A test of the nature of cosmic acceleration using galaxy redshift distortions (0802.1944v1)

Abstract: Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion the physical cause of which is a mystery. Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this "dark energy" corresponds to a cosmological constant, perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions. Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time. A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys. Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies, we measure the anisotropy parameter b = 0.70 +/- 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 +/- 0.36. This is consistent with the standard cosmological-constant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. This could be achieved with a further factor-of-ten increase in the sampled volume at similar redshift.

• The paper demonstrates that anisotropic galaxy clustering yields key measurements of cosmic growth, reporting β = 0.70 ± 0.26 and f = 0.91 ± 0.36 at z = 0.8.
• The paper utilizes data from over 10,000 galaxies in the VIMOS-VLT Deep Survey, with Millennium-based simulations ensuring robustness against observational uncertainties.
• The paper highlights that precise redshift distortion analysis can constrain gravitational theories and differentiate among competing cosmological models.

Analysis of Cosmic Acceleration via Galaxy Redshift Distortions

The paper conducted by L. Guzzo et al., published in Nature under the title "A test of the nature of cosmic acceleration using galaxy redshift distortions," addresses the persistent enigma of the Universe's accelerated expansion, primarily attributed to dark energy. A thorough examination of galaxy redshift distortions provides a direct avenue for measuring the coherent motions of galaxies, which act as critical indicators of the Universe's large-scale structure evolution and expansion dynamics.

Core Contributions

At the heart of this research lies the observation that different cosmological models, despite yielding similar expansion trajectories, predict distinct growth rates for cosmic structures. This work leverages the anisotropic distortion in the galaxy clustering pattern, discernible through galaxy redshift surveys, as a signal of this growth. The anisotropy parameter β, measured as β = 0.70 ± 0.26, corresponds to a growth rate f = 0.91 ± 0.36 at a redshift of z = 0.8. These findings are in alignment with a cosmological constant scenario combined with a low matter density and a flat geometric Universe, yet remain insufficiently precise to decisively differentiate among alternative cosmological models.

Implications and Methodology

The implications of these findings extend to both the foundation of cosmology and the theoretical underpinnings of gravity. By measuring redshift-space distortions, the paper evaluates the compression parameter, effectively quantifying the growth rate of structures in relation to the bias factor of galaxies. These measures are grounded in the linear approximation of density fluctuations described by perturbation theory. Such observations provide pivotal constraints on gravitational theories and alternative models of cosmic acceleration, including scalar fields, higher-dimensional gravity, and more.

Methodologically, the paper utilizes the VIMOS-VLT Deep Survey (VVDS), aggregating redshifts of over 10,000 galaxies, corroborating the results through state-of-the-art mock simulations derived from the Millennium simulation. These mock catalogues provide a robust framework for assessing statistical and systematic uncertainties, ensuring the reliability of the measurements amidst cosmic variance and observational artifacts.

Evaluation of Models and Future Prospects

The paper contextualizes its findings within the broader spectrum of cosmological models, comparing the derived growth rates with predictions from standard models, open models without cosmological constants, DGP braneworld theory, and models positing interactions between dark energy and dark matter. Despite the consistency of the findings with the cosmological constant model, larger samples and higher precision measurements are required to discern finer differences among these models.

The potential for future advancements in this research area is significant. As redshift surveys expand in scale and sophistication, the ability to more precisely measure the growth rate of cosmic structures promises to elucidate the fundamental nature of dark energy and test the assumptions underlying general relativity and its alternatives.

Conclusion

In sum, the paper by Guzzo et al. illustrates a crucial step forward in understanding cosmic acceleration through galaxy redshift distortions. While existing data aligns with the standard cosmological model, it underscores the necessity for more comprehensive surveys to further refine our understanding of the Universe's expansion dynamics. As such, future developments in redshift measurement and galaxy clustering analysis are poised to play a central role in addressing the profound cosmological questions surrounding dark energy and gravity.