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URL: http://talks.cam.ac.uk/show/index/54089
Updated: 26 min 14 sec ago

Mon 12 May 14:00: Title to be confirmed

Fri, 25/04/2025 - 10:22
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Mon 19 May 14:00: Title to be confirmed

Fri, 25/04/2025 - 10:22
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Tue 27 May 14:00: Title to be confirmed

Fri, 25/04/2025 - 10:21
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Thu 29 May 14:00: Title to be confirmed

Fri, 25/04/2025 - 10:21
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Tue 29 Apr 13:00: Formation of planetary cores in spontaneously generated dust traps during the secular evolution of magnetized protoplanetary disks

Fri, 25/04/2025 - 03:32
Formation of planetary cores in spontaneously generated dust traps during the secular evolution of magnetized protoplanetary disks

An outstanding gap in the current planet formation theory is about the first steps of the planet formation process; namely how, when and where the initially ISM like solid dust particles grow into pebbles and planetesimals, the building blocks of planetary cores. Protoplanetary disks provide the initial conditions for the planet formation process. They are weakly magnetized accretion disks that are subject to the magnetorotational instability (MRI), one of the main magnetized processes responsible for their angular momentum transport and gas turbulence. The nonideal magnetohydrodynamic (MHD) effects prevent the MRI from operating everywhere in PPDs, leading to a complex dichotomy between MRI active regions with higher gas turbulence and non-MRI regions with lower gas turbulence. In this talk,  I will present the first numerical framework that describes the evolution of PPDs over millions of years powered by the MRI . It captures the MRI driven gas evolution via nonideal MHD calculations, which accounts for the dynamics and growth of the solid dust particles. An MRI powered mechanism that can spontaneously generate short- and long-lived pressure maxima in the PPD is unveiled. Within the long-lived pressure maxima, solid dust particles can be efficiently trapped, grow into pebbles, and reach high enough dust-to-gas mass ratios to potentially trigger the formation of planetesimals via the streaming instability. These planetesimals and pebbles can further rapidly interact to form planetary cores.

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Fri 25 Apr 11:30: The Thesan-Zoom simulations: Studying high-redshift galaxy formation with cosmological radiation-hydrodynamics simulations

Wed, 23/04/2025 - 15:59
The Thesan-Zoom simulations: Studying high-redshift galaxy formation with cosmological radiation-hydrodynamics simulations

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Mon 09 Jun 13:00: Title to be confirmed

Wed, 23/04/2025 - 09:58
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Fri 25 Apr 13:00: Black hole radiation from non-vacuum initial states

Tue, 22/04/2025 - 23:10
Black hole radiation from non-vacuum initial states

Hawking derived the black hole thermal radiation state by comparing the vacuum state at the future null infinity I+ with that at the past null infinity I-. We revisit Hawking’s computation, considering a non-vacuum initial state at the past of the black hole geometry. We show in which cases the black hole radiation arising from the initial matter state differs from the well-known thermal state. We moreover classify what initial states are distinguishable from one another through measurements on the black hole radiation in this framework. Finally we provide a physical interpretation of the classification, using Algebraic Quantum Field Theory localisation.

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Mon 06 Oct 13:00: Title to be confirmed

Tue, 22/04/2025 - 14:23
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Fri 23 May 11:30: Between the extremes -- the physics of the first stars, galaxies, and black holes

Tue, 22/04/2025 - 13:36
Between the extremes -- the physics of the first stars, galaxies, and black holes

The emergence of the first sources 13.6 billion years ago had a profound effect on the Universe, initiating its last major phase-change and ending the so-called Cosmic Dark Ages. Unlocking the physics of those primordial sources thus represents a fundamental step towards a comprehensive understanding of the initial conditions that formed the building blocks for the Universe we see today. While Hubble painted a fairly straight forward picture, early JWST data revealed an infant Universe far more remarkable and exotic than previously thought, with hyper-luminous galaxies detected out to z=14, chemically-enriched and Nitrogen-enhanced interstellar media out to z=12, and apparently over-massive black holes to z=10. Are these sources representative of the global population, or do they reflect peculiar objects at a particular evolutionary phase? In this talk I will present efforts to address these questions through the spectroscopic study of statistical samples of high-redshift (z>5-14) galaxies with JWST /NIRSpec, establishing a benchmark for their chemical enrichment journeys, ISM conditions, (re)ionizing capabilities, and spectroscopic fingerprints. Additionally, I will showcase the importance of utilizing unbiased samples of galaxies to gain representative insight into the ISM conditions and evolutionary pathways of the most luminous populations uncovered by JWST .

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Fri 30 May 11:30: Chasing the Light: Shadowing, Collimation, and the Rapid Growth of Infant Black Holes

Mon, 21/04/2025 - 12:40
Chasing the Light: Shadowing, Collimation, and the Rapid Growth of Infant Black Holes

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift broad-line active galactic nuclei (AGNs) characterized by moderate luminosities, weak X-ray emissions, and faint high-ionization lines, challenging conventional models of AGN activity. In this talk I will propose that these sources are accreting at super-Eddington rates, and discuss how such accretion flows, shaped by thick disk geometries and anisotropic radiation fields, may provide new insights into black hole growth in the early Universe.

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Fri 02 May 11:30: Exploring high-redshift quenched galaxies: from low-mass to massive

Mon, 21/04/2025 - 12:39
Exploring high-redshift quenched galaxies: from low-mass to massive

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Fri 16 May 11:30: Towards understanding the epoch of reionization out to the cosmic dawn

Fri, 18/04/2025 - 08:54
Towards understanding the epoch of reionization out to the cosmic dawn

Work on understanding the epoch of reionization has been galvanized in recent years by a series of observational and theoretical breakthroughs. These include the recognition that spatial structure in the Lyman-α forest retains signatures of reionization history, the discovery of galaxies and quasars deep in the reionization era by JWST , and renewed efforts to detect the redshifted 21-cm signal from cosmic dawn and the epoch of reionization. In this talk, I will present a series of results from our group that address the goal of understanding the evolving ionization state of the Universe, from cosmic dawn to the final stages of reionization. This includes new simulations of reionization, updated measurements of the mean free path of ionizing photons, improved constraints on the neutral hydrogen fraction from quasar damping wings, and interpretations of AGNs and LAEs discovered by JWST . I will describe our ongoing attempts to directly detect the neutral parts of the IGM for the first time using the 21-cm forest, and discuss the implications of JWST data for quasar growth during this era. Deeper into the reionization epoch, I will present new approaches to charting reionization using LAEs. Closer to cosmic dawn, I will highlight new radiative transfer models of Lyman-α coupling and a model-agnostic framework for combining JWST and 21-cm observations, including results from REACH . I will conclude by reviewing where we are and outlining key challenges ahead.

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Mon 28 Apr 13:00: String axions: the hot and the fuzzy

Wed, 16/04/2025 - 13:34
String axions: the hot and the fuzzy

String axions have been proposed as candidates for solving several puzzles in cosmology. In this talk, I will focus on axions as dark matter. After reviewing how string axions can occur in our universe, I will provide a string theoretical explanation of dark matter as composed of axions coming from type IIB string theory. Based on the latest bounds, I will show how likely it is for dark matter to be composed of such particles and in which abundance, and I will provide predictions on the preferred ranges of masses and decay constants. On the contrary, requiring the axions to lie in a particular range of the parameter space imposes constraints on the UV theory. I will focus both on the role of moduli stabilization and the landscape of string vacua. Finally, I will discuss axion production at the end of inflation and the implications for the proposed cosmic axion background.

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Wed 14 May 16:00: Spacetime Singularities and Black Holes

Wed, 16/04/2025 - 08:40
Spacetime Singularities and Black Holes

After a brief introduction to Einstein’s theory of general relativity and its most profound prediction of black holes, I will focus on spacetime singularities, i.e., regions where general relativity breaks down and must be replaced by a quantum theory of gravity.  I first discuss singularities inside black holes. This is the usual case and is an old story, but there have been some recent developments. I will next describe some new results which show that some black holes have singularities on their surface. Finally, I will discuss the possibility of singularities outside black holes.

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Tue 15 Apr 11:00: Growing pains: the dining habits of stars, planets and black holes

Mon, 14/04/2025 - 12:24
Growing pains: the dining habits of stars, planets and black holes

To make planets, stars and supermassive black holes, one must rapidly accrete material onto central objects. But the tiniest tangential motion combined with angular momentum conservation sends material into orbit, rather than accreting. Since work at the IoA in the 1970s we have understood that Nature solves the angular momentum problem by forming accretion discs, but the angular momentum transport mechanism remains unclear. The past 10 years have given us spectacular resolved observations of discs around both young and old stars, bringing fresh clues. In this talk I’ll explain how pairing 3D simulations with observations helps us solve the problem of accretion, revealing how stars and planets form, black holes grow and how accretion powers tidal disruption events.

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Fri 09 May 11:30: How do the most luminous black holes accrete and expel gas?

Thu, 10/04/2025 - 11:40
How do the most luminous black holes accrete and expel gas?

The gravitational pull of a black hole attracts gas and forms an accretion disk where the interplay between hydromagnetic processes and the warping of space-time releases gravitational energy in the form of radiation, relativistic jets, and winds. Most gas falls into supermassive black holes when the accretion rate approaches the Eddington limit (L=Ledd), at which point radiation pressure overcomes gravity. To date, our knowledge of such `luminous’ black hole accretion disks mostly relies on semi-analytical models, supplemented by a limited set of numerical simulations. In my talk I will discuss new insights gained from state-of-the-art radiative general relativistic magnetohydrodynamics (GRMHD) simulations of accretion near the Eddington limit such as the formation of a hot corona, disk truncation, and other physical processes driving the spectral evolution of luminous black holes. I will finish my talk by discussing the challenges and opportunities the next-generation of GRMHD simulations will bring in developing a comprehensive understanding of black hole accretion across the luminosity spectrum.

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Tue 27 May 11:15: JAX-powered Bayesian anomaly detection for supernovae analysis

Thu, 10/04/2025 - 10:58
JAX-powered Bayesian anomaly detection for supernovae analysis

Abstract TBC

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Tue 15 Apr 11:00: Growing pains: the dining habits of stars, planets and black holes

Wed, 09/04/2025 - 15:30
Growing pains: the dining habits of stars, planets and black holes

To make planets, stars and supermassive black holes, one must rapidly accrete material onto central objects. But the tiniest tangential motion combined with angular momentum conservation sends material into orbit, rather than accreting. Since work at the IoA in the 1970s we have understood that Nature solves the angular momentum problem by forming accretion discs, but the angular momentum transport mechanism remains unclear. The past 10 years have given us spectacular resolved observations of discs around both young and old stars, bringing fresh clues. In this talk I’ll explain how pairing 3D simulations with observations helps us solve the problem of accretion, revealing how stars and planets form, black holes grow and how accretion powers tidal disruption events.

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Fri 13 Jun 13:00: TBC

Tue, 08/04/2025 - 11:29
TBC

Abstract not available

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