Kilo-DegreeSurvey Confirms Standard Model of Cosmology

An international research team has analyzeddata from 41 million galaxies in order to estimate the distribution of matterin the universe. The data confirm a long-established model – much to the team’ssurprise.

The Kilo-Degree Survey (KiDS) of the European Southern Observatory (ESO)was gathering light from large parts of the southern sky over a period of eightyears in order to gain valuable new insights into the distribution of matter inthe universe. On March 18, 2025, the finaldata set “KiDS-Legacy” was published under the leadership ofresearchers from several European institutes, including Center for TheoreticalPhysics PAS Warsaw. Previous KiDS analyses had cast doubt on the standard modelof cosmology: the data had suggested a more uniform distribution of matter thanthe standard model predicts based on measurements from the Planck satellite.However, after analyzing the now complete KiDS data set with improved methodsand calibration data, the results are consistent with the standard model, whichdescribes how the Universe and structures within evolved over cosmic time.

“We took great care to optimize all parts of our analysis, which was atime-consuming process,” says Dr. Angus Wright from Ruhr University Bochum, wholed the main cosmological study. “The fact that the result now deviates so muchfrom our previous analyses came as a surprise – but we were able to identifythe reasons behind these changes.” The final evaluation is described in fivepublications that have been published or submitted for publication in thejournal “Astronomy & Astrophysics”. All papers are accessible via thedocument server arXiv since March 26, 2025.

Determining the matter distribution withgravitational lensing

There are various methods for determining the density and structure ofmatter. The KiDS team uses gravitational lensing in this instance: massiveobjects deflect the light from distant galaxies so that, when viewed fromEarth, those galaxies appear in a slightly distorted shape than how theyactually look. Cosmologists can use these distortions to estimate the mass ofthe deflecting objects and, ultimately, the total matter content of theUniverse. “The major advantage over other methods is that gravitational lensingcan be used to detect and study any type of matter, including the mysteriousdark matter apparently dominating in the Universe”, explains Prof. MaciejBilicki from CTP PAS.

To this end, the researchers need to know quantities such as thedistances between the light source, the deflecting object and the observer.Astronomers make use of the redshift to calculate these factors; redshiftdescribes an effect where light shifts more and more towards longer wavelengthsas it travels from more distant galaxies through the expanding Universe beforeit reaches Earth.

Images of 41 million galaxies taken with the ESO VLT Survey Telescopewere included in the analysis. The KiDS data covers an area of about 1,350square degrees of the sky, i.e., almost ten percent of the southern sky visibleoutside of our own Galaxy.

Calculating the distance of galaxies based onredshift

In order to determine the redshift of such a large number of galaxies,the team used the photometric method: the researchers took nine images of eachgalaxy at different wavelengths and determined the brightness of the galaxiesin each image; from this they were able to infer the redshift. The redshift canbe measured more precisely by spectroscopy, but it would be too time-consumingto apply that method to millions of faint galaxies.

Still, for some galaxies both spectroscopic and photometric data areavailable, so that the KiDS team can calibrate its photometric measurements ofredshifts with these precise spectroscopic data. While the previous “KiDS-1000”analysis used spectroscopic data from approximately 25,000 galaxies for thecalibration, as many as 126,000 galaxies were available for KiDS-Legacy. “I wasleading the effort of putting this dataset together, starting from numerousexternal surveys.” - says Prof. Bilicki. “We carefully cleaned it up ofunreliable entries, which gave us state-of-the-art calibration catalog, greatlyimproving the robustness of our analysis.” In addition, the researchers usedoptimized methods and new computer simulations for the analysis to reduce systematicuncertainties in the final data set.

Following the optimized evaluation, the team was able to include moredistant galaxies in the final analysis than in the previous one. WhileKiDS-1000 was limited to galaxies with a maximum distance of 8.5 billion lightyears, KiDS-Legacy can now use galaxies up to 10.5 billion light years away.

Blind analysis to ensure unbiased results

In the field of cosmology, it is common practice to evaluate data sets“blindly”, to avoid any bias due to previous analyses or personal hypotheses.Before starting the analysis, researchers send the catalog of all galaxies to athird party, who changes a certain parameter for each galaxy, resulting inthree variants of the data set: one with the real measured values and two withslightly different ones. The astronomers analyzing the data sets don’t knowwhich is the true one. They carry out their analysis with all three data setsand only after getting the 3 sets of measurements, do they learn which is thecorrect result. Once this step is completed, the analysis method is no longerchanged.

KiDS team taken by surprise

According to the KiDS-Legacy data, the matter in space is distributedsomewhat more unevenly than KiDS-1000 had revealed. “Many tests of the internalconsistency of the data show that this final analysis is significantly morerobust than previous studies,” points out Dr. Benjamin Stölzner from RuhrUniversity Bochum. The team also compared the new results with those of othersurveys. Earlier KiDS analyses had indicated a discrepancy with the PlanckSurvey, which estimates the matter density based on the cosmic microwavebackground, a radiation that was emitted shortly after the Big Bang and canstill be measured today.

“The discrepancies between our previous KiDS data set and the Planckresults have caused quite a stir in the astronomical community in recentyears,” explains Professor Hendrik Hildebrandt from Ruhr University Bochum,coordinator of the KiDS team. “Ironically, we are now resolving thisdiscrepancy ourselves. To our surprise, the KiDS-Legacy data contain noevidence to suggest errors in the standard model of cosmology.” The researchersexplain how the differences in the various KiDS analyses come about in thecurrent publications.

Cooperation partners

The Kilo-Degree Survey was headed by Professor Hendrik Hildebrandt fromthe Chair of Observational Cosmology at Ruhr University Bochum (Germany),Professor Koen Kuijken from Leiden Observatory at Leiden University (TheNetherlands), Professor Catherine Heymans from the Institute for Astronomy atUniversity of Edinburgh (UK), Dr. Marika Asgari from Newcastle University (UK),and Professor Benjamin Joachimi from University College London (UK).

KiDS in Poland

The Polish node ofthe KiDS team, located in the Center for Theoretical Physics PAS, Warsaw, isled by Professor Maciej Bilicki, other members being Dr. Priyanka Jalan, Dr.Szymon Nakoneczny, as well as PhD students Anjitha John William andGursharanjit Kaur.

Funding

The research was supported by the National Science Center (NCN) withinthe grant PACIS: Precision and Accuracy for Cosmological Imaging Surveys(UMO-2020/38/E/ST9/00395).

Original publications

Angus Wright et al.: The Fifth Data Release of the Kilo Degree Survey:Multi-Epoch Optical/NIR Imaging Covering Wide and Legacy-Calibration Fields,in: Astronomy & Astrophysics, 2024, DOI: 10.1051/0004-6361/202346730, https://www.aanda.org/articles/aa/full_html/2024/06/aa46730-23/aa46730-23.html

paper download on arXiv: https://arxiv.org/abs/2503.19439

Robert Reischke et al.: KiDS-Legacy: Covariance Validation and theUnified OneCovariance Framework for Projected Large-Scale StructureObservables, paper download on arXiv: https://arxiv.org/abs/2410.06962

Benjamin Stölzner et al.: KiDS-Legacy: Consistency of Cosmic ShearMeasurements and Joint Cosmological Constraints with External Probes, paperdownload on arXiv: https://arxiv.org/abs/2503.19442

Angus Wright et al.: KiDS-Legacy: Redshift Distributions and TheirCalibration, paper download on arXiv: https://arxiv.org/abs/2503.19440

Angus Wright et al.: KiDS-Legacy: Cosmological Constraints from CosmicShear with the Complete Kilo-Degree Survey, paper download on arXiv: https://arxiv.org/abs/2503.19441

Press contact

Prof. Maciej Bilicki

Center for Theoretical Physics

Polish Academy of Sciences

al. Lotników 32/46, Warsaw

phone +48  573 823 493
email: bilicki@cft.edu.pl

Additional contacts

Prof. Dr. Hendrik Hildebrandt, Ruhr University Bochum, Germany, email: hendrik@astro.rub.de

Dr. Angus Wright, Ruhr University Bochum, email: awright@astro.rub.de

Dr. Benjamin Stölzner, Ruhr University Bochum, email: stoelzner@astro.rub.de

Dr. Robert Reischke, University of Bonn, email: rreischke@astro.uni-bonn.de

Prof. Koen Kuijken, Leiden Observatory, LeidenUniversity, the Netherlands, email: kuijken@strw.leidenuniv.nl

Prof. Catherine Heymans, Institute for Astronomy, University ofEdinburgh, UK, email: heymans@roe.ac.uk

Prof. Benjamin Joachimi, University College London, email: b.joachimi@ucl.ac.uk

Dr. Marika Asgari, NewcastleUniversity, email: marika.asgari@newcastle.ac.uk

Links

KiDS-Legacy data release

https://www.eso.org/sci/observing/phase3/news.html#KIDS_DR5

Color map of KiDS data set

https://alasky.cds.unistra.fr/KiDS/CDS_P_KiDS_DR5_color-gri/

Previous press release on KiDS-1000

https://kids.strw.leidenuniv.nl/pr_jul2020.php

‍