I'm reading the book "The theory of everything". In the second chapter it says the Universe is expanding, and we know this because the measurements made using the Doppler effect of other galaxies gives redshifted signals. My question is, There are no galaxies behind us in this expansion? I mean, if all the galaxies are moving farther away from us all the time, does this mean that the center of the universe is empty? or are the galaxies behind us slower so the redshift effect is smaller?
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4$\begingroup$ The universe doesn't have a centre. Please see astronomy.stackexchange.com/q/669/16685 and physics.stackexchange.com/q/25591/2451 $\endgroup$– PM 2RingCommented Oct 1, 2019 at 13:24
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1 Answer
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This seems unintuitive, but the Universe doesn't appear to have a center for galaxies to move away from. This is because the Big Bang seems not to be a single event at a particular place, but instead an event at every place. In other words, the Big Bang seems to have happened at every point in the Universe, possibly infinitely in all directions (note that "direction" didn't actually mean much back then). Because of this, there is no "center" for the Universe to spread from.
Instead, when it is said that the Universe is expanding, it is meant that the fabric of space-time itself is literally getting bigger. This is thought to be fueled by dark energy, but the mechanism is still unknown. In other words, we don't know why it happens.
This gives us the odd situation that two galaxies that are stationary relative to one another still end up "moving" farther away because the fabric of space in between them is getting bigger. Even two galaxies that are moving towards each other relative to some other object still end up receding from one another if they are far enough away, because the space in between them is getting bigger faster than they can traverse it.
Even light, if its source is far enough away from the observer, will fail to traverse a sufficient distance, as the space between the light's source and the observer will literally get bigger as time goes on. We already have one limit on how much of the Universe is observable, that being the distance beyond which light has had time to travel since the Universe began. The expansion of the Universe adds a second limit, which is the distance beyond which light will never reach us due to the space in between us and its source expanding faster than light could ever traverse it.
I am not particularly knowledgeable on this subject and may have made mistakes. Anyone who is more knowledgeable should feel free to offer corrections to this answer.
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$\begingroup$ Re your last paragraph: Kudos! Your answer is basically the same what I wrote in another forum in another (my) language a while ago. $\endgroup$ Commented Oct 1, 2019 at 23:29
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$\begingroup$ I once read (forget the source) that the universe's expansion can be visualized through baking raisin bread: raisins represent galaxies, and the dough represents space between the galaxies. Thus, as the dough rises, each individual raisin "sees" every other raisin moving away from it. In addition, distant raisins move away faster than closer raisins because there's more expanding dough in between. Although I prefer chocolate chips... $\endgroup$ Commented Oct 2, 2019 at 5:04
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$\begingroup$ If I understand the answers, The efect is like create space between the galaxies? Or in other way the galaxies are moving to farther empty zones of the universe? $\endgroup$– JuanCommented Oct 2, 2019 at 13:52
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1$\begingroup$ @Juan It's more like the space in between is literally getting bigger. The galaxies don't even have to move relative to one another, but they will still end up farther away from each other because space itself is literally getting bigger. So, it's not so much that galaxies are moving towards emptier zones of the universe, but that these empty zones are constantly being created and expanded, because "empty space" is getting bigger. $\endgroup$ Commented Oct 2, 2019 at 14:11
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$\begingroup$ To clarify, instead of galaxies getting pushed away from each other, think of it like more space is being created between them and where they're going. They aren't necessarily moving, but still end up farther away. This also means that if we were to travel through space and target a specific destination, we would actually travel across a greater distance than what was there when we began travelling, because that extra distance would have been created by the expansion of space-time as we traveled. We'd end up traversing a greater distance than what even existed when we started. $\endgroup$ Commented Oct 2, 2019 at 14:17