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List of talks of interest to members of the Institute of Astronomy.
URL: https://talks.cam.ac.uk/show/index/13732
Updated: 15 min 52 sec ago

Tue 21 May 11:15: Diffusion meets Nested Sampling

Thu, 16/05/2024 - 14:21
Diffusion meets Nested Sampling

Sampling techniques are a stalwart of reliable inference in the physical sciences, with the nested sampling paradigm emerging in the last decade(s) as a ubiquitous tool for model fitting and comparison. Parallel developments in the field of generative machine learning have enabled advances in many applications of sampling methods in scientific inference pipelines. This work explores the synergy of the latest developments in diffusion models and nested sampling. I will review the challenges of precise model comparison in high dimension, and explore how score based generative models can provide a solution. This work builds towards a public code that can apply out of the box to many established hard problems in fundamental physics, as well as providing potential to extend precise inference to problems that are intractable with classical methods. I will motivate some potential applications at the frontiers of inference that can be unlocked with these methods.

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Mon 20 May 14:00: A large-scale dynamo via MRI-driven alpha Omega dynamo in binary neutron star mergers

Wed, 15/05/2024 - 15:38
A large-scale dynamo via MRI-driven alpha Omega dynamo in binary neutron star mergers

We challenge the magnetar scenario for short gamma-ray bursts by performing a high-resolution neutrino magnetohydrodynamics numerical relativity simulation of a binary neutron star merger. We find the alpha Omega dynamo mediated by the magnetorotational instability generates the large-scale magnetic field in the merger remnant. The resultant large-scale field is efficiently wrapped and drives a relativistic Poynting-flux dominated outflow from the merger remnant.

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Tue 21 May 12:00: The dynamics of parsec-scale eccentric stellar disks around SMBHs in galactic nuclei: disk disruption mechanisms, tidal disruption events and milliparsec stars

Wed, 15/05/2024 - 12:02
The dynamics of parsec-scale eccentric stellar disks around SMBHs in galactic nuclei: disk disruption mechanisms, tidal disruption events and milliparsec stars

We study the dynamics of eccentric parsec-scale stellar disks around supermassive black holes (SMBHs) in galactic nuclei using our novel post-Newtonian N-body code BIFROST . The simulated eccentric disks (Mdisk

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Fri 17 May 13:00: Non-smooth horizons in Kerr black hole mergers

Fri, 10/05/2024 - 11:04
Non-smooth horizons in Kerr black hole mergers

Dynamical black holes are known to develop non-smooth structures on their horizon. We begin by reviewing a classification of all generic non-smooth structures that may appear on black hole horizons in four-dimensional spacetimes. Introducing a time function, we describe how two of these features – namely creases and caustics – evolve, and in particular discuss processes known as ‘perestroikas’, where the non-smooth structure on a horizon cross-section changes qualitatively. We then study the merger of two Kerr black holes in the extreme mass ratio limit, and focus on the creases and caustics that are present on the horizon. We explain how our results differ from an older analysis of the same system by Emparan et al., and show that these novel results are consistent with the properties of creases expected generically. This talk is based on work done with Harvey Reall and Robie Hennigar.

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Mon 13 May 14:00: Convection, waves and mixing in stars: insights and challenges from numerical simulations

Thu, 09/05/2024 - 13:44
Convection, waves and mixing in stars: insights and challenges from numerical simulations

The lifecycle of stars is profoundly shaped by the internal transport and mixing of chemical elements. Within most stars, regions of convective motions and stable stratification coexist, separated by so-called convective boundaries. While convective regions are very efficient at mixing chemical species, stably stratified regions mostly inhibit the vertical transport of elements. Current models suggest that mixing beyond convective regions is needed to explain observed stellar properties. Waves, excited by convection at convective boundaries, could play a crucial role by providing additional mixing in the vertical direction.

In this talk, I will highlight results from recent numerical simulations with the MUSIC code on the study of waves, convection, and mixing in stars. I will discuss challenges in measuring vertical mixing and transport by waves in numerical simulations, and present some prospects for improving our understanding of mixing mechanisms through numerical experiments.

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Fri 14 Jun 13:00: TBC

Tue, 07/05/2024 - 16:14
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Fri 07 Jun 13:00: TBC

Tue, 07/05/2024 - 16:12
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Fri 31 May 13:00: TBC

Tue, 07/05/2024 - 16:11
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Fri 24 May 13:00: TBC

Tue, 07/05/2024 - 16:10
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Fri 17 May 13:00: TBC

Tue, 07/05/2024 - 16:09
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Fri 10 May 13:00: Shall Bekenstein's Area Law Prevail?

Mon, 06/05/2024 - 12:57
Shall Bekenstein's Area Law Prevail?

According to Bekenstein’s area law, the black hole entropy is identified holographically—with one quarter of the horizon area. However, it is commonly believed that such a law is only valid in Einstein’s theory and that higher curvature corrections generically give rise to its modifications. This is for example the case of black holes in Lovelock gravities, or their four-dimensional cousins in the recently discovered 4D scalar-tensor Gauss-Bonnet gravity where one naively “finds” (classical) logarithmic corrections to Bekenstein’s law. We will argue that such logarithmic corrections originate from ignoring the shift symmetry of the 4D Gauss-Bonnet gravity. When this symmetry is properly taken into account, there is no longer any departure from the area law in this theory. Moreover, the first law remains valid upon modifying the black hole temperature, which can be derived via the Brown-York procedure but is no longer given by the surface gravity. Interestingly, we show that upon similar modification of the black hole temperature the area law can also prevail for black holes in higher-dimensional Lovelock gravities.

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Mon 17 Jun 14:00: Title to be confirmed

Mon, 06/05/2024 - 06:54
Title to be confirmed

Abstract not available

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Mon 03 Jun 14:00: Title to be confirmed

Mon, 06/05/2024 - 06:54
Title to be confirmed

Abstract not available

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Tue 21 May 14:00: Title to be confirmed

Mon, 06/05/2024 - 06:53
Title to be confirmed

Abstract not available

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Tue 07 May 14:00: Some questions and answers from protoplanetary disk observations: machine learning, hidden rings, and fake vortices

Mon, 06/05/2024 - 06:42
Some questions and answers from protoplanetary disk observations: machine learning, hidden rings, and fake vortices

In this talk, I will discuss some results from different observations of protoplanetary disks. I will first present our work combining disk models with artificial neural networks to model observed SEDs using a statistical approach, and what we learned by applying this to disks in Taurus. I will then discuss our surprising findings when studying the nearby protoplanetary disk MP Mus with ALMA , one of the best young solar analogues which remained relatively unexplored until recently. Finally, I will propose an alternative (and simpler) explanation for some of the disk azimuthal asymmetries that ALMA has revealed, explain how we can use this to gain information about the vertical structure of disks, and provide evidence for at least a number of these asymmetries likely having a geometric origin.

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Mon 13 May 10:00: Which universes does the no-boundary wave function favour?

Sun, 05/05/2024 - 21:32
Which universes does the no-boundary wave function favour?

I will provide an introduction to the no-boundary wave function, and review its implementation in simple inflationary models. Earlier treatments indicated that the no-boundary wave function would favour very short inflationary phases, in strong tension with observations. Recent investigations have shown the need for a more careful definition, especially with regards to the question of which metrics and field configurations should be summed over in gravitational path integrals. This leads both to a deeper understanding of gravitational path integrals, and to the consequence that, under certain circumstances, long inflationary phases are required. I will discuss the implications for observational predictions, in particular for the size of the universe, and I will comment on the relation to string theory.

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Tue 07 May 14:00: Some questions and answers from protoplanetary disk observations: machine learning, hidden rings, and fake vortices

Fri, 03/05/2024 - 21:52
Some questions and answers from protoplanetary disk observations: machine learning, hidden rings, and fake vortices

In this talk, I will discuss some results from different observations of protoplanetary disks. I will first present our work combining disk models with artificial neural networks to model observed SEDs using a statistical approach, and what we learned by applying this to disks in Taurus. I will then discuss our surprising findings when studying the nearby protoplanetary disk MP Mus with ALMA , one of the best young solar analogues which remained relatively unexplored until recently. Finally, I will propose an alternative (and simpler) explanation for some of the disk azimuthal asymmetries that ALMA has revealed, explain how we can use this to gain information about the vertical structure of disks, and provide evidence for at least a number of these asymmetries likely having a geometric origin.

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Wed 15 May 14:00: Dynamical Field Theories without Detailed Balance

Fri, 03/05/2024 - 06:35
Dynamical Field Theories without Detailed Balance

Many condensed matter problems, such as ordering of a classical ferromagnet or fluid-fluid phase separation, are described by dynamical field theories in which a scalar field obeys a noisy gradient flow governed by a quartic effective potential. These theories are called Model A and Model B for the cases of a non-conserved and conserved scalar, respectively. Traditionally, such models are constructed to obey detailed balance, so that the system evolves to the Boltzmann distribution, giving time-reversible fluctuations at stationarity. Reaching the equilibrium state can be nontrivial however: starting from a metastable uniform initial condition, it requires an instanton to nucleates a droplet large enough to then grow spontaneously. In recent years, attention has shifted to systems without detailed balance, whose stationary states are non-Boltzmann and involve continuous entropy production with time-asymmetric fluctuations. (One example is the study of phase separation among self-propelled particles such as swimming bacteria.) To describe such cases, we have recently introduced variants of Models A and B that break detailed balance explicitly. I will outline some of the qualitative and quantitative novelties that arise in the critical phenomena, steady states, and instantons of these new theories.

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Tue 07 May 11:15: Interferometric measurements of the 21-cm signal with SKA

Thu, 02/05/2024 - 15:09
Interferometric measurements of the 21-cm signal with SKA

The Cosmic Dawn marks the first star formations and preceded the Epoch-of-Reionization, when the Universe underwent a fundamental transformation propelled by the radiation from these first stars and galaxies. Interferometric 21-cm experiments aim to probe redshifted neutral hydrogen signals from these periods, constraining the conditions of the early Universe. The SKA -LOW instrument of the Square Kilometre Array telescope is envisaged to be the largest and most sensitive radio telescope at m and cm wavelengths. In this talk we present a full SKA pipeline that consist of forward modelling and data analysis that were also tested in the SKA Science Data Challenge 3a: Epoch of Reionisation (SKA SDC3a) to process the novel data products expected from the SKA . The forward modelling enables simulation of the astrophysical signals from the Epoch of Reionization and chosen systematic effects of the SKA -LOW. In the analysis part we implement predictive foreground and Bayesian Gaussian Process Regression models alongside a foreground avoidance strategy to isolate the 21-cm signal from that of the astrophysical radio frequency (RF) foregrounds. Together these will determine whether a successful 21-cm detection is possible with the envisaged SKA .

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Tue 26 Nov 11:15: Results of beamline testing at the MROI

Wed, 01/05/2024 - 11:14
Results of beamline testing at the MROI

TBC

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