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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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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arXiv:2306.09142v2 Announce Type: replace
Abstract: Recent observations with the \textit{James Webb} Space Telescope (JWST) have further refined the spectroscopic redshift of GN-z11, one of the most distant galaxies identified with the \textit{Hubble} Space Telescope (HST) at $z=10.603$. The presence of extremely dense gas ($>10^{10}$ cm$^{-3}$), the detection of high-ionisation lines and of CII*1335 emission, as well as the presence of an ionisation cone, indicate that GN-z11 also hosts an Active Galactic Nucleus (AGN). Further photometric and spectroscopic follow-up demonstrates that it lies in a large-scale, overdense structure with possible signatures of Population III (PopIII) stars in its halo. Surprisingly, Ly$\alpha$ has also been detected despite the expected largely neutral inter-galactic medium at such a redshift. We exploit recent JWST/NIRSpec IFU observations to demonstrate that the Ly$\alpha$ emission in GN-z11 is part of an extended halo with a minimum size of 0.8--3.2 kpc, depending on the definition used to derive the halo size. The surface brightness of the Ly$\alpha$ halo around GN-z11 appears consistent with Ly$\alpha$ halos observed around $z\sim6$ quasars. At the wavelength of Ly$\alpha$ at $z\sim$10.6, we identify three other emission line candidates within the IFU Field-of-View with no UV rest-frame counterpart visible in deep images from the JWST/NIRCam. If confirmed, this could be the first evidence that the local region of GN-z11 represents a candidate protocluster core, forming just 400 Myr after the Big Bang. We give a first estimate of the dark matter halo mass of this structure ($M_h$=2.96$^{+0.44}_{-0.39} \times$10$^{10}$ M$_{\odot}$), consistent with a Coma-like cluster progenitor.
The identification of an eleventh-century Islamic astrolabe bearing both Arabic and Hebrew inscriptions makes it one of the oldest examples ever discovered and one of only a handful known in the world. The astronomical instrument was adapted, translated and corrected for centuries by Muslim, Jewish and Christian users in Spain, North Africa and Italy.
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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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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Cosmic dust grains may be small, but they are mighty – it turns out dust is crucial to just about every process that occurs in space
Astronomers report an object that shines with a brightness equivalent to 500 trillion suns.
Scientists say they have discovered what lies at the heart of the cosmos's most famous explosion.
Positronium has the potential to revolutionise physics but the elusive substance had been too hot to handle.
arXiv:2310.07767v2 Announce Type: replace
Abstract: Confronting measurements of the Lyman-$\alpha$ forest with cosmological hydrodynamical simulations has produced stringent constraints on models of particle dark matter and the thermal and ionization state of the intergalactic medium. We investigate the robustness of such models of the Lyman-$\alpha$ forest, focussing on the effect of particle initial conditions on the Lyman-$\alpha$ forest statistics in cosmological SPH simulations. We study multiple particle initialization algorithms in simulations that are designed to be identical in other respects. In agreement with the literature, we find that the correct linear theory evolution is obtained when a glass-like configuration is used for initial unperturbed gas particle positions alongside a regular grid configuration for dark matter particles and the use of non-identical initial density perturbations for gas and dark matter. However, we report that this introduces a large scale-dependent distortion in the one-dimensional Lyman-$\alpha$ transmission power spectrum at small scales ($k > 0.05$ s/km). The effect is close to $50\%$ at $k\sim 0.1$ s/km, and persists at higher resolution. This can severely bias inferences in parameters such as the dark matter particle mass. By considering multiple initial conditions codes and their variations, we also study the impact of a variety of other assumptions and algorithmic choices, such as adaptive softening, background radiation density, particle staggering, and perturbation theory accuracy, on the matter power spectrum, the Lyman-$\alpha$ flux power spectrum, and the Lyman-$\alpha$ flux PDF. This work reveals possible pathways towards more accurate theoretical models of the Lyman-$\alpha$ forest to match the quality of upcoming measurements.
arXiv:2206.02729v2 Announce Type: replace
Abstract: Results of the mathematical treatment of the radiation by the superluminally moving current sheet in the magnetosphere of a neutron star, which was presented in Ardavan (2021, {\it MNRAS}, {\bf 507}, 4530), are explained here in more transparent physical terms with the aid of illustrations. Not only do these results provide an all-encompassing explanation for the salient features of the radiation received from pulsars (its brightness temperature, polarization, spectrum, profile with microstructure and with a phase lag between the radio and gamma-ray peaks, and the discrepancy between the energetic requirements of its radio and gamma-ray components), but they also shed light on the putative energetic requirements of magnetars and the sources of fast radio bursts and gamma-ray bursts.
arXiv:2402.18773v1 Announce Type: new
Abstract: Observations with the James Webb Space Telescope (JWST) have revealed active galactic nuclei (AGN) powered by supermassive black holes with estimated masses of $10^7-10^8$ M$_\odot$ at redshifts $z\sim7-9$. Some reside in overmassive systems with higher AGN to stellar mass ratios than locally. Understanding how massive black holes could form so early in cosmic history and affect their environment to establish the observed relations today are some of the major open questions in astrophysics and cosmology. One model to create these massive objects is through direct collapse black holes (DCBHs) that provide massive seeds ($\sim10^5-10^6$ M$_\odot$), able to reach high masses in the limited time available. We use the cosmological simulation code GIZMO to study the formation and growth of DCBH seeds in the early Universe. To grow the DCBHs, we implement a gas swallowing model that is set to match the Eddington accretion rate as long as the nearby gaseous environment, affected by stellar and accretion disk feedback, provides sufficient fuel. We find that to create massive AGN in overmassive systems at high redshifts, massive seeds accreting more efficiently than the fiducial Bondi-Hoyle model are needed. We assess whether the conditions for such enhanced accretion rates are realistic by considering limits on plausible transport mechanisms. We also examine various DCBH growth histories and find that mass growth is more sustained in overdense cosmological environments, where high gas densities are achieved locally. We discuss the exciting prospect to directly probe the assembly history of the first SMBHs with upcoming, ultra-deep JWST surveys.
arXiv:2402.18641v1 Announce Type: new
Abstract: The Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI) comprises spectroscopy obtained from 2020 December 14 to 2021 June 10. White dwarfs were targeted by DESI both as calibration sources and as science targets and were selected based on Gaia photometry and astrometry. Here we present the DESI EDR white dwarf catalogue, which includes 2706 spectroscopically confirmed white dwarfs of which approximately 1630 (roughly 60 per cent) have been spectroscopically observed for the first time, as well as 66 white dwarf binary systems. We provide spectral classifications for all white dwarfs, and discuss their distribution within the Gaia Hertzsprung-Russell diagram. We provide atmospheric parameters derived from spectroscopic and photometric fits for white dwarfs with pure hydrogen or helium photospheres, a mixture of those two, and white dwarfs displaying carbon features in their spectra. We also discuss the less abundant systems in the sample, such as those with magnetic fields, and cataclysmic variables. The DESI EDR white dwarf sample is significantly less biased than the sample observed by the Sloan Digital Sky Survey, which is skewed to bluer and therefore hotter white dwarfs, making DESI more complete and suitable for performing statistical studies of white dwarfs.
arXiv:2402.18624v1 Announce Type: new
Abstract: We use rest-frame optical and near-infrared (NIR) observations of 42 Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project at low-$z$ and 37 from the RAISIN Survey at high-$z$ to investigate correlations between SN Ia host galaxy dust, host mass, and redshift. This is the first time the SN Ia host galaxy dust extinction law at high-$z$ has been estimated using combined optical and rest-frame NIR data ($YJ$-band). We use the BayeSN hierarchical model to leverage the data's wide rest-frame wavelength range (extending to $\sim$1.0-1.2 microns for the RAISIN sample at $0.2\lesssim z\lesssim0.6$). By contrasting the RAISIN and CSP data, we constrain the population distributions of the host dust $R_V$ parameter for both redshift ranges. We place a limit on the difference in population mean $R_V$ between RAISIN and CSP of $-1.16
Nature, Published online: 29 February 2024; doi:10.1038/d41586-024-00623-6
Thirty Meter Telescope and Giant Magellan Telescope might need to compete for survival in face of federal spending limit.
Nature, Published online: 29 February 2024; doi:10.1038/d41586-024-00564-0
The surface of a white dwarf is marked with metallic patches — souvenirs of its encounter with an asteroid or planet.
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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arXiv:2402.18515v1 Announce Type: new
Abstract: We introduce a model for dust evolution in the {\sc ramses} code for simulations of galaxies with a resolved multiphase interstellar medium. Dust is modelled as a fluid transported with the gas component, and is decomposed into two sizes, 5 nm and $0.1 \, \mu\rm m$, and two chemical compositions for carbonaceous and silicate grains. Using a suite of isolated disc simulations with different masses and metallicities, the simulations can explore the role of these processes in shaping the key properties of dust in galaxies. The simulated Milky Way analogue reproduces the dust-to-metal mass ratio, depletion factors, size distribution and extinction curves of the Milky Way. Galaxies with lower metallicities reproduce the observed decrease in the dust-to-metal mass ratio with metallicity at around a few $0.1\,\rm Z_\odot$. This break in the DTM corresponds to a galactic gas metallicity threshold that marks the transition from an ejecta-dominated to an accretion-dominated grain growth, and that is different for silicate and carbonaceous grains, with $\simeq0.1\,\rm Z_\odot$ and $\simeq 0.5\,\rm Z_\odot$ respectively. This leads to more Magellanic Cloud-like extinction curves, with steeper slopes in the ultraviolet and a weaker bump feature at 217.5 nm, in galaxies with lower masses and lower metallicities. Steeper slopes in these galaxies are caused by the combination of the higher efficiency of gas accretion by silicate relative to carbonaceous grains, and by the low rates of coagulation that preserves the amount of small silicate grains. Weak bumps are due to the overall inefficient accretion growth of carbonaceous dust at low metallicity, whose growth is mostly supported by the release of large grains in SN ejecta. We also show that the formation of CO molecules is a key component to limit the ability of carbonaceous dust to grow, in particular in low-metallicity gas-rich galaxies.
