The assumption that the shape of the universe should be anything other than flat is based on several lines of evidence and theoretical considerations. While we cannot make definitive conclusions about the shape of the universe with absolute certainty, the observations and theoretical models suggest that the universe may have a non-flat geometry. Here are a few key reasons:
Observational Evidence: Studies of the cosmic microwave background (CMB) radiation, which is the residual radiation from the early universe, provide important clues about the geometry of the universe. The patterns and fluctuations observed in the CMB can be used to infer the overall curvature of space. Various experiments, including the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, have provided strong evidence for a universe with slight positive curvature, indicating a closed or "spherical" geometry.
Large-Scale Structure: The distribution of galaxies and galaxy clusters in the universe can also give us insights into its overall geometry. Observations of large-scale structures suggest that the distribution is consistent with a universe that has a non-flat geometry, where regions with higher matter density lead to gravitational attraction, causing clustering and the formation of galaxy filaments and voids.
General Relativity: Einstein's theory of general relativity, which describes gravity as the curvature of spacetime, allows for the possibility of a curved or non-flat universe. The theory suggests that the shape of the universe is determined by the amount of matter and energy it contains. Depending on the total energy density, the universe can have positive curvature (closed), negative curvature (open), or zero curvature (flat).
Inflationary Cosmology: The concept of cosmic inflation, which proposes a rapid expansion of the universe in its early stages, provides a theoretical framework that explains many observed properties of the universe. Inflationary models predict that the universe is more likely to have a non-flat geometry, with slight curvature arising from quantum fluctuations during the inflationary period.
It's important to note that the evidence supporting a non-flat universe is not conclusive, and further observations and research are needed to refine our understanding. The shape and geometry of the universe remain active areas of study in cosmology, and ongoing observations and experiments aim to provide more precise measurements and constraints on its overall structure.