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Constraining deviations from $Λ$CDM in the Hubble expansion rate

Yupeng Yang

TL;DR

The paper investigates whether the Hubble expansion rate can deviate from ΛCDM by introducing a phenomenological deviation $δ(z)$ into $H^{2}(z)=H^{2}_{\Lambda{\rm CDM}}(Ω_m,z)[1+δ(z)]$ with three parameterizations. Using DESI BAO, CMB distance priors, cosmic chronometers, and SNIa data, it constrains $δ_c$ and standard cosmological parameters through MCMC and assesses model preference with AIC. It finds that Models I and II are consistent with ΛCDM, whereas Model III shows statistically significant deviations in many dataset combinations, with AIC often favoring Model III over ΛCDM for the full datasets. The results hint at the possibility of dynamical dark energy or an effective interacting DM–DE scenario, while cautioning against over-interpretation and highlighting future work to include large-scale structure effects.

Abstract

The $Λ$CDM cosmolgical model has long been regarded as highly successful in accurately describing a wide range of astronomical observations. However, numerous observational findings have also provided hints of discrepancies from the predictions of the $Λ$CDM framework. We explore a phenomenological model that quantifies the deviation of the Hubble expansion rate from the standard scenario, which is expressed as $H^{2}(z) = H^{2}_{\rm ΛCDM}(Ω_m, z)[1+δ(z)]$. We consider three distinct forms for the deviation parameter $δ(z)$: in model I, $δ(z)=δ_c$; in model II, $δ(z)=δ_{c}z/(1+z)$, and in model III, $δ(z)=δ_{c}{\rm ln}(1+z)$. Here, $δ_c$ represents a constant value. We utilize a comprehensive set of observational data to constrain the models. Our results show that for most combined datasets, $δ_c$ tends to take on negative values for models I and II, while consistently taking positive values in model III. Furthermore, we find that both models I and II remain consistent with the standard $Λ$CDM model across all datasets examined. In contrast, model III exhibits a significant deviation from the $Λ$CDM model, exceeding $2σ$ for the full combined datastes. The AIC indicates that models I and II are consistent with the $Λ$CDM model, whereas model III is preferred over the standard $Λ$CDM model, with the $Λ$CDM model being disfavored for the combined datasets DESI BAO + CMB + CC + DESY5. These results suggest that the Hubble expansion rate likely deviates from the standard $Λ$CDM prediction.

Constraining deviations from $Λ$CDM in the Hubble expansion rate

TL;DR

The paper investigates whether the Hubble expansion rate can deviate from ΛCDM by introducing a phenomenological deviation into with three parameterizations. Using DESI BAO, CMB distance priors, cosmic chronometers, and SNIa data, it constrains and standard cosmological parameters through MCMC and assesses model preference with AIC. It finds that Models I and II are consistent with ΛCDM, whereas Model III shows statistically significant deviations in many dataset combinations, with AIC often favoring Model III over ΛCDM for the full datasets. The results hint at the possibility of dynamical dark energy or an effective interacting DM–DE scenario, while cautioning against over-interpretation and highlighting future work to include large-scale structure effects.

Abstract

The CDM cosmolgical model has long been regarded as highly successful in accurately describing a wide range of astronomical observations. However, numerous observational findings have also provided hints of discrepancies from the predictions of the CDM framework. We explore a phenomenological model that quantifies the deviation of the Hubble expansion rate from the standard scenario, which is expressed as . We consider three distinct forms for the deviation parameter : in model I, ; in model II, , and in model III, . Here, represents a constant value. We utilize a comprehensive set of observational data to constrain the models. Our results show that for most combined datasets, tends to take on negative values for models I and II, while consistently taking positive values in model III. Furthermore, we find that both models I and II remain consistent with the standard CDM model across all datasets examined. In contrast, model III exhibits a significant deviation from the CDM model, exceeding for the full combined datastes. The AIC indicates that models I and II are consistent with the CDM model, whereas model III is preferred over the standard CDM model, with the CDM model being disfavored for the combined datasets DESI BAO + CMB + CC + DESY5. These results suggest that the Hubble expansion rate likely deviates from the standard CDM prediction.

Paper Structure

This paper contains 10 sections, 28 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. The specific models examined throughout this research
  3. The datasets
  4. Baryon acoustic oscillation
  5. Cosmic microwave background
  6. Hubble rate from cosmic chronometers
  7. Type Ia supernova
  8. Constraints on considered models
  9. The potential cosmological model
  10. Conclusions

Figures (1)

  • Figure 1: One-dimensional marginalized posterior distributions and two-dimensional confidence contours (68% and 95% C.L.) for the three models, constrained by DESI BAO + CMB + CC (blue), and DESI BAO + CMB + CC + PantheonPlus (red)/DESY5 (green)/Union3 (orange). The vertical dashed line in the plots indicates the reference value of $\delta_{c} = 0$, which corresponds to the standard $\Lambda$CDM model.