For a long time, scientists have believed that the universe looks the same no matter where you are or which direction you look.
This idea, called the Cosmological Principle, is a key assumption in modern cosmology.
It states that the universe has no special center and that it is both homogeneous (matter is evenly distributed on large scales) and isotropic (it has no preferred direction).
These ideas form the foundation of the Standard Model of Cosmology, which explains how the universe began and evolved.
However, recent observations suggest there may be unexpected patterns or variations at extremely large scales—something that challenges the assumption of isotropy. If these variations, known as anisotropies, are real, it could mean that our current understanding of the universe is incomplete.
To investigate this possibility, scientists have developed a new method using data from the Euclid Space Telescope, a European Space Agency (ESA) mission launched in 2023. Euclid is designed to map the universe with incredible precision, capturing light from distant galaxies to help scientists study dark matter and dark energy.
A team led by astrophysicist James Adam from the University of the Western Cape in South Africa has proposed a way to use a phenomenon called weak gravitational lensing to test whether the universe is truly isotropic.
Gravitational lensing happens when the light from distant galaxies is slightly bent by massive objects, such as galaxies or dark matter, as it travels toward Earth. This bending causes the images of those galaxies to appear distorted. By studying these distortions, scientists can learn about the distribution of matter in the universe.
When analyzing weak lensing data, scientists divide the lensing effect into two types of signals:
- E-mode shear – This is expected in an isotropic universe and results from the normal distribution of matter.
- B-mode shear – This is much weaker and should not appear on large scales in an isotropic universe.
If scientists detect unexpected B-modes or find that E-modes and B-modes are correlated in a way that shouldn’t happen in an isotropic universe, it could be evidence of anisotropy. In other words, the universe may not be expanding equally in all directions.
However, small errors or secondary effects could also create B-modes, so researchers must be extremely careful when analyzing the data. If Euclid’s observations show a significant and consistent correlation between E- and B-modes, it would be a strong sign that the universe is not as uniform as we thought.
To prepare for real data, Adam and his team created computer simulations to predict how an anisotropic universe would affect weak lensing signals. Their models suggest that Euclid’s data should be precise enough to detect any large-scale anisotropies if they exist.
Now that the methodology is in place, the next step is to apply it to actual observations from Euclid and other upcoming space telescopes. If the results confirm the presence of anisotropies, it could mean that one of the most fundamental assumptions about the universe is wrong.
James Adam explains the importance of double- and triple-checking these findings: “Once you’ve 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.”
If anisotropies are confirmed, scientists would need to rethink some parts of the Standard Model of Cosmology. Some alternative models predict a universe with directional variations, but none are as widely accepted as the current model. The extent of the anisotropy—whether it’s a minor adjustment or a major change—remains unknown.
“It could be a serious revision,” says Adam, “or just adding a little term here or there. Who knows?”
Whatever the outcome, this research could help scientists better understand the true nature of the universe and its evolution.
The research findings can be found in arXiv.
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