School of Physical Sciences

ADIW03 - Climate Applications of Layering

• Speaker: Kasturi Shah (University of Cambridge)
• Tuesday 21 May 2024, 11:30-12:00
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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EMGW05 - Moduli stacks and enumerative geometry

• Speaker: Milla Kibble (Isaac Newton Institute)
• Monday 17 June 2024, 09:45-10:05
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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ADIW04 - Anti-Diffusion in Multiphase and Active Flows

• Speaker: Ulrike Tillmann (Isaac Newton Institute)
• Monday 10 June 2024, 10:00-10:20
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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FHTW02 - Fickle Heart: The intersection of UQ, AI and Digital Twins

• Speaker: Ulrike Tillmann (Isaac Newton Institute)
• Monday 03 June 2024, 09:45-10:00
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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Understanding how much mixing occurs in a stratified turbulent mixing event is a key question for understanding a variety of oceanic processes. However, directly measuring turbulent fluxes is difficult. Instead, parameterizations may be employed to estimate details of the mixing in terms of measurable quantities. One approach is to use characteristic lengthscales, such as the Thorpe and Ozmidov scales, to describe the state of the flow; relationships between such lengthscales can then in theory be used to estimate turbulent quantities of interest. While this approach has seen some success when applied to numerical simulations, there remain several challenges when it comes to using this approach in real observational datasets – for example, the underlying mechanism driving the mixing may not be known, or the measurements may only come from isolated profiles in the larger flow. In this work, we use a suite of direct numerical simulations of stratified shear instabilities as examples of both scouring and overturning mixing events. We subsample the data to mimic oceanographic measurements and compute lengthscales from the individual profiles. In doing so, we explore how well these profile-based estimates are able to characterize the state of the turbulent flow, particularly the mixing efficiency, and test how well fluxes can be computed from this limited data. 

ADIW03 - Climate Applications of Layering

• Speaker: Alexis Kaminski (University of California, Berkeley)
• Friday 24 May 2024, 10:00-11:00
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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Let k be a field of characteristic p>0 and let G be a topological group admitting an open pro-p subgroup. If H=k[I\G/I] denotes the corresponding Hecke algebra, then the functor of taking I-invariants provides an adjunction between the category of smooth k-linear representations of G and the category of H-modules. When H is the pro-p Iwahori-Hecke algebra of a p-adic reductive group, this adjunction is not well-understood. This is one of the main obstacles to understanding the smooth representation theory of these groups and to the mod-p Langlands program. I will describe a recent approach to studying this adjunction using the language and techniques from the theory of model categories. I will then finish by describing the homotopy classes of simple supersingular pro-p Iwahori-Hecke modules for GL_n.This is joint work with Jan Kohlhaase

• Speaker: Nicolas Dupré, University of Duisburg-Essen
• Wednesday 29 May 2024, 16:30-17:30
• Venue: MR12.
• Series: Algebra and Representation Theory Seminar; organiser: Adam Jones.

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Learning problems augmented with latent symmetries have attracted considerable interest in recent years. A significant class of such problems arises in experiments where a system is constrained to evolve in accordance with the rigid laws of nature, such as the celebrated technique of cryo electron microscopy (Cryo-EM). The constraint of such latent symmetries, given by group invariances or equivariances, precludes the possibility of having many repeated measurements of the exact same object, and poses a fundamental challenge for learning a signal in the presence of ambient noise. We will start with a gentle introduction to the problem of learning under latent symmetries, and explore its intriguing connections with a range of disparate topics — invariant theory, harmonic analysis, compressive sensing and Gaussian calculus. We will subsequently specialise to the Multi Reference Alignment (MRA) model, and explore the fundamental aspects of the recovery problem (such as sample complexity) in the presence of structural constraints on the signal (such as sparsity). In particular, we unveil a novel quartic dependence on noise level for the sample complexity of sparse MRA , leveraging a range of mathematical tools from uncertainty principles of Fourier analysis to techniques from combinatorial optimisation.

Based in part on the following works :

[1] Sparse Multi-Reference Alignment: Phase Retrieval, Uniform Uncertainty Principles and the Beltway Problem, S. Ghosh and P. Rigollet, Foundations of Computational Mathematics, 23(5), pp.1851-1898 (2023)

[2] Minimax-optimal estimation for sparse multi-reference alignment with collision-free signals, S. Ghosh, S.S. Mukherjee, J.B. Pan, arXiv preprint

• Speaker: Subhro Ghosh, National University of Singapore
• Friday 31 May 2024, 14:00-15:00
• Venue: MR12, Centre for Mathematical Sciences.
• Series: Statistics; organiser: Dr Sergio Bacallado.

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As theoretical physicists we know that Mathematics is unreasonably effective in describing our dynamical systems. This effectiveness singles out relevant mathematical structures in optimisation searches for models. In this talk I give an overview on where we can successfully apply this to search for symmetries or integrable structures of dynamical systems. Then I discuss how we use these methods to explore mathematical structures in beyond the Standard Model physics. In particular, we will use the string theory landscape as a benchmark for ML methods to search for new structures in this space.

• Speaker: Sven Krippendorf (LMU)
• Friday 17 May 2024, 16:00-17:00
• Venue: MR19 (Potter Room, Pavilion B), CMS.
• Series: HEP phenomenology joint Cavendish-DAMTP seminar; organiser: Benjamin Christopher Allanach.

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FHTW02 - Fickle Heart: The intersection of UQ, AI and Digital Twins

• Speaker: Linwei Wang (Rochester Institute of Technology)
• Tuesday 04 June 2024, 14:30-14:50
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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Abstract not available

• Speaker: Ce Zhang (University of Liverpool)
• Tuesday 11 June 2024, 11:00-12:00
• Venue: Ryle Seminar Room (930), Cavendish Laboratory, Department of Physics.
• Series: Cavendish HEP Seminars; organiser: Dr Steve Dennis.

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Co-Author: Mary-Louise Timmermans Diffusive staircases in the Arctic Ocean are structures that consist of mixed layers separated by thin interfaces in temperature and salinity. These staircases are prevalent in the Arctic and can occur when warm and salty water underlies cooler, fresher water. In this talk, I will focus on staircases in a particular region of the western Arctic Ocean, known as the Beaufort Gyre. Staircase layer thicknesses exhibit distinct differences across the Gyre, with a jump from thinner layers in the western Gyre to thicker layers on the eastern side. We will discuss how this sharp transition may be linked to a warm water influx into the Gyre and subsequent propagation across the region. The results may be of interest for interpreting how staircases may change under Arctic warming. 

ADIW03 - Climate Applications of Layering

• Speaker: Nicole Shibley (Princeton University)
• Monday 20 May 2024, 12:00-12:30
• Venue: Seminar Room 1, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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This seminar is hosted by the Institute of Astronomy, and will take place at the Madingley Road site.

The axion represents a well-motivated dark matter candidate with a relatively unexplored range of viable masses. Recent calculations argue for post-inflation axion mass ranges corresponding to frequencies of roughly 10-100 GHz. These frequency ranges offer challenges for the traditional cavity halscope which can be overcome through the use of metamaterial resonators that fill large volumes. The ALPHA (Axion Longitudinal Plasma HAloscope) experiment, located at Yale University, is an axion dark matter detector probing the 10-45 GHz frequency range. Axions can convert into photons in the tunable and cryogenically-cooled resonator within the 16-T magnet of the experiment, and be detected with the quantum-limited amplification and readout. In this talk, I will describe the general design parameters of the experiment, including the expected sensitivity, and discuss some of the challenges associated with resonator designs.

• Speaker: Dr Jón Guðmundsson (Stockholm)
• Tuesday 21 May 2024, 11:30-12:30
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Cavendish HEP Seminars; organiser: Dr Steve Dennis.

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Let G be a d-regular graph of growing degree on n vertices, and form a random subgraph G_p of G by retaining edge of G independently with probability p=p(d). Which conditions on G suffice to observe a phase transition at p=1/d, similar to that in the binomial random graph G(n,p), or, say, in a random subgraph of the binary hypercube Q^d?

We argue that in the supercritical regime p=(1+epsilon)/d, epsilon>0 being a small constant, postulating that every vertex subset S of G of at most n/2 vertices has its edge boundary at least C|S|, for some large enough constant C=C(\epsilon)>0, suffices to guarantee the likely appearance of the giant component in G_p. Moreover, its asymptotic order is equal to that in the random graph G(n,(1+\epsilon)/n), and all other components are typically much smaller.

We further give examples demonstrating the tightness of this result in several key senses.

This is joint work with Sahar Diskin, Joshua Erde and Mihyun Kang.

• Speaker: Michael Krivelevich (Tel Aviv)
• Thursday 30 May 2024, 14:30-15:30
• Venue: MR12.
• Series: Combinatorics Seminar; organiser: ibl10.

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We study colourings of the edges of the complete graph K_n. For some graph H, we say that a colouring contains a rainbow H, if there is an embedding of H into K_n, such that all edges of the embedded copy have pairwise distinct colours. Only two special types of rainbow trees of small diameter are forced by a high number of vertices of high colour degree, and then only two rainbow stars suffice. Whilst the optimal colour degree is known for one type of trees, we will discuss some bounds for the other type (of larger diameter).

• Speaker: Peter Wagner (Rostock)
• Thursday 23 May 2024, 14:30-15:30
• Venue: MR12.
• Series: Combinatorics Seminar; organiser: ibl10.

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Many Antarctic margin processes are changing including accelerated rates of ice sheet mass loss and a slowdown in the production of dense bottom waters. Although these changes are localised around the Antarctic continent, they have the potential to remotely disrupt downstream processes of climatic importance via advective connections along the shelf. In the first part of this talk I will present some work from my PhD thesis that investigates ocean connectivity around the Antarctic margins from a modelling perspective. The results from this work suggest there is widespread zonal connectivity between adjacent regions of the shelf, and that such connectivity is important to consider when interpreting and linking observed changes with upstream drivers. In the second part of the talk I will present results from simulations that investigate future climate-driven changes to Antarctic margin processes under different emission scenarios, with and without future freshwater contributions. Such changes are poorly constrained because many climate models fail to adequately resolve key features of the Antarctic margin including the narrow westward flowing currents, and the formation of both dense and abyssal water masses. Results from these simulations suggest that even under a mid-range emissions scenario without additional meltwater forcing, substantial changes in Antarctic continental shelf circulation and hydrography are possible by the end of this century.

• Speaker: Hannah Dawson, University of Tasmania
• Wednesday 22 May 2024, 13:00-14:00
• Venue: BAS Seminar Room 1; https://bas-ac-uk.zoom.us/j/91268978510.
• Series: British Antarctic Survey - Polar Oceans seminar series; organiser: Dr Birgit Rogalla.

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Through interactions between the modes of “waves” and “vortices”, stably stratified turbulence exhibits characteristic features both in 2D and 3D. Using DNS of the Navier-Stokes equations under the Boussinesq approximation, we investigate dynamical properties of stably stratified turbulence in relation with the mixed dimensionality. Also it is demonstrated that the temperature fluctuations make sharp fronts vertically which show sawtooth wave structures. Such sawtooth or cliff-ramp structures result in non-symmetric PDFs of the vertical derivative of temperature fluctuations. The reason for the mechanism of asymmetry will be considered.   

ADI - Anti-diffusive dynamics: from sub-cellular to astrophysical scales

• Speaker: Yoshifumi Kimura (Nagoya University)
• Thursday 16 May 2024, 16:00-17:00
• Venue: Seminar Room 2, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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We consider lower bounds on anti-concentration probabilities of the form P(|X| >= x), where X = a_1 ε_1 ... a_n ε_n is a Rademacher sum; ε_i are independent and uniform signs +1 or -1, and a_i > 0 are constants normalised so that Var(X) = 1. We determine the infimal value of P(|X| >= x) over Rademacher sums X for all values x >= 0, giving a partial answer to a question by Keller and Klein. In particular, for x = 1 we improve on a sequence of results to produce the optimal lower bound P(|X| >= 1) >= 7/32, confirming a conjecture of Hitczenko and Kwapień.

• Speaker: Lawrence Hollom (Cambridge)
• Thursday 16 May 2024, 14:30-15:30
• Venue: MR12.
• Series: Combinatorics Seminar; organiser: ibl10.

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A pseudo-automorphism f: X → X of a projective algebraic variety is a birational self-map inducing a regular automorphism from the complement of a codimension 2 open subset of X onto its image. Any surface pseudo-automorphism is regular. But It is a subtle problem to characterize pseudo-automorphisms in higher dimension that can be made regular in some birational model. This question is strongly related to the dynamics of f, and more specifically to its dynamical degrees. We shall discuss in more details the case of families of abelian varieties, which we studied in depth with Alexandra Kuznetsova.

• Speaker: Charles Favre, École Polytechnique
• Wednesday 15 May 2024, 14:15-15:15
• Venue: CMS MR13.
• Series: Algebraic Geometry Seminar; organiser: Holly Krieger.

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Velocity and magnetic shears are ubiquitous in tokamaks. Both of them play critical roles in plasma dynamics and confinement performance by affecting the coherence of fluctuations in space and time. In the past few decades, especially with the recent development of large-scale gyrokinetic numerical simulations, there has been a growing understanding of the shears in tokamaks.  However, theoretical discussions, which treat magnetic shear and E×B shear in tokamaks on an equal footing, have rarely been done. In this talk, I will start from the idea of resonance and demonstrate the analogies between velocity and magnetic shear. Shearing coordinates, introduced to eliminate the parallel gradient operator, serve as a useful tool to describe fluctuations in shear. With the adoption of shearing coordinates, we lose the normal mode description and instead gain a ‘quasi-mode’—an effective wave packet of spatially localized resistive interchange modes. Compared with resistive interchange mode, quasi-mode exhibits a broader mode structure, thus enhancing mixing. In addition, quasi-mode has many resemblances to the ballooning mode, which is a typical plasma instability arising from the toroidal geometry of tokamaks. Therefore, quasi-mode could be a bridge between cylindrical and toroidal geometries, and studying it helps us better understand the toroidicity effects in tokamaks. The talk ends with two traditional theoretical methods for ballooning mode studies: the Bloch eigenmode equation and the ballooning mode representation. This talk is an instructive resource for understanding the basics of shearing dynamics and toroidicity effects in tokamaks.

ADI - Anti-diffusive dynamics: from sub-cellular to astrophysical scales

• Speaker: Mingyun Cao (University of California, San Diego)
• Friday 17 May 2024, 11:30-12:30
• Venue: Seminar Room 2, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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Due to double diffusive processes below the thermocline in the Arctic Ocean, vertical density profiles exhibit a staircase structure in which successive layers of near-uniform density are separated by sharp density jumps.  Earlier work [Sutherland, PR Fluids (2016)] theoretically predicted the transmission and reflection of a vertically propagating internal gravity wave incident from above upon a density staircase with an arbitrary number of steps.   That work assumed a plane incident wave, being monochromatic in wavenumber and frequency.  For waves with given frequency, the transmission coefficient exhibited a series of spikes depending upon the depth of steps relative to the horizontal wavelength.  Here we use numerical simulations to examine the transmission of a vertically propagating internal wavepacket that interacts transiently with a staircase.  Rather than exhibit successive transmission spikes, the simulations show a smooth transition between perfect and zero transmission as the relative step size increases.  This results from internal waves exciting leaky internal modes of the staircase that retransmit waves above and below the staircase slowly over time, as can be predicted by theory.

ADI - Anti-diffusive dynamics: from sub-cellular to astrophysical scales

• Speaker: Bruce Sutherland (University of Alberta)
• Tuesday 14 May 2024, 16:00-17:00
• Venue: Seminar Room 2, Newton Institute.
• Series: Isaac Newton Institute Seminar Series; organiser: nobody.

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