arXiv subjects

Ryan E. Keeley

Publications and source records attributed to Ryan E. Keeley.

At least 19 recordsLinked to original sources

arXiv2026

$w_0$-probe: A new diagnostic of dark energy based on $Om$

Recent DESI data suggest that dark energy may be evolving and motivate the use of model-independent diagnostics such as $Om(z)$ and probes of the equation of state (EoS) of dark energy, $w(z)$. Traditional reconstructions of $w(z)$ rely on differentiating the expansion history, $h(z)=H(z)/H_0$, which amplifies noise and systematic uncertainties. In this work, we introduce a new diagnostic, the $w_0$-probe, which is constructed from $Om(z)$, and which enables a direct determination of the current EoS from $h(z)$ without any additional differentiation. While retaining the null-test capability of $Om(z)$ for $Λ$CDM, the $w_0$-probe also provides a direct estimate of $w_0$ -- the current EoS of dark energy. We demonstrate that this reconstruction of $w_0$ is robust for any smooth underlying $w(z)$. We apply this method to Gaussian-process (GP) reconstructions of $h(z)$ using current SNe Ia+BAO+CMB data. Both $Om(z)$ and the $w_0$-probe exclude $Λ$CDM at the $95\%$ confidence level (C.L.), with the latter favouring $w_0\simeq-0.62 \pm 0.03$ at $95\%$ C.L. To mitigate potential over-constraining from GP priors, we additionally analyze $χ^2$-limited reconstructions with likelihoods exceeding the $95\%$ CPL threshold. The $w_0$-probe obtained from these high-likelihood samples again predominantly excludes $Λ$CDM and yields $w_0\in(-0.8,-0.5)$ at $z\to 0$, demonstrating the robustness of our results. The $w_0$-probe therefore provides a simple, model-independent, and robust diagnostic of the current EoS of dark energy.

astro-ph.CO↗

arXiv2026

Reconstruction of the Quintessence Scalar Field Potential Using Gaussian Processes

Recent cosmological observations, including the latest Dark Energy Spectroscopic Instrument (DESI) data releases DR1 and DR2, have renewed interest in the possibility that dark energy may exhibit dynamical behavior rather than being a strict cosmological constant. In this work, we perform a fully model-independent reconstruction of the quintessence scalar field potential using Gaussian Process regression and current Hubble measurements. Instead of assuming a specific functional form for the scalar field potential, we reconstruct the quintessence potential and the corresponding kinetic energy directly from observational data. Our analysis is based on Hubble parameter measurements obtained from cosmic chronometers and the latest high-precision DESI DR2 baryon acoustic oscillation (BAO) data, together with Type Ia supernova data from the Pantheon+ compilation. Gaussian Processes provide a nonparametric and model-independent framework that allows the data to guide the reconstruction. We employ two covariance functions, namely the squared exponential and the Matern ($ν= 9/2$) kernels, in order to assess the sensitivity of the reconstruction to the kernel choice. We further explore the impact of background cosmological assumptions by considering different priors on the matter density and spatial curvature. Finally, we compare the reconstructed scalar field potential with two theoretically motivated benchmark models: a power law potential and an exponential potential. We find that both models remain consistent with the reconstructed potential within the inferred confidence intervals.

astro-ph.CO↗

arXiv2025

The Preference for Evolving Dark Energy from Cosmological Distance Measurements and Possible Signatures in the Growth Rate of Perturbations

In this study, we use a flexible parametrization of the equation of state of dark energy to explore its possible evolution with datasets from the Dark Energy Spectroscopic Instrument (DESI), Planck cosmic microwave background, and either the 5-year Dark Energy Survey (DES) or the Pantheon+ (PP) supernova (SN) compilation. This parametrization, called transitional dark energy, allows for rapid changes in the equation of state but also changes like that in the Chevallier-Polarski-Linder parametrization. We find a 3.8σ preference for evolving dark energy over ΛCDM with the DES dataset and a weaker 2.4σ preference when using the PP dataset. This corroborates the finding of the DESI Collaboration, who found that their baryon acoustic oscillation data preferred evolving dark energy when fit with the CPL parametrization of the equation of state. Our analysis reveals no significant outliers in the DESI data around the TDE best-fit, while the data is asymmetrically distributed around the ΛCDM best-fit model such that the measured distances are on average smaller. The DESI and SN data both prefer an expansion history that implies a higher dark energy density around z=0.5 than in the Planck-ΛCDM model, with the inferred equation of state being greater than -1 around z=0 and close to or below -1 at z>0.5. We show that when the expansion rate is greater than that in the Planck-ΛCDM model (around z=0.5), the growth rate calculated assuming General Relativity is suppressed relative to the Planck-ΛCDM model, and it rebounds as the expansion rate differences between the models become smaller closer to the present time. The resulting flattening of the $fσ_8(z)$ curve compared to the ΛCDM model could be an independent signature of the temporal evolution of dark energy.

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↗

arXiv2025

Litmus tests of the flat $Λ$CDM model and model-independent measurement of $H_0r_\mathrm{d}$ with LSST and DESI

In this analysis we apply a model-independent framework to test the flat $Λ$CDM cosmology using simulated SNIa data from the upcoming Legacy Survey of Space and Time (LSST) and combined with simulated Dark Energy Spectroscopic Instrument (DESI) five-years Baryon Acoustic Oscillations (BAO) data. We adopt an iterative smoothing technique to reconstruct the expansion history from SNIa data, which, when combined with BAO measurements, facilitates a comprehensive test of the Universe's curvature and the nature of dark energy. The analysis is conducted under four different mock true cosmologies: Two curvatures ($Ω_{k,0}=0$ and 0.1) and two models of dark energy: a cosmological constant $Λ$ and the phenomenologically emergent dark energy. We forecast that our reconstruction technique can constrain cosmological parameters, such as the curvature ($Ω_{k,0}$) and $c/(H_0 r_\mathrm{d})$, with spread due to the SNIa uncertainties up to $\pm 4\%$ and $\pm 0.1$ respectively, without assuming any form of dark energy.

astro-ph.CO↗

arXiv2025

Redshift evolution of the X-ray and UV luminosity relation of quasars: calibrated results from SNe Ia

Quasars could serve as standard candles if the relation between their ultraviolet and X-ray luminosities can be accurately calibrated. Previously, we developed a model-independent method to calibrate quasar standard candles using the distances-redshift relation reconstructed from Type Ia supernova at z<2 using Gaussian process regression. Interestingly, we found that the calibrated quasar standard candle dataset preferred a deviation from $Λ$CDM at redshifts above z>2. One interpretation of these findings is that the calibration parameters of the quasar UV-X-ray luminosity relationship evolves with redshift. In order to test the redshift dependence of the quasar calibration in a model-independent manner, we divided the quasar sample whose redshift overlap with the redshift coverage of Pantheon+ Type Ia supernova compilation into two sub-samples: a low-redshift quasar sub-sample and a high-redshift quasar sub-sample. Our present results show that there is about a 4$σ$ inconsistency between the quasar parameters inferred from the high-redshift quasar sub-sample and from the low-redshift sub-sample if no evolution of the quasar relation is considered. This inconsistency suggests the necessity of considering redshift evolution for the relationship between the quasars$'$ ultraviolet and X-ray luminosities. We then test an explicit parametrization of the redshift evolution of the quasar calibration parameters via $γ(z) = γ_0+γ_1(1+z)$ and $β(z)=β_0+β_1(1+z)$. Combining this redshift-dependent calibration relationship with the distance-redshift relationship reconstructed from Pantheon+ supernova compilation, we find the high-redshift sub-sample and low-redshift sub-sample become consistent at the 1$σ$ level, which means that the parameterized form of $γ(z)$ and $β(z)$ works well at describing the evolution of the quasar calibration parameters.

astro-ph.CO↗

arXiv2025

Detection of Unresolved Strongly Lensed Supernovae with 7-Dimensional Telescope

Gravitationally lensed supernovae (glSNe) are a powerful tool for exploring the realms of astronomy and cosmology. Time-delay measurements and lens modeling of glSNe can provide a robust and independent method for constraining the expansion rate of the universe. The study of unresolved glSNe light curves presents a unique opportunity for utilizing small telescopes to investigate these systems. In this work, we investigate diverse observational strategies for the initial detection of glSNe using the 7-Dimensional Telescope (7DT), a multitelescope system composed of twenty 50-cm telescopes. We implement different observing strategies on a subset of 5807 strong lensing systems and candidates identified within the Dark Energy Camera Legacy Survey (DECaLS), as reported in various publications. Our simulations under ideal observing conditions indicate the maximum expected annual detection rates for various glSNe types (Type Ia and core-collapse (CC)) using the 7DT target observing mode in the $r$-band at a depth of 22.04 mag, as follows: 7.46 events for type Ia, 2.49 for type Ic, 0.8 for type IIb, 0.52 for type IIL, 0.78 for type IIn, 3.75 for type IIP, and 1.15 for type Ib. Furthermore, in the case of medium-band filter observations (m6000) at a depth of 20.61 in the Wide-field Time-domain Survey (WTS)program, the predicted detection rate for glSNe Ia is 2.53 $yr^{-1}$. Given targeted follow-up observations of these initially detected systems with more powerful telescopes, we can apply a model-independent approach to forecast the ability to measure $H_{0}$ using a Gaussian process from Type Ia Supernovae (SNe Ia) data and time-delay distance information derived from glSNe systems, which include both Ia and CC types. We forecast that the expected detection rate of glSNe systems can achieve a $2.7\%$ precision in estimating the $H_{0}$.

astro-ph.HE↗

arXiv2024

JWST Lensed quasar dark matter survey II: Strongest gravitational lensing limit on the dark matter free streaming length to date

This is the second in a series of papers in which we use JWST MIRI multiband imaging to measure the warm dust emission in a sample of 31 multiply imaged quasars, to be used as a probe of the particle nature of dark matter. We present measurements of the relative magnifications of the strongly lensed warm dust emission in a sample of 9 systems. The warm dust region is compact and sensitive to perturbations by populations of halos down to masses $\sim 10^6$ M$_{\odot}$. Using these warm dust flux-ratio measurements in combination with 5 previous narrow-line flux-ratio measurements, we constrain the halo mass function. In our model, we allow for complex deflector macromodels with flexible third and fourth-order multipole deviations from ellipticity, and we introduce an improved model of the tidal evolution of subhalos. We constrain a WDM model and find an upper limit on the half-mode mass of $10^{7.6} M_\odot$ at posterior odds of 10:1. This corresponds to a lower limit on a thermally produced dark matter particle mass of 6.1 keV. This is the strongest gravitational lensing constraint to date, and comparable to those from independent probes such as the Ly$α$ forest and Milky Way satellite galaxies.

astro-ph.CO↗

arXiv2024

A Model-independent Method to Determine $H_0$ Using Time-Delay Lensing, Quasars and Type Ia Supernovae

Absolute distances from strong lensing can anchor Type Ia Supernovae (SNe Ia) at cosmological distances giving a model-independent inference of the Hubble constant ($H_0$). Future observations could provide strong lensing time-delay distances with source redshifts up to $z\,\simeq\,4$, which are much higher than the maximum redshift of SNe Ia observed so far. In order to make full use of time-delay distances measured at higher redshifts, we use quasars as a complementary cosmic probe to measure cosmological distances at redshifts beyond those of SNe Ia and provide a model-independent method to determine $H_0$. In this work, we demonstrate a model-independent, joint constraint of SNe Ia, quasars, and time-delay distances from strong lensed quasars. We first generate mock data sets of SNe Ia, quasar, and time-delay distances based on a fiducial cosmological model. Then, we calibrate the quasar parameters model independently using Gaussian process (GP) regression with mock SNe Ia data. Finally, we determine the value of $H_0$ model-independently using GP regression from mock quasars and time-delay distances from strong lensing systems. As a comparison, we also show the $H_0$ results obtained from mock SNe Ia in combination with time-delay lensing systems whose redshifts overlap with SNe Ia. Our results show that quasars at higher redshifts show great potential to extend the redshift coverage of SNe Ia and thus enable the full use of strong lens time-delay distance measurements from ongoing cosmic surveys and improve the accuracy of the estimation of $H_0$ from $2.1\%$ to $1.3\%$ when the uncertainties of the time-delay distances are $5\%$ of the distance values.

astro-ph.CO↗

arXiv2023

Pushing the Limits of Detectability: Mixed Dark Matter from Strong Gravitational Lenses

One of the frontiers for advancing what is known about dark matter lies in using strong gravitational lenses to characterize the population of the smallest dark matter halos. There is a large volume of information in strong gravitational lens images -- the question we seek to answer is to what extent we can refine this information. To this end, we forecast the detectability of a mixed warm and cold dark matter scenario using the anomalous flux ratio method from strong gravitational lensed images. The halo mass function of the mixed dark matter scenario is suppressed relative to cold dark matter but still predicts numerous low-mass dark matter halos relative to warm dark matter. Since the strong lens signal is a convolution over a range of dark matter halo masses and since the signal is sensitive to the specific configuration of dark matter halos, not just the halo mass function, degeneracies between different forms of suppression in the halo mass function, relative to cold dark matter, can arise. We find that, with a set of lenses with different configurations of the main deflector and hence different sensitivities to different mass ranges of the halo mass function, the different forms of suppression of the halo mass function between the warm dark matter model and the mixed dark matter model can be distinguished with $40$ lenses with Bayesian odds of 29.4:1.

astro-ph.CO↗

arXiv2023

Ruling Out New Physics at Low Redshift as a solution to the $H_0$ Tension

We make the case that there can be no low-redshift solution to the $H_0$ tension. To robustly answer this question, we use a very flexible parameterization for the dark energy equation of state such that every cosmological distance still allowed by data exists within this prior volume. To then answer whether there exists a satisfactory solution to the $H_0$ tension within this comprehensive parameterization, we constrained the parametric form using different partitions of the Planck cosmic microwave background, SDSS-IV/eBOSS DR16 baryon acoustic oscillation, and Pantheon supernova datasets. When constrained by just the cosmic microwave background dataset, there exists a set of equations of state which yields high $H_0$ values, but these equations of state are ruled out by the combination of the supernova and baryon acoustic oscillation datasets. In other words, the constraint from the cosmic microwave background, baryon acoustic oscillation, and supernova datasets together does not allow for high $H_0$ values and converges around an equation of state consistent with a cosmological constant. Thus, since this very flexible parameterization does not offer a solution to the $H_0$ tension, there can be no solution to the $H_0$ tension that adds physics at only low redshifts. This is directly related to the expansion history of the Universe and its geometrical properties and would include models beyond those parametrized by $w(z)$.

astro-ph.CO↗

arXiv2022

Early or phantom dark energy, self-interacting, extra, or massive neutrinos, primordial magnetic fields, or a curved universe: An exploration of possible solutions to the $H_0$ and $σ_8$ problems

In this paper we explore the existing tensions in the local cosmological expansion rate, $H_0$, and amplitude of the clustering of large-scale structure at $8\, h^{-1}\mathrm{Mpc}$, $σ_8$, as well as models that claim to alleviate these tensions. We consider seven models: evolving dark energy ($w$CDM), extra radiation ($N_\mathrm{eff}$), massive neutrinos, curvature, primordial magnetic fields (PMF), self-interacting neutrino models, and early dark energy (EDE). We test these models against three data sets that span the full range of measurable cosmological epochs, have significant precision, and are well-tested against systematic effects: the Planck 2018 cosmic microwave background data, the Sloan Digital Sky Survey baryon acoustic oscillation scale measurements, and the Pantheon catalog of Type Ia supernovae. We use the recent SH0ES $H_0$ measurement and several measures of $σ_8$ (and its related parameter $S_8=σ_8\sqrt{Ω_\mathrm{m}/0.3}$). We find that four models are above the "strong" threshold in Bayesian model selection, $w$CDM, $N_\mathrm{eff}$, PMF, and EDE. However, only EDE also relieves the $H_0$ tension in the full data sets to below 2$σ$. Contrarily, no model alleviates the $S_8/σ_8$ tension in the full data set, nor does better than $Λ$CDM in the combined case of both $H_0$ and $S_8/σ_8$ tensions.

astro-ph.CO↗

arXiv2022

On The Distribution of Bayesian Evidences

We look at the distribution of the Bayesian evidence for mock realizations of supernova and baryon acoustic oscillation data. The ratios of Bayesian evidences of different models are often used to perform model selection. The significance of these Bayes factors are then interpreted using scales such as the Jeffreys or Kass \& Raftery scale. First, we demonstrate how to use the evidence itself to validate the model, that is to say how well a model fits the data, regardless of how well other models perform. The basic idea is that if, for some real dataset a model's evidence lies outside the distribution of evidences that result when the same fiducial model that generates the datasets is used for the analysis, then the model in question is robustly ruled out. Further, we show how to assess the significance of a hypothetically computed Bayes factor. We show that the range of the distribution of Bayes factors can greatly depend on the models in question and also the number of data points in the dataset. Thus, we have demonstrated that the significance of Bayes factors needs to be calculated for each unique dataset.

astro-ph.CO↗

arXiv2022

How to use GP: Effects of the mean function and hyperparameter selection on Gaussian Process regression

Gaussian processes have been widely used in cosmology to reconstruct cosmological quantities in a model-independent way. However, the validity of the adopted mean function and hyperparameters, and the dependence of the results on the choice have not been well explored. In this paper, we study the effects of the underlying mean function and the hyperparameter selection on the reconstruction of the distance moduli from type Ia supernovae. We show that the choice of an arbitrary mean function affects the reconstruction: a zero mean function leads to unphysical distance moduli and the best-fit LCDM to biased reconstructions. We propose to marginalize over a family of mean functions and over the hyperparameters to effectively remove their impact on the reconstructions. We further explore the validity and consistency of the results considering different kernel functions and show that our method is unbiased.

astro-ph.CO↗

arXiv2022

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

In this paper we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant $H_0$, the $σ_8$--$S_8$ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the $5.0\,σ$ tension between the {\it Planck} CMB estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the {\it Planck} CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the $H_0$--$S_8$ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals.[Abridged]

astro-ph.CO↗

arXiv2021

Testing the Mutual Consistency of the Pantheon and SDSS/eBOSS BAO Data Sets with Gaussian Processes

We test the mutual consistency between the baryon acoustic oscillation measurements from the eBOSS SDSS final release, as well as the Pantheon supernova compilation in a model independent fashion using Gaussian process regression. We also test their joint consistency with the $Λ$CDM model, also in a model independent fashion. We also use Gaussian process regression to reconstruct the expansion history that is preferred by these two datasets. While this methodology finds no significant preference for model flexibility beyond $Λ$CDM, we are able to generate a number of reconstructed expansion histories that fit the data better than the best-fit $Λ$CDM model. These example expansion histories may point the way towards modifications to $Λ$CDM. We also constrain the parameters $Ω_k$ and $H_0r_d$ both with $Λ$CDM and with Gaussian process regression. We find that $H_0r_d =10030 \pm 130$ km/s and $Ω_k = 0.05 \pm 0.10$ for $Λ$CDM and that $H_0r_d = 10040 \pm 140$ km/s and $Ω_k = 0.02 \pm 0.20$ for the Gaussian process case.

astro-ph.CO↗

arXiv2021

Bayesian vs Frequentist: Comparing Bayesian model selection with a frequentist approach using the iterative smoothing method

We have developed a frequentist approach for model selection which determines the consistency between any cosmological model and the data using the distribution of likelihoods from the iterative smoothing method. Using this approach, we have shown how confidently we can conclude whether the data support any given model without comparison to a different one. In this current work, we compare our approach with the conventional Bayesian approach based on the estimation of the Bayesian evidence using nested sampling. We use simulated future Roman (formerly WFIRST)-like type Ia supernovae data in our analysis. We discuss the limits of the Bayesian approach for model selection and show how our proposed frequentist approach can perform better in the falsification of individual models. Namely, if the true model is among the candidates being tested in the Bayesian approach, that approach can select the correct model. If all of the options are false, then the Bayesian approach will select merely the least incorrect one. Our approach is designed for such a case and we can conclude that all of the models are false.

astro-ph.CO↗

arXiv2021

Hubble diagram at higher redshifts: Model independent calibration of quasars

In this paper, we present a model-independent approach to calibrate the largest quasar sample. Calibrating quasar samples is essentially constraining the parameters of the linear relation between the $\log$ of the ultraviolet (UV) and X-ray luminosities. This calibration allows quasars to be used as standardized candles. There is a strong correlation between the parameters characterizing the quasar luminosity relation and the cosmological distances inferred from using quasars as standardized candles. We break this degeneracy by using Gaussian process regression to model-independently reconstruct the expansion history of the Universe from the latest type Ia supernova observations. Using the calibrated quasar dataset, we further reconstruct the expansion history up to redshift of $z\sim 7.5$. Finally, we test the consistency between the calibrated quasar sample and the standard $\rmΛCDM$ model based on the posterior probability distribution of the GP hyperparameters. Our results show that the quasar sample is in good agreement with the standard $\rmΛCDM$ model in the redshift range of the supernova, despite of mildly significant deviations taking place at higher redshifts. Fitting the standard $\rmΛCDM$ model to the calibrated quasar sample, we obtain a high value of the matter density parameter $Ω_m = 0.382^{+0.045}_{-0.042}$, which is marginally consistent with the constraints from other cosmological observations.

astro-ph.CO↗