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A new study suggests that our universe may be expanding unevenly in different directions, challenging a key principle of cosmology known as the Cosmological Principle. This principle states that the universe has no center or preferred directions, especially when viewed on large enough scales. But a series of puzzling observations in recent years suggest that the cosmos may be more complex than previously thought. The study was led by astrophysicist James Adam of the University of the Western Cape in South Africa.
Assumptions about the expansion of the Universe
The Standard Model of Cosmology, which describes the expansion, structure, and evolution of the Universe, is currently based on two assumptions: that the Universe is homogeneous (the same everywhere on large scales) and isotropic (the same in all directions). However, recent cosmic measurements, including conflicting estimates of the rate of expansion of the Universe and anomalies in the cosmic microwave background, challenge the assumption of isotropicity. If confirmed, these results could change our understanding of the structure of the cosmos. Adam notes that hints of anisotropy are not yet universally accepted, as the methods for detecting them have certain limitations. More data from independent sources are needed to confirm these anomalies and rule out possible errors.
Signs of uneven expansion
Adam and his colleagues have proposed a new approach to testing the uniformity of the universe's expansion using weak gravitational lensing, an effect that occurs when massive objects bend the light of distant galaxies, causing slight distortions in their shapes.
“We investigated a new way to constrain anisotropy, based on so-called weak gravitational lensing,” Adam explained.
The weak lensing data can be divided into two main components: the E-type shift, which corresponds to an isotropic Universe, and the B-type shift, which should be minimal on large scales if there are no preferential directions. However, detecting the B-shift alone is not enough to confirm anisotropy, as these signals can be weak and sensitive to measurement errors. Instead, the researchers showed that if anisotropic expansion does exist, it would cause some correlation between the E- and B-shifts, which is impossible in an isotropic Universe. The team used computer simulations to predict how such anisotropy might appear in the Euclid data, and concluded that future observations could detect these signals if they do exist.
Data from the Euclid Space Telescope
The European Space Agency (ESA) launched the Euclid telescope in 2023 to map billions of galaxies in detail. Euclid’s instruments are designed to measure cosmic structures with extreme precision, helping to study dark energy, dark matter, and now possible anisotropy. Adam and his team hope that once Euclid has data, they will be able to apply their method to test the correlation between E- and B-shifts. If the results confirm the uneven expansion, it will seriously challenge the notion that there are no preferred directions in the universe.
Alternative models of the expansion of the Universe
If future data confirm that the expansion rate of the universe depends on direction, scientists will be forced to revise the Standard Model of cosmology. This model successfully explains many phenomena, including fluctuations in the cosmic microwave background and the formation of galaxies over billions of years. However, anomalies such as the Hubble strain (discrepancies in measurements of the expansion rate) or the nonuniformity of the cosmic microwave background may receive a new explanation if isotropy is not confirmed. A confirmed deviation from isotropy will force scientists to consider alternative theoretical models or modify existing concepts to account for the new complexity.
"Centerless" does not mean "symmetry"
The cosmological principle states not only that the universe has no center, but also that it should look the same in all directions on large scales. An analogy is often used with the surface of an inflatable balloon: as it expands, all points move away from each other evenly, with no defined center. If observations confirm real anisotropies, this analogy will need to be revised or expanded to explain the differences found.
A new chapter in the evolution of space
Adam emphasizes the importance of testing anomalies with a variety of methods. Even if the Euclid data show signs of anisotropy, it will be necessary to rule out possible systematic errors, sampling biases, or technical artifacts. If other studies, such as the Legacy Survey of Space and Time (LSST) at the Vera Rubin Observatory, confirm the same signals, it will be strong evidence that isotropy is not a fundamental principle.
The cosmological principle remains a cornerstone of modern cosmology, but if new data confirm anisotropy, it could prove unstable. Even a small deviation from isotropy would open a new chapter in understanding the evolution of the cosmos and the fundamental laws that govern the fate of our universe. The study is published in the journal Journal of Cosmology and Astroparticle Physics.
