What, Precisely, Is Expanding
The Universe is expanding. Does this also mean that you and all your molecules are also expanding? That everything around you is expanding but it is not noticeable because it is all expanding together at the same rate?
It's quite the fanciful image, but it is not true. The Universe and everything in it is not expanding. You are not expanding. What is expanding is the Universe at large, the space within which everything in the Universe resides. It might be more clear to explain to people that the spaces between objects in the universe are stretching, causing galaxies to lie farther apart than they did initially.
Objects in the Universe that are held together by gravity, such as humans, the Earth, the solar system, the galaxy, and galaxy clusters, will not expand.
The Raisin Bread Analogy
A common and helpful illustration is to imagine a lump of raising bread in dough form. If you put it on a tray and put it in the oven and watch as it bakes, you will see the bread itself expand. Think of this as "space." The raisins are the objects in space, the galaxies and galaxy clusters. The raisins do not grow in proportion to the loaf of bread. They simply accumulate more space between each other.
A History of Expansion
It was Edwin Hubble in the early 1900s who realized the Universe was expanding. He first discerned that the "nebula," or misty patches and swirls he saw through the 100-inch telescope on Mount Wilson, were not all gas and dust regions in our Milky Way but some of them were incredible distant. By measuring the distances to these "nebulae" using Cepheid variable stars, he was able to determine that these patches were other galaxies like our own, sprinkled all about space. This revelation was a first, figurative expansion of the Universe when we realized that the Cosmos was much more than we had initially imagined.
The next, literal, expansion of the Universe found by Hubble was when he compared the velocities, or speed, of these galaxies to each other. He found that the galaxies that were closest to ours were moving at slower speeds than galaxies that were father away. This discovery showed that the Universe is racing outward. We must assume that we do not have a special point of view in the Universe, and that what we see here is what beings in any other galaxy would see, therefore any galaxy would see that the Universe is spreading away from them, at increasing speeds the farther out you look.
Going back to the raisin bread analogy, one raisin would see nearby raisins moving away, and farther raisins moving even father away because of the larger amount of stretching dough between them.
It was these observations by Hubble that gave the Big Bang Theory, proposed by Alexander Friedman and Georges Lemaitre, a solid foundation. If the Universe is expanding, then theoretically reverse it in time to when it was one "point," from which it exploded and expanded outward, which is what we see today.
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