arXiv subjects

William L. Matthewson

Publications and source records attributed to William L. Matthewson.

5 recordsLinked to original sources

arXiv2025

Braneworld Dark Energy in light of DESI DR2

Recent observational results from the DESI collaboration reveal tensions with the standard $Λ$CDM model and favour a scenario in which dark energy (DE) decays over time. The DESI DR2 data also suggest that the DE equation of state (EoS) may have been phantom-like ($w < - 1$) in the past, evolving to $w > - 1$ at present, implying a recent crossing of the phantom divide at $w = - 1$. Scalar field models of DE naturally emerge in ultraviolet-complete theories such as string theory, which is typically formulated in higher dimensions. In this work, we investigate a broad class of $thawing~scalar~field~models$, including the simple quadratic, quartic, exponential, symmetry-breaking and axion potentials, propagating on a (4+1)-dimensional ghost-free phantom braneworld, and demonstrate that their effective EoS exhibits a phantom-divide crossing. Alongside the Hubble parameter and EoS of DE, we also analyse the evolution of the $Om$ diagnostic, and demonstrate that the time dependence of these quantities is in excellent agreement with the DESI DR2 observations. Furthermore, we perform a comprehensive parameter estimation using Markov Chain Monte Carlo sampling, and find that the $χ^2$ values for all our models are remarkably close to that of the widely used CPL parametrisation, indicating that our models fit the data very well.

astro-ph.CO↗

arXiv2025

Star-Crossed Labours: Checking Consistency Between Current Supernovae Compilations

We make use of model-independent statistical methods to assess the consistency of three different supernova compilations: Union3, Pantheon+ and DES 5-year supernovae. We expand the available model space of each, using Crossing Statistics, and test the compatibility of each dataset, against the other two. This is done using (I) a Flat $Λ$CDM fitting to, and (II) Iterative Smoothing from, one particular dataset, and determining the level of deformation by required to fit the other two. This allows us to test the mutual consistency of the datasets both within the standard model and in the case of some extended model, motivated by features present in a particular dataset. We find that, in both these cases, the data are only consistent with the point in the parameter space corresponding to zero deformation, at around a $2σ$ level, with the DES compilation showing the largest disagreement. However, all three datasets are still found to be consistent to within $1-2σ$ for some subset of the extended model space implied by the deformations.

astro-ph.CO↗

arXiv2025

Could We Be Fooled about Phantom Crossing?

Recent data from DESI Year 2 BAO, Planck CMB, and various supernova compilations suggest a preference for evolving dark energy, with hints that the equation of state may cross the phantom divide line ($w = -1$). While this behavior is seen in both parametric and non-parametric reconstructions, comparing reconstructions that support such behavior (such as the best fit of CPL) with those that maintain $w>-1$ (like the best fit algebraic quintessence) is not straightforward, as they differ in flexibility and structure, and are not necessarily nested within one another. Thus, the question remains as to whether the crossing behavior that we observe, suggested by the data, truly represents a dark energy model that crosses the phantom divide line, or if it could instead be a result of data fluctuations and the way the data are distributed. We investigate the likelihood of this possibility. For this analysis we perform 1,000 Monte Carlo simulations based on a fiducial algebraic quintessence model. We find that in $3.2 \% $ of cases, CPL with phantom crossing not only fits better, but exceeds the real-data $χ^2$ improvement. This Monte Carlo approach quantifies to what extent statistical fluctuations and the specific distribution of the data could fool us into thinking the phantom divide line is crossed, when it is not. Although evolving dark energy remains a robust signal, and crossing $w=-1$ a viable phenomenological solution that seems to be preferred by the data, its precise behavior requires deeper investigation with more precise data.

astro-ph.CO↗

arXiv2022

Small scale effects in the observable power spectrum at large angular scales

In this paper we show how effects from small scales can enter the angular-redshift power spectrum $C_\ell(z,z')$. In particular, we show that spectroscopic surveys with high redshift resolution are already affected on large angular scales, i.e. at low multipoles, by features from small scales. When considering the angular power spectrum with spectroscopic redshift resolution, it is therefore important to account for non-linearities relevant on small scales, even at low multipoles. This may also motivate the use of the correlation function in relatively wide redshift bins, which is not affected by non-linearities on large scales, instead of the angular power spectrum. The extent to which small-scale effects become visible on large scales, which is more relevant for bin auto-correlations than for cross-correlations, is quantified in detail.

astro-ph.CO↗

arXiv2021

The Flat Sky Approximation to Galaxy Number Counts

We derive and test an approximation for the angular power spectrum of galaxy number counts in the flat sky limit. The standard density and redshift space distortion (RSD) terms in the resulting approximation are distinct to the Limber approximation, providing an accurate result for multipoles as low as $\ell\simeq10$, where the corresponding Limber approximation is completely inaccurate. At equal redshift the accuracy of the density and RSD (standard) terms is around 0.2% for $z<3$ and 0.5% at $z=5$, even to $\ell<50$. At unequal redshifts, if we consider the total power spectrum, the precision is better than 5% only for very small redshift differences, $δ<δ_0 (\simeq 3.6\times10^{-4}(1+z)^{2.14})$ where the standard terms are well-approximated, or for large enough redshift differences $δ>δ_1 (\simeq 0.33(r(z)H(z))/(z+1))$ where the lensing terms dominate. The flat sky expressions for the pure lensing and the lensing-density cross-correlation terms are equivalent to the Limber approximation. For arbitrary redshift differences, the Limber approximation achieves an accuracy of 0.5% (above $\ell\simeq 40$ for pure lensing and $\ell\simeq 80$ for density-lensing). Besides being very accurate, the flat sky approximation is computationally much simpler and can therefore be very useful for data analysis and forecasts with MCMC methods. This will be particularly crucial for upcoming galaxy surveys that will measure the power spectrum of galaxy number counts.

astro-ph.CO↗