"Atoms, molecules, and human beings aren't expanding; our sizes remain static. Similarly, the Earth's, Sun's, and Solar System's distances don't expand. Even in the Milky Way's outskirts, stellar orbits remain stable; they don't expand. Other Local Group galaxies don't recede; they're gravitationally bound. What determines the dividing line between expanding versus non-expanding? It's whether any local region of space became "bound" before dark energy began dominating the overall expansion. If something becomes bound through: the electromagnetic force, or even the gravitational force, then locally, the spacetime surrounding it won't expand."
"For nearly 100 years, we've known our Universe is expanding. Einstein's equations forbid static, stable, uniform solutions. Observations of galactic distances and redshifts validate an expanding Universe. The evidence supporting our cosmic expansion has been overwhelming since the 1920s. The Universe evolved for 7.8 billion years before dark energy's effects became dominant. Over that time, galaxies, galaxy groups, and galaxy clusters all formed bound structures. Filaments and superclusters created structures up to 1.4 billion light-years across. Beyond that, all apparent structures are merely phantasmal."
Einstein's equations exclude static, stable, uniform cosmological solutions, and observations of galactic distances and redshifts confirm cosmic expansion since the 1920s. Small-scale systems such as atoms, molecules, humans, planets, the Solar System, stellar orbits, and Local Group galaxies remain non-expanding when they became bound by electromagnetic or gravitational forces. The transition to dark-energy-dominated expansion occurred after about 7.8 billion years, allowing bound structures—galaxies, groups, clusters, filaments, and superclusters—up to roughly 1.4 billion light-years to form. Structures larger than that are not truly bound and are being driven apart by dark energy, leaving isolated bound "islands" in an expanding cosmos.
Read at Big Think
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