arXiv subjects

Tamara M. Davis

Publications and source records attributed to Tamara M. Davis.

At least 55 records · Page 3Linked to original sources

arXiv2022

Full-Shape Galaxy Power Spectra and the Curvature Tension

With recent evidence for a possible "curvature tension" among early and late universe cosmological probes, Effective Field Theories of Large Scale Structure (EFTofLSS) have emerged as a promising new framework to generate constraints on $Ω_k$ that are independent of both CMB measurements, and some of the assumptions of flatness that enter into other large-scale structure analyses. In this work we use EFTofLSS to simultaneously constrain measurements from the 6dFGS, BOSS, and eBOSS catalogues, representing the most expansive full-shape investigation of curvature to date. Fitting the full-shape data with a BBN prior on $Ω_b h^2$ and fixed $n_s$, we measure $Ω_k = -0.089^{+0.049}_{-0.046}$, corresponding to a $\sim 2 σ$ preference for curvature. We argue that this result cannot be biased towards flatness by assumptions in the fitting methodology. Using the Bayesian evidence ratio our full-shape data assigns betting odds of 2:1 in favour of curvature, indicating present measurements remain broadly compatible with both flat and curved cosmological models. When our full-shape sample is combined with Planck 2018 CMB measurements, we break the geometric degeneracy and recover a joint fit on $Ω_k$ of $-0.0041^{+0.0026}_{-0.0021}$. Using the suspiciousness statistic (built on the standard Bayes factor), we find evidence for a moderate tension between Planck 2018 and our suite of full-shape measurements, at a significance of $1.76 ^{+0.14}_{-0.11} σ$ ($p \sim 0.08 \pm 0.02$). These results demonstrate the usefulness of full-shape clustering measurements as a CMB independent probe of curvature in the ongoing curvature tension debate.

astro-ph.CO↗

arXiv2022

The Cosmic Web Crystal: Ising model for Large Scale Structures

If Dark Matter halos possess the gravitational equivalent of an intrinsic magnetic spin, a formal analogy exists between the low redshift behaviour of the Cosmic Web in a flat FLRW background, and a crystal of spins submerged in a thermal reservoir with temperature $T \propto H(t)$. We argue that, within the use of the Bianchi type IX geometry to describe the gravitational collapse of matter inhomogeneities, the spins are nothing but the heritage of its underlying $SU(2)$ symmetry. Therefore, just like electrons in quantum mechanics, these structures may have spin independently from their orbital angular momentum. We explore the phenomenological implications on cosmological scales of a possible late time phase transition of the Cosmic Web towards (the gravitational equivalent of) a ferromagnetic state, described qualitatively using the Ising model in the mean field approximation.

gr-qc↗

arXiv2022

The Pantheon+ Analysis: The Full Dataset and Light-Curve Release

Here we present 1701 light curves of 1550 spectroscopically confirmed Type Ia supernovae (SNe Ia) that will be used to infer cosmological parameters as part of the Pantheon+ SN analysis and the SH0ES (Supernovae and H0 for the Equation of State of dark energy) distance-ladder analysis. This effort is one part of a series of works that perform an extensive review of redshifts, peculiar velocities, photometric calibration, and intrinsic-scatter models of SNe Ia. The total number of light curves, which are compiled across 18 different surveys, is a significant increase from the first Pantheon analysis (1048 SNe), particularly at low redshift ($z$). Furthermore, unlike in the Pantheon analysis, we include light curves for SNe with $z<0.01$ such that SN systematic covariance can be included in a joint measurement of the Hubble constant (H$_0$) and the dark energy equation-of-state parameter ($w$). We use the large sample to compare properties of 151 SNe Ia observed by multiple surveys and 12 pairs/triplets of "SN siblings" - SNe found in the same host galaxy. Distance measurements, application of bias corrections, and inference of cosmological parameters are discussed in the companion paper by Brout et al. (2022b), and the determination of H$_0$ is discussed by Riess et al. (2022). These analyses will measure w with $\sim3\%$ precision and H$_0$ with 1 km/s/Mpc precision.

astro-ph.CO↗

arXiv2021

The Effect of Systematic Redshift Biases in BAO Cosmology

With the remarkable increase in scale and precision provided by upcoming galaxy redshift surveys, systematic errors that were previously negligible may become significant. In this paper, we explore the potential impact of low-magnitude systematic redshift offsets on measurements of the Baryon Acoustic Oscillation (BAO) feature, and the cosmological constraints recovered from such measurements. Using 500 mock galaxy redshift surveys as our baseline sample, we inject a series of systematic redshift biases (ranging from +/-0.2% to +/-2%), and measure the resulting shift in the recovered isotropic BAO scale. When BAO measurements are combined with CMB constraints (in both ΛCDM and wCDM cosmologies), plausible systematics introduce a negligible offset on combined fits of H0 and Ωm, and systematics must be an order of magnitude greater than this plausible baseline to introduce a 1-σ shift on such combined fits. We conclude that systematic redshift biases are very unlikely to bias constraints on parameters such as H0 provided by BAO cosmology, either now or in the near future. We also detail a theoretical model that predicts the impact of uniform redshift systematics on α, and show this model is in close alignment with the results of our mock survey analysis.

astro-ph.CO↗

arXiv2020

Preliminary Target Selection for the DESI Quasar (QSO) Sample

The DESI survey will measure large-scale structure using quasars as direct tracers of dark matter in the redshift range $0.9 2.1$. We present two methods to select candidate quasars for DESI based on imaging in three optical ($g, r, z$) and two infrared ($W1, W2$) bands. The first method uses traditional color cuts and the second utilizes a machine-learning algorithm.

astro-ph.CO↗

arXiv2020

Candidate Periodically Variable Quasars from the Dark Energy Survey and the Sloan Digital Sky Survey

Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg$^2$ overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color ($griz$) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to $r{\sim}$23.5 mag) powered by less massive black holes (with masses $\gtrsim10^{8.5}M_{\odot}$) at high redshift for the first time. We find five candidates with significant (at $>$99.74% single-frequency significance in at least two bands with a global p-value of $\sim$7$\times10^{-4}$--3$\times10^{-3}$ accounting for the look-elsewhere effect) periodicity with observed periods of $\sim$3--5 years (i.e., 1--2 years in rest frame) having $\sim$4--6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of ${\sim}0.8^{+0.5}_{-0.3}$% or ${\sim}1.1^{+0.7}_{-0.5}$ quasar per deg$^2$. Our detection rate is 4--80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. We discuss implications on the future direct detection of low-frequency gravitational waves. Continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins.

astro-ph.HE↗

arXiv2020

Are the spiral arms in the MWC 758 protoplanetary disc driven by a companion inside the cavity?

Spiral arms in protoplanetary discs are thought to be linked to the presence of companions. We test the hypothesis that the double spiral arm morphology observed in the transition disc MWC 758 can be generated by an $\approx 10$ M$_{\rm Jup}$ companion on an eccentric orbit internal to the spiral arms. Previous studies on MWC 758 have assumed an external companion. We compare simulated observations from three dimensional hydrodynamics simulations of disc-companion interaction to scattered light, infrared and CO molecular line observations, taking into account observational biases. The inner companion hypothesis is found to explain the double spiral arms, as well as several additional features seen in MWC 758 -- the arc in the northwest, substructures inside the spiral arms, the cavity in CO isotopologues, and the twist in the kinematics. Testable predictions include detection of fainter spiral structure, detection of a point source south-southeast of the primary, and proper motion of the spiral arms.

astro-ph.EP↗

arXiv2020

Barry and the BAO Model Comparison

We compare the performance of four state-of-the-art models for extracting isotropic measurements of the Baryon Acoustic Oscillation (BAO) scale. To do this, we created a new, public, modular code Barry, which contains datasets, model fitting tools, and model implementations incorporating different descriptions of non-linear physics and algorithms for isolating the BAO feature. These are then evaluated for bias, correlation, and fitting strength using mock power spectra and correlation functions developed for the Sloan Digital Sky Survey Data Release 12. Our main findings are as follows: 1) All of the models can recover unbiased constraints when fit to the pre- and post-reconstruction simulations. 2) Models that provide physical descriptions of the damping of the BAO feature (using e.g., standard perturbation or effective-field theory arguments) report smaller errors on average, although the distribution of mock $χ^{2}$ values indicates these are underestimated. 3) Allowing the BAO damping scale to vary can provide tighter constraints for some mocks, but is an artificial improvement that only arises when noise randomly sharpens the BAO peak. 4) Unlike recent claims in the literature when utilising a BAO Extractor technique, we find no improvement in the accuracy of the recovered BAO scale. 5) We implement a procedure for combining all models into a single consensus result that improves over the standard method without obviously underestimating the uncertainties. Overall, Barry provides an excellent framework for performing the cosmological analyses for upcoming surveys, and for rapidly testing and validating new models.

astro-ph.CO↗

arXiv2020

Standard Siren Speeds: Improving velocities in gravitational-wave measurements of $H_{0}$

We re-analyze data from the gravitational-wave event GW170817 and its host galaxy NGC4993 to demonstrate the importance of accurate total and peculiar velocities when measuring the Hubble constant using this nearby Standard Siren. We show that a number of reasonable choices can be made to estimate the velocities for this event, but that systematic differences remain between these measurements depending on the data used. This leads to significant changes in the Hubble constant inferred from GW170817. We present Bayesian Model Averaging as one way to account for these differences, and obtain $H_{0}=66.8^{+13.4}_{-9.2}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. Adding additional information on the viewing angle from high resolution imaging of the radio counterpart refines this to $H_{0}=64.8^{+7.3}_{-7.2}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. During this analysis we also present an alternative Bayesian model for the posterior on $H_{0}$ from Standard Sirens that works more closely with observed quantities from redshift and peculiar velocity surveys. Our results more accurately capture the true uncertainty on the total and peculiar velocities of NGC4993 and show that exploring how well different datasets characterize galaxy groups and the velocity field in the local Universe could improve this measurement further. These considerations impact any low-redshift distance measurement, and the improvements we suggest here can also be applied to standard candles like type Ia supernovae. GW170817 is particularly sensitive to peculiar velocity uncertainties because it is so close. For future standard siren measurements the importance of this error will decrease as (a) we will measure more distant standard sirens, and (b) the random direction of peculiar velocities will average out with more detections.

astro-ph.CO↗

arXiv2019

The WiggleZ Dark Energy Survey: Final Data Release and the Metallicity of UV-Luminous Galaxies

The WiggleZ Dark Energy Survey measured the redshifts of over 200,000 UV-selected (NUV<22.8 mag) galaxies on the Anglo-Australian Telescope. The survey detected the baryon acoustic oscillation signal in the large scale distribution of galaxies over the redshift range 0.2<z<1.0, confirming the acceleration of the expansion of the Universe and measuring the rate of structure growth within it. Here we present the final data release of the survey: a catalogue of 225415 galaxies and individual files of the galaxy spectra. We analyse the emission-line properties of these UV-luminous Lyman-break galaxies by stacking the spectra in bins of luminosity, redshift, and stellar mass. The most luminous (-25 mag < MFUV <-22 mag) galaxies have very broad H-beta emission from active nuclei, as well as a broad second component to the [OIII] (495.9 nm, 500.7 nm) doublet lines that is blue shifted by 100 km/s, indicating the presence of gas outflows in these galaxies. The composite spectra allow us to detect and measure the temperature-sensitive [OIII] (436.3 nm) line and obtain metallicities using the direct method. The metallicities of intermediate stellar mass (8.8<log(M*/Msun)<10) WiggleZ galaxies are consistent with normal emission-line galaxies at the same masses. In contrast, the metallicities of high stellar mass (10<log(M*/Msun)<12) WiggleZ galaxies are significantly lower than for normal emission-line galaxies at the same masses. This is not an effect of evolution as the metallicities do not vary with redshift; it is most likely a property specific to the extremely UV-luminous WiggleZ galaxies.

astro-ph.GA↗

arXiv2019

Can redshift errors bias measurements of the Hubble Constant?

Redshifts have been so easy to measure for so long that we tend to neglect the fact that they too have uncertainties and are susceptible to systematic error. As we strive to measure cosmological parameters to better than 1% it is worth reviewing the accuracy of our redshift measurements. Surprisingly small systematic redshift errors, as low as 0.0001, can have a significant impact on the cosmological parameters we infer, such as $H_0$. Here we investigate an extensive (but not exhaustive) list of ways in which redshift estimation can go systematically astray. We review common theoretical errors, such as adding redshifts instead of multiplying by (1+z); using v=cz; and using only cosmological redshift in the estimates of luminosity and angular-diameter distances. We consider potential observational errors, such as rest wavelength precision, air to vacuum conversion at altitude, and spectrograph wavelength calibration. Finally, we explore physical effects, such as peculiar velocity corrections, galaxy internal velocities, gravitational redshifts, and overcorrecting within a bulk flow. We conclude that it would be quite easy for small systematic redshift errors to have infiltrated our data and be impacting our cosmological results. While it is unlikely that these errors are large enough to resolve the current $H_0$ tension, it remains possible, and redshift accuracy may become a limiting factor in near future experiments. With the enormous efforts going into calibrating the vertical axis of our plots (standard candles, rulers, clocks, and sirens) we argue that it is now worth paying a little more attention to the horizontal axis (redshifts).

astro-ph.CO↗

arXiv2018

Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

We present a detailed overview of the cosmological surveys that will be carried out with Phase 1 of the Square Kilometre Array (SKA1), and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5,000 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 3 with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to z = 6. These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical surveys like LSST and Euclid, leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys, and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

astro-ph.CO↗

arXiv2018

Can the fluctuations of the quantum vacuum solve the cosmological constant problem?

The cosmological constant problem arises because the magnitude of vacuum energy density predicted by quantum mechanics is about 120 orders of magnitude larger than the value implied by cosmological observations of accelerating cosmic expansion. Recently, some of the current authors proposed that the stochastic nature of the quantum vacuum can resolve this tension [Q, Wang, Z. Zhu, and W. G. Unruh, Phys. Rev. D 95, 103504, 2017]. By treating the fluctuations in the vacuum seriously and allowing fluctuations up to some high-energy cutoff at which Quantum Field Theory is believed to break down, a parametric resonance effect arises that leads to a slow expansion and acceleration. In this work, we thoroughly examine the implications of this proposal by investigating the resulting dynamics. First, we improve upon numerical calculations in the original work and show that convergence issues had overshadowed some important effects. Correct calculations reverse some of the conclusions in [Q. Wang, Z. Zhu, and W. G. Unruh, Phys. Rev. D 95, 103504, 2017], however the premise that parametric resonance can explain a very slowly accelerating expansion appears to remain sound. After improving the resolution and efficiency of the numerical tests, we explore a wider range of cutoff energies, and examine the effects of multiple particle fields. We introduce a simple model using the Mathieu equation (a prototypical example of parametric resonance), and find that it closely matches numerical results in regimes where its assumptions are valid. Using this model, we extrapolate to find that in a universe with 28 bosonic fields and a high-energy cutoff 40 times higher than the Planck energy, the acceleration would be comparable to what is observed.

gr-qc↗

arXiv2018

Updating the MACHO fraction of the Milky Way dark halo with improved mass models

Recent interest in primordial black holes as a possible dark matter candidate has motivated the reanalysis of previous methods for constraining massive astrophysical compact objects in the Milky Way halo and beyond. In order to derive these constraints, a model for the dark matter distribution around the Milky Way must be used. Previous microlensing searches have assumed a semi-isothermal density sphere for this task. We show this model is no longer consistent with data from the Milky Way rotation curve, and test two replacement models, namely NFW and power-law. The power-law model is the most flexible as it can break spherical symmetry, and best fits the data. Thus, we recommend the power-law model as a replacement, although it still lacks the flexibility to fully encapsulate all possible shapes of the Milky Way halo. We then use the power-law model to rederive some previous microlensing constraints in the literature, while propagating the primary halo-shape uncertainties through to our final constraints. Our analysis reveals that the microlensing constraints towards the Large Magellanic Cloud weaken somewhat for MACHO masses around $10\, M_\odot$ when this uncertainty is taken into account, but the constraints tighten at lower masses. Exploring some of the simplifying assumptions of previous constraints we also study the effect of wide mass distributions of compact halo objects, as well as the effect of spatial clustering on microlensing constraints. We find that both effects induce a shift in the constraints towards smaller masses, and can effectively remove the microlensing constraints from $M \sim 1-10 M_\odot$ for certain MACHO populations.

astro-ph.CO↗

arXiv2018

The Taipan Galaxy Survey: Scientific Goals and Observing Strategy

Taipan is a multi-object spectroscopic galaxy survey starting in 2017 that will cover 2pi steradians over the southern sky, and obtain optical spectra for about two million galaxies out to z<0.4. Taipan will use the newly-refurbished 1.2m UK Schmidt Telescope at Siding Spring Observatory with the new TAIPAN instrument, which includes an innovative 'Starbugs' positioning system capable of rapidly and simultaneously deploying up to 150 spectroscopic fibres (and up to 300 with a proposed upgrade) over the 6-deg diameter focal plane, and a purpose-built spectrograph operating from 370 to 870nm with resolving power R>2000. The main scientific goals of Taipan are: (i) to measure the distance scale of the Universe (primarily governed by the local expansion rate, H_0) to 1% precision, and the structure growth rate of structure to 5%; (ii) to make the most extensive map yet constructed of the mass distribution and motions in the local Universe, using peculiar velocities based on improved Fundamental Plane distances, which will enable sensitive tests of gravitational physics; and (iii) to deliver a legacy sample of low-redshift galaxies as a unique laboratory for studying galaxy evolution as a function of mass and environment. The final survey, which will be completed within 5 years, will consist of a complete magnitude-limited sample (i<17) of about 1.2x10^6 galaxies, supplemented by an extension to higher redshifts and fainter magnitudes (i<18.1) of a luminous red galaxy sample of about 0.8x10^6 galaxies. Observations and data processing will be carried out remotely and in a fully-automated way, using a purpose-built automated 'virtual observer' software and an automated data reduction pipeline. The Taipan survey is deliberately designed to maximise its legacy value, by complementing and enhancing current and planned surveys of the southern sky at wavelengths from the optical to the radio.

astro-ph.GA↗

arXiv2017

A blinded determination of $H_0$ from low-redshift Type Ia supernovae, calibrated by Cepheid variables

Presently a ${>}3σ$ tension exists between values of the Hubble constant $H_0$ derived from analysis of fluctuations in the Cosmic Microwave Background by Planck, and local measurements of the expansion using calibrators of type Ia supernovae (SNe Ia). We perform a blinded reanalysis of Riess et al. 2011 to measure $H_0$ from low-redshift SNe Ia, calibrated by Cepheid variables and geometric distances including to NGC 4258. This paper is a demonstration of techniques to be applied to the Riess et at. 2016 data. Our end-to-end analysis starts from available CfA3 and LOSS photometry, providing an independent validation of Riess et al. 2011. We obscure the value of $H_0$ throughout our analysis and the first stage of the referee process, because calibration of SNe Ia requires a series of often subtle choices, and the potential for results to be affected by human bias is significant. Our analysis departs from that of Riess et al. 2011 by incorporating the covariance matrix method adopted in SNLS and JLA to quantify SN Ia systematics, and by including a simultaneous fit of all SN Ia and Cepheid data. We find $H_0 = 72.5 \pm 3.1$ (stat) $\pm 0.77$ (sys) km s$^{-1}$ Mpc$^{-1}$ with a three-galaxy (NGC 4258+LMC+MW) anchor. The relative uncertainties are 4.3% statistical, 1.1% systematic, and 4.4% total, larger than in Riess et al. 2011 (3.3% total) and the Efstathiou 2014 reanalysis (3.4% total). Our error budget for $H_0$ is dominated by statistical errors due to the small size of the supernova sample, whilst the systematic contribution is dominated by variation in the Cepheid fits, and for the SNe Ia, uncertainties in the host galaxy mass dependence and Malmquist bias.

astro-ph.CO↗

arXiv2017

Accounting for sample selection in Bayesian analyses

Astronomers are often confronted with funky populations and distributions of objects: brighter objects are more likely to be detected; targets are selected based on colour cuts; imperfect classification yields impure samples. Failing to account for these effects leads to biased analyses. In this paper we present a simple overview of a Bayesian consideration of sample selection, giving solutions to both analytically tractable and intractable models. This is accomplished via a combination of analytic approximations and Monte Carlo integration, in which dataset simulation is efficiently used to correct for issues in the observed dataset. This methodology is also applicable for data truncation, such as requiring densities to be strictly positive. Toy models are included for demonstration, along with discussions of numerical considerations and how to optimise for implementation. We provide sample code to demonstrate the techniques. The methods in this paper should be widely applicable in fields beyond astronomy, wherever sample selection effects occur.

astro-ph.CO↗

arXiv2017

The effects of velocities and lensing on moments of the Hubble diagram

We consider the dispersion on the supernova distance-redshift relation due to peculiar velocities and gravitational lensing, and the sensitivity of these effects to the amplitude of the matter power spectrum. We use the MeMo lensing likelihood developed by Quartin, Marra & Amendola (2014), which accounts for the characteristic non-Gaussian distribution caused by lensing magnification with measurements of the first four central moments of the distribution of magnitudes. We build on the MeMo likelihood by including the effects of peculiar velocities directly into the model for the moments. In order to measure the moments from sparse numbers of supernovae, we take a new approach using Kernel Density Estimation to estimate the underlying probability density function of the magnitude residuals. We also describe a bootstrap re-sampling approach to estimate the data covariance matrix. We then apply the method to the Joint Light-curve Analysis (JLA) supernova catalogue. When we impose only that the intrinsic dispersion in magnitudes is independent of redshift, we find $σ_8=0.44^{+0.63}_{-0.44}$ at the one standard deviation level, although we note that in tests on simulations, this model tends to overestimate the magnitude of the intrinsic dispersion, and underestimate $σ_8$. We note that the degeneracy between intrinsic dispersion and the effects of $σ_8$ is more pronounced when lensing and velocity effects are considered simultaneously, due to a cancellation of redshift dependence when both effects are included. Keeping the model of the intrinsic dispersion fixed as a Gaussian distribution of width 0.14 mag, we find $σ_8 = 1.07^{+0.50}_{-0.76}$.

astro-ph.CO↗