What is meant by the word “expanding” when scientists say that the Universe is expanding? Is this about physical matter, gasses, or creation of new galaxies, or something else?
In: Physics
It literally means growing larger in three dimensions from the “inside-out.” Simply that existing space creates more space, literally from nothing (well, nothing except itself). Space itself contains a certain amount of energy (the vacuum energy) and that energy is repulsive – it forces space apart, and as it is pushed apart, more space spontaneously pops into existence. This new space also has vacuum energy, so even more space is created. Rinse and repeat and space expands and expands. Note that while space everywhere expands in this way, objects in space that are gravitationally bound will (at least for some time) remain close together as the gravitational force overcomes the repulsive force of the vacuum energy. Some physicists posit that eventually even local gravitational attraction will be overcome and all macroscopic objects will move apart. Repeat this on ever smaller scales, until all gravitational attraction is overcome, leaving only atoms and other subatomic particles. Even these will be torn apart eventually in what is known as the Big Rip (according to some) as the strong nuclear force is overwhelmed by expansion.
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