Does the universe behave the same way everywhere?
by Erica Marchand
Rome, Italy (SPX) Feb 18, 2025

"The cosmological principle is like an ultimate kind of statement of humility," says James Adam, an astrophysicist at the University of the Western Cape in Cape Town, South Africa, and lead author of a new study. The Cosmological Principle posits that no specific location in the Universe is unique or central, and that on the largest scales, the cosmos exhibits uniformity in all directions. This assumption forms the foundation of the Standard Model of Cosmology, which describes the Universe's evolution and structure. While widely accepted and supported by extensive observations, the model is not without its challenges.

Some recent findings indicate that the Universe may exhibit anisotropies-variations in structure that contradict the assumed isotropy. These deviations have been observed through different means, such as inconsistencies in measurements of the Universe's expansion rate, analyses of the cosmic microwave background radiation, and other cosmological data discrepancies. However, these results remain inconclusive, and verifying them requires additional independent data. If multiple observational techniques detect the same anomalies, they would become difficult to dismiss as mere measurement errors.

In a new study published in JCAP, Adam and his team developed an innovative approach to test isotropy using gravitational lensing observations, particularly from ESA's Euclid space telescope. Launched in 2023, Euclid has recently begun capturing high-resolution images of the cosmos, offering an unprecedented level of detail.

"We explored an alternative way to constrain anisotropy by analyzing weak gravitational lensing," Adam explains. Weak lensing occurs when matter between a distant galaxy and an observer subtly bends the galaxy's light, distorting its apparent shape. This effect provides clues about the Universe's structure. In an isotropic Universe, weak lensing data should predominantly show E-mode shear, which is associated with matter distribution. Conversely, B-mode shear, which is generally weak, should not manifest on large scales if isotropy holds.

Detecting B-modes alone would not confirm anisotropies, as these signals could result from measurement inaccuracies or secondary effects. However, if an anisotropy truly exists, it would influence both E-modes and B-modes in a correlated manner. The key test is whether Euclid's data reveal a significant correlation between these signals-such a finding would suggest the Universe expands anisotropically.

Next Steps and Potential Impact

Adam and his colleagues simulated how an anisotropic Universe would alter weak lensing signals and formulated a model predicting these changes. By computing the E-B cross-correlation, they demonstrated that anisotropic expansion would induce a measurable correlation between the two signals. Their model suggests that Euclid's data will be precise enough to detect such effects if they exist.

With Euclid already delivering early observations, Adam's team plans to apply their methodology to real data. "Once you've kind of quadruple-checked your work, then you have to seriously consider whether this fundamental assumption is actually true or not, particularly in the late Universe. Or perhaps it just was never true," Adam reflects.

If verified, these anomalies could reshape cosmology. However, any theoretical revisions would depend on the magnitude of the detected anisotropies. Some alternative cosmological models predict such variations, but none match the predictive power and observational support of the Standard Model. The extent of any necessary modifications remains uncertain. "It could be a serious revision," Adam concludes, "or just adding a little term here or there. Who knows?"

Research Report:Probing the Cosmological Principle with weak lensing shear

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