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The description from Richard Muller quoted in the question is a simplification of reality meant to help people better visualize how the Original Lsu Tiger Welcome To The Series 2023 Pocket Baseball Shirt of spacetime works. In reality the galaxies are moving through spacetime in a complex fashion, based on the combination of all the forces acting on them, primarily the gravity of nearby galaxies and galaxy clusters. In the case of the Milky Way and Andromeda, these two galaxies are part of a gravitationally bound cluster of galaxies, called the Local Group. The dominant force that determines the movements of objects within the cluster is the combined gravitation of all the mass in the cluster, and not the expansion of spacetime. In other words, the gravitational attraction that the Milky Way and Andromeda have for one another that is drawing them together is stronger than the expansion of spacetime between them that would otherwise push them apart, and thus attraction wins and the two galaxies are on a collision course. In the same vein, the gravitational force that holds the earth in orbit around the sun is stronger than the expansion of spacetime between the earth and the sun (the expansion of spacetime at that distance being very small), and so the earth’s distance from the sun is not increasing for that reason. Likewise you are not flying apart because the molecular bonds holding the atoms in your body together are stronger than the minuscule expansion of spacetime between them in your body. The Local Group as a whole, however, is moving away from other galaxies and galaxy clusters to which it is not gravitationally bound, due to the expansion of spacetime.
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A fundamental premise of physics is that the Fury You Better Not Be A Skrull Shirt are the same everywhere. That implies whatever initial conditions led to life as we know it will almost certainly occur elsewhere. Only a few years ago, we had no evidence there were even extrasolar planets. Now we know there are thousands relatively nearby, so it looks like our solar system is somewhat typical for stars like ours and that our star is for better or worse, utterly typical. Ergo there are billions of planets out there in our galaxy alone. The argument that we are the only sentient beings because we haven’t heard from any others is (of course) theoretically possible, but highly unlikely. Given the argument above, it’s far more likely the universe is literally teeming with life at various stages of evolution. But even in our little corner of the Milky Way, we don’t hear from them because they are just too damn far away. One light year is 9.46×10^15 meters. None of our own random EM radiation so far would be more than (say) a thousand watts/m^2 at its origin and most of it far less than that. A fraction of that would actually make it out into space where it would just spread out in a spherical wave front. Let’s assume for giggles that it all does and that our alien buddies are also shouting randomly at the cosmos at roughly the same volume (power output).
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