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| Researchers carry out a geometric test for the Universe | ||
Astronomers use the faint light from distant galaxies to understand the history and makeup of our universe. As light from these “background” galaxies travels toward Earth, it passes by closer “foreground” galaxies, and the gravity of the closer galaxies bends and distorts the light, a phenomenon called weak gravitational lensing. Gravitational lensing is used to make cosmic maps of matter, most of which is dark matter, and study how structure in the Universe grows. For this, scientists need to know exactly how much the light from background galaxies has stretched as it traveled across the expanding universe, a concept known as redshift. The uncertainty in redshift of the galaxy can result in a biased understanding of the cosmological parameters that describe the Universe.
Because it takes too much time to use precise instruments to measure the exact redshift of every single galaxy, astronomers estimate redshifts by looking at the galaxies' colors. However, this estimation method isn't perfect and can introduce small, systematic errors or “biases”. If the redshifts are wrong, the final map of the universe will be inaccurate, as the redshift errors directly impact the inferred geometry and distances of the Universe.
To fix this problem, Divya Rana, a postdoctoral student at Leiden University and former student at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Surhud More, Professor at IUCAA and their international collaborators used a clever technique called shear ratios in a recently published article in Physical Review D. Here is how it works:
They applied this technique to data from the Subaru Hyper Suprime-Cam (HSC) survey, a massive project mapping the matter distribution in the sky. They successfully corrected the redshift estimation biases and inferred cosmological parameters related to the matter density and amplitude of inhomogeneities in the Universe.
This study proves that the shear ratio method is a powerful, independent way to cross-check redshift measurements. The redshift measurement uncertainties are expected to be a leading source of systematic uncertainties in future cosmological surveys. In the near future, next-generation observatories like the Vera C. Rubin Observatory (LSST), the Roman Space Telescope, and the Euclid mission will map hundreds of millions of galaxies to understand dark energy and the evolution of the universe. This technique will be an essential tool to ensure the redshift data in those massive future surveys is precise and reliable.
Because it takes too much time to use precise instruments to measure the exact redshift of every single galaxy, astronomers estimate redshifts by looking at the galaxies' colors. However, this estimation method isn't perfect and can introduce small, systematic errors or “biases”. If the redshifts are wrong, the final map of the universe will be inaccurate, as the redshift errors directly impact the inferred geometry and distances of the Universe.
To fix this problem, Divya Rana, a postdoctoral student at Leiden University and former student at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Surhud More, Professor at IUCAA and their international collaborators used a clever technique called shear ratios in a recently published article in Physical Review D. Here is how it works:
- Instead of just looking at the lensing effect on one set of background galaxies, they looked at how the same set of foreground galaxies bent the light of background galaxies located at different redshifts.
- By comparing (taking the ratio of) these different bending effects, a lot of the messy, complicated physics, like the behavior of gas, stars, and complex galaxy formation, naturally cancels out.
- What is left over is a clean, pure measurement of geometry of the Universe. This geometry acts like a cosmic ruler, allowing the researchers to calculate exactly how far off their original redshift estimates were.
They applied this technique to data from the Subaru Hyper Suprime-Cam (HSC) survey, a massive project mapping the matter distribution in the sky. They successfully corrected the redshift estimation biases and inferred cosmological parameters related to the matter density and amplitude of inhomogeneities in the Universe.
This study proves that the shear ratio method is a powerful, independent way to cross-check redshift measurements. The redshift measurement uncertainties are expected to be a leading source of systematic uncertainties in future cosmological surveys. In the near future, next-generation observatories like the Vera C. Rubin Observatory (LSST), the Roman Space Telescope, and the Euclid mission will map hundreds of millions of galaxies to understand dark energy and the evolution of the universe. This technique will be an essential tool to ensure the redshift data in those massive future surveys is precise and reliable.
Publication Details
“Hyper Suprime-Cam Y3 results: Photo-𝑧 bias calibration with lensing shear ratios and cosmological constraints from cosmic shear”
Rana, et al, 2026.
Published in Physics Review D. [ DOI: 10.1103/mfj8-vt3w ]
Published in Physics Review D. [ DOI: 10.1103/mfj8-vt3w ]
Research contacts:
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Dr. Divya Rana Leiden University E-mail: rana_at_strw.leidenuniv.nl |
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Prof. Surhud More IUCAA, Pune E-mail: surhud_at_iucaa.in |