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DESI DR2 Constraints on the $R_h=ct$ Universe: Model Viability and Comparison with $Λ$CDM

Amritansh Mehrotra, S. K. J. Pacif, A. F. Santos

TL;DR

The paper confronts the viability of the linear-expansion $R_h=ct$ cosmology against the standard $\Lambda$CDM model using a joint analysis of cosmic chronometers, Pantheon$^+$ supernovae, and DESI DR2 BAO data. It employs a unified Bayesian framework with nested sampling to derive posterior distributions and model evidences, computing $\chi^2$, AIC, BIC, and Bayes factors, and propagates full posteriors to obtain cosmic-age estimates. The results show that $\Lambda$CDM provides a substantially better fit and higher likelihood, correctly capturing the transition from early-time deceleration to late-time acceleration, while $R_h=ct$ yields a constant $q(z)=0$ and underestimates $H(z)$ at higher redshifts, predicting an older Universe. The findings reinforce $\Lambda$CDM as the preferred cosmology given current data, though they contextualize ongoing debates about cosmic evolution, JWST high-redshift observations, and BAO-derived constraints, highlighting the importance of comprehensive model comparison frameworks.

Abstract

We carry out a comparative analysis of the standard $Λ$CDM cosmological model and the alternative $R_h=ct$ framework using recent observational data from cosmic chronometers, Type Ia supernova, and baryon acoustic oscillations. The study evaluates the ability of each model to reproduce the observed expansion history of the Universe through a joint statistical assessment based on the chi-squared statistics, Akaike Information Criterion (AIC), Bayesian Information Criteria (BIC), and Bayes factor. While both models yield acceptable fits, $Λ$CDM consistently attains lower information-criterion values and higher likelihood, indicating a superior overall performance. An examination of the redshift evolution of the Hubble parameter $H(z)$ and the deceleration parameter $q(z)$ shows that $Λ$CDM naturally captures the transition from early-time deceleration to late-time acceleration, where as $R_h=ct$ predicts a strictly linear expansion. We also estimate the age of the Universe within both models, finding that $Λ$CDM prediction agrees with the Planck 2018 result, while the linear expansion in $R_h=ct$ leads to an older cosmic age. Recent JWST observations of unexpectedly mature high-redshift galaxies have reopened the discussion regarding whether the Universe may be older than implied by the standard model; although these results remain under active investigation, they underscore that fully resolving cosmic evolution may require refinements beyond the concordance paradigm.

DESI DR2 Constraints on the $R_h=ct$ Universe: Model Viability and Comparison with $Λ$CDM

TL;DR

The paper confronts the viability of the linear-expansion cosmology against the standard CDM model using a joint analysis of cosmic chronometers, Pantheon supernovae, and DESI DR2 BAO data. It employs a unified Bayesian framework with nested sampling to derive posterior distributions and model evidences, computing , AIC, BIC, and Bayes factors, and propagates full posteriors to obtain cosmic-age estimates. The results show that CDM provides a substantially better fit and higher likelihood, correctly capturing the transition from early-time deceleration to late-time acceleration, while yields a constant and underestimates at higher redshifts, predicting an older Universe. The findings reinforce CDM as the preferred cosmology given current data, though they contextualize ongoing debates about cosmic evolution, JWST high-redshift observations, and BAO-derived constraints, highlighting the importance of comprehensive model comparison frameworks.

Abstract

We carry out a comparative analysis of the standard CDM cosmological model and the alternative framework using recent observational data from cosmic chronometers, Type Ia supernova, and baryon acoustic oscillations. The study evaluates the ability of each model to reproduce the observed expansion history of the Universe through a joint statistical assessment based on the chi-squared statistics, Akaike Information Criterion (AIC), Bayesian Information Criteria (BIC), and Bayes factor. While both models yield acceptable fits, CDM consistently attains lower information-criterion values and higher likelihood, indicating a superior overall performance. An examination of the redshift evolution of the Hubble parameter and the deceleration parameter shows that CDM naturally captures the transition from early-time deceleration to late-time acceleration, where as predicts a strictly linear expansion. We also estimate the age of the Universe within both models, finding that CDM prediction agrees with the Planck 2018 result, while the linear expansion in leads to an older cosmic age. Recent JWST observations of unexpectedly mature high-redshift galaxies have reopened the discussion regarding whether the Universe may be older than implied by the standard model; although these results remain under active investigation, they underscore that fully resolving cosmic evolution may require refinements beyond the concordance paradigm.
Paper Structure (7 sections, 11 equations, 3 figures, 1 table)

This paper contains 7 sections, 11 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. The $R_h=ct$ Cosmology
  3. Datasets and Statistical Methods
  4. Redshift evolution of $H(z)$ and $q(z)$
  5. Cosmic age
  6. Results
  7. Conclusion

Figures (3)

  • Figure 1: The left panel of this figure shows the combined MCMC corner plot for $\Lambda$CDM model and the right panel for $R_h=ct$ model. Diagonal panels show 1D posteriors; off-diagonal panels show 2D correlations with 68% and 95% confidence contours.
  • Figure 2: The top panel presents the redshift evolution of Hubble parameter $H(z)$ with observational data comparing $R_h=ct$ model to that of $\Lambda$CDM model. The bottom panel shows the corresponding deceleration parameter $q(z)$.
  • Figure 3: Age-Redshift comparison drawn between $\Lambda$CDM model and $R_h=ct$ model with 68% confidence level.