[Paper Review] Is the present cosmic expansion decelerating?

Puxun Wu, Hongwei Yu|arXiv (Cornell University)|Dec 14, 2010

Cosmology and Gravitation Theories3 citations

TL;DR

This study directly reconstructs the deceleration parameter $q(z)$ using low-redshift SNIa and BAO data without assuming a dark energy model, finding that current observations favor a slowing cosmic acceleration, with $q(0) > 0$ in $z < 0.1$ and $z < 0.2$ regions, suggesting the accelerating expansion may be transient. The results are robust across multiple data sets and reconstruction methods, though a currently accelerating universe remains possible at 1σ confidence level.

ABSTRACT

We probe the recent cosmic expansion by directly reconstructing the deceleration parameter $q(z)$ at recent times with a linear expansion at $z=0$ using the low redshift SNIa and BAO data. Our results show that the observations seem to favor a slowing down of the present cosmic acceleration. Using only very low redshift SNIa data, for example, those within $z&lt;0.1$ or $0.2$, we find that our Universe may have already entered a decelerating expansion era since a positive $q(0)$ seems to be favored. This result is further supported by a different approach which aims to reconstruct $q(z)$ in the whole redshift region. So, the accelerating cosmic expansion may be just a transient phenomenon.

Motivation & Objective

• To determine whether the current cosmic acceleration is slowing down or speeding up using observational data without assuming a specific dark energy model.
• To test the robustness of the cosmic acceleration trend using low-redshift SNIa and BAO data, minimizing model dependence.
• To assess the impact of different light curve fitters (SALT2, MLCS2k2) on the reconstruction of $q(z)$ and the resulting conclusions.
• To explore the full redshift evolution of $q(z)$ using segmented constant $q$ models, incorporating CMB shift parameter data for consistency.
• To resolve discrepancies in prior studies by comparing results from different data combinations and parametrizations.

Proposed method

• Reconstruct the deceleration parameter $q(z)$ using a linear expansion $q(z) = q_0 + q_1 z$ at low redshifts ($z \leq 0.5$), focusing on $z \leq 0.1$, $0.2$, $0.35$, and $0.5$.
• Use the Union2 SNIa compilation (557 data points) and BAO distance ratio measurements at $z=0.2$ and $z=0.35$ from 2dF and SDSS surveys.
• Apply a combined likelihood analysis of SNIa and BAO data to constrain $q_0$ and $q_1$ at the 1σ confidence level.
• Test the sensitivity of results to light curve fitting methods by analyzing Constitution and SDSS-II SNIa data with both SALT2 and MLCS2k2 fitters.
• Reconstruct $q(z)$ in five redshift bins ($0-0.05$, $0.05-0.2$, $0.2-0.5$, $0.5-1.0$, $1.0-$) assuming constant $q$ in each bin, and include CMB shift parameter from WMAP7.
• Compare results from SNIa-only, SNIa+BAO, and SNIa+BAO+CMB to assess consistency and resolve tensions with high-redshift data.

Experimental results

Research questions

• RQ1 Is the present cosmic expansion accelerating or decelerating, based on low-redshift SNIa and BAO data?
• RQ2 Does the deceleration parameter $q(0)$ favor a positive value, indicating current deceleration?
• RQ3 How robust are the results to different light curve fitters (SALT2 vs. MLCS2k2) used in SNIa data analysis?
• RQ4 Can a transient accelerating universe be supported by a full-redshift reconstruction of $q(z)$ without assuming a dark energy model?
• RQ5 Do tensions between low- and high-redshift data (e.g., SNIa+BAO vs. CMB) persist when using a non-parametric reconstruction approach?

Key findings

• For $z \leq 0.1$, the Union2 SNIa data favor a positive $q(0)$, indicating that the Universe may have already entered a decelerating expansion phase.
• The addition of BAO data strengthens the trend, making the decelerating expansion more evident, especially in the $z \leq 0.1$ and $z \leq 0.2$ regions.
• The best-fit $q_0$ values from Constitution and SDSS-II SNIa data with SALT2 fitter are $1.107$ and $0.389$, respectively, both indicating $q(0) > 0$ and favoring deceleration.
• The SDSS-II data with MLCS2k2 fitter yield $q_0 = -1.23$, but this is an outlier; all other combinations favor $q_1 < 0$, indicating slowing acceleration.
• In the segmented $q(z)$ model, SNIa+BAO+CMB data show that $q(z)$ is positive in both low and high redshift regions, supporting a transient accelerating phase.
• The $1\sigma$ confidence interval still allows for a currently accelerating universe, but the best-fit results do not favor it, indicating a possible transition to deceleration.