)
• Tuesday 20 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR12, Pav. D basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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Abstract not available

• Speaker: Arnaud de Mattia (IRFU, CEA, Université Paris-Saclay)
• Monday 12 May 2025, 13:00-14:00
• Venue: SPECIAL LOCATION - CMS, MR5, Pav A basement.
• Series: Cosmology Lunch; organiser: Louis Legrand.

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