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Message: <blockquote data-quote="WhiteWalker" data-source="post: 21057750" data-attributes="member: 548558">I don't really get how the space between objects in the universe is increasing faster than the speed of light. Could you explain it in more detail?Wait. Grab two laser pointers. Shoot two laser beams in two opposite directions. Measure how long it would take from the moment you press the button until the beams hit an object, say, 300 meters in each direction. (OK, such a measurement requires elaborate instrumentation, but it is eminently doable.) You’ll notice that the time required is 1 microsecond. But you shot the two beams in opposite directions, so now the two laser pulses are actually 600 meters apart. That is, the distance between them increased from 0 to 600 meters in one microsecond… it was increasing at twice the speed of light. How about that! You just made two things fly apart at twice the speed of light!Of course I am cheating a little, because I am measuring the distance between the two things while I am not traveling with either of them. I am just standing in the middle. Relativity theory does not say that I cannot measure a speed between two things that’s greater than the speed of light. It only says that neither of those things can travel faster than the speed of light relative to me, the observer.And when it comes to cosmic expansion, we are talking about things very far from here that, supposedly, do travel faster than light relative to me, the observer.But here’s the clinch. I’m not really observing those things, do I. They are safely tucked away behind a cosmological event horizon. The only things I can actually observe are things that do move slower than the speed of light relative to me.And this cosmological horizon exists, ultimately, because we live in curved spacetime. Sure, nothing can move faster than the speed of light at its location. And when you are co-located with a ray of light, its speed will always be (in a vacuum) 299,792,458 m/s. But when you look at a distant ray of light, its speed will be different, the same way the observed speed of light in a gravitational field will appear different to a distant observer. It has to do with relativistic time dilation, among other things.At the cosmological horizon, time dilation becomes infinite. That is, if you could observe anything at the horizon, it would appear to have slowed down to a complete halt. (We cannot observe the horizon, but we can observe the cosmic microwave background. It is at a “redshift of about 1,100”, which means that if we were to observe any processes taking place at the location where the CMB originates, they would appear to have slowed down by a factor of 1,100. So if some space aliens had a laboratory there set up to measure the speed of light, and we watched it with some incredible telescope, we’d measure a speed of light of only about 270 km/s in that distant laboratory.)Beyond the cosmic horizon… well, that part of the cosmos is not accessible to us because, you guessed it, it is moving away from us faster than the speed of light. Time dilation is now beyond infinite, which is kind of meaningless, but then again, since we cannot observe these things it doesn’t really matter.As back in the distant past, cosmic expansion was still slowing down, these distant objects do slow down and eventually may come into sight. But when that happens, we get to see their entire history from the very beginning, or rather, from the moment they become visible… when they were still just hot, primordial gas, emitting what would become the cosmic microwave background.So think about it. Any chunk of gas we observe with a radio telescope today, emitting that CMB, was, a few billion years prior, still moving faster than the speed of light relative to us and was not yet visible, hidden behind an event horizon of sorts. Eventually, it slowed down and became visible, and we are now seeing its history unfold from the beginning. For all we know, that chunk of gas at present is a mature galaxy, hosting numerous civilizations. But we are seeing it when it was still just primordial gas.By the way, something similar also happens with black holes. An object that crosses the black hole event horizon is moving faster than light relative to us. But we don’t get to see it. In fact, the very event of it crossing the event horizon is forever in our future, unless we decide to fall into the black hole along with the object.So yes, general relativity does allow all these things when spacetime is curved, and its rules remain self-consistent and always satisfied.If all this is a little confusing, I am sorry. These things are really difficult to explain without the math, as the concepts themselves are highly mathematical and non-intuitive.</blockquote>

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