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Kavli Institute for Cosmology, Cambridge

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

Wed, 06/03/2024 - 13:32
Which universes does the no-boundary wave function favour?

Abstract not available

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Thu 07 Mar 16:00: The Dawn of Galaxy-scale Gravitational Wave Astronomy

Wed, 06/03/2024 - 11:58
The Dawn of Galaxy-scale Gravitational Wave Astronomy

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.

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Tue 05 Mar 11:15: New Space & the CubeSat Revolution

Tue, 05/03/2024 - 19:34
New Space & the CubeSat Revolution

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.

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Fri 08 Mar 11:30: Obscured AGN across cosmic time

Mon, 04/03/2024 - 16:31
Obscured AGN across cosmic time

The bulk of the growth of supermassive black holes over cosmic time takes place behind heavy obscuration. I will present observational evidence that much, or even most, of these AGN are very heavily obscured (Compton-thick) and thus hidden from many previous multiwavelength surveys. I will discuss the implications for cosmic black hole growth and the populations of AGN now detected in the early Universe with JWST .

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Fri 08 Mar 13:00: The Fate of Matter Fields in Metric-Affine Gravity

Mon, 04/03/2024 - 15:25
The Fate of Matter Fields in Metric-Affine Gravity

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.

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Tue 12 Mar 13:00: Relative contribution from comets and carbonaceous asteroids to the Earth's volatile budget

Mon, 04/03/2024 - 14:31
Relative contribution from comets and carbonaceous asteroids to the Earth's volatile budget

Recent models of solar system formation suggest that a dynamical instability among the giant planets happened within the first 100 Myr after disk dispersal, perhaps before the Moon-forming impact. As a direct consequence, a bombardment of volatile-rich impactors may have taken place on Earth before internal and atmospheric reservoirs were decoupled. However, such a timing has been interpreted to potentially be at odds with the disparate inventories of Xe isotopes in Earth’s mantle compared to its atmosphere. In this seminar, I will talk about the dynamical effects of an Early Instability on the delivery of carbonaceous asteroids and comets to Earth, and address the implications for the Earth’s volatile budget.

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Wed 06 Mar 13:15: The Nature of Dark Matter with Lyman-Alpha Forest

Mon, 04/03/2024 - 09:41
The Nature of Dark Matter with Lyman-Alpha Forest

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.

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Tue 05 Mar 13:00: Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7b

Fri, 01/03/2024 - 09:20
Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7b

We reassess the claimed detection of variability in the atmosphere of the hot Jupiter HAT -P-7 b, reported by Armstrong et al. (2016). Although astronomers expect hot Jupiters to have changing atmospheres, variability is challenging to detect. We looked for time variation in the phase curves of HAT -P-7 b in Kepler data using similar methods to Armstrong et al. (2016), and identified apparently significant variations similar to what they found. Numerous tests show the variations to be mostly robust to different analysis strategies. However, when we injected unchanging phase curve signals into the light curves of other stars and searched for variability, we often saw similar levels of variations as in the HAT -P-7 light curve. Fourier analysis of the HAT -P-7 light curve revealed background red noise from stellar supergranulation on timescales similar to the planet’s orbital period. Tests of simulated light curves with the same level of noise as HAT -P-7’s supergranulation show that this effect alone can cause the amplitude and phase offset variability we detect for HAT -P-7 b. Therefore, the apparent variations in HAT -P-7 b’s atmosphere could instead be caused by non-planetary sources, most likely photometric variability due to supergranulation on the host star.

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Tue 05 Mar 11:15: New Space & the CubeSat Revolution

Thu, 29/02/2024 - 09:24
New Space & the CubeSat Revolution

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.

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Wed 06 Mar 14:00: Applying Quantum Computing to HEP

Thu, 29/02/2024 - 09:23
Applying Quantum Computing to HEP

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.

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Fri 10 May 11:30: Title to be confirmed

Tue, 27/02/2024 - 17:23
Title to be confirmed

Abstract not available

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Mon 04 Mar 14:00: The First Ionization Potential Effect in Solar/Stellar Coronae and Winds

Tue, 27/02/2024 - 12:57
The First Ionization Potential Effect in Solar/Stellar Coronae and Winds

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.

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Mon 04 Mar 13:00: Fifth forces in and around galaxies

Mon, 26/02/2024 - 22:22
Fifth forces in and around galaxies

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.

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Thu 14 Mar 16:00: How Did Cassiopeia A Explode?

Mon, 26/02/2024 - 16:45
How Did Cassiopeia A Explode?

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.

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Mon 26 Feb 14:00: Dynamics in gas is different

Mon, 26/02/2024 - 15:09
Dynamics in gas is different

Gaseous environments are abundant in the Universe and include AGN disks, star-forming regions, protoplanetary disks and regions of late star formation in globular clusters. While the dynamics of binaries in gas-free environments have been studied extensively, gas-rich environments are fertile ground for phenomena that are still largely unexplored, and in this talk, we will discuss some of the unique phenomena of dynamics in gas. Binaries tend statistically to get softer as they encounter other stars, according to Heggie’s law. However, in gaseous environments, this law should be modified as gas-hardening could lead to a significant energy dissipation that could dominate over stellar softening. Here we explore the effect of gas hardening on the softening rate of binaries and its implications. We will also discuss gas-assisted binary formation. In gaseous media, interactions between two initially unbound objects could result in gas-assisted binary formation, induced by a loss of kinetic energy to the ambient gas medium. Here, we derive analytically the criteria for gas-assisted binary capture through gas dynamical friction dissipation. In some environments, these captures could occur more than once per object, leading to multicaptures. We will discuss that and further implications.

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Thu 07 Mar 16:00: Eddington lecture 2024: The Dawn of Galaxy-scale Gravitational Wave Astronomy

Mon, 26/02/2024 - 13:02
Eddington lecture 2024: The Dawn of Galaxy-scale Gravitational Wave Astronomy

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.

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Fri 01 Mar 13:00: AdS Black Holes and Their Microstates

Mon, 26/02/2024 - 12:26
AdS Black Holes and Their Microstates

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.

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Fri 24 May 11:30: Title to be confirmed

Sat, 24/02/2024 - 21:17
Title to be confirmed

Abstract not available

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Tue 05 Mar 11:15: New Space & the CubeSat Revolution

Fri, 23/02/2024 - 19:59
New Space & the CubeSat Revolution

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 of 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 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.

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Fri 01 Mar 11:30: The chemical abundance pattern of a nitrogen-loud AGN in the early Universe

Fri, 23/02/2024 - 17:25
The chemical abundance pattern of a nitrogen-loud AGN in the early Universe

Recent JWST /NIRSpec observations have revealed several interesting galaxies with particularly enhanced nitrogen abundances at redshifts of z > 6. These galaxies are featured by strong nitrogen emission lines in the rest-frame UV, similar to the ’’nitrogen-loud’’ quasars previously observed at lower redshifts. Among these nitrogen-loud galaxies, there is an AGN candidate, GN-z11, at a redshift of z = 10.6, also showing potentially exotic abundances of carbon and iron. The chemical enrichment history of the high-redshift nitrogen-loud galaxies and their connection with nitrogen-loud quasars remain unclear despite that many theories have been proposed. In this talk, I present our recent analyses on a nitrogen-loud Type-1 AGN at z = 5.55 observed within the JWST /NIRSpec IFS GTO Program, GA-NIFS. By combining the ground-based observations and JWST observations, we confirmed the nitrogen loudness of this AGN and further constrained its carbon abundance and iron abundance. We found evidence of a chemically stratified narrow-line region (NLR) in this AGN , where the denser and more highly ionized part of the NLR is more nitrogen enhanced. Our results suggest the peculiar chemical enrichment in this AGN is confined to the inner part of the NLR and potentially traces a specific stage of chemical evolution in a dense environment.

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