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Kavli Institute for Cosmology, Cambridge


Informing antenna design for sky-averaged 21-cm experiments using a simulated Bayesian data analysis pipeline. (arXiv:2106.10193v1 [astro-ph.IM])

KICC papers - Mon, 21/06/2021 - 09:10

Global 21cm experiments aim to measure the sky averaged HI absorption signal from cosmic dawn and the epoch of reionisation. However, antenna chromaticity coupling to bright foregrounds can introduce distortions into the observational data of such experiments. We demonstrate a method for guiding the antenna design of a global experiment through data analysis simulations. This is done by performing simulated observations for a range of inserted 21cm signals, then attempting to identify the signals with a data analysis pipeline. We demonstrate this method on five antennae that were considered as potential designs for the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH); a conical log spiral antenna, an inverted conical sinuous antenna and polygonal-, square- and elliptical-bladed dipoles. We find that the log spiral performs significantly better than the other antennae tested, able to correctly and confidently identify every inserted 21cm signal. In second place is the polygonal dipole antenna, which was only unable to detect signals with both very low amplitudes of 0.05K and low centre frequency of 80MHz. The conical sinuous antenna was found to perform least accurately, only able to detect the highest amplitude 21cm signals, and even then with biases. We also demonstrate that, due to the non-trivial nature of chromatic distortion and the processes of correcting for it, these are not the results that could have been expected superficially from the extent of chromatic variation in each antenna.

Sounding out the Universe

Cosmology Papers - Fri, 18/06/2021 - 08:33

Mystery of Galaxy's Missing Dark Matter Deepens

Cosmology Papers - Fri, 18/06/2021 - 08:28
Portal origin URL: Mystery of Galaxy's Missing Dark Matter DeepensPortal origin nid: 471822Published: Thursday, June 17, 2021 - 11:00Featured (stick to top of list): noPortal text teaser: When astronomers using NASA's Hubble Space Telescope uncovered an oddball galaxy that looked like it didn't have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation.Portal image: This Hubble Space Telescope snapshot reveals an unusual "see-through" galaxy. The giant cosmic cotton ball is so diffuse and its ancient stars so spread out that distant galaxies in the background can be seen through it.

Fri 18 Jun 13:00: Calm after the storm: black hole ringdown beyond linear perturbation theory.

Upcoming Talks - 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

Upcoming Talks - 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

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

Abstract not available

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A measurement of the Lyman-beta forest power spectrum and its cross with the Lyman-alpha forest in X-Shooter XQ-100. (arXiv:2106.04837v1 [astro-ph.CO])

KICC papers - Fri, 11/06/2021 - 08:54

The Lyman-alpha forest is the large-scale structure probe for which we appear to have modeling control to the highest wavenumbers, which makes it of great interest for constraining the warmness/fuzziness of the dark matter and the timing of reionization processes. However, the standard statistic, the Lyman-alpha forest power spectrum, is unable to strongly constrain the IGM temperature-density relation, and this inability further limits how well other high wavenumber-sensitive parameters can be constrained. With the aim of breaking these degeneracies, we measure the power spectrum of the Lyman-beta forest and its cross correlation with the coeveal Lyman-alpha forest using the one hundred spectra of z=3.5-4.5 quasars in the VLT/X-Shooter XQ-100 Legacy Survey, motivated by the Lyman-beta transition's smaller absorption cross section that makes it sensitive to somewhat higher densities relative to the Lyman-alpha transition. Our inferences from this measurement for the IGM temperature-density relation appear to latch consistently onto the recent tight lower-redshift Lyman-alpha forest constraints of arXiv:2009.00016v1 [astro-ph.CO]. The z=3.4-4.7 trends we find using the Lyman-alpha--Lyman-beta cross correlation show a flattening of the slope of the temperature-density relation with decreasing redshift. This is the trend anticipated from ongoing HeII reionization and there being sufficient time to reach the asymptotic temperature-density slope after hydrogen reionization completes. Furthermore, our measurements provide a consistency check on IGM models that explain the Lyman-alpha forest, with the cross correlation being immune to systematics that are uncorrelated between the two forests, such as metal line contamination.

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

Upcoming Talks - 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

Upcoming Talks - 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

Upcoming Talks - 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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Wed 09 Jun 13:30: The Invention of Dark Matter and Dark Energy.

Upcoming Talks - Tue, 08/06/2021 - 10:43
The Invention of Dark Matter and Dark Energy.

High quality photometry and spectroscopy of disk galaxies has shown that the mass of a galaxy is the only parameter that determines the dark matter attributed to a galaxy. Together with other results on elliptical galaxies we look carefully at the fundamentals that led to the idea that Dark Matter was needed to account for the motions of stars and gas in galaxies. It turns out that the problem is with the use of Newtonian dynamics to explain motions in galaxies. Einstein showed that the field of the gravitating object must not be neglected and that assumptions of homogeneity, isotropy and spherical symmetry are the root cause. Gravitational field lines are affected by other masses in the same way that light is affected. This changes the way that gravitational forces act upon material orbiting in the disk of a galaxy and are proportional to 1/r as observed rather than 1/r^2 predicted by Newton. This error led to the invention of Dark Matter. Gravitational field interaction also reduces gravitational forces outside an object, reducing the forces between galaxies and giving effects that could be mistaken for Dark Energy. Dark Matter and Dark Energy were each introduced as concepts in order to make specific astronomical observations fit in with the then understanding of the physics involved. We demonstrate that by replacing Newtonian dynamics which essentially forces symmetries on objects with Einstein’s general relativistic dynamics is very likely to make the need for either Dark Matter or Dark Energy to go away.

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Wed 16 Jun 14:00: EDI Committee

Upcoming Talks - Mon, 07/06/2021 - 10:39
EDI Committee

Abstract not available

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Thermal WIMPs and the Scale of New Physics: Global Fits of Dirac Dark Matter Effective Field Theories. (arXiv:2106.02056v1 [hep-ph])

KICC papers - Mon, 07/06/2021 - 08:33

We assess the status of a wide class of WIMP dark matter (DM) models in light of the latest experimental results using the global fitting framework $\textsf{GAMBIT}$. We perform a global analysis of effective field theory (EFT) operators describing the interactions between a gauge-singlet Dirac fermion and the Standard Model quarks, the gluons and the photon. In this bottom-up approach, we simultaneously vary the coefficients of 14 such operators up to dimension 7, along with the DM mass, the scale of new physics and 8 nuisance parameters that reflect uncertainties in the local DM halo, nuclear form factors and the top quark mass. We include the renormalization group evolution of all operator coefficients and perform an automated matching to the non-relativistic EFT relevant for DM scattering. Our up-to-date likelihood functions include all relevant experimental constraints based on the latest data from $\mathit{Planck}$, direct and indirect detection experiments, and the LHC, in particular a very recent ATLAS monojet search based on the full run 2 dataset. For light DM ($\lesssim 100$ GeV), we find that it is impossible to satisfy all constraints simultaneously unless the particle under consideration constitutes only a DM sub-component and the scale of the new physics is so low that the EFT breaks down for the calculation of LHC constraints. At intermediate values of the new physics scale ($\approx 1$ TeV), we find that our results are significantly influenced by several small excesses in the LHC data such that the best-fit parameter regions depend on the precise prescription that we adopt to ensure EFT validity. In addition to these interesting features, we find a large region of viable parameter space where the EFT is valid and the relic density can be reproduced, implying that WIMPs can still account for the DM of the universe while being consistent with the latest data.

Wed 09 Jun 14:15: Celestial Holography and w(1+infinity)

Upcoming Talks - Sat, 05/06/2021 - 10:15
Celestial Holography and w(1+infinity)

Abstract not available

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The first 5 years of gravitational-wave astrophysics

Cosmology Papers - Fri, 04/06/2021 - 07:29

Gravitational waves are ripples in spacetime generated by the acceleration of astrophysical objects; a direct consequence of general relativity, they were first directly observed in 2015. Here, I review the first 5 years of gravitational-wave detections. More than 50 gravitational-wave events have been found, emitted by pairs of merging compact objects such as neutron stars and black holes. These signals yield insights into the formation of compact objects and their progenitor stars, enable stringent tests of general relativity, and constrain the behavior of matter at densities higher than that of an atomic nucleus. Mergers that emit both gravitational and electromagnetic waves probe the formation of short gamma-ray bursts and the nucleosynthesis of heavy elements, and they measure the local expansion rate of the Universe.

Mon 14 Jun 16:00: Tides in the high-eccentricity migration of hot Jupiters: Triggering diffusive growth by nonlinear mode interactions PLEASE NOTE THE DIFFERENT START TIME OF **4 PM**!

Upcoming Talks - Wed, 02/06/2021 - 23:41
Tides in the high-eccentricity migration of hot Jupiters: Triggering diffusive growth by nonlinear mode interactions

High eccentricity migration is a possible formation channel for hot Jupiters. However, in order for it to be consistent with the observed population of planets, tides must circularize the orbits in an efficient way. A potential mechanism for such rapid circularization is the diffusive growth of the tidally driven planetary f-mode. Such growth occurs if the f-mode’s phase at pericenter varies chaotically from one pericenter passage to the next. Previous studies focused on the variation of the orbital period due to tidal back-reaction on the orbit as the source of chaos. Here we show that nonlinear mode interactions can also be an important source. Specifically, we show that nonlinear interactions between a parent f-mode and daughter f-/p-modes induce an energy-dependent shift in the oscillation frequency of the parent. This frequency shift varies randomly from orbit to orbit because the parent’s energy varies. As a result, the parent’s phase at pericenter varies randomly, which we find can trigger it to grow diffusively. We show that the phase shift induced by nonlinear mode interactions in fact dominates the shift induced by tidal back-reaction and significantly lowers the one-kick energy threshold for diffusive growth by about a factor of 5 compared to the linear theory’s prediction. Nonlinear interactions could thus enhance the formation rate of hot Jupiters through the high-eccentricity migration channel and potentially mitigate the discrepancy between the observed and predicted occurrence rates for close-in gas giants as compared to those further from the star.


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