A 6% measurement of the Hubble parameter at $z\sim0.45$: direct evidence of the epoch of cosmic re-acceleration (1601.01701v2)

Abstract: Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 to provide new constraints on the Hubble parameter $H(z)$ using the em cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent $H(z)$ measurements in the redshift range $0.3<z<0.5$, with an accuracy of $\sim$11-16\% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6\% accuracy constraint of $H(z=0.4293)=91.8\pm5.3$ km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring $z_{t}=0.4\pm0.1$, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9\% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyond -and including- the $\Lambda$CDM model.

• The paper reports a 6% measurement of H(z=0.4293)=91.8±5.3 km/s/Mpc using the cosmic chronometers approach on a sample of over 130,000 massive galaxies.
• The analysis achieves 11–16% precision in H(z) without relying on cosmological model assumptions, emphasizing the robustness of the technique.
• The findings robustly detect the transition from deceleration to acceleration at z_t=0.4±0.1 with 99.9% confidence, offering significant insights into dark energy and cosmic expansion.

Overview of Measurements and Findings on Cosmic Re-acceleration

The paper "A 6% measurement of the Hubble parameter at $z\sim0.45$: direct evidence of the epoch of cosmic re-acceleration" offers an empirical assessment of the Hubble parameter, $H(z)$, at redshift $z \sim 0.45$, implementing a cosmology-independent technique known as the cosmic chronometers approach. This paper leverages data from the Baryon Oscillation Spectroscopic Survey (BOSS) to derive new constraints on the cosmic expansion history without relying on model-dependent assumptions, thus contributing significantly to the understanding of the Universe's accelerated expansion.

Key Findings and Methodology

1. Cosmic Chronometers Technique: The authors exploit the cosmic chronometers methodology, which uses the relative age of old, massive, and passively evolving galaxies as a standard for measuring the gradient $dz/dt$. This approach uses galaxy ages determined from spectroscopic features to yield direct measurements of $H(z)$, ensuring independence from assumptions tied to specific cosmological models. This is crucial because it provides a direct examination of re-acceleration epoch without model reservations.
2. High-Precision Measurement: The analysis utilizes a sample of over 130,000 massive galaxies obtained from BOSS, leading to five new determinations of $H(z)$ in the redshift range $0.3 < z < 0.5$, achieving approximately 11–16% precision, including both statistical and systematic errors. The aggregated result gives $H(z=0.4293)=91.8\pm5.3$ km/s/Mpc with a 6% accuracy, which is competitive with traditional probes like Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO).
3. Transition Redshift: These measurements have significant cosmological implications, particularly in validating the existence of a transition from decelerated to accelerated expansion, marked by the transition redshift $\rm{z_t}=0.4\pm0.1$. This transition is detected at a 99.9% confidence level, exceeding the null hypothesis of no transition, thereby firmly establishing the epoch of re-acceleration— a pivotal event that has previously been postulated by SNe and CMB analyses.
4. Model Independence and Systematics: The independence from specific cosmological models adds robustness to these findings. Despite achieving high precision, the authors note that systematic errors outweigh statistical ones, suggesting that further refinement—especially regarding stellar metallicity—could reduce uncertainties further. Additionally, the results exhibit consistency across different evolutionary population synthesis (EPS) models tested, confirming the stability of the derived $H(z)$.

Implications for Cosmology and Future Work

The paper advances our understanding of the Universe's expansion dynamics by providing a methodology that relinquishes dependence on cosmological model assumptions, useful for testing theories beyond the $\Lambda$CDM framework. Notably, this work underscores the practical utility of the cosmic chronometers in complementing existing cosmological probes, suggesting that future research could tackle systematic uncertainties more rigorously to achieve even tighter constraints.

Going forward, increased high-redshift observational data from upcoming astronomical surveys like Euclid, WFIRST, and LSST could enhance the chronometers approach, enabling a fuller exploration of cosmic history. Such data, combined with even more refined metallicity determinations, could potentially elucidate the nuanced dynamics of late-time cosmic acceleration.