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Thread: Is everything really expanding faster the further out we look into space.. or time?

  1. #1 Is everything really expanding faster the further out we look into space.. or time? 
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    I was watching the science channel and they were talking about the expansion of the universe and how it can be explained through the light shift and hubble's law.

    Our understanding of Hubble's law explains that the further out we look, the faster everything seems to be expanding away from us. We also know that the further out we look into space, the more we look back in time because of the lapse needed in order for the light to reach our eyes.

    My thought is..Now perhaps things aren't expanding more quickly because of their distance away from us, but perhaps things have been expanding faster in the past and not so fast closer to the present. In which would suggest that the expansion of space is slowing down.

    In other words, it only seems that the more distant objects are traveling faster because they were in fact moving faster in the past. But if we were to observe them as they existed today, they would be moving just as fast (or slower) as the closer objects.




    Could someone explain why this wouldn't be the case? Thanks and Cheers!


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    Forum Professor Daecon's Avatar
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    From how I think I understand it, it's like this: If something's moving away from you at 10mph, and something else is moving away from that at 10mph too, then you would see that second thing moving away from you at 20mph. It's the same with stars. Maybe?


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    To simplify things let's consider an elastic string. We have placed objects on this string at various points. We start to stretch the string at a constant rate, let's say 0.0001 meters/sec/meter. In other words, For every meter of distance separating two objects, they are moving apart at 0.0001 meters per sec. And for the sake of argument, that it takes 1 sec for information to pass between objects 1 meter apart.

    If you are on one of these objects and looking at the other objects, what do you see? For instance, you recieve information from an object that tells you that it is 1 meter away and moving away at 0.0001 m/s. You know that this information is 1 sec old. By the time you got this info, the object is really further away and receding faster. IOW, what the info is telling you is that when it was 1 meter away, it was moving away at 0.0001 m/s. If you see recieve nfo from an object that says it is 2m away and moving at 0.0002 m/s, you know that this is 2 sec old info, but it still tells you how fast it was moving away from you when it was 2 meters away. Even if the speed of inforemation exchange was sped up or slowed down, while it would change how old the info was when it got to you, you would still get the same info as to how fast it was moving when it was that far away.

    Thus when you have a situation where the expansion is constant, we get the observation pattern that shows observed speed increasing linearly with observed distance.

    Now consider what happens it, as you suggest, the expansion rate decreased over time, with it being faster at earlier times.

    We look out at the other objects. Again, we note that the first object is 1 meter away and moving at 0.0001 m/s. This by itself doesn't tell us anything new. But now we get info from the second object which was 2 meters away when the info left it. The info is 2 sec old. So we are getting info from the expansion rate 2 sec ago. So we see this object receding faster than 0.0001 m/s . The same happens, as we look further and further away, we don't see a linear realtionship between dist and speed, the speed increases with distance.

    If the speed of expansion has increased over time, we see the opposite, the speed increases slower than the distance.

    So basically: If you see a linear pattern of distance to speed, this indicates a constant rate of expansion. If speed increases faster than distance, you have an decreasing rate of expansion, and if speed increases slower than distance, you have an increasing rate of expansion.

    Up until the 1990's it was assumed that we should see the second case. The mutual gravitational attraction of the universe should have been slowing the expansion with time. The big question at the time was whether or not the expansion would ever slow down to the point where the universe would begin to collapse back in on itself. A team of researchers set out to try and answer this question by very carefully measuring and comparing distances and speed of distant galaxies to measure just how much the distance/speed ratio varied. What happened was one of great moments in science where the results turned out to be completely different than what anyone expected. Instead of measuring speeds that increased faster than distance indicating a deccelerating expansion, they found that speeds increased slower than distance, indicating that the expansion of the universe is accelerating. And thus opened a whole new chapter in cosmology.

    Looking back in time as we look further out does give us a handle on how the universe expanded in the past, just not in the way you envisioned it.
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    Quote Originally Posted by Daecon View Post
    From how I think I understand it, it's like this: If something's moving away from you at 10mph, and something else is moving away from that at 10mph too, then you would see that second thing moving away from you at 20mph. It's the same with stars. Maybe?
    But if the light to tell you that has taken you 2 hours to get here, what speed is it going now? For the speeds you calculate are really averaged speeds. Isn't that what Dave87 is saying? It is the rate of expansion of space too rather than changes in speeds.
    At what part of the journey of light across the Universe did the stretching of the wavelengths occur? Was it earlier or later? Same ultimate redshift but when did it occur? That is his question.
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    KJW
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    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by Daecon View Post
    From how I think I understand it, it's like this: If something's moving away from you at 10mph, and something else is moving away from that at 10mph too, then you would see that second thing moving away from you at 20mph. It's the same with stars. Maybe?
    But if the light to tell you that has taken you 2 hours to get here, what speed is it going now? For the speeds you calculate are really averaged speeds. Isn't that what Dave87 is saying? It is the rate of expansion of space too rather than changes in speeds.
    At what part of the journey of light across the Universe did the stretching of the wavelengths occur? Was it earlier or later? Same ultimate redshift but when did it occur? That is his question.
    Suppose a distant object emits two very short pulses of light 1 second apart in the frame of reference of the object. Suppose we receive those two pulses 2 seconds apart in our frame of reference. Then this corresponds to a redshift that halves the frequency. What happened to those two pulses in the intervening spacetime is irrelevant because it is the relationship between the times at the source and at the destination that give the redshift.
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    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by Daecon View Post
    From how I think I understand it, it's like this: If something's moving away from you at 10mph, and something else is moving away from that at 10mph too, then you would see that second thing moving away from you at 20mph. It's the same with stars. Maybe?
    But if the light to tell you that has taken you 2 hours to get here, what speed is it going now? For the speeds you calculate are really averaged speeds. Isn't that what Dave87 is saying? It is the rate of expansion of space too rather than changes in speeds.
    At what part of the journey of light across the Universe did the stretching of the wavelengths occur? Was it earlier or later? Same ultimate redshift but when did it occur? That is his question.
    Suppose a distant object emits two very short pulses of light 1 second apart in the frame of reference of the object. Suppose we receive those two pulses 2 seconds apart in our frame of reference. Then this corresponds to a redshift that halves the frequency. What happened to those two pulses in the intervening spacetime is irrelevant because it is the relationship between the times at the source and at the destination that give the redshift.
    Is the Milky Way an old galaxy? Wouldn't all galaxies have to be formed about the same time? Ater that they could disappear or collide and join but could a galaxy form in the Universe today from just from interstellar dust?
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    Quote Originally Posted by KJW View Post
    Suppose a distant object emits two very short pulses of light 1 second apart in the frame of reference of the object. Suppose we receive those two pulses 2 seconds apart in our frame of reference. Then this corresponds to a redshift that halves the frequency. What happened to those two pulses in the intervening spacetime is irrelevant because it is the relationship between the times at the source and at the destination that give the redshift.
    Thanks KLW. Your mention of time and redshift made me wonder... given how easy it is to shift a spectrum by adjusting the rate of flow of time, and given how susceptible the rate of flow of time is to energy density, and given the sort of energy densities implied by quantum field theories for the "vacuum of space", and given that standard model cosmology is happy for time to be abused sufficiently for inflation to have occurred, why do researchers still assume that redshift is a reliable way to measure cosmological distances? Apart from the redshift of quasar spectra, do we have any other way to estimate (non-local) distances?

    And all this makes me wonder... is it really possible for a Higgs-type field to be homogeneous across space? If not, what repercussions for our electroweak assumptions?

    The next decade may be a great time to be in the game.
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    Actually, the rate the universe expands at has changed over time. And we have measured this change.

    Right after the Big Bang, the universe was expanding. This gradually slowed over time as all the matter spread out and the original expansion died out. But when the universe was about half the age it is now, about seven billion years ago, the expansion suddenly started to speed up again. This was due to Λ, the cosmological constant in the Einstein Field Equations. Λ is a property of space, and the expansion of space made more space, so it made more Λ. Λ causes expansion to accelerate, so once there was enough space, then the expansion from Λ took over from the slowing impulse of the Big Bang.

    We know this from multiple lines of evidence, including Type 1a supernovae, and studies of gravitational lensing by objects at various distances.

    Does that help, or do you want more?
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    Quote Originally Posted by Schneibster View Post
    Actually, [...] has changed over time.
    Is there any way of determining if the rate of flow of that "time" has changed over time? Or across space?
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    Quote Originally Posted by nnunn View Post
    Quote Originally Posted by Schneibster View Post
    Actually, [...] has changed over time.
    Is there any way of determining if the rate of flow of that "time" has changed over time? Or across space?
    Time is a constant. If the speed of time changed you could not notice. Notice I say "could not" and not "would not." It is theoretically impossible for you to detect a change in the speed of time just as it is for you to detect any speed of light but c. In fact it is for the same reason.
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    What if we include General Relativity, and two observers?
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    A remote observer could of course observe a time change; but it would not be sensible for the local observer. The local observer will see local proper time.

    Remember also that GR does not deny any part of SR. GR merely extends SR to mathematical realms it did not previously address, using an extra postulate.
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    Quote Originally Posted by nnunn View Post
    Apart from the redshift of quasar spectra, do we have any other way to estimate (non-local) distances?
    Cosmic distance ladder
    nnunn and Schneibster like this.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    I didn't know there was one online. +1 and an entry in my favorites.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by nnunn View Post
    Apart from the redshift of quasar spectra, do we have any other way to estimate (non-local) distances?
    Cosmic distance ladder
    Thanks KJW. That sort of ladder is helpful for getting a feel for how we estimate relative distance to a subset of galaxies. But since this thread is about whether "everything is really expanding faster the further out we look..., " and since the idea of an accelerated cosmic expansion sits on the measured redshift of a population of supernovae, isn't that ladder a bit short for extrapolating assumptions about a universal action?

    From that link:

    Hubble's Law is the primary means we have for estimating the distances of quasars and distant galaxies in which individual distance indicators cannot be seen.
    Since redshift is so sensitive to (local and global) variations in the rate of flow of time, and since a Big Banged universe is now seen to depend upon that period of inflation (when a rate of flow of time was not well-defined) I'm left wondering if all the assumptions embedded in "Hubble's Law" are still valid?

    One thing that's really got me re-thinking all this... is the background condensate of weak hypercharge (required to make all this work) really homogeneous? If not, then there's lots of scope for variation in the properties of the old "vacuum of space" through which those measured photons fly.
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