Is cosmic acceleration slowing down? (0903.5141v4)

Abstract: We investigate the course of cosmic expansion in its recent past' using the Constitution SN Ia sample (which includes CfA data at low redshifts), jointly with signatures of baryon acoustic oscillations (BAO) in the galaxy distribution and fluctuations in the cosmic microwave background (CMB). Earlier SN Ia data sets could not address this issue because of a paucity of data at low redshifts. Allowing the equation of state of dark energy (DE) to vary, we find that a coasting model of the universe (q_0=0) fits the data about as well as LCDM. This effect, which is most clearly seen using the recently introducedOm' diagnostic, corresponds to an increase of Om(z) and q(z) at redshifts z \lleq 0.3. In geometrical terms, this suggests that cosmic acceleration may have already peaked and that we are currently witnessing its slowing down. The case for evolving DE strengthens if a subsample of the Constitution set consisting of SNLS+ESSENCE+CfA SN Ia data is analysed in combination with BAO+CMB using the same statistical methods. The effect we observe could correspond to DE decaying into dark matter (or something else). A toy model which mimics this process agrees well with the combined SN Ia+BAO+CMB data.

• The paper introduces a novel approach by incorporating low-redshift SN Ia data to uncover a peaking acceleration and subsequent deceleration in cosmic expansion.
• The analysis employs the Om diagnostic and CPL ansatz to reconstruct dark energy’s evolving equation of state and deceleration parameter q(z) with greater accuracy.
• The findings suggest that dark energy may decay into dark matter, challenging the static ΛCDM model and highlighting the need for further SN Ia surveys.

Insights into Cosmic Acceleration and Dark Energy Evolution

The paper "Is cosmic acceleration slowing down?" by Arman Shafieloo, Varun Sahni, and Alexei A. Starobinsky investigates recent dynamics in the universe’s expansion, leveraging the Constitution type Ia supernova (SN Ia) sample alongside baryon acoustic oscillations (BAO) and cosmic microwave background (CMB) data. The essential focus lies in exploring dark energy (DE) evolution and evaluating the possibility that cosmic acceleration, traditionally attributed to $\Lambda$CDM, might be decelerating.

Key Findings:

• Data Inclusion & Novel Approach: The authors incorporate low-redshift SN Ia data, enhancing previous datasets that lacked sufficient low-redshift information, thanks to the Constitution set.
• Diagnostics Utilized: The analysis introduces the Om diagnostic, a novel approach compared to the conventional $\Lambda$CDM model, providing insights into DE’s behavior with the potential for evolving characteristics.

Analytical Approach:

The data was scrutinized using various statistical methods to determine DE's equation of state (EOS). The authors challenged the prevalent assumption of a constant EOS, considering instead a variable EOS parameterized with the CPL (Chevallier-Polarski-Linder) ansatz. This approach allowed for more refined reconstructions of key cosmological parameters such as the deceleration parameter $q(z)$ and the Om diagnostic $Om(z)$.

Significant Observations:

• Peaking Acceleration: Contrary to traditional interpretations, findings suggest cosmic acceleration might be peaking, followed by a deceleration phase, particularly noticeable at redshifts $z \leq 0.3$.
• Dark Energy Evolution: Evidence potentially indicates a transition in DE behavior, possibly due to DE decaying into dark matter. A toy model simulating this decay fits the SN Ia+BAO+CMB combined dataset well, hinting at evolving DE dynamics.

Statistical Implications:

• The analysis introduces statistical tension between low redshift data (SN Ia + BAO) and high redshift data (CMB) under the CPL framework, implying that current models may not be optimally flexible to interpret evolving DE.
• Experimenting with a modified EOS closely mimicking rapid DE evolution showed improvements in data compatibility, suggesting alternative scenarios to static $\Lambda$CDM could indeed be viable.

Future Considerations and Implications:

The findings broaden the discourse on dark energy by challenging the primacy of the $\Lambda$CDM model. The authors advocate for continued investigation into DE's nature, particularly at lower redshifts, where the potential evolutionary characteristics of DE could be further defined. The robustness of these observations could be reinforced through extensive future SN Ia surveys, deepening the understanding of cosmic dynamics. The implication is that enhanced datasets might reshape existing cosmological models, readjusting our theoretical understanding of the universe's fate.

This paper is pivotal in opening new discussions regarding cosmic acceleration, underscoring the necessity for flexible models that accommodate potential DE evolution, hence prompting deeper investigations and potential revisions in theoretical cosmology.