Scope: Lectures 09-10 · Purpose: Translate DES Y3 S8 and Omega_m summaries into conditional sigma8 values and quantify how fixed-Omega_m assumptions affect interpretation.
Setup and data
- Use the DES Y3 3x2pt flat-LCDM summaries; do not invent a joint covariance.
- The relation is S8=sigma8 sqrt(Omega_m/0.3).
- Reported central values and asymmetric Omega_m errors are literature summaries, not a raw shear vector.
Procedure
- Load DES Y3 S8 and Omega_m values and preserve asymmetric bounds.
- Invert the S8 definition at the central Omega_m to obtain sigma8.
- Evaluate sigma8 across the Omega_m lower and upper endpoints while holding S8 fixed; label this sensitivity, not a confidence interval.
- Draw the S8 constant contour and explain the compressed degeneracy direction.
- List the missing covariance and nuisance information needed for a proper propagated posterior.
Deliverables
Deliver: calculation, a parameter-space plot, asymmetric endpoint table, and an interpretation separating conditional transformation from posterior inference.
Assessment criteria
Rubric: correct algebra 25%; bounds and assumptions 25%; plot readability 15%; missing-covariance reasoning 25%; provenance 10%.
Uncertainty and interpretation
Report numerical precision, input uncertainty, model assumptions, and covariance availability separately. Label every output as directly measured, published summary, derived, or model/synthetic. A successful script run is not evidence that the scientific interpretation is valid.
Revisions and resubmissions are encouraged. Credit is based on defensible reasoning, validation, provenance, and clear limitations.