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URL: http://talks.cam.ac.uk/show/index/54089
Updated: 53 min 13 sec ago

Mon 12 May 13:00: DESI DR2: Survey overview and cosmological constraints from DR2 Baryon Acoustic Oscillation measurements

Tue, 06/05/2025 - 16:58
DESI DR2: Survey overview and cosmological constraints from DR2 Baryon Acoustic Oscillation measurements

The Dark Energy Spectroscopic Instrument (DESI) is undertaking a five-year survey spanning 14,000 square degrees of the sky, with the goal of mapping 40 million extragalactic redshifts. These observations aim to refine our understanding of the universe’s expansion history through Baryon Acoustic Oscillations (BAO) and the growth of cosmic structure via Full Shape analyses. In 2025, the DESI collaboration released BAO cosmology results from the Data Release 2 (DR2) sample, assembled from the first three years of data taking (2021 – 2024). This presentation will introduce the instrument and the survey and review the BAO measurements derived from DR2 . I will discuss the consistency of BAO constraints with other probes—-CMB (including the latest ACT DR6 CMB data) and supernovae—-and present cosmological constraints on dark energy and neutrino masses. I will conclude by providing an outlook on upcoming DESI analyses.

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Fri 09 May 13:00: Dynamical Formation of Regular Black Holes

Tue, 06/05/2025 - 15:01
Dynamical Formation of Regular Black Holes

I will discuss recent work where it was demonstrated that regular black holes emerge as the unique spherically symmetric solutions to certain gravitational actions that incorporate infinite towers of higher-derivative corrections. I will then illustrate what happens when one considers the collapse of spherical thin shells and dust in these theories, showing that the collapse is generically non-singular. This is based on work with Pablo Bueno, Pablo Cano and Ángel Murcia.

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Tue 13 May 11:15: Testing the HARPS3 Data Reduction Pipeline with Synthetic Spectra to achieve Earth-Twin RV Precision

Tue, 06/05/2025 - 13:53
Testing the HARPS3 Data Reduction Pipeline with Synthetic Spectra to achieve Earth-Twin RV Precision

The High Accuracy Radial velocity Planet Searcher-3 (HARPS3) is being developed for the Terra Hunting Experiment, a 10-year observing campaign to conduct nightly observations of a carefully selected group of solar-like stars to detect long-period, low-mass exoplanets. The goal is to achieve extremely-precise radial velocity (EPRV) measurements at the level of 10 cm/s to enable the detection of an Earth-twin. Attaining this precision requires a deep understanding of all error sources: instrumental systematics, astrophysical noise, and data reduction algorithms.

To address the latter, I have developed a novel method to test the data reduction pipeline (DRP) using synthetic data. Rather than attempting to replicate the instrument’s response exactly, the method is designed to systematically probe the DRP ’s performance, identify potential biases, and validate the reduction algorithms. By injecting known inputs into the DRP and tracing their propagation, I can control all aspects of the data, test specific algorithms, and verify the accuracy of the reduction products. The aim is to use simulated data to identify systematic biases and inaccuracies that could impact EPRV measurements.

In this talk I will present my work, currently in preparation for publication, describing how I simulate the data and discussing the first results of passing the synthetic echellogram through the DRP . This approach provides a framework to assess the performance of HARPS3 during commissioning and early operations – when it comes on-sky in late 2025 – enabling us to identify issues and refine data processing techniques.

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Tue 20 May 13:00: Title to be confirmed

Tue, 06/05/2025 - 12:13
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Mon 12 May 13:00: Title to be confirmed

Tue, 06/05/2025 - 12:11
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Mon 30 Jun 13:00: Title to be confirmed

Tue, 06/05/2025 - 10:33
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Wed 07 May 13:15: The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

Mon, 05/05/2025 - 16:34
The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth’s anoxic atmosphere to one where molecular oxygen (O2) was abundant. This rise in O2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. In this study we use WACCM6 , a three-dimensional Earth System Model to simulate Earth’s atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O2 concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 PAL to 150 PAL , where PAL is the present atmospheric level of 21 % by volume. O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O2 leads to decreased O3 densities, reducing local temperatures by up to 5 K, which increases H2O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH4 mixing ratio setting a minimum diffusion escape rate of ≈ 2 × 10^10 mol H/yr. These numerical predictions support geological evidence that the majority of Earth’s hydrogen escape occurred prior to the GOE .

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Wed 07 May 13:40: On the Road to the Radius Valley

Mon, 05/05/2025 - 16:33
On the Road to the Radius Valley

Twenty years after their initial discovery, the nature of super-Earths and sub-Neptunes remains largely unknown. In this talk, I will discuss recent work addressing their interior compositions and formation pathways. In particular, I will show how the detection of young transiting exoplanets may provide a route to revealing their interior compositions.

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Tue 06 May 13:15: The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

Fri, 02/05/2025 - 10:55
The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth’s anoxic atmosphere to one where molecular oxygen (O2) was abundant. This rise in O2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. In this study we use WACCM6 , a three-dimensional Earth System Model to simulate Earth’s atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O2 concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 PAL to 150 PAL , where PAL is the present atmospheric level of 21 % by volume. O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O2 leads to decreased O3 densities, reducing local temperatures by up to 5 K, which increases H2O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH4 mixing ratio setting a minimum diffusion escape rate of ≈ 2 × 10^10 mol H/yr. These numerical predictions support geological evidence that the majority of Earth’s hydrogen escape occurred prior to the GOE .

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Tue 17 Jun 11:15: Love Bites: The Deadly Romance of Spider Pulsars

Thu, 01/05/2025 - 10:25
Love Bites: The Deadly Romance of Spider Pulsars

Pulsars in binary systems are fantastic physics laboratories, primarily because their orbital dynamics allow us to probe binary evolution, test gravity theories, measure neutron star masses, etc. Among them are the “black widows” and “redbacks”, which are nicknamed after the deadly arachnids because the millisecond pulsar they contain gradually destroys their low mass companion. The strongly irradiated dayside displayed by the low-mass companions in these systems is reminiscent of what is observed in exoplanets called “hot jupiters”. In the last decade, the number of known spiders has grown exponentially to the point of becoming the most prevalent type of fast rotating binary pulsars. In this talk, I will present some of the recent efforts undertaken with the MeerKAT telescope to uncover these pulsars and review some of the key advances they have provided for our understanding of binary evolution, stellar physics under extreme irradiation, and measurement of neutron star masses.

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Fri 20 Jun 13:00: Well-posed initial value formulation of general effective field theories of gravity

Wed, 30/04/2025 - 16:38
Well-posed initial value formulation of general effective field theories of gravity

In this talk, I will show that all higher-derivative effective field theories (EFTs) of vacuum gravity admit a well-posed initial value formulation when augmented by suitable regularising terms. These regularising terms can be obtained by field redefinitions and do not affect the dynamics in the regime of validity of EFT . I will explain how our result applies to the quadratic, cubic, and quartic truncations of the EFT of gravity and to various truncations of a simple EFT of a scalar field. Finally, I will also discuss some numerical results on the non-linear dynamics of this simple scalar field theory.

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Tue 06 May 13:00: The Dynamic Chemistry of Planet-Forming Disks

Tue, 29/04/2025 - 21:15
The Dynamic Chemistry of Planet-Forming Disks

The chemical composition of a planet’s atmosphere is intimately tied to the volatile inventory of the protoplanetary disk in which it forms. Establishing this connection requires detailed measurements of elemental abundances in disks at small spatial scales relevant to planet formation. In this talk, I will present two targeted studies of well-known Herbig Ae/Be systems, combining ALMA observations with chemical modelling to probe disk chemistry. In HD 100546 , we detect complex molecular asymmetries, interpreted as the result of shadowing from planet-induced structures within the inner cavity, generating azimuthal temperature variations that drive chemical diversity. In HD 169142 , we investigate the first detection of SiS emission from a protoplanetary disk—nearly a billion times brighter than predicted under typical conditions—indicative of planet-induced shocks that release silicon from dust grains into the gas phase. These findings reveal that planet formation can significantly reshape the chemical environment of disks, with direct implications for how emerging planets accrete their atmospheres. Together, these studies emphasise the dynamic and heterogeneous nature of disk chemistry and provide new insights into the origins of the wide diversity observed in exoplanetary atmospheres.

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Tue 06 May 13:00: Updates on fundamental science from the secondary CMB

Tue, 29/04/2025 - 15:19
Updates on fundamental science from the secondary CMB

A major frontier in cosmic microwave background (CMB) science is the study of secondary anisotropies—temperature and polarization anisotropies induced by the gravitational, electromagnetic, or beyond-standard-model (BSM) interactions of CMB photons with large-scale structure (LSS) over cosmic history. Leveraging their distinct statistical properties and cross-correlations with LSS enables us to isolate these secondary anisotropies from the primary CMB and extract new astrophysical and cosmological information. In this talk, I discuss how secondary anisotropies from electromagnetic interactions (Sunyaev-Zel’dovich effects) and hypothetical BSM particles (dark screening) can serve as probes of fundamental physics. I present a general formalism for capturing the information content of secondary anisotropies. I then give a summary of existing measurements of the kinetic Sunyaev-Zel’dovich (kSZ), polarized Sunyaev-Zel’dovich (pSZ), and dark screening effects. Next I provide an update on how these measurements constrain large-scale homogeneity, primordial non-Gaussianity, isocurvature, and BSM particles (axions and dark photons). Looking ahead to the high-resolution, low-noise, large-volume frontier, I discuss how upcoming observations from the Simons Observatory, combined with LSS surveys like DESI and LSST , will significantly improve these results and allow for novel tests of fundamental physics.

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Thu 15 May 16:00: Title to be confirmed

Tue, 29/04/2025 - 13:01
Title to be confirmed

Abstract not available

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Fri 30 May 13:00: Gravitational Wave Signatures of Dark Matter in Neutron Star Mergers

Tue, 29/04/2025 - 12:38
Gravitational Wave Signatures of Dark Matter in Neutron Star Mergers

Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology – a dense core or a diluted halo – may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from standard models when the tidal deformability is calculated in a two-fluid framework neglecting the dilute and extended nature of the halo. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter’s role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

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Fri 16 May 13:00: TBC

Tue, 29/04/2025 - 12:36
TBC

Abstract not available

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Fri 09 May 13:00: TBC

Tue, 29/04/2025 - 12:34
TBC

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Fri 06 Jun 13:00: A Spacetime Interpretation of the Confluent Heun Functions in Black Hole Perturbation Theory

Tue, 29/04/2025 - 12:32
A Spacetime Interpretation of the Confluent Heun Functions in Black Hole Perturbation Theory

In Black Hole Perturbation Theory, confluent Heun functions appear as solutions to the radial Teukolsky equation, which governs perturbations in black hole spacetimes. While these functions are typically studied for their analytic properties, their connection to the underlying spacetime geometry has received less attention. In this talk, I will propose a spacetime interpretation of the confluent Heun functions, demonstrating how their behaviour near their singular points reflects the structure of key surfaces in Kerr spacetimes. By interpreting homotopic transformations of these functions as changes in the spacetime foliation, I will establish a connection between these solutions and various regions of the black hole’s global structure. I will also explore their relationship with the hyperboloidal formulation of the radial Teukolsky equation.

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Fri 30 May 13:00: Gravitational Wave Signatures of Dark Matter in Neutron Star Mergers

Mon, 28/04/2025 - 19:30
Gravitational Wave Signatures of Dark Matter in Neutron Star Mergers

Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology – a dense core or a diluted halo – may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from standard models when the tidal deformability is calculated in a two-fluid framework neglecting the dilute and extended nature of the halo. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter’s role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

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

Mon, 28/04/2025 - 15:16
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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Upcoming Talks

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