The discrepancy appears again in cosmological observations

The discrepancy appears again in cosmological observations

    Meijin Yoon

    • Leiden Observatory, Leiden University, Leiden, Netherlands

Physics 16, 193

New analysis of the distribution of matter in the universe continues to find a discrepancy in the clumping of dark matter in the late and early universe, suggesting a fundamental flaw in the standard cosmological model.

Figure 1: Weak gravitational lensing surveys, such as those conducted by HSC-SSP, can reveal the presence of invisible clumps of dark matter. The effect of gravitational lensing is imprinted as a coherent distortion of the shape of galaxies beyond the same mass of dark matter. Estimating the shapes and distances to galaxies is a crucial step in extracting weak gravitational signals and in measuring the distribution of matter in the universe.

Cosmologists study the universe by making a wide range of observations using a variety of modern techniques. Each observation can reveal different details about the formation of the universe during a certain period of its history. An astronomical survey – a map of a region of the sky – is an effective way to survey a large area of ​​the universe and the things it contains. For example, weak lens scanning does this by obtaining clear images of galaxies, which can then be used to map the distribution of the universe’s matter throughout history. The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is one of these weak-lens surveys, and has the highest resolution and deepest depth of all current weak-lens surveys. Over the past six years, the HSC-SSP survey team has spent 330 Clear the nights 3% of the entire spherical sky, capturing light emitted by even galaxies 10 A billion years ago. The team has now done an analysis 40% of his data (1-5), and found results inconsistent with predictions of cosmological models derived from Planck satellite data for the early universe, such as measurements of the first light of the universe. This discrepancy has repeatedly appeared in weak lensing surveys, suggesting a fundamental flaw in the standard cosmological model, known as ΛCDM.

Traditionally, astronomers use light emitted directly from an astrophysical object to investigate the properties of that object. Instead, gravitational lensing surveys use light emitted from behind an object to infer the object’s presence. When light is emitted from a distant object, it can be deflected by the strong gravitational force of matter between the object and the observer. This curvature can distort the shape of a distant galaxy, an effect called weak gravitational lensing (Figure 1). This shape distortion is consistent for nearby galaxies because the light from them travels through the same region of space as it makes its way to the observer. Hence, gravitational lensing surveys can detect objects that do not emit light themselves, such as clumps of dark matter, and weak lensing primarily detects dark matter along the line of sight of the galaxy. Although the effect is small for most galaxies, when measurements are made of millions of galaxies, the general properties of the weak lensing signal can determine cosmological parameters with high precision (see Viewpoint: Weak lensing becomes high-resolution surveying science).

Among ongoing weak lensing surveys, the HSC-SSP survey explores the furthest reaches of the universe, meaning it can capture information about more of the universe’s past. However, the farther away a galaxy is, the more difficult it is to precisely determine its shape and distance, two pieces of information necessary to determine the amount of weak lensing. Accurate distance measurements can be obtained using spectroscopic observations. However, current publicly available spectroscopic data lack information about the most distant galaxies in the HSC-SSP survey. The HSC-SSP team works to mitigate this distance problem through “self-calibration” technology. But the calibration process adds an additional degree of freedom to the modeling, which ultimately reduces the accuracy of cosmological measurements.

Using measurements of galaxy shape and distance, the HSC-SSP collaboration provides an estimate of the distribution of matter in the universe. As in previous surveys, the researchers analyzed this distribution to determine how the density of matter fluctuates across space. One way to describe these fluctuations of matter is using a parameter called s8It is a measure of the clumping of matter – visible and dark – in the universe. The HSC-SSP collaboration reports four cosmological parameter values s8, each of which is calculated via a different analysis protocol. Values ​​range from 0.763 to 0.776 ( ±0.033). These values s8 They are consistent with, but lower than, values ​​obtained by other weak lens surveys (6, 7). 0.832±0.013 It was measured by the Planck Collaboration, which monitors the early universe. When placed in ΛCDM, s8 to 0.776 It leads to the prediction that the universe today is much less clumpy than it would be if another planet existed s8 to 0.832 Which indicates the possibility of failure of ΛCDM. If other measurements support this conclusion, cosmologists will need to build new cosmological models that can explain early (Planck) and later (HSC-SSP) universe data.

To make these additional measurements, cosmologists need improved observing capabilities. Some of these capabilities come, for example, through the European Space Agency’s recently launched Euclid Telescope, and the soon-to-be operational Vera C. Rubin Observatory in Chile. The Nancy Grace Roman Space Telescope and the China Space Survey Telescope are also being developed. Each of these facilities will provide the potential to observe billions if not millions of galaxies in and around them 10 Many times more from the sky than observed by HSC-SSP. Future space telescopes will also have resolutions about… 7 Times better than HSC. By observing billions of galaxies with such unprecedented celestial coverage and precision, these upcoming surveys will dramatically expand our view of the universe. The timely release of the new HSC-SSP results and the knowledge developed by the collaboration in building robust lines of analysis will be valuable to the success of these upcoming surveys. An exciting era of cosmology is about to begin.


  1. R. Dalal et al.“Hyper Suprime-Cam Year 3 Results: Cosmology of Cosmic Shear Force Spectra,” Phys. Rev. Dr 108123519 (2023).
  2. X Lee et al.“Hyper Suprime-Cam Year 3 Results: Cosmology of Two-Point Correlation Functions for Cosmological Shear,” Phys. Rev. Dr 108123518 (2023).
  3. s. More et al.“Third year results from Hyper Suprime-Cam: SDSS-BOSS galaxy clustering, galaxy lensing, galaxies, and cosmic shear measurements,” fi. Rev. Dr 108123520 (2023).
  4. S. Sugiyama et al.“Hyper Suprime-Cam Year 3 Results: Cosmology of Galaxy Clustering and Weak Lensing with HSC and SDSS Using Minimum Bias Model,” Phys. Rev. Dr 108123521 (2023).
  5. H. Miyatake et al.“Third year results from Hyper Suprime-Cam: Cosmology of galaxy clustering and weak lensing using HSC and SDSS using a simulator-based halo model,” Phys. Rev. Dr 108123517 (2023).
  6. C. Hemans et al.“KiDS-1000 cosmology: multiprobe weak gravitational lensing and spectroscopic galaxy clustering constraints,” Astron. Astronomy. 646A140 (2021).
  7. TMC Abbott et al. (DES Collaboration), “Third Year Dark Energy Survey Results: Cosmological Constraints Due to Galaxy Clustering and Weak Lensing,” Phys. Rev. Dr 105023520 (2022).

About the author

Photo by Mejin Yoon

Meijin Yuen is a postdoctoral researcher at the Leiden Observatory at Leiden University in the Netherlands. Since receiving her PhD in cosmology from the University of Michigan in 2016, Yun has worked on various aspects of weak gravitational lensing surveys. She is currently studying the impact of calibration and astrophysical effects on cosmological inference.

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