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Title to be confirmed

Abstract not available

• Speaker: Fred Alford (Imperial College)
• Friday 26 November 2021, 13:00-14:00
• Venue: Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Justin Ripley.

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Title to be confirmed

Abstract not available

• Speaker: Stefan Hollands (University of Leipzig/DAMTP)
• Friday 08 October 2021, 13:00-14:00
• Venue: Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Justin Ripley.

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Quasi-local mass of the Kerr black hole horizon

Abstract not available

• Speaker: Maciej Dunajski (DAMTP)
• Friday 05 November 2021, 13:00-14:00
• Venue: Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Justin Ripley.

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Title to be confirmed

Abstract not available

• Speaker: Elisa Chisari (Utrecht)
• Monday 01 November 2021, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: James Bonifacio.

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Title to be confirmed

Abstract not available

• Speaker: Uros Seljak (Berkeley)
• Monday 25 October 2021, 16:00-17:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: James Bonifacio.

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Title to be confirmed

Abstract not available

• Speaker: Matthijs Hogervorst (EPFL)
• Monday 18 October 2021, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: James Bonifacio.

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Nature, Published online: 17 September 2021; doi:10.1038/d41586-021-02531-5

Hints of a previously unknown, primordial form of the substance could explain why the cosmos now seems to be expanding faster than theory predicts.

Title to be confirmed

Abstract not available

• Speaker: Amelia Hankla - University of Colorado Boulder
• Monday 22 November 2021, 14:00-15:00
• Venue: Online.
• Series: DAMTP Astro Mondays; organiser: Dr Can Cui.

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Linear stability of disk galaxies

An attractive theory for the occurrence of spiral patterns in simulated and, hopefully, real disc galaxies is based on the idea that the underlying stellar disc is linearly unstable and spontaneously grows eigenmodes. These rotating, overlapping modes then form the changing, transient patterns that are observed in simulated discs (Sellwood & Carlberg 2014). This obviously begs the question why these discs are linearly unstable to begin with. Using the linearized Boltzmann equation, I investigate how grooves carved in the phase space of a stellar disc can trigger the vigorous growth of two-armed spiral eigenmodes (De Rijcke, Fouvry, Pichon 2019). Such grooves result from the self-induced dynamics of a disc subject to finite-N shot noise, as swing-amplified noise patterns push stars towards lower angular momentum orbits at their inner Lindblad radius (Sellwood 2012, Fouvry et al. 2015). I provide evidence that the depletion of near-circular orbits, and not the addition of radial orbits, is the crucial physical ingredient that causes these new eigenmodes. Thus, it is possible for an isolated, linearly stable stellar disc to spontaneously become linearly unstable via the self-induced formation of phase-space grooves through finite-N dynamics. In order to be able to compare the linear stability computations directly with N-body simulations, they were equipped with gravitational softening (De Rijcke, Fouvry, Dehnen 2019). I also show some first results obtained using this linear stability code with the inclusion of the gravitational coupling between a stellar disc and a cooling gas disc, which enables the search for eigenmodes in the star+gas system.

• Speaker: Sven De Rijcke - Ghent University
• Monday 11 October 2021, 14:00-15:00
• Venue: Online.
• Series: DAMTP Astro Mondays; organiser: Cleo Loi.

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We use the Sherwood-Relics suite of hybrid hydrodynamical and radiative transfer simulations to model the effect of inhomogeneous reionisation on the 1D power spectrum of the Lyman-$\alpha$ forest transmitted flux at redshifts $4.2\leq z \leq 5$. Relative to models that assume a homogeneous UV background, reionisation suppresses the power spectrum at small scales, $k \sim 0.1\rm\,km^{-1}\,s$, by $\sim 10$ per cent because of spatial variations in the thermal broadening kernel and the divergent peculiar velocity field associated with over-pressurised intergalactic gas. On larger scales, $k<0.03\rm\,km^{-1}\,s$, the power spectrum is instead enhanced by $10$-$50$ per cent by large scale spatial variations in the neutral hydrogen fraction. The effect of inhomogeneous reionisation must therefore be accounted for in analyses of forthcoming high precision measurements. We provide a correction for the Lyman-$\alpha$ forest power spectrum at $4.1\leq z \leq 5.4$ in a form that can be easily applied within other parameter inference frameworks. We perform a Bayesian analysis of mock data to assess the extent of systematic biases that may arise in measurements of the intergalactic medium if ignoring this correction. At the scales probed by current high resolution Lyman-$\alpha$ forest data at $z>4$, $0.006 \rm \,km^{-1}\,s\leq k \leq 0.2 \rm\, km^{-1}\,s$, we find inhomogeneous reionisation does not introduce any significant bias in thermal parameter recovery for the current measurement uncertainties of $\sim 10$ per cent. However, for $5$ per cent uncertainties, $\sim 1\sigma$ shifts between the estimated and true parameters occur.

RV follow-up of planets around young stars

TBC

• Speaker: Mario Damasso (Torino)
• Tuesday 23 November 2021, 13:00-14:00
• Venue: ONLINE - Details to be sent by email.
• Series: Exoplanet Meetings; organiser: Dr Annelies Mortier.

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The EXPRES Stellar Signals Project (ESSP): Establishing the State of the Field in Disentangling Photospheric Velocities

Measured spectral shifts due to intrinsic stellar variability (e.g. pulsations, granulation) and activity (e.g. spots, plages) are the largest source of error for extreme precision radial velocity (EPRV). Several methods have been developed to disentangle stellar signals from true center-of-mass shifts due to planets. The EXPRES Stellar Signals Project (ESSP) presents a self-consistent comparison of 21 different methods tested on the same extreme-precision spectroscopic data from EXPRES . Methods either derived new activity indicators, constructed new models for mapping an indicator to the needed RV correction, or separated out shape- and shift-driven RV components. Method results were compared based on the total and nightly scatter of returned RVs, agreement with other methods, and correlation with activity indicators. I will give an overview of the project, the submitted methods, and the final results. This comparison allows me to highlight commonalities between the different approaches and propose recommendations for moving forward.

• Speaker: Lily Zhao (Flatiron Institute, NY)
• Tuesday 02 November 2021, 13:00-14:00
• Venue: ONLINE - Details to be sent by email.
• Series: Exoplanet Meetings; organiser: Dr Annelies Mortier.

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Title to be confirmed

Abstract not available

• Speaker: Maria Okounkova (Flatiron Institute)
• Friday 29 October 2021, 14:00-15:00
• Venue: Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Justin Ripley.

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We present a new investigation of the intergalactic medium (IGM) near the end of reionization using "dark gaps" in the Lyman-alpha (Ly$\alpha$) forest. Using spectra of 55 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Ly$\alpha$ forest where the transmission averaged over 1 comoving $h^{-1}\,{\rm Mpc}$ bins falls below 5%. Nine ultra-long ($L > 80~h^{-1}\,{\rm Mpc}$) dark gaps are identified at $z<6$. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than $30~h^{-1}\,{\rm Mpc}$, $F_{30}$, as a function of redshift. We measure $F_{30} \simeq 0.9$, 0.6, and 0.15 at $z = 6.0$, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to $z \simeq 5.3$. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at $z \lesssim 6$ are also potentially consistent with the data. Overall, our results suggest that signature of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least $z \simeq 5.3$.

Nature, Published online: 15 September 2021; doi:10.1038/s41586-021-03802-x

The diffuse, isotropic background of gamma rays comes mainly from star-forming galaxies, according to a physical model of gamma-ray emission.

A new study, led by researchers at the University of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. The researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world – from tiny moons to massive galaxies, from ants to blue whales – makes up less than five percent of the universe. The rest is dark. About 27% is dark matter – the invisible force holding galaxies and the cosmic web together – while 68% is dark energy, which causes the universe to expand at an accelerated rate.

“Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn’t discovered until 1998,” said Dr Sunny Vagnozzi from Cambridge’s Kavli Institute for Cosmology, the paper’s first author. “Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter ‘hitting’ ordinary matter, but dark energy is even more elusive.”

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the Universe’s expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. “These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said Dr Luca Visinelli, a researcher at Frascati National Laboratories in Italy, a co-author of the study. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”

At the time, the most popular explanation for the excess were axions – hypothetical, extremely light particles – produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can’t be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein’s theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be ‘hidden’ or ‘screened’ when it comes to small scales, and can only operate on the largest scales where Einstein's theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the XENON1T excess.

“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said Vagnozzi. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”

The researchers used their model to show what would happen in the detector if the dark energy was produced in a particular region of the Sun, called the tachocline, where the magnetic fields are particularly strong.

“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. “We first need to know that this wasn’t simply a fluke,” said Visinelli. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”

If the excess was the result of dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect dark energy within the next decade.

 

Reference:
Sunny Vagnozzi et al. ‘Direct detection of dark energy: the XENON1T excess and future prospects.’ Physical Review D (2021). DOI: 10.1103/PhysRevD.104.063023

Dark energy, the mysterious force that causes the universe to accelerate, may have been responsible for unexpected results from the XENON1T experiment, deep below Italy’s Apennine Mountains.

It was surprising that this excess could in principle have been caused by dark energy rather than dark matter. When things click together like that, it’s really special.Sunny VagnozzibetmariSun

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Title to be confirmed

Abstract not available

• Speaker: Will Biggs (DAMTP)
• Friday 19 November 2021, 13:00-14:00
• Venue: Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Justin Ripley.

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A giant molecular halo around a z ∼ 2 quasar

Abstract not available

• Speaker: Claudia Cicone (Oslo)
• Friday 03 December 2021, 11:30-12:30
• Venue: via zoom .
• Series: Institute of Astronomy Galaxies Discussion Group; organiser: Martin Haehnelt.

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Aims. The catalog of Stars With ExoplanETs (SWEET-Cat) was originally introduced in 2013. Since then many more exoplanets have been confirmed, increasing significantly the number of host stars listed there. A crucial step toward a comprehensive understanding of these new worlds is the precise and homogeneous characterization of their host stars. Better spectroscopic stellar parameters along with new results from Gaia eDR3 provide updated and precise parameters for the discovered planets. A new version of the catalog, whose homogeneity in the derivation of the parameters is key to unraveling star-planet connections, is available to the community. Methods. We made use of high-resolution spectra for planet-host stars, either observed by our team or collected through public archives. The spectroscopic stellar parameters were derived for the spectra following the same homogeneous process using ARES and MOOG (ARES+MOOG) as for the previous SWEET-Cat releases. We re-derived parameters for the stars in the catalog using better quality spectra and/or using the most recent versions of the codes. Moreover, the new SWEET-Cat table can now be more easily combined with the planet properties listed both at the Extrasolar Planets Encyclopedia and at the NASA exoplanet archive to perform statistical analyses of exoplanets. We also made use of the recent GAIA eDR3 parallaxes and respective photometry to derive consistent and accurate surface gravity values for the host stars. Results. We increased the number of stars with homogeneous parameters by more than 40\% (from 645 to 928). We reviewed and updated the metallicity distributions of stars hosting planets with different mass regimes comparing the low-mass planets (< 30M$_{\oplus}$) with the high-mass planets. The new data strengthen previous results showing the possible trend in the metallicity-period-mass diagram for low-mass planets.

We construct a parametric SED model which is able to reproduce the average observed SDSS-UKIDSS-WISE quasar colours to within one tenth of a magnitude across a wide range of redshift $(0<z<5)$ and luminosity $(-22>M_i>-29)$. This model is shown to provide accurate predictions for the colours of known quasars which are less luminous than those used to calibrate the model parameters, and also those at higher redshifts $z>5$. Using a single parameter, the model encapsulates an up-to-date understanding of the intra-population variance in the rest-frame ultraviolet and optical emission lines of luminous quasars. At fixed redshift, there are systematic changes in the average quasar colours with apparent i-band magnitude, which we find to be well explained by the contribution from the host galaxy and our parametrization of the emission-line properties. By including redshift as an additional free parameter, the model could be used to provide photometric redshifts for individual objects. For the population as a whole we find that the average emission line and host galaxy contributions can be well described by simple functions of luminosity which account for the observed changes in the average quasar colours across $18.1<i_\textrm{AB}<21.5$. We use these trends to provide predictions for quasar colours at the luminosities and redshifts which will be probed by the Rubin Observatory LSST and ESA-Euclid wide survey. The model code is applicable to a wide range of upcoming photometric and spectroscopic surveys, and is made publicly available.