arXiv subjects

Paula Boubel

Publications and source records attributed to Paula Boubel.

7 recordsLinked to original sources

arXiv2025

The Local Distance Network: a community consensus report on the measurement of the Hubble constant at 1% precision

The direct, empirical determination of the local value of the Hubble constant (H0) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance estimators. However, combining determinations from different estimators is non-trivial, due to correlated calibrations and different analysis methodologies. Using covariance weighting and leveraging the broad and comprehensive community of experts, we constructed a rigorous and transparent Distance Network (DN) to find a consensus value and uncertainty for the local H0. All critically reviewed the available data sets, spanning parallaxes, detached eclipsing binaries, masers, Cepheids, the TRGB, Miras, JAGB stars, SN Ia, Surface Brightness Fluctuations, SN II, the Fundamental Plane, and Tully-Fisher relations and voted for indicators to define a `baseline' DN and others to assess robustness and sensitivity of the results. We provide open-source software and data products to support full transparency and future extensions of this effort. Our conclusions: 1) Local H0 is robustly determined, with first-rank indicators internally consistent within their uncertainties; 2) A covariance-weighted combination yields an uncertainty of 1.1% (baseline) or 0.9% (all estimators); 3) The contribution from SNe Ia is consistent across four current compilations of optical magnitudes or using NIR-only magnitudes; 4) Removing either Cepheids or TRGB has minimal effect; 5) Replacing SNe Ia with galaxy-based indicators changes H0 by less than 0.1 km/s/Mpc, while doubling its uncertainty; 6) The baseline result is H0=73.50+/-0.81 km/s/Mpc. Compared to early Universe results, our result differs by 7.1sigma from flat ΛCDM with Planck+SPT+ACT and 5.0 sigma with BBN+BAO (DESI2). A networked approach is invaluable for enabling further progress in accuracy and precision without overreliance on any single method, sample or group.

astro-ph.CO↗

arXiv2025

Testing anisotropic Hubble expansion

The cosmological principle asserting the large-scale uniformity of the Universe is a testable assumption of the standard cosmological model. We explore the constraints on anisotropic expansion provided by measuring directional variation in the Hubble constant, $H_0$, derived from differential zeropoint measurements of the Tully-Fisher distance estimator. We fit various models for directional variation in $H_0$ using the Tully-Fisher dataset from the all-sky Cosmicflows-4 catalog. The best-fit dipole variation has an amplitude of 0.063 $\pm$ 0.016 mag in the direction ($\ell,b$) = (142 $\pm$ 30$^{\circ}$, 52 $\pm$ 10$^{\circ}$). If this were due to anisotropic expansion it would imply a 3% variation in $H_0$, corresponding to $ΔH_0$ = 2.10 $\pm$ 0.53 km/s/Mpc if $H_0$ = 70 km/s/Mpc, with a significance of 3.9$σ$. A model that includes this $H_0$ dipole is only weakly favored relative to a model with a constant $H_0$ and a bulk motion of the volume sampled by Cosmicflows-4 that is consistent with the standard $Λ$CDM cosmology. However, we show that with the expected Tully-Fisher data from the WALLABY and DESI surveys it should be possible to detect a 1% $H_0$ dipole anisotropy at 5.8$σ$ confidence and to distinguish it from the typical bulk flow predicted by $Λ$CDM over the volume of these surveys.

astro-ph.CO↗

arXiv2024

Calibrating the Tully-Fisher Relation to Measure the Hubble Constant

Boubel et al. 2024 (B24) recently used the Tully-Fisher (TF) relation to measure calibrated distances in the Hubble flow and found $H_0= 73.3 \pm 2.1 (stat) \pm 3.5 (sys)$ km/s/Mpc. The large systematic uncertainty was the result of propagating the conflict between two sources of empirical distance calibration: a difference in zeropoint when calibrating the TF relation with Type Ia supernovae (SNe Ia) versus Cepheids and Tip-of-the-Red-Giant-Branch (TRGB) and an apparent difference in zeropoint between two distinct TRGB datasets. We trace the SN Ia-based calibration used in the TF analysis to a study where $H_0$ was fixed to 70 km/s/Mpc rather than measured, (with host distances derived from redshifts and the Hubble law), thus introducing a discrepancy with the other empirically calibrated indicators. In addition, we trace the difference in TRGB zeropoints to a miscalibration of $0.14$ mag that should be $\sim0.01-0.04$ mag. Using the consistent Cepheid and TRGB calibration from B24 while removing the problematic data reduces the systematic error by a factor of two and results in $H_0 = 76.3 \pm 2.1 \textrm{(stat)} \pm 1.5 \textrm{(sys)}$ km/s/Mpc. This measurement is consistent with previous determinations of $H_0$ using the TF relation. We also show that most determinations of $H_0$ measurements that replace Type Ia supernovae measurements with another far-field distance indicator yield $H_0>73$ km/s/Mpc, reinforcing previous findings that the Hubble tension is not tied to any one distance indicator.

astro-ph.CO↗

arXiv2024

An improved Tully-Fisher estimate of $H_0$

We propose an improved comprehensive method for determining the Hubble constant ($H_0$) using the Tully-Fisher relation. By fitting a peculiar velocity model in conjunction with the Tully-Fisher relation, all available data can be used to derive self-consistent Tully-Fisher parameters. In comparison to previous approaches, our method offers several improvements: it can be readily generalised to different forms of the Tully-Fisher relation and its intrinsic scatter; it uses a peculiar velocity model to predict distances more accurately; it can account for all selection effects; it uses the entire dataset to fit the Tully-Fisher relation; and it is fully self-consistent. The Tully-Fisher relation zero-point is calibrated using the subset of galaxies with distances from absolute distance indicators. We demonstrate this method on the Cosmicflows-4 catalogue $i$-band and $W1$-band Tully-Fisher samples and show that the uncertainties from fitting the Tully-Fisher relation amount to only 0.2 km s$^{-1}$Mpc$^{-1}$. Using all available absolute distance calibrators, we obtain $H_0=73.3$ $\pm$ 2.1 (stat) $\pm$ 3.5 (sys) km s$^{-1}$Mpc$^{-1}$, where the statistical uncertainty is dominated by the small number of galaxies with absolute distance estimates. The substantial systematic uncertainty reflects inconsistencies between various zero-point calibrations of the Cepheid period-luminosity relation, the tip of the red giant branch standard candle, and the Type Ia supernova standard candle. However, given a reliable set of absolute distance calibrators, our method promises enhanced precision in $H_0$ measurements from large new Tully-Fisher samples such as the WALLABY survey.

astro-ph.CO↗

arXiv2024

Large-scale motions and growth rate from forward-modelling Tully-Fisher peculiar velocities

Peculiar velocities are an important probe of the mass distribution in the Universe and the growth rate of structure, directly measuring the effects of gravity on the largest scales and providing a test for theories of gravity. Comparing peculiar velocities predicted from the density field mapped by a galaxy redshift survey with peculiar velocities measured using a distance estimator such as the Tully-Fisher relation yields the growth factor for large-scale structure. We present a method for forward-modelling a sample of galaxy magnitudes and velocity widths that simultaneously determines the parameters of the Tully-Fisher relation and the peculiar velocity field. We apply this to the Cosmicflows-4 (CF4) Tully-Fisher dataset, using the peculiar velocities predicted from the 2M++ redshift survey. After validating the method on mock surveys, we measure the product of the growth rate and mass fluctuation amplitude to be $f\!σ_8$ = 0.35$\pm$0.03 at an effective redshift of $z$ = 0.017. This is consistent at 3$σ$ with the Planck CMB prediction, even though the uncertainty does not fully account for all sources of sample variance. We find the residual bulk flow from gravitational influences outside the 2M++ survey volume to be $|V|$ = 227$\pm$11 km/s, $(l,b)$ = (303$^\circ$,$-$1$^\circ$) in Galactic polar coordinates and the CMB frame. Using simulations, we show that applying our methodology to the large new sample of Tully-Fisher peculiar velocities expected from the WALLABY HI survey of the southern sky can improve the constraints on the growth rate by a factor of 2-3.

astro-ph.CO↗

arXiv2019

Observations of Fast Radio Bursts at Frequencies down to 400 Megahertz

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio flashes likely arriving from far outside the Milky Way galaxy. This phenomenon was discovered at radio frequencies near 1.4 GHz and to date has been observed in one case at as high as 8 GHz, but not below 700 MHz in spite of significant searches at low frequencies. Here we report detections of FRBs at radio frequencies as low as 400 MHz, on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) using the CHIME/FRB instrument. We present 13 FRBs detected during a telescope pre-commissioning phase, when our sensitivity and field-of-view were not yet at design specifications. Emission in multiple events is seen down to 400 MHz, the lowest radio frequency to which we are sensitive. The FRBs show a variety of temporal scattering behaviours, with the majority significantly scattered, and some apparently unscattered to within measurement uncertainty even at our lowest frequencies. Of the 13 reported here, one event has the lowest dispersion measure yet reported, implying it is among the closest yet known, and another has shown multiple repeat bursts, as described in a companion paper. Our low-scattering events suggest that efforts to detect FRBs at radio frequencies below 400 MHz will eventually be successful. The overall scattering properties of our sample suggest that FRBs as a class are preferentially located in environments that scatter radio waves more strongly than the diffuse interstellar medium (ISM) in the Milky Way.

astro-ph.HE↗

arXiv2018

Constraints on direction-dependent cosmic birefringence from Planck polarization data

Cosmic birefringence is the process that rotates the plane of polarization by an amount, $α$, as photons propagate through free space. Such an effect arises in parity-violating extensions to the electromagnetic sector, such as the Chern-Simons term common in axion models, quintessence models, or Lorentz-violating extensions to the standard model. Most studies consider the monopole of this rotation, but it is also possible for the effect to have spatial anisotropies. Paying particular attention to large scales, we implement a novel pixel-based method to extract the spherical harmonics for $L \le 30$ and a pseudo-$C_L$ method for $L > 30$. Our results are consistent with no detection and we set 95% upper limits on the amplitude of a scale-invariant power spectrum of $L(L+1)C_L/2π< [2.2\, (\mathrm{stat.})\, \pm 0.7\, (\mathrm{syst.})]\times10^{-5} = [0.07\, (\mathrm{stat.}) \pm 0.02\, (\mathrm{syst.})] \,\mathrm{deg}^2$, on par with previous constraints. This implies specific limits on the dipole and quadrupole amplitudes to be $\sqrt{C_1/4π} < 0.2^\circ$ and $\sqrt{C_2/4π} < 0.1^\circ$, at 95% CL, respectively, improving previous constraints by an order of magnitude. We further constrain a model independent $M=0$ quadrupole in an arbitrary direction to be $α_{20} = 0.02^\circ \pm 0.21^\circ$, with an unconstrained direction. However, we find an excess of dipolar power with an amplitude $\sqrt{3C_1/4π} = 0.32^\circ \pm 0.10^\circ\, (\mathrm{stat.})\, \pm 0.08^\circ\, (\mathrm{syst.})$, in the direction $(l, b) = (295^\circ, 17^\circ) \pm (22^\circ, 17^\circ)\, (\mathrm{stat.})\, \pm (5^\circ, 16^\circ)\, (\mathrm{syst.})$, larger than 1.4% of simulations with no birefringence. We attribute part of this signal to the contamination of residual foregrounds not accounted for in our simulations, although this should be further investigated.

astro-ph.CO↗