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Thread: question about star location

  1. #1 question about star location 
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    its a little tricky keeping track of time and distance. just to clarify, when we look at an object a billion light years away, that object is not in that location now...but rather it was in that location a billion years ago. is that correct? if so, do astronomers make a correction when they map the universe, and do other calculations. this is going to sound idiotic, but when when we look at objects further away from us, they are moving faster away. but thats not the speed that they are travelling at now...thats the speed they were travelling in the past. maybe they have slowed down since then. could this explain why we believe that the universe is expanding faster than it actually is? thanks for the help.


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    Yes you are right. In special relativity this phenomenon is incorporated by term "relative present". When astronomers say some star exploded or something they really mean signal o that event reached us. Thing is you cannot really establish something like absolute present because two events that are relatively present (that means that they cannot reach each other by signal with speed of light) can "happen" in different orders based on inertial frame.

    I recommend these short videos: Special Relativity: 2 - Spacetime Diagrams - YouTube
    Special Relativity: 4 - Past, Future and Causality - YouTube

    or if you like reading more Light cone - Wikipedia, the free encyclopedia.

    Dont worry about expansion. Guys that do that are aware of these effects.


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    Quote Originally Posted by mrjc99 View Post
    but thats not the speed that they are travelling at now...thats the speed they were travelling in the past. maybe they have slowed down since then. could this explain why we believe that the universe is expanding faster than it actually is? thanks for the help.
    It the very fact that we are looking at these galaxies as they were in the past that allows us to determine that the the rate at which the Universe is expanding is accelerating. The further we look into the universe, the further we look back in time. By measuring the distances of the galaxies and how fast they were receding, and doing this for galaxies at different distances from us, its like taking a series of snapshots of the universe over a range of time. A pattern is formed,and that pattern gives us a picture of how the universes expansion has changed over time.
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    The red-shift taken in account will determine the speed of recession of say a galaxiy, that is the 'z' compnent, the universal rate of expansion is about 74/sec/km/megaparsec (Spitzer). But my wondering is, is this expansion the same in ALL direction at x -distance. And my intuition & some physics says, that there will be warps due to localized differences in gravity. So the universe is not a smooth sphere expanding endlessly (I think this is an open universe), but an uneven shell, as not all matter in all directions have reached the same distance away from, say Earth.
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    Quote Originally Posted by mrjc99 View Post
    but thats not the speed that they are travelling at now...thats the speed they were travelling in the past. maybe they have slowed down since then. could this explain why we believe that the universe is expanding faster than it actually is? thanks for the help.
    You have presented two problems here that are related.
    You are right about the redshift being the speed they were travelling in the past.
    Then you pose a hypothesis that they might have slowed down. It is a fair hypothesis, but there is no way to test it. There is no real way to know if the universe is expanding faster, slower or even if it has decided to stop expanding and start contracting again.
    Without being able to know what the present state of the universe is there no way to say if it is expanding faster or slower than people think it is.

    About the only thing I can say about it is that the universe we see is not the universe that exists out there, but we have nothing but our observations of what we can see.
    All of our ideas have to account for what we can actually see right now, the "now"which is the apparent present to us.

    What Janus said is right about how looking at the past gives us an idea about the trend the universe is following, but it requires us to believe the past trends are continuing now, and again there is no real way to test that.
    Last edited by dan hunter; February 18th, 2014 at 01:31 AM.
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    Universal Mind John Galt's Avatar
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    Quote Originally Posted by dan hunter View Post
    What Janus said is right about how looking at the past gives us an idea about the trend the universe is following, but it requires us to believe the past trends are continuing now, and again there is no real way to test that.
    Since we can view the universe at all times since shortly after the Big Bang, till just a microsecond ago, then we can readily see trends. Your argument would only work if we were in a preferred location in the universe. Axiomatically and observationally and theoretically that is thought to be false.
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