Outside of the human visible universe, about 95% of its composition comes from invisible dark matter and dark energy. They neither emit light nor are directly detectable, but they quietly influence the evolution of galaxies and large-scale structures through gravity and cosmic expansion. Recently, an international team composed of the University of Chicago and other institutions used the Dark Energy Camera (DECam) to conduct statistical analysis of weak distortions in the shapes of hundreds of millions of galaxies, and drew a new map of the "invisible universe" covering about one-third of the entire sky, bringing key evidence to test the current mainstream cosmological models.
This study is based on the Dark Energy Survey (DES) observations from 2013 to 2019. The latter's Dark Energy Camera installed on the 4-meter Blanco Telescope at the Cerro Tololo InterAmerican Observatory in Chile has accurately measured the shapes of more than 150 million galaxies, covering about 5,000 square degrees of the sky, which is equivalent to one-eighth of the entire sky. Previously, these data have played a central role in testing the standard cosmological model of "Λcold dark matter" (ΛCDM), and the latest results further incorporate into the analysis a large number of images captured by the camera but not originally within the official DES survey area, almost doubling the number of galaxies that can be used for weak gravitational lensing research.
In a new round of analysis, the research team measured the shapes of more than 100 million galaxies and estimated their distances from the Earth through the redward shift of their spectra, thereby simultaneously grasping the projection distribution and three-dimensional depth information of galaxies in the sky. When the light of these galaxies travels through the universe, it will be slightly "pulled" by the gravity of the matter along the way, and appears as extremely fine shape shearing in the imaging of Earth telescopes. This effect is called "weak gravitational lensing" and is a key tool for inferring the distribution of matter in the universe and examining the degree of dark matter clumps and the role of dark energy.
In this work, called the "DECADE" cosmic shear project, scientists used these shape and distance data to fit the ΛCDM model, focusing on whether the rate of growth of cosmic structure over time is consistent with model predictions. The research results show that the "clumpiness" parameters of the universe obtained using this independent data set are consistent with previous weak lensing measurements, and are also in good agreement with the early universe parameters deduced from the cosmic microwave background radiation. No evidence of "obvious tension between the late universe and the early universe" that has been controversial in the past few years has been found.
The team also combined DECADE's weak lens measurements with original DES data to construct a galaxy lens analysis sample that covers the largest number of galaxies and the widest sky area so far, totaling about 270 million galaxies, covering about 13,000 square degrees of sky, which is about one-third of the entire sky. Thanks to such a large sample, researchers can adopt a more conservative quality control strategy in their analysis, using only the most credible data points with the smallest systematic errors, and still obtain high-precision constraints that are sufficient to compare head-on with the cosmic microwave background results.
This work was described by researchers as "an unconventional weak lens survey" because it used a large number of historical images previously taken by the astronomical community for other scientific targets and scattered in archives, rather than a long-term dedicated observation plan designed for weak lenses from the beginning. In traditional solutions, a large number of photos that do not meet certain strict imaging quality standards will be discarded. However, the DECADE team adopted looser screening conditions under the premise of strictly testing systematic errors, proving that even data that is not "born for lenses" can still support robust cosmological analysis as long as it undergoes careful calibration and quality assessment.
The dark energy camera itself is equipped with 62 ultra-high-sensitivity CCD detectors, which can photograph the deep space of the universe at unprecedented depth. It is one of the core facilities for current weak lens and dark energy research. In this project, scientists from the University of Chicago, Fermilab, the National Supercomputing Applications Center at the University of Illinois, as well as Argonne National Laboratory, the University of Wisconsin-Madison and other institutions collaborated to form a joint force in data retrieval, manual review of image quality, shape measurement methods and cosmological statistical analysis, turning these "rediscovered" archival observations into new cosmological weapons.
The newly released catalog of shaped galaxies has been opened to the scientific community and has quickly attracted the attention of the cosmology and astronomy communities. It has been used in many topics such as studying dwarf galaxies and mapping the mass distribution of the universe. The researchers said that as larger-scale projects such as the Vera Rubin Observatory's "Heritage Survey of Time and Space" (Rubin LSST) are launched in the future, this experience shows that making good use of all available images instead of just "perfect" samples is expected to significantly improve the measurement accuracy of the properties of dark matter and dark energy, providing clearer clues for understanding this "95% invisible universe."
Compiled from /ScitechDaily