Upcoming talks

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.

• 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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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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It has been almost ten years since CARMENES opened its two spectroscopic eyes at the Calar-Alto observatory. Here’s an up-to-date account of the findings: more than 40 new planets in a sample of 354 M dwarfs; mass estimates of 32 transiting planets; and more than 120 papers, also covering topics such as stellar magnetic activity, binaries, and atmospheric characterization of exoplanets. So, what’s next? Stellar activity is still the main factor limiting the detection of many more planets or estimating the mass of transiting planets around low-mass stars. But for CARMENES , stellar activity is a signal, not just correlated noise. In its spectroscopic time series, it is manifested as a quasiperiodic wavelength-dependent variability, which induces activity-related radial velocity (ARV) variations of at least 2 m/s. For many stars, ARV variability is >10 m/s. Fortunately, ARV variability differs from Doppler shifts: it is usually incoherent, wavelength-dependent, and accompanied by spectral shape variations. These differences can help us distinguish between activity-related and planetary signals and model both phenomena simultaneously.

• Speaker: Lev Tal-Or (Ariel)
• Tuesday 20 May 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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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 Dark Energy Spectroscopic Instrument (DESI) has published BAO measurements from one year of data (DR1) in 2024 and 3 years of data (DR2) in 2025. The DESI collaboration argue that their measurements suggest that dark energy is evolving and that this evidence is stronger using the DR2 data. This result would have major implications for fundamental physics if true. I will present a new way of looking at BAO data which shows that the DR2 data are more consistent with the Planck LCDM cosmology than the DR1 data. The evidence for evolving dark energy from DESI BAO has therefore weakened as the data have improved. I will also discuss the impact of systematic errors if DESI BAO data are combined with Type Ia supernovae. In summary, I find very little evidence to suggest that dark energy is evolving.

KICC Special Seminar

• Speaker: George Efstathiou (IoA/KICC)
• Thursday 22 May 2025, 11:30-12:00
• Venue: Ryle Meeting Room, KICC.
• Series: Kavli Institute for Cosmology Seminars; organiser: Steven Brereton.

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Abstract not available

• Speaker: Speaker to be confirmed
• Friday 20 June 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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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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Abstract not available

• Speaker: Anne Verhamme (University of Geneva)
• Friday 11 July 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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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 multitude of detected exoplanets and their diversity never cease to fascinate us, while the statistical trends emerging from these detections present promising opportunities to delve into the past of planetary systems, all the way back to their formation. In this talk, I will give an overview of my group’s recent observational and theoretical results on the formation of gas giants. Owing to their large gravitational influence these planets cannot be overlooked in the evolution of planetary systems towards a life-harbouring system such as our own. Results of RV and direct imaging surveys in recent years revealed that gas giants are not a common outcome of planet formation, and that their most frequent hosts – the intermediate-mass stars (IMSs) seem to hold the answers to their formation.

We investigate the formation of giant planets using the pebble-accretion driven planet formation simulations, exploring a range of different formation conditions. In this work, and in contrast to common approaches in the literature, we implement stellar-mass dependent time evolution of luminosity on the pre-main sequence, and find that this makes a significant difference to giant planet formation outcomes. We successfully reproduce the giant planet occurrence rates as a function of stellar mass, found by RV surveys. This work revealed that mass accretion rate is the key parameter in determining whether a star will likely host a giant planet in its future planetary system.

Our large surveys of pre-main sequence star candidates led to the first unbiased sample of such IMSs, and the result that their protoplanetary discs are dispersed faster than discs around low mass stars, a devastating prospect for giant planet formation unless it happens very fast (e.g., via GI). This is in stark contrast with the observational examples of massive discs actively forming planets at 5-6Myr of age. Our work shows that late gas accretion, as seen in some of those sources, must be the dominant mechanism that sustains the mass reservoir of these older protoplanetary discs. Our surveys, and follow-up with ALMA also allowed a unique insight in the elusive transition state from protoplanetary to debris discs and origin of gas in debris discs.

• Speaker: Olja Panic (Leeds)
• Tuesday 13 May 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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