Cosmic expansion manifests as a change in the large-scale geometry of space described by the Friedmann-Lemaître-Robertson-Walker metric, but its effect on systems bound by strong forces is effectively negligible. The expansion rate is encoded in the Hubble parameter, which sets how comoving distances change on scales where matter is not gravitationally or electromagnetically bound. In General Relativity this is a statement about the global metric rather than a direct force that pulls bound objects apart.
Relativistic framework
In relativistic terms, freely falling test particles follow geodesics of the global metric; bound systems do not follow those geodesics because internal forces constrain relative motion. The competition can be framed in a Newtonian approximation where an additional cosmological acceleration term scales roughly like H^2 times separation and acts as a tidal perturbation. If internal binding—gravitational for galaxies and star systems, electromagnetic for atoms—produces accelerations much larger than that tidal term, the system remains effectively immune to expansion. This distinction is emphasized in standard texts by Steven Weinberg University of Texas at Austin and in accessible reviews by Sean M. Carroll California Institute of Technology, which explain that cosmic expansion is meaningful for unbound, large-scale flows but not for tightly bound structures.
Observational constraints and consequences
Observations support the theoretical separation of scales. Precision measurements such as lunar laser ranging have not detected any secular outward drift of the Moon attributable to Hubble expansion, consistent with analyses by James G. Williams Jet Propulsion Laboratory and colleagues showing local orbital dynamics are dominated by near-field forces. On galactic scales, internal dynamics and dark matter halos set the structure; the expansion of the Universe shapes the background within which galaxies form and interact, but it does not stretch individual galaxies apart.
The relevance of this separation has human and cultural resonance: everyday and territorial stability—from atomic chemistry to planetary orbits—depends on local binding physics, not cosmic stretching. Environmentally, cosmic expansion influences climate only indirectly through cosmic history (for example, by setting conditions for star and planet formation), not via ongoing mechanical effects on ecosystems. Consequences for cosmology are significant: expansion determines large-scale structure growth and the future fate of unbound matter, while the integrity of bound systems provides the local scaffolding for observers and civilizations.