Turning cosmic shear into a litmus test for the standard model of cosmology

Project Information

COSMIC-LITMUS

Grant agreement ID: 770935

DOI

10.3030/770935

Project closed

EC signature date 11 April 2018

Start date 1 June 2018

End date 31 May 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 931 493,00

EU contribution

€ 1 931 493,00

Coordinated by

RUHR-UNIVERSITAET BOCHUM
Germany

Periodic Reporting for period 5 - COSMIC-LITMUS (Turning cosmic shear into a litmus test for the standard model of cosmology)

Reporting period: 2023-06-01 to 2024-05-31

Summary of the context and overall objectives of the project

Recent cosmological observations show an interesting discrepancy in the clustering strength of matter. Simply put, the early Universe as observed by the cosmic microwave background (CMB) suggests more structure than is actually observed in the late Universe with a technique called Gravitational Lensing. These Gravitational Lensing observations hence show a smoother Universe than is predicted from the exquisite CMB observations conducted with the Planck satellite. This tension might be a first sign of a breakdown of the otherwise highly successful standard model of cosmology or due to unknown systematic errors.

The aim of this project was to push the envelope of these Gravitational Lensing measurements and potentially falsify the standard model. A focus was put on the calibration of redshifts, which yield the third dimension of the inherently two-dimensional images of the sky. Only a very robust calibration enables a conclusive answer on the question, which kind of Universe we live in.

The main part of the project was about exploiting an existing data set, KiDS+VIKING. The KiDS+VIKING data were successfully calibrated with auxiliary data sets like PAUS and C3R2 and more data were used for the cosmological measurements. The results confirm the tension at higher significance strengthening the case for a problem with the standard model. This statement is now based on a more robust calibration leaving less room for an alternative explanation of the tension being due to systematic errors.

In the last phase of the project, the lessons learned from KiDS+VIKING were successfully transferred to the next generation of cosmological surveys, Euclid (launched in 2023) and LSST (first light in 2025). Only with the preparatory work on KiDS+VIKING will those projects be able to reach their full potential of solving the mystery that is dark energy, the hypothetical substance that is responsible for the accelerating expansion of the Universe.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The first half of the project was carried out very successfully. The KiDS optical and VIKING infrared data have been fully integrated and first cosmological measurements have been performed. This confirms the tension seen previously from optical-only data that still suffered from an inferior redshift calibration. With the KiDS-1000 analyses in 2021, the survey area was more than doubled and this more precise analysis again confirmed the previous results. The odds that the tension is a pure noise fluctuations was reported to be below 1%. In order to make such a strong statement we developed and carried out the different calibration techniques described in the proposal taking advantage of the newest calibration data that became available. A strong emphasis has been put on the detailed simulation of the whole Gravitational Lensing analysis, further supporting the validity of the approach and increasing trust in the robustness of the results.

During the second half of the project, the final cosmological analysis of KiDS+VIKING was prepared. This was almost entirely led by project members. The increasing precision gained by a larger data set and by pushing to higher redshifts with the cosmic shear measurements than ever before required a great effort in understanding systematic errors. Supported by new image simulations and a greatly increased calibration data set, a new level of robustness in the redshift calibration was achieved. Not only does this conclusively calibrate KiDS out to redshifts of z~2 but also represent a blueprint for calibrating Euclid and LSST. As such, the coherence between the complementary calibration techniques applied to the KiDS data and the parallel agreement with different sets of independent simulations sets a new standard in the field.

In parallel, the lessons learned from the KiDS+VIKING work flowed into the preparation for the next generation of weak lensing experiments, Euclid and LSST. The group got increasingly involved in the planning for those upcoming analyses and decisively influenced the pipelines that will be put to use very soon.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project pushed the envelope by integrating optical and infrared data from the KiDS and VIKING surveys, respectively, for the first time in a cosmic shear measurement. This was done to improve photometric redshifts, select galaxies at higher redshift (i.e. earlier cosmic times), and push down the statistical uncertainties. This was complemented by a coherent effort to also increase our knowledge of the most important systematic errors, again with a particular focus on the redshift calibration. Final results from KiDS+VIKING (to be published before the end of 2024) will be 30-40% more constraining than the best current results thanks to the work of the COSMIC-LITMUS team. If the results don't change significantly, the odds for the standard model being correct will be vanishingly small by that point.

At the same time, we increased our efforts within the Euclid and LSST consortia, applying the techniques that have been pioneered on KiDS+VIKING on these new data sets as they come online. No matter what the final KiDS+VIKING analysis will tell us about the S8 tension in the end, the lessons learned there will be invaluable to make sure that Euclid and LSST meet their primary scientific goal of determining the physical nature of dark energy.

Sketch of the cosmic shear effect; © Ruhr University Bochum