Speaker
Description
Increasingly precise observations are beginning to expose potential cracks in the standard cosmological model, from the Hubble constant tension to emerging hints of evolving dark energy. Resolving whether these point to new physics, and uncovering the nature of dark matter and dark energy, requires precise measurements of cosmic geometry and the growth of structure. The Dark Energy Survey (DES) was designed precisely to address these questions.
In this talk, I will present the final cosmological results from DES Year 6, covering $\sim 5000 \text{ deg}^2$ with 140 million source galaxies and 9 million lens galaxies. Through the combination of cosmic shear, galaxy clustering, and galaxy–galaxy lensing ($3\times2$pt), DES Y6 yields constraints on the amplitude of matter fluctuations $S_8$ with a factor of two improvement over DES Y3. For the first time, we combine all four DES dark energy probes - $3\times2$pt, Type Ia supernovae, baryon acoustic oscillations (BAO), and galaxy clusters - into a single coherent analysis, presenting constraints in both $\Lambda$CDM and wCDM.
I will then discuss lensing ratios, an exciting geometric probe for weak-lensing cosmology. By comparing galaxy-galaxy lensing signals around the same lens sample but different source samples, the dependence on the lens mass distribution cancels, leaving a signal governed by angular diameter distances. This makes lensing ratios a clean probe of cosmic geometry and expansion history, while providing an internal validation of redshift and shear calibrations. I will show how this has already been realised in DES Y6, where lensing ratios have been used to validate photometric redshift and multiplicative shear calibrations.
Stage-IV surveys such as Euclid, Rubin/LSST, Roman and Simons Observatory, will dramatically increase statistical power, making calibration systematics ever more critical while simultaneously unlocking the full potential of lensing ratios. I will close by showing how their geometric sensitivity on small angular scales positions lensing ratios as both a precision calibration tool and a cosmological probe in their own right.