Geraint F. Lewis's research while affiliated with The University of Sydney and other places

Publications (47)

Preprint
Whilst the underlying assumption of the Friedman-Lema\^itre-Robertson-Walker (FLRW) cosmological model is that matter is homogeneously distributed throughout the universe, gravitational influences over the life of the universe have resulted in mass clustered on a range of scales. Hence we expect that, in our inhomogeneous universe, the view of an o...
Preprint
Full-text available
Redshift-space distortions (RSD) generically affect any spatially-dependent observable that is mapped using redshift information. The effect on the observed clustering of galaxies is the primary example of this. This paper is devoted to another example: the effect of RSD on the apparent peculiar motions of tracers as inferred from their positions i...
Article
An unprecedented number of exoplanets are being discovered by the Transiting Exoplanet Survey Satellite (TESS). Determining the orbital parameters of these exoplanets, and especially their mass and radius, will depend heavily upon the measured physical characteristics of their host stars. We have cross-matched spectroscopic, photometric, and astrom...
Preprint
Full-text available
An unprecedented number of exoplanets are being discovered by the Transiting Exoplanet Survey Satellite (TESS). Determining the orbital parameters of these exoplanets, and especially their mass and radius, will depend heavily upon the measured physical characteristics of their host stars. We have cross-matched spectroscopic, photometric, and astrom...
Article
Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present complete results for planet-candidate hosts from the K2-HERMES survey, which uses the HERMES multi-object spectrograph on...
Preprint
Due to differing gravitational potentials and path lengths, gravitational lensing induces time delays between multiple images of a source which, for solar mass objects, is of order $\sim10^{-5}$ seconds. If an astrophysically compact source, such as a Fast Radio Burst (FRB), is observed through a region with a high optical depth of such microlensin...
Preprint
We investigate the properties of 1306 red giant stars with high photospheric abundances of lithium observed by the GALAH, K2-HERMES and TESS-HERMES surveys, and discuss them in the context of proposed mechanisms for lithium enrichment in giant stars. We confirm that Li-rich giants are rare, making up only 1.1 per cent of our giant star sample. We u...
Preprint
Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present complete results for planet-candidate hosts from the K2-HERMES survey, which uses the HERMES multi-object spectrograph on...
Article
Gravitational microlensing within the Galaxy offers the prospect of probing the details of distant stellar sources, as well as revealing the distribution of compact (and potentially non-luminous) masses along the line of sight. Recently, it has been suggested that additional constraints on the lensing properties can be determined through the measur...
Preprint
Gravitational microlensing within the Galaxy offers the prospect of probing the details of distant stellar sources, as well as revealing the distribution of compact (and potentially non-luminous) masses along the line-of-sight. Recently, it has been suggested that additional constraints on the lensing properties can be determined through the measur...
Article
Full-text available
It has recently been claimed that relativity’s most famous equation, \(E=mc^2\), has a cosmological basis, representing the gravitational binding energy for a particle to escape from the origin to a gravitational horizon of the universe. In this paper, I examine these claims in detail, concluding that they result from a misinterpretation of motion...
Preprint
It has recently been claimed that relativity's most famous equation, E = mc^2, has a cosmological basis, representing the gravitational binding energy for a particle to escape from the origin to a gravitational horizon of the universe. In this paper, I examine these claims in detail, concluding that they result from a misinterpretation of motion of...
Preprint
The standard model of cosmology is based on two unknown dark components that are uncoupled from each other. In this paper we investigate whether there is evidence for an interaction between these components of cold dark matter (CDM) and dark energy (DE). In particular, we reconstruct the interaction history at low-redshifts non-parametrically using...
Article
Observations of the large-scale structure (LSS) implicitly assume an ideal Friedmann-Lemaitre-Robertson-Walker (FLRW) observer with the ambient structure having no influence on the observer. However, due to correlations in the LSS, cosmological observables are dependent on the position of an observer. We investigate this influence in full generalit...
Preprint
The redshift drift, the change of cosmological redshift with time, is a direct consequence of the expansion of the Universe. Thus the measurement of the cosmological redshift drift will offer a direct test of our models of cosmology. The magnitude of the effect is very small, i.e. the spectral shift is of order of $10^{-10} - 10^{-9}$ over the peri...
Preprint
Full-text available
(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale s...
Preprint
Observations of the large-scale structure (LSS) implicitly assume an ideal FLRW observer with the ambient structure having no influence on the observer. However, due to correlations in the LSS, cosmological observables are dependent on the position of an observer. We investigate this influence in full generality for a weakly non-Gaussian random fie...
Preprint
We utilise the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii $R_p=25-150$ kpc. In this region the cluster surface density profile exhibits a power-law decline with index $\Gamma=-2.37\pm0.17$, matching that for the metal-poor stellar...
Article
The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHBstars from the overall population, we de...
Article
We compare the evolution of voids formed under the standard cosmological model and two alternative cosmological models. The two models are a quintessence model and a Coupled Dark Matter-Dark Energy model, both of which have evolving and interacting dark sectors. From N-body adiabatic hydrodynamical simulations of thesemodels, we measure the statist...
Preprint
We use data from the second data releases of the ESA Gaia astrometric survey and the high-resolution GALAH spectroscopic survey to analyse the structure of our Galaxy's discs. With GALAH, we can separate the alpha-rich and alpha-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distribution...
Article
GALAH is a large-scale magnitude-limited southern stellar spectroscopic survey. Buder et al. provide values of stellar parameters and abundances of 23 elements for 342,682 stars. Here we add a description of the public release of radial velocities with a typical accuracy of 0.1 km/s for 199,628 of these stars. This accuracy is achievable due to the...
Article
The primary goal of Galactic archaeology is to learn about the origin of the Milky Way from the detailed chemistry and kinematics of millions of stars. Wide-field multi-fibre spectrographs are increasingly used to obtain spectral information for huge samples of stars. Some surveys (e.g. GALAH) are attempting to measure up to 30 separate elements pe...
Article
Full-text available
Models of the very early universe, including inflationary models, are argued to produce varying universe domains with different values of fundamental constants and cosmic parameters. Using the cosmological hydrodynamical simulation code from the \eagle collaboration, we investigate the effect of the cosmological constant on the formation of galaxie...
Article
The technique of chemical tagging uses the elemental abundances of stellar atmospheres to `reconstruct' chemically homogeneous star clusters that have long since dispersed. The GALAH spectroscopic survey --which aims to observe one million stars using the Anglo-Australian Telescope -- allows us to measure up to 30 elements or dimensions in the stel...
Article
Full-text available
We present the chemical distribution of the Milky Way, based on 2,900$\, {\rm deg^2}$ of $u$-band photometry taken as part of the Canada-France Imaging Survey. When complete, this survey will cover 10,000$\, {\rm deg^2}$ of the Northern sky. By combing the CFHT $u$-band photometry together with SDSS and Pan-STARRS $g,r,$ and $i$, we demonstrate tha...
Article
Full-text available
The Canada-France Imaging Survey (CFIS) will map the northern high Galactic latitude sky in the $u$-band ("CFIS-u", 10,000$\, {\rm deg^2}$) and in the $r$-band ("CFIS-r", 5,000$\, {\rm deg^2}$), enabling a host of stand-alone science investigations, and providing some of the ground-based data necessary for photometric redshift determination for the...
Article
Stellar nucleosynthesis proceeds via the deuteron (D), but only a small change in the fundamental constants of nature is required to unbind it. Here, we investigate the effect of altering the binding energy of the deuteron on proton burning in stars. We find that the most definitive boundary in parameter space that divides probably life-permitting...
Article
A vast wealth of literature exists on the topic of rocket trajectory optimisation, particularly in the area of interplanetary trajectories due to its relevance today. Studies on optimising interstellar and intergalactic trajectories are usually performed in flat spacetime using an analytical approach, with very little focus on optimising interstell...
Article
The Galactic Archaeology with HERMES (GALAH) Survey is a massive observational project to trace the Milky Way's history of star formation, chemical enrichment, stellar migration and minor mergers. Using high-resolution (R$\simeq$28,000) spectra taken with the High Efficiency and Resolution Multi-Element Spectrograph (HERMES) instrument at the Anglo...
Article
Despite being a major component in the teaching of special relativity, the twin `paradox' is generally not examined in courses on general relativity. Due to the complexity of analytical solutions to the problem, the paradox is often neglected entirely, and students are left with an incomplete understanding of the relativistic behaviour of time. Thi...
Article
Full-text available
MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) reso...
Article
Primordial nucleosynthesis is rightly hailed as one of the great successes of the standard cosmological model. Here we consider the initial forging of elements in the recently proposed Rh = ct universe, a cosmology that demands linear evolution of the scale factor. Such a universe cools extremely slowly compared to standard cosmologies, considerabl...
Article
High redshift star-forming galaxies are likely responsible for the reionization of the Universe, yet direct detection of their escaping ionizing (Lyman continuum) photons has proven to be extremely challenging. In this study, we search for escaping Lyman continuum of the Cosmic Horseshoe, a gravitationally lensed, star-forming galaxy at z=2.38 with...
Article
Full-text available
The Australian Dark Energy Survey (OzDES) is a 100-night spectroscopic survey underway on the Anglo-Australian Telescope using the fibre-fed 2-degree-field (2dF) spectrograph. We have developed a new redshifting application Marz with greater usability, flexibility, and the capacity to analyse a wider range of object types than the runz software pac...
Article
Cosmological inflation generates primordial density perturbations on all scales, including those far too small to contribute to the cosmic microwave background. At these scales, isolated ultracompact minihalos of dark matter can form, well before standard structure formation, if the small-scale perturbations have a large enough amplitude. Such mini...
Chapter
The clues to galaxy formation and evolution should lie all about us in the form of dismembered remains of dwarf galaxies littering galaxy halos. The faintness and extent of this debris has, however, presented a significant observational challenge. The Pan-Andromeda Archeological Survey (PAndAS) has targeted the halos of our nearest cosmic companion...
Article
Full-text available
Ultracompact minihaloes (UCMHs) have been proposed as a type of dark matter substructure seeded by large-amplitude primordial perturbations and topological defects. UCMHs are expected to survive to the present era, allowing constraints to be placed on their cosmic abundance using observations within our own Galaxy. Constraints on their number densi...
Article
Small-scale dark matter structure within the Milky Way is expected to affect pulsar timing. The change in gravitational potential induced by a dark matter halo passing near the line of sight to a pulsar would produce a varying delay in the light travel time of photons from the pulsar. Individual transits produce an effect that would either be too r...
Article
We introduce a Bayesian solution to the problem of inferring the density profile of strong gravitational lenses when the lens galaxy may contain multiple dark or faint substructures. The source and lens models are based on a superposition of an unknown number of non-negative basis functions (or ‘blobs’) whose form was chosen with speed as a primary...
Article
Full-text available
The nature of the dark sector of the Universe remains one of the outstanding problems in modern cosmology, with the search for new observational probes guiding the development of the next generation of observational facilities. Clues come from tension between the predictions from {\Lambda}CDM and observations of gravitationally lensed galaxies. Pre...
Article
It is generally agreed that there is matter in the universe, and in this paper, we show that the existence of matter is extremely problematic for the proposed Rh = ct universe. Considering a dark energy component with an equation of state of w = −1/3, it is shown that the presence of matter destroys the strict expansion properties that define the e...
Conference Paper
We are using the DEIMOS multi-object spectrograph on the Keck II 10 m telescope to conduct a spectroscopic survey of red giant branch stars in the outskirts of M31. To date, velocities have been obtained for most of the major substructures in the halo as well as at several positions in the far outer disk and inner halo. First results concerning the...

Citations

... The time difference, either at the lens or source, is less than 100 yr. Considering the result from Amendola et al. (2008) and Dam et al. (2021), the peculiar acceleration is less than 1 cm s −1 per decade for a cosmic object, which means a redshift difference around ∼10 −10 . ...
... The GALAH catalogue (Buder et al. 2018) fits these criteria and is regarded as a reliable data source of stellar abundances. It is also an independent dataset, as it shares only five stars with the Hypatia catalogue (Clark et al. 2021). Target selection for the GALAH survey was based on the 2MASS catalogue (De Silva et al. 2015), which had no available abundance measurements for Hypatia to include. ...
... There are 18 K2 candidates that have problematic radii with 13 having been found to be false positives in previous work (Wittenmyer et al. 2018(Wittenmyer et al. , 2020 ...
... Their outcomes suggest that the observation of WL signals associated with under-dense regions is a promising tool to constrain the law of gravitation on largescales. Reischke et al. (2019) have theoretically investigated how the cosmological observables depend on the locality of an observer. After critically examining the analysis of Reischke et al. (2019), Hall (2020) has concluded that there is no evidence of significant bias depending on the observer's position at different regions on the LSS. ...
... Both signals place an exquisite bound on the speed of gravity to be the same as the speed of light. This constraint rejected many modifications to general relativity [16][17][18][19][20][21] and also many unifications between dark energy and dark matter. ...
... Many new examples of specific stellar classes have been discovered, such as Wolf-Rayet stars (Morello et al., 2018), blue horizontal branch stars (Wan et al., 2018), hot sub dwarf stars (Bu et al., 2017), and rare hypervelocity stars (Marchetti et al., 2017). ML/AI have also led to the discovery of unresolved binary stars in simulated catalogues using RF and ANN algorithms (Kuntzer & Courbin, 2017), and new pulsars, and fewer false positives, from the LOFAR Tied-Array All-Sky Survey (Michilli et al., 2018;Tan et al., 2018). ...
... Cosmic voids are the under-dense patches of the universe surrounded by filaments and sheets of galaxies that make up the cosmic web. There are a variety of void-finding techniques in the literature, including Voronoi tessellation and watershed methods (Platen et al. 2007;Neyrinck 2008;Sutter et al. 2012a;Nadathur et al. 2019a), Delaunay density estimation methods (Schaap & van de Weygaert 2000;Zhao et al. 2016), 2D projections (Clampitt & Jain 2015), Hessian-based methods (Adermann et al. 2017(Adermann et al. , 2018, tunnel methods Davies et al. 2021), spherical underdensity estimation methods (Colberg et al. 2005;Padilla et al. 2005;Li 2011;Pan et al. 2012;Hoyle et al. 2012;Villaescusa-Navarro et al. 2018). We recommend the readers to these papers (Colberg et al. 2008;Cautun et al. 2018) for further information on the comparison of void finding methodologies. ...
... 4MOST can observe 1462 stars at low spectroscopic resolution (R = λ/δλ ≈ 4000 − 7500) and 812 stars in high resolution (R ≈ 18000 − 21000) mode. The expected 4MOST limit 3 for a 2 hour exposure in low resolution is 1 km s −1 (1σ) at V ∼ 18 increasing to 3 km s −1 at V ∼ 20, which are feasible with proper consideration of which spectral features are not affected by stellar winds (Zwitter et al. 2018). In fact, the 4MOST low-resolution (R ∼ 4000 -7500) halo survey (Helmi et al. 2019) is planning to observe almost all halo giants with G < 20 mag over 10,000 square degrees, which is about 1.5 million halo stars. ...
... It seems that with photonic combs an alternative forward modelling approach is possible, which convolves a list of spectral templates with assumed values of stellar parameters with known aberrations in the spectrograph to produce a fit to the original CCD image. This approach may yield much better results in the future ( Kos et al. 2018). ...
... In this paper, we therefore present cosmological hydrodynamic box simulations of counterfactual universes, with the principal specific aim of studying the resulting cosmic star-formation history in galaxies as a function of Λ. This question has previously been addressed with the EAGLE numerical code by Barnes et al. (2018), and there have also been a number of semianalytic studies of the same problem (Bousso & Leichenauer 2009Sudoh et al. 2017;Sorini 2022). But the importance of the issue justifies a diversity of approaches; as we will see, there are significant differences between our results and this existing work. ...