Raising the bar: new constraints on the Hubble parameter with cosmic chronometers at z$\sim$2 (1503.01116v1)

Abstract: One of the most compelling tasks of modern cosmology is to constrain the expansion history of the Universe, since this measurement can give insights on the nature of dark energy and help to estimate cosmological parameters. In this letter are presented two new measurements of the Hubble parameter H(z) obtained with the cosmic chronometer method up to $z\sim2$. Taking advantage of near-infrared spectroscopy of the few very massive and passive galaxies observed at $z>1.4$ available in literature, the differential evolution of this population is estimated and calibrated with different stellar population synthesis models to constrain H(z), including in the final error budget all possible sources of systematic uncertainties (star formation history, stellar metallicity, model dependencies). This analysis is able to extend significantly the redshift range coverage with respect to present-day constraints, crossing for the first time the limit at $z\sim1.75$. The new H(z) data are used to estimate the gain in accuracy on cosmological parameters with respect to previous measurements in two cosmological models, finding a small but detectable improvement ($\sim$5 %) in particular on $\Omega_{M}$ and $w_{0}$. Finally, a simulation of a Euclid-like survey has been performed to forecast the expected improvement with future data. The provided constraints have been obtained just with the cosmic chronometers approach, without any additional data, and the results show the high potentiality of this method to constrain the expansion history of the Universe at these redshifts.

• The paper employs the cosmic chronometer method to deliver new H(z) measurements at z=1.36 and z=1.97, improving parameter accuracy by 5%.
• It calibrates systematic uncertainties using diverse stellar population synthesis models, extending H(z) constraints robustly to z~2.
• The study lays the groundwork for future surveys like Euclid to tighten constraints on dark energy and matter density, complementing other cosmological probes.

Constraints on the Hubble Parameter at High Redshifts Using Cosmic Chronometers

The paper under review offers an investigation into the Hubble parameter $H(z)$ utilizing cosmic chronometers at redshifts approaching $z \sim 2$. The paper aims to contribute to the understanding of the Universe's expansion by introducing and refining methods for measuring $H(z)$ and hence constraining cosmological parameters such as the matter density $\Omega_{M}$ and the dark energy equation of state parameter $w_{0}$.

Methodology

The research employs the cosmic chronometer method to derive two new measurements of the Hubble parameter at $z = 1.36$ and $z = 1.97$. This method capitalizes on the relative ages of passive galaxies as standard clocks to infer $H(z)$, focusing specifically on the differential age evolution. Recognizing systematic uncertainties like star formation history and metallicity, the authors calibrate their measurements using different stellar population synthesis (SPS) models. This methodology allows for a broader redshift coverage, surpassing previous constraints and addressing the universe's expansion during epochs not well covered by other observational techniques.

Results and Analysis

A significant achievement of this paper is the successful extension of $H(z)$ measurements to a redshift of approximately $z = 2$. The results highlight a modest yet detectable improvement in parameter constraints, notably a 5% enhancement in accuracy for $\Omega_{M}$ and $w_{0}$ in comparison to prior results. The authors demonstrate the utility of this approach absent of complementary observational datasets, underscoring the robustness of cosmic chronometers in this context.

Implications

The findings exhibit the potency of cosmic chronometers in augmenting our comprehension of the Universe's expansion history, particularly at high redshifts. Anticipated future surveys, such as Euclid, are expected to leverage this methodology significantly, offering more stringent constraints on cosmological parameters by capitalizing on larger datasets of distant passive galaxies. These endeavors will be critical as they provide a valuable cross-check against other cosmological probes like Baryon Acoustic Oscillations (BAO) and Supernovae Type Ia (SNe).

Conclusions and Future Work

The results presented signify an important step towards refining high-redshift cosmological models, enhancing our grasp of the parameters governing the Universe's large-scale dynamics. The research paves the way for future studies to synergize various cosmological probes, promising tighter constraints on theoretical models of dark energy and matter density. As further surveys expand the available data pool, the cosmic chronometer approach could prove indispensable in revealing new insights into the properties of dark energy and the mechanics of cosmic expansion.

In conclusion, the paper effectively contributes to the ongoing efforts in cosmology to refine the constraints on the universe's expansion history through innovative utilization of cosmic chronometers, providing a foundation for future empirical and theoretical explorations in the field.