"Here in our Universe, as soon as you open your eyes to the vastness of the cosmos beyond our own world, you see just how full of structure - and particularly, light-emitting and light-absorbing structures - it is. But looks can often be deceiving, as points of light that are clustered closely together on the sky aren't necessarily part of the same system, the same structure, or even close together in three-dimensional space."
"It seems to me (for example) that the space between stars on the opposite sides of the Milky Way is expanding, just like any other space, and that the "tug of war" between gravity and the expansion of space would decide if those two stars remained bound together forever, or if they would get separated... If you allow the cosmological constant to continually increase, accelerating the expansion of space,"
Light from distant objects can appear clustered on the sky while being widely separated in three-dimensional space. Determining whether nearby-seeming objects are gravitationally bound requires comparing their mutual gravitational attraction to the effective expansion between them. Local gravitational binding prevents cosmic expansion from increasing separations within galaxies, solar systems, and many galaxy clusters. The cosmological constant drives large-scale accelerated expansion but does not unbind systems whose internal gravitational potential dominates. Only if expansion or external perturbations inject enough energy to overcome binding energy will a bound system become unbound. Binding depends on dynamics, mass distribution, and the scale of competing expansion.
Read at Big Think
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