arXiv subjects

A. N. Ormondroyd

Publications and source records attributed to A. N. Ormondroyd.

4 recordsLinked to original sources

arXiv2026

Dynamic or Systematic? Bayesian model selection between dark energy and supernova biases

The Dark Energy Survey 5-year (DES-5Y) supernovae, combined with Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillations, appear to favour Chevallier-Polarski-Linder $(w_0, w_a)$ dynamical dark energy over $Λ$CDM. It has been suggested in other work that this is driven by a systematic in the DES pipeline, which particularly affects the low-redshift supernovae brought in from legacy surveys. It is difficult to investigate these data in isolation, however, as the complicated supernovae pipelines must properly account for selection effects. In this work, we discover that a magnitude offset between the low- and high-redshift supernovae is favoured by the Bayesian evidence over the flexknot dark energy found in our previous work. In addition, we find that the possible tension between DES-5Y and DESI is significantly reduced by such an offset. The recent DES-Dovekie recalibration partially alleviates this tension, but does not eliminate it. We also take the opportunity to trial Nested Bridge Sampling with Sequential Monte Carlo as an alternative method for calculating Bayes factors.

astro-ph.CO↗

arXiv2026

Tilting the scales: weighing prior dependency and global tensions of CMB lensing

We provide a nested sampling analysis of the combination of CMB lensing experiments with other cosmological measurements. Nested samples can be used to compute global consistency statistics between datasets. This is demonstrated for CMB lensing and Baryon Acoustic Oscillations which are uncorrelated, and the correlated case between ACT DR6 and NPIPE lensing. We investigate the effect of the prior widths of the spectral tilt $n_\mathrm s$ used in CMB lensing analyses, which quantitatively, but not qualitatively, affect headline constraints. In the absence of informative priors, SPT-3G performs more similarly to ACT and NPIPE. Bayes factors and the suspiciousness statistic are used to quantify the possibility of tension, and we find the Gaussian assumptions inherent in calculating the suspiciousness tension probability to be unsuitable in the case of strong agreement between CMB lensing experiments.

astro-ph.CO↗

arXiv2025

Comparison of dynamical dark energy with ΛCDM in light of DESI DR2

We present an updated reconstruction of the dark energy equation of state, $w(a)$, using the newly released DESI DR2 Baryon Acoustic Oscillation (BAO) data in combination with Pantheon+ and DES5Y Type Ia supernovae measurements, respectively. Building on our previous analysis in arXiv:2503.08658, which employed a nonparametric flexknot reconstruction approach, we examine whether the evidence for dynamical dark energy persists with the improved precision of the DESI DR2 dataset. We find that while the overall qualitative structure of $w(a)$ remains consistent with our earlier findings, the statistical support for dynamical dark energy is reduced when considering DESI DR2 data alone, particularly for more complex flexknot models with higher numbers of knots. However, the evidence for simpler dynamical models, such as $w$CDM and CPL (which correspond to $n=1$ and $n=2$ knots respectively), increases relative to $Λ$CDM with DESI DR2 alone, with CPL being the preferred dynamical model, consistent with previous DESI analyses. When combined with Pantheon+ data, the conclusions remain broadly consistent with our earlier work, but when instead combined with DES5Y supernovae data, there is an increased preference for flexknot models for all values of $n$ considered. This results in all such models being preferred over $Λ$CDM, with the CPL model being the most favoured by a Bayes factor of $\sim 2.3$ relative to $Λ$CDM.

astro-ph.CO↗

arXiv2025

Nonparametric reconstructions of dynamical dark energy via flexknots

Recent cosmological surveys have provided unprecedented datasets that can be used to reconstruct the history of the dark energy equation of state. In this work, a free-form "flexknot'' parameterisation is employed to represent $w(a)$ as a linear spline between free-moving nodes, the number of which may vary. By combining DESI Baryon Acoustic Oscillation measurements with Pantheon+ or DES5Y supernovae, the functional posteriors of $w(a)$ reveal an unexpected W-shaped structure. While the Bayesian evidence may still favour $Λ$CDM, the robustness of these results suggests the structure is indeed present in the data. The tension $R$-statistic and suspiciousness have been marginalised over models, and demonstrate that while the reconstructions from DESI and Pantheon+ agree, DESI and DES5Y do not. We conclude that, while there is no smoking gun for dynamical dark energy, the structure unearthed in this work is generally too complex to be captured by the restrictive $w$CDM or CPL parameterisations.

astro-ph.CO↗