Next External Talks

Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology – a dense core or a diluted halo – may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from standard models when the tidal deformability is calculated in a two-fluid framework neglecting the dilute and extended nature of the halo. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter’s role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

• Speaker: Violetta Sagun, University of Southampton
• Friday 30 May 2025, 13:00-14:00
• Venue: MR9/Zoom https://cam-ac-uk.zoom.us/j/87235967698.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Accretion disks power many of the universe’s most luminous phenomena, acting as intermediaries that enable matter to shed angular momentum and accrete onto stars or compact objects. While angular momentum transport in disks has been extensively studied, especially in the context of magneto-rotational turbulence, significant challenges remain. These include reconciling simulation results with observed accretion rates and understanding state transitions in cataclysmic variables, x-ray binaries, and quasars.

In this talk, I explore how strongly magnetised disks — where azimuthal magnetic fields dominate, with energies exceeding the plasma’s thermal energy — may help resolve these issues. Interest in this regime is motivated by recent “hyper-refined” cosmological simulations, in which such a disk forms self-consistently around a black hole and supports super-Eddington accretion rates. Using local shearing-box simulations, we identify two distinct turbulent states: the previously known “high-β” state with modest accretion stresses (α << 1) and weak magnetic fields, and a new “low-β” state with strong, self-sustaining azimuthal magnetic fields, supersonic turbulence, and rapid accretion (α ≈ 1). The transition between these states is abrupt and occurs when sufficiently strong azimuthal fields are present, allowing the system to sustain a Parker-instability-driven dynamo. While many aspects of this behaviour remain uncertain, it offers a promising pathway to reconcile simulations and observations, with interesting implications for quasars and other rapidly accreting systems.

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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In recent years, free floating planets, i.e. those planets not found to be in a planetary system and with no observable companions, have begun to be found in microlensing and direct imaging surveys. Observations have shown that they have a wide variety of masses, ranging from terrestrial-like to giant planets. Microlensing surveys predict that there could be on order tens of free floating planets per star in the Milky Way. How these planets form and arrive on their observed trajectories remains a very open and intriguing question.

Whilst there are many mechanisms for forming free floating planets, e.g. ejections from planet-planet interactions or gravitational collapse of gas within molecular clouds, very few models have predicted the properties of free floating planets on a global scale. In this talk I will present the outcomes of state-of-the-art circumbinary planet formation models, that naturally produce a large abundance free floating planets per system. I will show the resulting mass and velocity distributions arising from the models, which will then be extended to include stellar populations of both single and binary stars, taking into binary fractions, and separations. The population distributions show clear observable features that can be investigated by future missions such as Roman, where evidence of these features will directly point to the specific formation pathways of specific planets, as well as informing on the processes of the planet forming environment in which they originated.

• Speaker: Gavin Coleman [Queen Mary University London]
• Monday 16 June 2025, 14:00-15:00
• Venue: Venue to be confirmed.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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Abstract not available

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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In person.

Epistemic pessimism, the idea that there are fundamental barriers to the possibility of explaining an event, has been expressed under various guises in the context of the origin of life since the inception of the field. In this talk, I unpack three ways in which the epistemic pessimists’ argument has been mounted. The first claims that the origin of life cannot be explained because it is a unique event, which hinders researchers’ ability  to formulate generalizations about it. The second claims that the origin of life cannot be explained because it left no traces. Unlike palaeobiological research, origins of life researchers have no direct fossil evidence to work as ‘smoking guns’ (i.e., to verify one hypothesis about the origin of life over another). The third claims that the origin of life was a highly unlikely combination of events, making it impossible to recover the sequence of events leading up to life. I show how each argument fails. An upshot is that appeal to god-of-the-gaps or alien-of-the-gaps style arguments as possible explanations for the origin of life is unnecessary and unwarranted. 

• Speaker: Jules Macome (Cambridge History and Philosophy of Science)
• Thursday 29 May 2025, 11:00-12:00
• Venue: Battcock, Room F17.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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We introduce a unified Bayesian anomaly-detection framework for Cosmology, applied to the REACH global 21cm probe and also Type Ia supernovae. This approach embeds data-integrity beliefs directly into the inference process. Rather than excising contaminated or anomalous data points, the method employs a piecewise likelihood constrained by a Bernoulli prior and an Occam penalty, allowing anomalies to be down-weighted automatically while performing numerical sampling for parameter inference. When applied to supernova light curves, the framework yields precise estimates of brightness scaling, stretch, and colour, while also automating supernova sample and band selection. In the context of global 21 cm cosmology, it offers a principled way to mitigate radio-frequency interference (RFI), particularly within the band of interest. We also discuss additional potential applications of this methodology.

• Speaker: Samuel Leeney (University of Cambridge)
• Tuesday 27 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: Lukas Rossmanith
• Tuesday 03 June 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: David Buscher.

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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)
• Monday 02 June 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Thomas Colas.

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In this talk I will describe the results of the PI2FA project focused on creating and applying tools for multi-dimensional modeling of partially ionized chromospheric plasma based on the single-fluid and two-fluid multi-species formalism. Scientific questions include clarifying chromospheric heating mechanisms, creating multi-dimensional realistic models of the solar chromosphere incorporating ion-neutral effects, and understanding neutrals’ role in prominence dynamics. The research focused on fundamental mechanisms of energy propagation and exchange in complex plasmas, such as waves, instabilities, and plasma-radiation interactions, seeking the transition from one-dimensional idealized models to multi-dimensional simulations, and observational support. Among the main conclusions, our research unveiled that multi-fluid effects become pronounced for waves with frequencies lower than typical inter-particle collisional frequencies, unlike suggested by theory of waves in homogeneous plasmas; we showed that ambipolar heating is most significant in the quietest regions, characterized by small-scale dynamo fields; we found that multi-fluid effects hold great importance within transition layers between cool and hot materials, such as the solar transition region and prominence-corona interface. Multi-fluid effects operate at scales beyond the resolution capabilities of even our most advanced instrumentation, necessitating specialized observational initiatives. Our initial steps in this direction allowed the detection of subtle differences in velocities between ions and neutrals, in line with theoretical predictions.

• 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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Fast inward migration of planetary cores embedded in gaseous disks is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been considered until recently. In this talk, I will show that a planetesimal embedded in a dusty disk leads to an asymmetric dust-density distribution that can exert a net torque under conditions relevant to planetary embryos up to several Earth masses. Building on the results or a large suite of numerical simulations for measuring this dust torque under a wide range of conditions, I will present the first study showing that dust torques can have a significant impact on the migration and formation history of planetary embryos.

• Speaker: Martin Pessah [NBI Copenhagen]
• Thursday 29 May 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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In weakly ionized regions of protoplanetary disks, hydrodynamic instabilities likely play a key role in the development of turbulence, the formation of structures, and the transport of angular momentum. Among these, the vertical shear instability (VSI) stands out as a robust mechanism, requiring only baroclinic stratification and short thermal relaxation timescales to operate. In this talk, I will present results from axisymmetric radiation-hydrodynamical simulations of the VSI in passive, irradiated T Tauri disks, focusing on angular momentum redistribution, the emergence of secondary instabilities, and their role in VSI saturation. I will also discuss how dust and molecular cooling shape the regions where the VSI can operate, and compare these results with current observations of protoplanetary disks.

• Speaker: David Melon-Fuksman [MPIA Heidelberg]
• Tuesday 27 May 2025, 14:00-15:00
• Venue: MR4 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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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.

• 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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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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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 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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