Buzz
What does the universe expand into if it's constantly growing?Richard Muller:
The universe doesn't need to be expanding into a specific space for it to grow. That might seem strange, but let's consider a simpler example to clarify.
Imagine a line that stretches infinitely. Along that line, you place a mark every inch. There are an infinite number of inches. Now, spread each marker apart by two inches. The pattern has grown, but it still reaches infinity, only now the markers are farther apart. The expansion occurs, but the total length remains infinite.
Here’s a new example: Imagine you have an endless piece of rubber stretching to infinity. (This rubber symbolizes the universe.) There are marks every inch along the rubber. Now, stretch it until the markers are spaced two inches apart. It still extends to infinity—but it has expanded.
Physicists envision "space" not as an empty void, but more like a piece of rubber. (Though they don’t call it rubber; they refer to it as the "vacuum.") In physics, "particles" are essentially vibrations in the vacuum. Just like the rubber, the vacuum can stretch. However, because it extends infinitely, there’s no need for more space. A clever way to put it is, "there’s plenty of room at infinity." (Though that sounds smart, it doesn’t really explain everything.)
Now here’s something that might either confuse or clarify things. In standard physics theory, galaxies are receding from each other; this is seen as the universe expanding. However, as described in General Relativity Theory, none of the galaxies are physically moving. What’s actually happening is the space (vacuum) between them is increasing.
No, you won’t learn this in school or even in college (unless you’re lucky enough to have an extraordinary professor). It’s generally taught in graduate school when pursuing a Ph.D. At that level, the terminology changes: "In the Big Bang Theory, all galaxies have fixed coordinates (meaning they aren’t moving). The 'expansion' is explained by the 'metric tensor,' which defines the distances between these fixed coordinates. In the Big Bang Theory, it’s the metric tensor that changes, which represents the expansion of the universe, even though the galaxies themselves aren’t shifting. The recent discovery of accelerated expansion indicates the rate at which the universe is expanding is increasing."
Perhaps you’ve heard about space curvature. Place a black hole between two stationary objects, and the distance between them will increase—without either of them moving. Thus, "distance" is more complex than once believed. It was Einstein who proposed the brilliant idea that "space" (the vacuum) is flexible; it can bend and stretch.
I anticipate that this may seem quite perplexing to you. That’s actually a positive sign; it means you’re on the right track. When you encounter new ideas that challenge your previous understanding, "confusion" is simply the first step towards learning.
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