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What is the difference between top-down and bottom-up models of galaxy formation?
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Cosmology
Learn about defining and measuring the observable universe within the “whole” universe.
During the latter half of the 20th century, astronomers debated two primary models regarding how galaxies formed in the early universe:
- Top-down model: This theory proposed that galaxies originated from the gravitational collapse of massive, large-scale gas clouds.
- Bottom-up model: This theory suggested that galaxies were built through the hierarchical merger and amalgamation of smaller entities, roughly the size of globular clusters.
Modern understanding has largely shifted toward the bottom-up perspective. Observations from deep-space surveys—such as those conducted by the Hubble Space Telescope—reveal that the early universe contained many small, irregular fragments that acted as building blocks for larger structures. Furthermore, evidence of ongoing collisions and the capture of smaller galaxies by larger ones, such as those observed in the Milky Way Galaxy and the Andromeda Galaxy, supports the view that galaxies grow and evolve through continuous accretion and merging processes.1