If the Universe Is Expanding, What Does It Expand Into?

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If the universe is expanding, then what does it expand into?Richard Muller:

The universe doesn't have to be expanding into anything in order to expand. I know that sounds ridiculous, so let me give you a different example that is easier to understand. 

Imagine that you have a line that goes on forever. On that line, you have a mark every inch. There are an infinite number of inches. Now move each marker so that they are separated by two inches. The whole pattern has expanded. It still goes to infinity, but the markers are further apart. The pattern has expanded, but the length is still infinite. 

Now a new example: Suppose you have a long piece of rubber, going all the way to infinity. (That piece of rubber represents the universe.) The rubber has marks on it every inch. Now stretch the rubber, until the markers are two inches apart. It still goes to infinity—but it has expanded.

Physicists think of "space" not as emptiness, but similar to a piece of rubber. (But they don't call it rubber; they call it the "vacuum." "Particles," in physics, are just vibrations of the vacuum.) The vacuum can expand, just like the piece of rubber. But because it goes all the way to infinity, it doesn't need more space. A clever way to say it is that "there's lots of room at infinity". (That's clever, but it doesn't really explain anything.)

Now here is something new that might confuse you, or might help. In the standard physics theory, the galaxies are all getting farther apart; that is the expansion of the universe. Yet in the way the theory describes it (I mean in General Relativity Theory), none of the galaxies are actually moving. All that is happening is that the amount of space (vacuum) in between them is increasing.

No, you will not learn this in school, or even in college (unless you have an extraordinary professor). It is usually taught in graduate school, when you are earning a Ph.D. degree. At that point the language you will encounter is this: "In the Big Bang Theory, all galaxies have fixed coordinates. (That means they are not moving.) The 'expansion' is described by the 'metric tensor,' which describes the distances between those fixed coordinates. In the Big Bang Theory, it is the metric tensor which is changing; that represents the expansion of the universe, even though the galaxies aren't moving. The recent discovery of accelerated expansion means that the rate of expansion is increasing."

Maybe you've read about the curvature of space. Put a black hole between two unmoving objects, and the distance between them will suddenly increase—even though they haven't moved. So "distance" is not as simple as people thought. It was Einstein who came up with the remarkable idea that "space" (that is, vacuum) is flexible; it can curve and stretch.

I expect you will find this to be very confusing. That's not a bad sign; it is a good one. When you learn new things that are completely different than you ever imagined, then "confusion" is the first step.

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Why Did Noon Used to Mean 3 p.m.?

3 p.m. is basically noon for people who wake up at 12 p.m.
3 p.m. is basically noon for people who wake up at 12 p.m.
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If you’re a late sleeper, you might find yourself thinking 12 p.m. seems way too early to be considered midday, and the word noon would much better describe, say, 3 p.m. It turns out that ancient Romans would have agreed with you, if only for etymological reasons.

As Reader’s Digest explains, the days in ancient Rome were split into four periods of three hours each. The first hour was at sunrise around 6 a.m.—called prime, for first—followed by 9 a.m. (terce, denoting the third hour), 12 p.m. (sext, for sixth), and 3 p.m. (none, for ninth).

According to Merriam-Webster, Middle and Old English borrowed the time-keeping tradition, along with the Latin word for ninth, which was changed to nōn and eventually noon. Though we’re not sure exactly when or why noon started referring to 12 p.m. instead of 3 p.m., it could have something to do with Christian prayer traditions. In the Bible, Jesus’s crucifixion is said to have taken place at the ninth hour, and that’s when worshippers partook in their second of three daily prayers; the others were in the morning and evening. It’s possible that hungry monks were behind noon’s gradual shift from 3 p.m. to 12 p.m.—since their daily fast didn’t end until after the midday prayer, they had a built-in motive for moving it earlier.

While we didn’t exactly stay true to the original Latin meaning of noon, there’s another important remnant of ancient Rome hiding in the way we tell time today. Romans referred to 12 p.m. as meridiem, for midday, and so do we. A.M. is an abbreviation for ante meridiem, or before midday, and P.M. means post meridiem, or after midday.

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