Cosmic Chronometers: Constraining the Equation of State of Dark Energy. I: H(z) Measurements (0907.3149v1)

Abstract: We present new determinations of the cosmic expansion history from red-envelope galaxies. We have obtained for this purpose high-quality spectra with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with high-quality, publicly available archival spectra from the SPICES and VVDS surveys. We improve over our previous expansion history measurements in Simon et al. (2005) by providing two new determinations of the expansion history: H(z) = 97 +- 62 km/sec/Mpc at z = 0.5 and H(z) = 90 +- 40 km/sec/Mpc at z = 0.8. We discuss the uncertainty in the expansion history determination that arises from uncertainties in the synthetic stellar-population models. We then use these new measurements in concert with cosmic-microwave-background (CMB) measurements to constrain cosmological parameters, with a special emphasis on dark-energy parameters and constraints to the curvature. In particular, we demonstrate the usefulness of direct H(z) measurements by constraining the dark- energy equation of state parameterized by w0 and wa and allowing for arbitrary curvature. Further, we also constrain, using only CMB and H(z) data, the number of relativistic degrees of freedom to be 4 +- 0.5 and their total mass to be < 0.2 eV, both at 1-sigma.

• The paper introduces a differential age technique using red-envelope galaxies to measure H(z), reporting 97 ± 62 km/s/Mpc at z ≈ 0.5 and 90 ± 40 km/s/Mpc at z ≈ 0.8.
• It constrains dark energy parameters, including w0 and wa, and allows for arbitrary cosmic curvature through high-quality spectroscopic data from Keck-LRIS, SPICES, and VVDS surveys.
• The study refines limits on relativistic species and neutrino mass, offering a more sensitive method compared to traditional integrated distance measurements.

Overview of Cosmic Expansion History through Cosmic Chronometers

The paper "Cosmic Chronometers: Constraining the Equation of State of Dark Energy. I: $H(z)$ Measurements" offers a detailed exploration into understanding the cosmic expansion history utilizing red-envelope galaxies as cosmic chronometers. The research leverages high-quality spectra from the Keck-LRIS spectrograph alongside archival spectra from SPICES and VVDS surveys, focusing on galaxy clusters in the redshift range $0.2 < z < 1.0$. This paper provides improved expansions of the expansion history compared to previous measurements, like those reported in Simon et al. (2005), and explores constraining cosmological parameters, emphasizing dark energy equations of state and curvature.

Methodology and Results

The research utilized passively evolving galaxies as standard clocks to directly measure the differential age, $dt/dz$, as a function of redshift, providing insight into the Hubbard $H(z)$ directly rather than integrated quantities like luminosity distance. The main results consist of two new determinations of cosmic expansion history: $H(z) = 97 \pm 62$ km s$^{-1}$ Mpc$^{-1}$ at $z \approx 0.5$ and $H(z) = 90 \pm 40$ km s$^{-1}$ Mpc$^{-1}$ at $z \approx 0.8$. These results are integrated with cosmic microwave background (CMB) measurements for a comprehensive constraint framework on cosmological parameters.

Notably, the paper details the constraints obtained on dark energy equation-of-state parameters $w_0$ and $w_a$, allowing also for arbitrary curvature, which direct $H(z)$ measurements facilitated. Additional findings include constraints on the number of relativistic species, determined to be $4 \pm 0.5$, and their total mass, constrained to $< 0.2$ eV at 1$\sigma$.

Implications and Theoretical Considerations

The implications of this research are substantial for the field of cosmology. Accurately determining the expansion history is pivotal for understanding dark energy dynamics. The differential age technique demonstrated in this paper stands out for directly probing $H(z)$, as opposed to relying on integrated properties, thus potentially offering a more sensitive and efficient method to constrain cosmic expansion.

From a theoretical perspective, this paper's findings help in assessing models beyond the standard $\Lambda$CDM, supporting constraints on neutrino properties and refining dark energy parameters. The analysis showed improved capabilities in constraining the dark-energy equation of state and exploring the curvature of the Universe, providing reinforcement and alternative approaches to supernova, BAO, and lensing investigations.

Future Directions

The outcomes set precedent for expanding the observational campaigns to assess the cosmic chronometer approach further, potentially improving the observational techniques, data collection, and theoretical modeling aspects. The approach undertakes high-quality observational data on early-type galaxies, which presents a spectrum of possibilities for future research, especially in refining the cosmic chronometer method and extending observational reach to $z > 1$. Evolution in modeling of stellar populations will likely enhance accuracy and reliability in future iterations, thereby further solidifying constraints on the dark energy potential dynamics.

In summary, this work propels a promising method to directly test cosmic expansion mechanisms, fosters significant cosmological parameter constraints, and invites further investigation into the underlying mechanics of universe acceleration and dark energy behavior.