Here is the second paragraph from the article on metric expansion of space from Wikipedia Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the FLRW metric, and is a generic property of the Universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence ofgravitational attraction and such things do not expand at the metric expansion rate as the Universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the Universe given the matter density and average expansion rate. 1.Why is the model only valid on large scales? This is worded in a way to suggest that gravity somehow influences expansion. My understanding is that the 'coordinate system' expands and gravity is irrelevant? So if two particles stay bound or are separated is only dependent on whether the force between them overcomes this expansion or not. Is it not the case that the expansion continues equally everywhere (so even between two atoms in my arm), but gravitationally bound objects are in this perpetual 'tug of war' with expansion?