Pune: Two astronomers from Pune are part of a research team that has developed a method to correct errors in measuring galaxy distances, a crucial step toward improving our understanding of the Universe’s structure, dark matter, and cosmic evolution.
Prof. Surhud More from the Inter-University Centre for Astronomy and Astrophysics, Pune and Divya Rana, a former student of the centre currently at Leiden University, along with their international collaborators proposed a novel geometrical approach called the ‘shear ratio method’. Their findings have been published in the international journal, Physical Review D.
A central concept in measuring galaxy distances is ‘redshift’: the measure of how much light from distant galaxies has been stretched due to the Universe’s expansion. However, measuring precise redshifts for every galaxy is time-intensive, so astronomers often estimate redshifts based on galaxy colours. While efficient, this method introduces small but systematic errors that can distort measurements of key cosmological parameters.
The ‘shear ratio method’ addresses this challenge because unlike traditional approaches that analyse a single set of background galaxies, it compares how the same set of foreground galaxies distorts light from multiple background galaxies at different redshifts. By taking ratios of these distortions, complex astrophysical factors such as the influence of gas, stars, and galaxy formation effectively cancel out.
This leaves a clean measurement of the Universe’s geometry, acting as a ‘cosmic ruler’ to accurately identify and correct biases in redshift estimates.
{{/usCountry}}This leaves a clean measurement of the Universe’s geometry, acting as a ‘cosmic ruler’ to accurately identify and correct biases in redshift estimates.
{{/usCountry}}Prof. More explained, “Redshift uncertainties are emerging as one of the most significant sources of systematic error in modern cosmology. Our study shows that the shear ratio method provides a powerful and independent way to validate and correct these measurements, ensuring more reliable cosmological conclusions.”
The researchers applied this method to data from the Subaru Hyper Suprime-Cam Survey, one of the most extensive galaxy surveys aimed at mapping matter distribution across the sky.
Using this dataset, they successfully corrected redshift biases and derived improved estimates of key cosmological parameters, including the Universe’s overall matter density and the amplitude of inhomogeneities (variations in matter distribution).
Rana added that their method is especially valuable for large-scale surveys. “By isolating the geometric signal, we can directly test and calibrate redshift estimates without relying heavily on complex astrophysical modeling,” she said.
The study comes at a critical time, as several next-generation observatories like the Vera C. Rubin Observatory, Nancy Grace Roman Space Telescope, and the Euclid Mission are preparing to map hundreds of millions of galaxies. But their success depends on precise and unbiased redshift measurements.
“Future surveys will rely on extremely accurate data to answer fundamental questions about the Universe,” Prof. More said. “Techniques like shear ratios will be indispensable in ensuring that the redshift measurements they depend on are both precise and trustworthy.”