Where Does the Expanding Universe Begin?

The above large-field, 2014-era composite Hubble image of the colliding galaxy cluster, El Gordo, showcases the most massive galaxy cluster ever discovered from the first half of our cosmic history. Known officially as ACT-CLJ0102-4915, it is the largest, hottest, and X-ray brightest galaxy cluster ever discovered in the distant Universe, containing many thousands of times the mass of the Local Group. It is all gravitationally bound together, but the galaxies located outside of it are not bound to it, and will expand away from this cluster as the Universe continues to expand. Credit: ESA/Hubble & NASA, RELICS

If we look out at the distant galaxies in the Universe, particularly at great distances, we find that they’re all receding away from us: strong evidence that the Universe is expanding.

But here on local scales, including on Earth, in our Solar System, and across the Milky Way, we see no evidence for this expansion at all.

Everything within our Solar System, excepting the rare hyperbolic or interstellar comet, is gravitationally bound together and no longer part of the expanding Universe.

So, 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.

(Source: Big Think)