An international team co-led by UCL researchers has estimated the distribution of matter in the universe and found that it supports the standard model of cosmology - much to the team's surprise.
The Kilo-Degree Survey (KiDS) observed large parts of the southern sky over eight years to gain insights into the distribution of matter in the universe.
The final data set, "KiDS-Legacy", has now been published. Previous KiDS analyses had cast doubt on the standard model of cosmology. The data had suggested a more uniform distribution of matter than the standard model predicts based on measurements of the cosmic microwave background (light left over from the Big Bang).
However, after analysing the now complete KiDS data set with improved methods and calibration data, the results are consistent with the standard model, which describes how the Universe and structures within evolved over cosmic time.
KiDS co-lead Professor Benjamin Joachimi (UCL Physics & Astronomy) said: "Our results highlight the importance of being solely guided by the data and carefully avoiding any undue influence of the researchers' expectations or intuition. This allowed us to revise a trend seen in gravitational lensing experiments for more than a decade, including in our own previous analyses.
"The final KiDS results may be another confirmation of the cosmological standard model, but our work does not stop here. For this standard model to work, 95% of all matter and energy in the Universe today has to consist of strange, as yet unidentified ingredients.
"We will gain new insights from the next generation of galaxy surveys, in particular the European Space Agency Euclid mission to which UCL is making major contributions. I am proud that the KiDS team has set high standards for the interpretation of cosmological surveys which will help Euclid achieve its goals."
Dr. Angus Wright from Ruhr University Bochum said: "We took great care to optimise all parts of our analysis, which was a time-consuming process.
"The fact that the result now deviates so much from our previous analyses came as a surprise - but we were able to identify the reasons behind these changes."
The final evaluation is described in five publications that have been published or submitted for publication in the journal "Astronomy & Astrophysics". All papers are accessible via the document server arXiv.
Determining the matter distribution with gravitational lensing
There are various methods for determining the density and structure of matter. The KiDS team used gravitational lensing in this instance. Massive objects deflect the light from distant galaxies so that these galaxies appear in a distorted shape and in a different place than they actually are when viewed from Earth.
Cosmologists can use these distortions to estimate the mass of the deflecting objects and, ultimately, the total mass of the Universe. This includes dominant dark matter.
To this end, the researchers need to know quantities such as the distances between the light source, the deflecting object and the observer. The researchers make use of the redshift to calculate these factors; redshift describes an effect where light shifts more and more towards longer wavelengths as it travels from more distant galaxies through the expanding Universe before it reaches Earth.
Images of 41 million galaxies taken with the Very Large Telescope Survey Telescope were included in the analysis. The KiDS data covers an area of 1,347 square degrees of the sky, i.e., almost 10% of the sky where we can see past our own galaxy.
Calculating the distance of galaxies based on redshift
In order to determine the redshift of such a large number of galaxies, the team used the photometric method. They took nine images of each galaxy at different wavelengths and determined the brightness of the galaxies in each image; from this they were able to infer the redshift. The redshift can be measured more precisely by spectroscopy, but it would be too time-consuming to apply that method to millions of faint galaxies.
Still, for some galaxies both spectroscopic and photometric data are available, so that the KiDS team can calibrate its photometric measurements of redshifts with these precise spectroscopic data.
While the previous analysis KiDS-1000 used spectroscopic data from approximately 25,000 galaxies for the calibration, data from as many as 126,000 galaxies were available for KiDS-Legacy.
In addition, the researchers used optimised methods and new computer simulations for the analysis to reduce systematic uncertainties in the final data set.
Following the optimised evaluation, the team was able to include more distant galaxies in the final analysis than in the previous one. While KiDS-1000 was limited to galaxies with a maximum distance of 8.5 billion light years, KiDS-Legacy can now observe galaxies 10.4 billion light years away.
Blind analysis to ensure unbiased results
In cosmology, it is common to evaluate data sets blindly to avoid any bias due to previous analyses or personal hypotheses. Before starting the analysis, researchers send the catalogue of all galaxies to a third party, who changes a certain parameter for each galaxy, resulting in three variants of the data set: one with the real measured values and two with slightly different values.
The researchers analysing the data set don't know which is the real data. They carry out their analysis with all data sets and only then learn which is the correct result. Once this step is completed, the analysis method is no longer changed.
KiDS team taken by surprise
According to the KiDS-Legacy data, the matter in space is distributed somewhat more unevenly than KiDS-1000 had revealed. The researchers say this final analysis is significantly more robust than previous analyses.
The team also compared the new results with those of other surveys. Earlier KiDS analyses had indicated a discrepancy with the Planck survey, which estimates the matter density based on the cosmic microwave background.
The Kilo-Degree Survey was led by UCL's Professor Joachimi, Professor Hendrik Hildebrandt from Ruhr University Bochum (Germany), Professor Koen Kuijken from Leiden University (The Netherlands), Professor Catherine Heymans from the University of Edinburgh, and Dr Marika Asgari from Newcastle University.
