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Kavli Institute for Cosmology, Cambridge

 
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Mon 18 Oct 14:00: Title to be confirmed

Fri, 03/09/2021 - 10:24
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Mon 15 Nov 14:00: Title to be confirmed

Thu, 02/09/2021 - 17:14
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Fri 15 Oct 11:30: Title to be confirmed

Mon, 30/08/2021 - 16:00
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Fri 03 Sep 11:30: The causal origin of the angular momentum and of the properties of galaxies

Mon, 23/08/2021 - 10:14
The causal origin of the angular momentum and of the properties of galaxies

Understanding the origin of galaxy and dark matter properties, and in particular, their angular momentum, is a crucial open problem. Studying statistical correlations in large simulated populations sheds some light, but this approach is intrinsically limited to correlations rather than causal links. In this presentation, I will present recent numerical developments that allow to uncover the causal origin of galaxy and dark matter properties. In particular, I will show that the dark matter angular momentum can be predicted from the initial conditions in the very early Universe. I will also show that half the variation in the concentration of dark matter halos can be attributed to interactions with its cosmological environment. In a second part, I will discuss early results about the causal origin of galactic properties obtained from simulations that also include baryonic physics (cooling, star formation, supernova and AGN feedback).

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Fri 01 Oct 11:30: First 21cm Results from HERA Phase I

Thu, 19/08/2021 - 19:18
First 21cm Results from HERA Phase I

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Fri 24 Sep 11:30: The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

Thu, 22/07/2021 - 16:54
The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

ALMA observations of z>6 galaxies have revealed abnormally high [OIII]/[CII] ratios and [CII] deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionisation parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, I will introduce SPHINX20 , a new state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization and discuss the physics that drives high [OIII]/[CII] ratios.

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Fri 24 Sep 11:30: The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

Thu, 22/07/2021 - 16:50
The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

ALMA observations of $z>6$ galaxies have revealed abnormally high [OIII]$$/[CII]${\rm 158\mu m}$ ratios and [CII]$$ deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionisation parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ {\small SPHINX $^{20}$}, a state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization. We find that the observed $z>6$ [OIII]${\rm 88\mu m}$-SFR and [CII]$$-SFR relations can only be reproduced when the C/O abundance ratio is $\sim8\times$ lower than Solar and the total metal production is $\sim5.7\times$ higher than that of a Kroupa IMF . This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core-collapse supernovae with a more top-heavy IMF . As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [CII]${\rm 158\mu m}$-SFR and [CII]$$ luminosity functions are predicted to converge to observed values at $z\sim4.5$. While we demonstrate that ionisation parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [OIII]${\rm 88\mu m}$/[CII]$$, observed values at $z>6$ can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [CII]${\rm 158\mu m}$ and [OIII]$$ can be used to predict the values of ionisation parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed $z>6$ galaxies. Finally, we demonstrate that [OI]${\rm 63\mu m}$ can be used as a replacement for [CII]$$ in high-redshift galaxies where [CII]${\rm 158\mu m}$ is unobserved and argue that more observation time should be used to target [OI]$_{\rm 63\mu m}$ at $z>6$.

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Fri 24 Sep 11:30: Title to be confirmed

Thu, 22/07/2021 - 12:52
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Fri 24 Sep 11:30: Title to be confirmed

Thu, 22/07/2021 - 10:03
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Fri 18 Jun 13:00: Calm after the storm: black hole ringdown beyond linear perturbation theory.

Wed, 16/06/2021 - 12:27
Calm after the storm: black hole ringdown beyond linear perturbation theory.

Recent analyses have shown that the gravitational-wave signal emitted by merging black holes is surprisingly simple in its final stages. It has long been known that the very end stage of a black hole merger—the late ringdown—is well-described by black hole perturbation theory. The latter, however, has been found to be in excellent agreement with numerical simulations even at much earlier times, immediately following the merger. In this talk, I will describe efforts to understand this phenomenon by characterizing the size and nature of backreaction effects in black-hole ringdowns. Our analysis, which we validate against numerical relativity simulations, is based on a novel perturbation theory incorporating a bilinear form between quasinormal modes. From a toy-model system (an asymptotically anti-de Sitter black hole with a ringing scalar field), to gravitational perturbations of Schwarzschild, the lessons we learn can help explain the simplicity of ringdown signals. I will also discuss ongoing efforts to extend these studies to more realistic (Kerr) black holes.

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Thu 17 Jun 16:00: New results on galaxy clusters from the eROSITA early data release

Mon, 14/06/2021 - 21:46
New results on galaxy clusters from the eROSITA early data release

The eROSITA X-ray telescope is the soft X-ray instrument on the Spectrum-Roentgen-Gamma (SRG) mission, launched successfully in 2019. Before the start of its four-year all-sky survey, eROSITA observed a number of calibration and performance verification targets. The German eROSITA team will shortly be releasing its data in an early data release. I will report on a number of results from these data focusing on galaxy clusters. These observations include a 140 square-degree survey of the sky containing a number of clusters and superclusters, a detailed mosaic of a galaxy cluster pair, and deep observations of relaxed and merging clusters. With its wide field of view and well-characterised background eROSITA is perfect for detecting clusters and other faint structures which would be hard to find using other X-ray telescopes. These results provide a number of insights about cluster populations and astrophysical processes.

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Wed 16 Jun 14:00: Results of the 2020 IoA Annual Staff Survey

Mon, 14/06/2021 - 14:56
Results of the 2020 IoA Annual Staff Survey

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Wed 16 Jun 16:00: Neutron stars as gravitational-wave sources: dense matter and stellar mass

Thu, 10/06/2021 - 16:04
Neutron stars as gravitational-wave sources: dense matter and stellar mass

Astronomical observations of neutron stars inform our understanding of matter at the highest densities. Already, we have used the gravitational-wave data of GW170817 - the first signal from merging neutron stars – to constrain the equation of state of dense matter in neutron stars. The heavy neutron-star merger GW190425 indicated that the gravitational-wave population may include heavier stars not previously observed in galactic double neutron star binaries. For distant sources, the distribution of masses in neutron-star mergers will be a key observable in the coming years of gravitational-wave astronomy. In this talk, I will discuss methods being used to explore matter and mass properties for LIGO /Virgo neutron stars. I will discuss how these results fit with other neutron-star observations, outline prospects of learning about matter in the current Advanced-detector era, and extrapolate to the potential of next-generation gravitational-wave observatories to map the phase diagram of dense neutron-rich matter and the endpoints of stellar evolution.

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Fri 11 Jun 13:00: Oscillations in the Stochastic Gravitational Wave Background

Wed, 09/06/2021 - 16:15
Oscillations in the Stochastic Gravitational Wave Background

Observational constraints and prospects for detection of features, i.e. oscillations in the primordial power spectrum, have so far concentrated on the CMB and Large Scale Structure surveys. After a broad introduction, I will show how different features in the primordial spectrum lead to characteristic oscillatory patterns in the cosmological stochastic gravitational wave background. Probing these features could, for instance, establish the existence of heavy particles beyond the reach of terrestrial experiments, and even test the inflationary paradigm or point to alternatives to it. Further, high energy embeddings of inflation often lead to departures from the single-field slow-roll paradigm, resulting in features in the primordial scalar power spectrum. This provides a clear target for gravitational wave observatories as well as a challenge for developing dedicated data analysis techniques to look for this unique insight into the physics of the early universe.

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Mon 14 Jun 15:00: Love and Naturalness

Tue, 08/06/2021 - 17:37
Love and Naturalness

Tidal deformability coefficients (“Love numbers”) are key parameters of compact objects, which can be probed with gravitational wave measurements. Also they are Wilson coefficients of the effective field theory (EFT) of inspiraling binaries. Remarkably, black hole Love numbers vanish identically in general relativity in four dimensions, which represents a major naturalness problem in the EFT context. In my talk, I will present a new hidden SL(2,R)xU(1) symmetry of general relativity (“Love symmetry”), which elegantly resolves the naturalness paradox. In particular, I will demonstrate that all known properties of black hole tidal Love numbers follow from the SL(2,R) representation theory. Finally, I will discuss implications of the Love symmetry for gravitational wave science and its relation to near-horizon isometries of extremal black holes.

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