One of the basic principles of cosmology is the Cosmological Principle. It states that, no matter where you go in the Universe, it will always be broadly the same. Given that we have only explored our own Solar System there is currently no empirical way to measure this. A new study proposes that we can test the Cosmological Principle using weak gravitational lensing. The team suggests that measuring tiny distortions in light as it passes through the lenses, it may just be possible to find out if there are differences in density far away.
The Cosmological Principle is a fundamental assumption stating that the universe is homogeneous on a large scale. In other words regardless of location or direction, the universe appears uniform and it underpins many cosmological models, including the Big Bang theory. Taking the assumption that physical laws apply consistently everywhere makes calculations and predictions about the universe’s structure and evolution far simpler, but research has been testing its validity by searching for potential anomalies.
A paper has been published by a team of astrophysicists, led by James Adam from the University of Western Cape in South Africa and explains that the Standard Model of Cosmology predicts the Universe has no centre and has no preferred directions (isotropy.) The paper, which was published in the Journal of Cosmology and Astroparticle Physics, articulates a new way to test the isotropy of the Universe using the Euclid space telescope.
The Euclid telescope is a European Space Agency mission to explore dark matter and dark energy. It was launched in 2023 and maps the positions and movements of billions of galaxies. It’s using this instrument that the team hope to search for variations in the structure of the Universe that might challenge the Cosmological Principle.
Previous studies have found such anomalies before but there are conflicting measurements of the expansion rate of the Universe, in the microwave background radiation and in various cosmological data. Further independent observations are required though, providing more data to see if the observations were the result of measurement errors.
The team explore using weak gravitational lenses, which occur when matter sits between us and a distant galaxy, slightly bending the galaxies light. Analysis of this distortion can be separated into two components; E-mode shear (caused by the distribution of matter in an isotropic and homogenous Universe) and B-mode shear which is weak and would not appear in an isotropic Universe at large scale.
If the team can detect large scale B-modes this in itself wouldn’t be enough to confirm the anisotropies since the measurements are tiny and prone to measurement errors. To confirm, and finally test the Cosmological Principles, E-mode shear needs to be detected as well. Such discovery and correlation of E-mode and B-mode shear would suggest the expansion of the Universe is anisotropic.
Ahead of the Euclid observations, the team simulated the effects of an anisotropic universe expansion on a computer. They were able to use the model to describe the effect of the weak gravitational force and predict that Euclid data would be sufficient to complete the study.
Source : Does the universe behave the same way everywhere? Gravitational lenses could help us find out
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