Dark Energy Survey Year 1 Results: Cosmological Constraints from Cosmic Shear (1708.01538v2)

Abstract: We use 26 million galaxies from the Dark Energy Survey (DES) Year 1 shape catalogs over 1321 deg$^2$ of the sky to produce the most significant measurement of cosmic shear in a galaxy survey to date. We constrain cosmological parameters in both the flat $\Lambda$CDM and $w$CDM models, while also varying the neutrino mass density. These results are shown to be robust using two independent shape catalogs, two independent \photoz\ calibration methods, and two independent analysis pipelines in a blind analysis. We find a 3.5\% fractional uncertainty on $\sigma_8(\Omega_m/0.3)^{0.5} = 0.782^{+0.027}_{-0.027}$ at 68\% CL, which is a factor of 2.5 improvement over the fractional constraining power of our DES Science Verification results. In $w$CDM, we find a 4.8\% fractional uncertainty on $\sigma_8(\Omega_m/0.3)^{0.5} = 0.777^{+0.036}_{-0.038}$ and a dark energy equation-of-state $w=-0.95^{+0.33}_{-0.39}$. We find results that are consistent with previous cosmic shear constraints in $\sigma_8$ -- $\Omega_m$, and see no evidence for disagreement of our weak lensing data with data from the CMB. Finally, we find no evidence preferring a $w$CDM model allowing $w\ne -1$. We expect further significant improvements with subsequent years of DES data, which will more than triple the sky coverage of our shape catalogs and double the effective integrated exposure time per galaxy.

• The paper achieves a 3.5% uncertainty in the S8 parameter, demonstrating a significant improvement in precision for cosmic shear constraints.
• The paper employs two independent shape measurement pipelines and rigorous photometric redshift calibration to effectively mitigate systematic biases.
• The paper’s results are consistent with KiDS and Planck findings, reinforcing cosmic shear as a reliable probe for cosmological and dark energy studies.

Cosmological Constraints from Cosmic Shear in the Dark Energy Survey Year 1

The paper titled "Dark Energy Survey Year 1 Results: Cosmological Constraints from Cosmic Shear" presents a detailed analysis of cosmic shear measurements from the first year of the Dark Energy Survey (DES Y1), covering an extensive area of 1321 deg$^2$ with data from approximately 26 million galaxies. It provides a rigorous statistical treatment to constrain cosmological parameters within both the flat $\Lambda$CDM and $w$CDM models, notably with a consideration of neutrino mass density variations, showcasing the robustness of these constraints through dual methodologies for shape and photometric redshift calibrations.

Methodology

Two independent shape measurement pipelines, Metacalibration and im3shape, were utilized to ensure the reliability of the cosmic shear measurements, both of which underwent extensive null tests for systematic effects. These pipelines processed the imaging data from DES Y1, which featured improved PSF calibration and shape catalogues compared to previous DES Science Verification data. The statistical method employed attempted to address potential biases due to shape selection and redshift distribution, adopting both Bayesian photometric redshift estimation and cross-correlation with photometric samples of more precise redshift calibration.

Results

The most notable outcome of this analysis is the derived 3.5% uncertainty in the parameter $$S_8 = \sigma_8(\Omega_m/0.3)^{0.5} = 0.782^{+0.027}_{-0.027}$$ for the $\Lambda$CDM model, representing a substantial improvement over earlier results from DES's Science Verification phase. In the $w$CDM scenario, $w=-0.95^{+0.33}_{-0.39}$ was measured, indicating no significant evidence to favor $w\ne -1$. A central point of the paper is that the constraints derived from DES Y1 cosmic shear are consistent with previous results from KiDS data and do not contradict CMB data from Planck, reducing concerns about tension between low-redshift measurements and CMB predictions.

Systematic Analysis and Robustness Tests

The robustness of these results was affirmed through numerous tests, including:

1. Intrinsic Alignment (IA): Various models were considered, including tidal alignment and a more generalized 'mixed' model accounting for additional alignment mechanisms. Constraints on IA parameters indicate a significant detection of tidal alignment effects.
2. Photometric Redshift Calibration: Systematic biases were mitigated by leveraging data from both COSMOS and clustering redshifts, confirming robustness against potential redshift distribution shape variations.
3. Scale Selection and Baryon Effects: Scale cuts were implemented to minimize the impact of uncertainties in nonlinear matter clustering due to baryonic effects, ensuring that model predictions remained unbiased.
4. Comparison of Shape Measurement Pipelines: Both pipelines provided consistent constraints on cosmological parameters, reinforcing the reliability of the results independent of methodological biases.

Implications and Future Directions

The results presented offer meaningful insight into the cosmological model parameters, particularly in confirming the consistency of shear-derived parameters with those from other cosmic probes. The paper sets a precedent for future surveys in managing systematic uncertainties through meticulous methodological redundancy and cross-validation. With further DES data, particularly from subsequent years which will enhance the dataset, significant improvements in precision and potentially novel insights into late-time cosmology are expected.

This paper stands as a pivotal addition to the body of cosmological research, providing substantial evidence for the reliability of cosmic shear as a tool to probe the underlying physics of the universe at large scales. In anticipation of further advances, these methodologies may serve as a foundational approach in upcoming surveys like LSST and Euclid, promising even greater precision in the exploration of dark energy and structure formation.