Talks

Abstract not available

• Speaker: Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics)
• Thursday 02 May 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy (and online - details to be sent by e-mail).
• Series: The Kavli Lectures; organiser: Alison Wilson.

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I will present new VLA observations that reveal the structure of a new population of low-luminosity FRII radio galaxies discovered in LoTSS. Fanaroff and Riley (1974) identified a luminosity break between their two morphological classes. FRIs are defined to be low-luminosity, centre bright jets and the higher luminosity FRI Is have jets that are edge brightened and terminate in hotspots. Using LoTSS DR1 , Mingo et al (2019) demonstrated an overlap in luminosity between FRI and FRII morphology rather than a clear divide, discovering a sub-sample of FRI Is with luminosities up to 3 magnitudes lower than the typical FR break. A population of low-luminosity FRI Is raises questions about their origins; are they older, fading FRI Is, or hosted by lower mass galaxies? Our new VLA observations of a sample of LoTSS-selected low-luminosity FRI Is allow us to make comparisons between the two FRII luminosity populations on the prevalence of hotspots, as well as morphological and spectral differences.

• Speaker: Dr. Bonny Barkus (University of Hertfordshire)
• Tuesday 19 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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In addition to being at the core of the widely employed AdS/CFT correspondence, asymptotically anti-de Sitter (AdS) spacetimes with reflective boundary conditions can serve as the arena for a range of strong gravity effects: arbitrarily small perturbations are bound to remain in the bulk, where they continue to interact and typically become sizeable. Initially, I will review a numerical scheme that enables the Cauchy evolution of these spacetimes to be performed in full generality. Then, I will present the first simulations of the trapping mechanism occurring in the exterior of a Kerr-AdS black hole. This mechanism has been conjectured to cause a non-linear instability, the existence of which remains a topic of debate. The simulations offer insights into this debate, providing new perspectives on the non-linear instability of Kerr-AdS.

• Speaker: Lorenzo Rossi, Queen Mary University of London
• Friday 15 March 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Convection is an essential process for transporting heat and moisture in planetary atmospheres. The standard Earth picture of moist convection rising from the surface is only one of a number of modes of convection. Notably, convection in atmospheres with a high condensible mass fraction (non-dilute atmospheres), or with a lighter background gas than the condensible species (e.g. water convection in a hydrogen dominated atmosphere) – acts very differently and can be much weaker or even shut down entirely in the latter case. Here I present a new mass-flux scheme which can capture these variations and simulate convection in a wide range of parameter space for use in 3D climate models. A validation using the case of Trappist-1 e is presented.

• Speaker: Edouard Barrier (University of Cambridge)
• Wednesday 13 March 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Hannah Uebler.

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The CMB anisotropies are measured in several microwave frequency bands. Having this frequency information allows us to separate signals that are due to different sources. We can easily make maps that are sensitive to specific frequency combinations, and in this way isolate the contribution from the primary CMB (early-Universe) and various other CMB interactions such as the Sunyaev—Zel’dovich (SZ) effect (the scattering of the CMB from electrons in the late Universe). I will talk about constraints on the SZ effect from Planck data using a new frequency-separation code, pyilc, which we use to isolate the signal while removing other late-Universe biases, in particular the infrared emission from star-forming galaxies. I will also show an application to beyond standard model interactions between the CMB and a non-trivial dark sector, and how we can use the CMB to constrain beyond-standard-model particle physics.

• Speaker: Fiona McCarthy (DAMTP)
• Wednesday 13 March 2024, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Hannah Uebler.

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For more than 15 years, NANO Grav and other pulsar-timing array collaborations have been carefully monitoring networks of pulsars across the Milky Way. The goal was to find a tell-tale correlation signature amid the data from all those pulsars that would signal the presence of an all-sky background of nanohertz-frequency gravitational waves, washing through the Galaxy. At the end of June 2023, the global pulsar-timing array community finally announced its evidence for this gravitational-wave background, along with a series of studies that interpreted this signal as either originating from a population of supermassive black-hole binary systems, or as relics from cosmological processes in the very early Universe. I will describe the journey up to this point (including the integral role that the IoA played), what led to the ultimate breakthrough, how this affects our knowledge of supermassive black holes and the early Universe, and what lies next for gravitational-wave astronomy at light-year wavelengths.

• Speaker: Dr Stephen Taylor, Vanderbilt University, Nashville, Tennessee, USA
• Thursday 07 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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Abstract not available

• Speaker: Alvaro Ribas (IOA Cambridge)
• Tuesday 07 May 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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Parity symmetry is known to be violated in the weak interaction. Do the physical laws behind the unsolved problems of modern cosmology – cosmic inflation, dark matter, and dark energy – also violate parity symmetry? In this talk, we will discuss theoretical and observational possibilities of parity violation in cosmology, a topic that has received much attention in recent years.

• Speaker: Eiichiro Komatsu (MPA Garching)
• Monday 11 March 2024, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Inigo Zubeldia.

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Abstract not available

• Speaker: Jean-Luc Lehners (MPI for Gravitational Physics, Potsdam)
• Monday 13 May 2024, 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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For more than 15 years, NANO Grav and other pulsar-timing array collaborations have been carefully monitoring networks of pulsars across the Milky Way. The goal was to find a tell-tale correlation signature amid the data from all those pulsars that would signal the presence of an all-sky background of nanohertz-frequency gravitational waves, washing through the Galaxy. At the end of June 2023, the global pulsar-timing array community finally announced its evidence for this gravitational-wave background, along with a series of studies that interpreted this signal as either originating from a population of supermassive black-hole binary systems, or as relics from cosmological processes in the very early Universe. I will describe the journey up to this point (including the integral role that the IoA played), what led to the ultimate breakthrough, how this affects our knowledge of supermassive black holes and the early Universe, and what lies next for gravitational-wave astronomy at light-year wavelengths.

• Speaker: Stephen Taylor, Vanderbilt University
• Thursday 07 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, IoA (tea at 3:30 pm).
• Series: The Eddington Lectures ; organiser: .

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CubeSats stand at the forefront of the New Space Revolution, a paradigm shift in space exploration characterised by reduced launch costs and increased accessibility to space. These miniature satellites, defined by their standardised dimensions and modular design, have emerged as a pivotal technology with some implications for research in astronomy. With their standardised dimensions and modular design, these Nanosatellites enable a wide range of experiments that were previously the domain of larger, more costly missions. In my talk, I aim to introduce you to valuable opportunities that can emerge by leading a CubeSat project with a special interest in payloads dedicated to astrophysics research. As a cost-effective space instrument, CubeSats unlock observational windows across the ultraviolet, far-infrared, and low-frequency radio spectra, which are inaccessible from Earth’s surface. Beyond their technical capabilities, these satellites enable sustained observations of celestial bodies over extended periods, free from the scheduling constraints of larger telescopes.

• Speaker: Dr. Dolev Bashi
• Tuesday 05 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Dr. Dolev Bashi.

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General relativity (GR) exists in different formulations. They are equivalent in pure gravity but generically lead to distinct predictions once matter is included. After a brief overview of various versions of GR, I will focus on metric-affine gravity, which avoids any assumption about the vanishing of curvature, torsion, or nonmetricity. With a view toward the Standard Model, we can construct a generic model of (complex) scalar, fermionic, and gauge fields coupled to GR and derive an equivalent metric theory, which features numerous new interaction terms. There are multiple phenomenological consequences, which I will detail: an improved setting for Higgs inflation, a new (purely gravitational) production channel for fermionic dark matter, and an outlook on axion inflation.

• Speaker: Claire Rigouzzo (KCL)
• Friday 08 March 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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The existence of dark matter, which constitutes 85% of the matter density and 26% of the total energy density, is clearly demonstrated by cosmological observations of the Universe. And yet, very little is known about the nature of dark matter. The observations support the ‘cold dark matter’ (CDM) paradigm, in which the dark matter is a heavy particle, with little to no interactions through fundamental forces other than gravity. The cosmological and astrophysical observations of dark matter’s gravitational interaction currently provide the only robust evidence of dark matter. These observations typically rely on characterising the distribution of matter in the Universe. A dark matter particle that is lighter than the standard CDM paradigm predicts imprints a suppression of structure in the matter distribution. The exact scale where this happens is most often linked to the mass of the dark matter particle. I will present new results on the thermal relic warm dark matter constraints using the high-redshift cosmic web as traced by the Lyman-alpha forest.

• Speaker: Vid Iršič
• Wednesday 06 March 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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CubeSats stand at the forefront of the New Space Revolution, a paradigm shift in space exploration characterised by reduced launch costs and increased accessibility to space. These miniature satellites, defined by their standardised dimensions and modular design, have emerged as a pivotal technology with some implications for research in astronomy. With their standardised dimensions and modular design, these Nanosatellites enable a wide range of experiments that were previously the domain of larger, more costly missions. In my talk, I aim to introduce you to valuable opportunities that can emerge by leading a CubeSat project with a special interest in payloads dedicated to astrophysics research. As a cost-effective space instrument, CubeSats unlock observational windows across the ultraviolet, far-infrared, and low-frequency radio spectra, which are inaccessible from Earth’s surface. Beyond their technical capabilities, these satellites enable sustained observations of celestial bodies over extended periods, free from the scheduling constraints of larger telescopes.

• Speaker: Dolev Bashi
• Tuesday 05 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Dolev Bashi.

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In this talk I will discuss several studies in which quantum computers have been proposed as tools for studying particle theories. The first study uses quantum annealers to implement simple scalar field theories and observe quantum tunnelling, which follows the expected analytic behaviour: for example the decay rate follows the WKB approximation. The second more recent idea uses photonic devices to implement quantum tunnelling. In principle in both of these studies we are observing non-perturbative effects, suggesting that these and similar methods may be of interest in understanding a wide range of phenomena. The talk will be pedagogical.

• Speaker: Steve Abel (IPPP)
• Wednesday 06 March 2024, 14:00-15:00
• Venue: MR2.
• Series: Theoretical Physics Colloquium; organiser: Hannah Banks.

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Since 1963, we have known, or at least suspected, that element abundances in the solar corona and wind are different to those in the photosphere. Elements that are predominantly ionized in the photosphere and chromosphere, e.g. Fe, Si, Mg, are observed to be enhanced in abundance once transported to the corona and wind by a factor of typically 3-4, while elements that are mainly neutral are relatively unaffected. Due to the dependence on the ionization potential, this phenomenon has been dubbed the First Ionization Potential (FIP) Effect. A model capturing the FIP and effect and the various modifications to it seen in different coronal regions and the solar wind invokes ion-neutral separation by the ponderomotive force due to Alfven and magnetosonic waves propagating through the chromosphere. This acts on ions, but not neutrals, and depends on the interaction of the waves with the magnetic geometry of the solar atmosphere.

The launch of the Extreme Ultraviolet Explorer (EUVE) in 1992 afforded us the first observations of abundances in stellar coronae. This, and succeeding missions like Chandra, XMM -Newton and Suzaku, have shown that solar-like stars show a similar FIP effect, which transitions to Inverse FIP (IFIP), i.e. a coronal depletion of Fe, Si, Mg, etc., as the activity level increases.

In this presentation, I will review the theoretical basis of FIP and IFIP fractionation by the ponderomotive force. Fractionation by such means is new to solar physics and astrophysics, but is not new to science. Manipulation of atoms, molecules, biological samples, etc. by the forces due to refraction of photons from lasers, known as “optical tweezers”, has a rather long history in optical sciences, and won Nobel Prizes for Steven Chu (in 1997) and Arthur Ashkin (in 2018). Our model is a precise analog of this work, but with magnetohydrodynamic waves instead of optical photons. This connection leads to a better physical understanding of the mechanisms at work in FIP fractionated plasma.

Work supported by NASA Heliophysics Supporting Research Program (80HQTR20T0076), and by Basic Research Funds of the Office of Naval Research.

• Speaker: Martin Laming (Naval Research Laboratory, Washington DC)
• Monday 04 March 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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Light scalar fields are often considered as part of explanations for dark energy and dark matter. They also appear in theories in which gravity is modified on cosmological scales. If the scalar field theory has a screening mechanism, the fifth forces that the scalar mediates can be significant in the cosmological vacuum but suppressed on Earth and in the solar system. It has been claimed that current data from galaxy surveys is sufficient to exclude the possibility that these theories could be relevant on astrophysical or cosmological scales. In this talk I will consider how such scalar fields behave in galaxies, and show that they may not be excluded after all.

• Speaker: Clare Burrage (Nottingham)
• Monday 04 March 2024, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Dr Dong-Gang Wang.

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Cassiopeia A is the best-observed core-collapse supernova remnant in our galaxy. Analyses of the 1 Million second Chandra Very Large Project X-ray observation and the data from infrared spectroscopy by Spitzer lead to a “complete” (within the limitations of the data quality) assessment of the elemental composition of the explosion ejecta, comprising both the reverse shocked X-ray emitting plasma and the photoionized unshocked ejecta emitting primarily in the infrared. This is the first time such a detailed census of supernova ejecta has ever been accomplished. More recently, Cassiopeia A has been observed with the James Webb Space Telescope. A first look suggests that these data corroborate and extend our previous analysis. Hard X-ray observations by NUSTAR reveal the mass and location of the radioactive nucleus 44Ti and optical imaging reveals a natal kick imparted to the compact central object (presumed to be a neutron star), anti-correlated with the 44Ti location, as expected. However, X-ray imaging reveals almost “pure” Fe knots on the east limb, presumably the ashes of alpha rich freeze out, which do not correlate so well. All these observables carry information about processes at the core of the supernova and allow us (and others) to speculate about the nature of the explosion, in ways that complement conclusions drawn from the prompt observations of supernovae.

• Speaker: Martin Laming (US Naval Research Laboratory)
• Thursday 14 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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For more than 15 years, NANO Grav and other pulsar-timing array collaborations have been carefully monitoring networks of pulsars across the Milky Way. The goal was to find a tell-tale correlation signature amid the data from all those pulsars that would signal the presence of an all-sky background of nanohertz-frequency gravitational waves, washing through the Galaxy. At the end of June 2023, the global pulsar-timing array community finally announced its evidence for this gravitational-wave background, along with a series of studies that interpreted this signal as either originating from a population of supermassive black-hole binary systems, or as relics from cosmological processes in the very early Universe. I will describe the journey up to this point (including the integral role that the IoA played), what led to the ultimate breakthrough, how this affects our knowledge of supermassive black holes and the early Universe, and what lies next for gravitational-wave astronomy at light-year wavelengths.

• Speaker: Dr Stephen Taylor, Vanderbilt University, Nashville, Tennessee, USA
• Thursday 07 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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Investigating the fundamental origins of Bekenstein-Hawking entropy is a profound challenge in theoretical physics. This investigation is especially relevant for anti de Sitter (AdS) black holes, where it is proposed that the entropy can be explained through the microstates of the holographic dual quantum field theory, in line with the AdS/CFT correspondence. In this talk, I will review recent advancements in deriving the Bekenstein-Hawking entropy for various supersymmetric AdS black holes across different dimensions.

• Speaker: Seyed Morteza Hosseini, Imperial College London
• Friday 01 March 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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