zapatos Posted March 2, 2017 Share Posted March 2, 2017 Is it safe to assume that since distant galaxies recede from us at a rate greater than c that they eventually leave the gravity well of our galaxy? If so, does this imply that Dark Energy will have a greater impact on recession rates over time as gravity wells are no longer an influence on distant galaxies; that recession rates increase over time and not just over distance? Link to comment Share on other sites More sharing options...
Strange Posted March 2, 2017 Share Posted March 2, 2017 Is it safe to assume that since distant galaxies recede from us at a rate greater than c that they eventually leave the gravity well of our galaxy? Gravity extends to infinity, so no. Link to comment Share on other sites More sharing options...
zapatos Posted March 2, 2017 Author Share Posted March 2, 2017 Gravity extends to infinity, so no. But gravity travels at c, no? If I am receding at a rate greater than c won't I eventually outpace the gravity of the galaxy I am receding from and move into an area that the gravity of said galaxy has not yet reached? Link to comment Share on other sites More sharing options...
swansont Posted March 2, 2017 Share Posted March 2, 2017 But gravity travels at c, no? If I am receding at a rate greater than c won't I eventually outpace the gravity of the galaxy I am receding from and move into an area that the gravity of said galaxy has not yet reached? The gravity from the earth already exists where you are. How will go get to an area where "gravity hasn't yet reached" if you move slower than c? Link to comment Share on other sites More sharing options...
zapatos Posted March 2, 2017 Author Share Posted March 2, 2017 I am receding from galaxy 'x' at a rate greater than c right now. It is the reach of gravity from galaxy 'x' that I believe I will eventually pass. Link to comment Share on other sites More sharing options...
swansont Posted March 2, 2017 Share Posted March 2, 2017 I am receding from galaxy 'x' at a rate greater than c right now. It is the reach of gravity from galaxy 'x' that I believe I will eventually pass. But you are not moving faster than c. Link to comment Share on other sites More sharing options...
zapatos Posted March 2, 2017 Author Share Posted March 2, 2017 But I am receding from galaxy 'x' faster than c, right? For any galaxy we see receding from us faster than c, we are receding from them faster than c. (I am unsure whether or not you are trying to make a distinction between 'receding' and 'moving'.) Presumably there are some galaxies that are beyond our ability to ever communicate with because we are receding from each other at rates faster than c. Are we still in their gravity well? Link to comment Share on other sites More sharing options...
Strange Posted March 2, 2017 Share Posted March 2, 2017 I am receding from galaxy 'x' at a rate greater than c right now. It is the reach of gravity from galaxy 'x' that I believe I will eventually pass. Only changes in gravity travel at c. At that distance, the distribution of mass in our galaxy (and the galaxy cluster and supercluster we are part of) will be roughly constant. So I don't see how receding at more than c would make any difference. Link to comment Share on other sites More sharing options...
zapatos Posted March 2, 2017 Author Share Posted March 2, 2017 Many years from now if things continue as they are, our supercluster (or whatever we are gravitationally bound to) will be the only thing any future earthling will be capable of detecting. All other large structures will eventually have receded from us, at speeds greater than c, into the unobservable universe. Are you saying that we will forever be under the gravitational influence of all other superclusters, whether in our observable universe or not, and no matter how long we've receded from them at > c? And if we are, can those superclusters really be considered 'unobservable'? I've heard said on this forum that if the sun were to suddenly disappear that the earth would continue to revolve around the spot where the sun previously existed for an additional eight minutes or so, after which there would be no gravity to bind us and we would fly off into space. Looking at that from another perspective, if Earth was out in intergalactic space and a sun sized star suddenly appeared out of nothing, 93 million miles from us, would we be immediately attracted to it, or would it take eight minutes or so before we began to be influenced by its gravity? If it took eight minutes or so to affect us, that tells me that if at the moment it appeared I began to recede from it at greater than c, that its gravitational influence would never affect me. This is what leads me to believe that there is an ever expanding gravitational sphere of influence around any given mass (expanding at the rate of c) and that if we are inside that sphere we can eventually leave the sphere of influence by receding away from the source of gravity at a rate greater than c. I have no idea if what I'm suggesting is correct, I'm just trying to see if I understand correctly. If I do, then that leads me to believe that structures receding from each other at rates > c will eventually leave the gravitational sphere of influence of each other and will begin to recede even faster as Dark Energy continues to act on them without the opposing force of gravity. Link to comment Share on other sites More sharing options...
Strange Posted March 2, 2017 Share Posted March 2, 2017 The trouble is, those examples are based on something impossible (a mass suddenly appearing or disappearing). Are you saying that we will forever be under the gravitational influence of all other superclusters, whether in our observable universe or not, and no matter how long we've receded from them at > c? And if we are, can those superclusters really be considered 'unobservable'? That mass can never disappear and so the question is irrelevant. However, if you suppose the same unrealistic though experiment (our supercluster just vanishes) then, yes, that would never be detectable by those galaxies outside our observable universe, whether by visual observation or detection of the change in gravity. But as mass cannot disappear like that, the question doesn't really arise. Although, a more realistic example might be: the gravity waves from the merger of two black holes in our galaxy could never be detected by a galaxy outside the observable universe. Does that answer the question? Link to comment Share on other sites More sharing options...
zapatos Posted March 2, 2017 Author Share Posted March 2, 2017 The trouble is, those examples are based on something impossible (a mass suddenly appearing or disappearing).The purpose of the impossible question was simply a thought question to help me understand the principles of how gravity works. I don't seem to be making myself clear so let me just start with a question. You earlier said that gravity extends to infinity. Does that mean that if our universe simply popped into existence from nothing, that at that moment gravity from our universe had infinite reach? This is not a question about how our universe began, it is a question about the behavior of gravity. Link to comment Share on other sites More sharing options...
Strange Posted March 3, 2017 Share Posted March 3, 2017 You earlier said that gravity extends to infinity. Does that mean that if our universe simply popped into existence from nothing, that at that moment gravity from our universe had infinite reach? This is not a question about how our universe began, it is a question about the behavior of gravity. Sorry, I thought that had been answered. the gravity from something magically appearing would propagate at the speed as light. So if the Sun (or a distant galaxy) suddenly disappeared we would not the change in gravity at the same time we would see the event. So, if that happened outside our observable universe then we would never see it and never notice the change in gravity. Link to comment Share on other sites More sharing options...
swansont Posted March 3, 2017 Share Posted March 3, 2017 If it took eight minutes or so to affect us, that tells me that if at the moment it appeared I began to recede from it at greater than c, that its gravitational influence would never affect me. Which is a different scenario from the original one. Here, gravity from the object does not exist. There are a number of times I seem to recall where the people well-versed in GR have admonished that one should not conflate recession with a velocity. You can be receding without moving. But I am receding from galaxy 'x' faster than c, right? For any galaxy we see receding from us faster than c, we are receding from them faster than c. (I am unsure whether or not you are trying to make a distinction between 'receding' and 'moving'.) Yes, that's exactly the distinction I am making. We can still see galaxies that are receding faster than c, and yet light moves at c. Think about how that is possible. Link to comment Share on other sites More sharing options...
zapatos Posted March 3, 2017 Author Share Posted March 3, 2017 Yes, that's exactly the distinction I am making. We can still see galaxies that are receding faster than c, and yet light moves at c. Think about how that is possible.I believe that is possible because we are seeing photons that left the distant galaxy prior to achieving a recession rate greater than c. And if I understand correctly, all photons that left the distant galaxy prior the the recession rate exceeding c will eventually pass us, and at that point that distant galaxy will become invisible to us, never to be heard from again. Which sort of brings me back to the reason I opened this thread, which has to do with the consequences on gravity at > c recession. 1. If photons and the propagation of gravity travel at c, 2. and a recession rate > c means that eventually photons from a distant galaxy will no longer reach us, 3. then would it also be true that at basically the same time photons are longer able to reach us, that the propagation of gravity will no longer be able to 'reach us'? Link to comment Share on other sites More sharing options...
Strange Posted March 3, 2017 Share Posted March 3, 2017 3. then would it also be true that at basically the same time photons are longer able to reach us, that the propagation of gravity will no longer be able to 'reach us'? Only changes in gravity will no longer be able to reach us. Link to comment Share on other sites More sharing options...
zapatos Posted March 3, 2017 Author Share Posted March 3, 2017 Only changes in gravity will no longer be able to reach us.Okay. So any gravitational impact the distant galaxy had on us prior to crossing over into the unobservable universe would remain indefinitely? Link to comment Share on other sites More sharing options...
Strange Posted March 3, 2017 Share Posted March 3, 2017 Okay. So any gravitational impact the distant galaxy had on us prior to crossing over into the unobservable universe would remain indefinitely? Yes. Link to comment Share on other sites More sharing options...
zapatos Posted March 3, 2017 Author Share Posted March 3, 2017 Okay, thank you. That is the part I did not understand. Thank you both for helping me through that. Link to comment Share on other sites More sharing options...
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