Does the Universe Have Physical Boundaries?

08/30/2017 02:56 pm ET
Hemera Getty Images Plus/Getty Images
Hemera Getty Images Plus/Getty Images

Why couldn't the boundary of the observable universe be where antimatter and matter collide, creating "more" universe? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Viktor T. Toth, IT pro, part-time physicist, on Quora:

The universe (observable or otherwise) has no boundary in the physical sense.

This is one of the most commonly seen misunderstandings about physical cosmology. The universe is not a bubble expanding into a preexisting volume. The universe exists everywhere and and is approximately the same everywhere. The expansion means not that more universe is being created, but that the distance between things and other things, on average, increases. (I said “on average” because there are exceptions, things that stopped receding from each other: our Milky Way galaxy is not expanding, our solar system is not expanding, the distance between your head and your toe is not increasing either, at least if you’re an adult who stopped growing.)

Now on to this business with the observable universe and its effective boundary. The boundary of the observable universe is defined by the age of the universe: You cannot observe parts of the universe that are too far away for light to have reached you given the finite age of the universe.

Note that I said “you”. That is because your observable universe is not the same as my observable universe. The difference is very small, because we are not that far apart, living on the same planet and all. But your observable universe (not to mention its boundary) is definitely very different from the observable universe of someone living on a distant galaxy.

In particular, there are very distant places in the universe where there may be creatures for whom the boundary of the observable universe is, well, us. Or rather, the lumps of matter in a very early stage of existence (a primordial plasma, not even yet a quark-gluon plasma) that would eventually become us nearly 14 billion years later.

Does this make you feel any special? Do you feel like some “matter and antimatter colliding, creating ‘more’ universe”? I’m guessing not. Again, that’s because this boundary business is subjective. It is not a physical property of the universe; the only relevant physical property is its age, which defines, for each of us, the farthest things that we can see in principle in an otherwise infinite universe. (In practice, we see a lot less, because the early universe was hot, dense and opaque. So the earliest light that we see was emitted not when the universe was 0 seconds old but when it was roughly 385,000 years old, just becoming transparent. This “light” is the cosmic microwave background radiation.)

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