Comments by "EebstertheGreat" (@EebstertheGreat) on "How big is the universe ... compared with a grain of sand?" video.

  1. The edge of the observable universe is NOT 13 billion light years away. It is closer to 47 billion light years away. This is because the universe has been expanding while the light was travelling.
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  2. TheWireFan2002 No, of course not. We're in the center of our observable universe, because that's the portion of the universe we can observe.
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  3. TheWireFan2002 That's the diameter of the universe, not the radius. The distance to the edge of the observable universe is the radius, which is half the diameter.
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  4. TheWireFan2002 We are in the center of the observable universe, or at least very close to it (technically were are not in the exact geometrical center because the universe is not exactly flat locally (it is warped by gravity), but we are extremely close). The observable universe is merely the part of the universe we can observe because light has had time to reach us. So its edge is the same distance in all directions. That's literally the second sentence in the Wikipedia article, and it is also fairly obvious.
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  5. Pat McCann Space is expanding at the Hubble constant, which is in the ballpark of 2.3 aHz = 2.3 * 10^-18 Hz = 70. (km/s)/Mpc, where aHz is attohertz, Hz is hertz, and Mpc is megaparsec (approx. 3.1 * 10^19 km). What this means is that every second, distance (any length of space anywhere of any size) increases by a factor of roughly 1 + (2.3 * 10^-18). Of course, objects in that space get pulled along with it, unless they are subject to forces like gravity or chemical bonds. So yes, if two points in space are far enough away, that factor of 10^-18 will be large enough so that the total expansion of space (or if you prefer, the "amount of new space created") between those points each second will be significant. For instance, objects ~4.4 * 10^18 km away would become another km further away from each other each second, not because they are moving but because space between them is expanding. From our point of view, this makes it appear that objects that far away are receding from us at a speed of 1 km/sec. Objects even further away, for instance 1 megaparsec, have an even higher apparent speed (70 km/sec). And objects far enough away will appear to recede faster than light. But they aren't really moving, it's just that the space between us is so vast, that even that minute expansion factor results in the distance between us increasing very fast. Indeed, even if it emits light toward us, so that the light really is moving at us at the speed of light, the distance between us and the leading edge of the light will not be decreasing, but in fact still be increasing because space is expanding so quickly. That is what we mean when we say that space is "expanding faster than light." Note that the Hubble constant is actually extremely small. However, for any nonzero constant, however small, we can find a distance far enough such that its apparent speed is greater than the speed of light.
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  6. ***** There is no definition of "THE observable universe" that is so specific. It could just as easily be the center of the Sun or the center of the Solar System or the galactic center. It doesn't really make any difference when you're talking about such large scales. The actual definition of the observable universe is the part of the universe that is within your past light cone, and obviously that depends on where "you" are.
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  7. ***** Really, you want me to look it up on "a wiki page"? OK, how does this sound? https://en.wikipedia.org/wiki/Observable_universe "[T]he observable universe is a spherical volume (a ball) centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth." Nothing in there about the core.
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  8. ***** The two definitions are the same to within our ability to measure. In fact, they are many orders of magnitude away from being different enough to measure. Nevertheless, it is pretty obvious from the article that the second sentence clarifies the first. Since people all live on Earth, the first sentence is still true. Nothing anywhere indicates the center of the Earth has any significance. You still haven't provided any kind of source. It wouldn't even make mathematical sense. Your distinction between "an observable universe" and "the observable universe" is not made anywhere I can find. I am certain you made it up, and the way you keep trying to defend it does not help your case.
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  9. ***** The center of the "observable universe" depends on how you are defining that term. If you mean the center of the universe observable to, say, your left eye, then your left eye is the center (assuming the universe is homogeneous, which is reasonably close to accurate). There is almost certainly no center to the "entire universe." Even if the universe is finite, it is closed, and therefore still has no center. It's similar to how a 2-sphere (the surface of a 3-ball) has no center.
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  10. ***** That may be the definition you like to use, but can you show any actual reliable source using that definition? Or did you just make it up and decide it sounded good?
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  11. ***** Please stop making facile arguments that your definition is the best and give me any actual source indicating that it is used by anybody but you.
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  12. ***** Yeah, you keep saying this shit that is blatantly not true. It has become obvious that you have no sources, nobody who agrees with you. You made it up yourself and are now trying to justify it by typing and capital letters and insulting me.
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  13. ***** There is a difference between the observable universe and the universe. We obviously know some things about the universe, such as what is on the table in front of me at the moment (beef), because those things are close to us and easily observed. We don't know much if anything about the entire universe, which is why we focus instead on the part of it that we can observe.
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  14. humanbeingill Actually the mass of the observable universe is currently growing. That is because the observable universe is increasing in radius faster than objects at its edge are retreating. However, this won't be true forever. As the universe expands and its density decreases, it becomes increasingly dominated by dark energy, and eventually the mass of the observable universe will indeed shrink as we lose sight of distant galaxies. When this will happen is not yet known, though it could be within a few billion years.
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  15. And what is 'truth'? Is truth unchanging law? We both have truths. Are mine the same as yours?
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  16. ***** Aliens in the Andromeda Galaxy would disagree with you on that point.
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  17. ***** [citation needed]
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  18. ***** And the searches I performed quite convincingly supported my claim.
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  19. ***** It's all we can see, period. It is the literal, exact English meaning of "observable universe"--the part of the universe that is observable. Not observable from Earth, not observable on J2000.0, not observable to David Davidson, just observable. Any more specific a definition requires using more specific qualifiers.
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  20. ***** Right, Wikipedia is not written by a single author. The rest of the paragraph reads "Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical volume (a ball) centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth." This is true; the first sentence is not wrong, because everyone reading the article is necessarily on Earth. Of course hypothetically someone could eventually read it from somewhere else, but the article is a living document and could change to accommodate that. More to the point, it does not say that the definition of the center of the observable universe is the center of the Earth, because there is no such standard. Anybody may use the standard of their present point of observation or the present center of the Earth or the barycenter of the Solar System or whatever they like. There is no standard because it does not currently appear necessary because the differences between these are trivial anyway and the various reference points are effectively indistinguishable at such a large scale. Let me put it another way. There are many ways one could define the "present." For most fields it makes no sense to define it because it is constantly changing, but for purposes that require a single standard like radiometric dating, the "present" is simply agreed to mean January 1, 1950. Even then, there is no further clarification as to time zone or whatnot because it simply doesn't matter. Any such standard would give a different but indistinguishable definition. This is all arbitrary, and none of it reflects the underlying physics at all, but still a standard does exist. There is no "standard" center of "the" observable universe. It could be a particular spacetime event, or it could be a function of space over time (like "the center of the Earth"), or other things, but there just isn't one. The only definition is the one of basic physics, which is observer-centric. The observable universe is the part of the universe that you can observe. And by that definition, clearly mine is not the same as yours.
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  21. ***** Yes, the observable universe from Andromeda is not the same as the observable universe from Earth. That's the point. Neither one is any more or less "the" universe. You can say you "gave the definition" of whatever, but it's not a definition anybody actually uses.
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  22. ***** Can you give an example?
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