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Author | Topic: Expansion of the Universe | |||||||||||||||||||||||||||
slevesque Member (Idle past 4662 days) Posts: 1456 Joined: |
I don't know how what I said could seem like post hoc rationalization.
The part of the OP was clearly badly worded (or is it my english comprehension?), and I simply misunderstood it to be what I wrote in my post no11. This is what I was responding to in my message no3. But then again, your message no12 seems to clearly contradict Nonukes statement, which you now quote and agree with. All this tells me that there is a misunderstanding, and that in fact we all agree with each other (but disagree with what Cavediver said)
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
Hi Slevesque,
I think NoNukes and I agree with Cavediver that as observed from Earth, two distant objects at radial distances x1 < x2 will have a smaller measured radial Δv than two closer objects of the same radial separation. But that's just because of relativistic effects from our observer position. Ignoring local motion, two objects at separation x from each other will be retreating from one another at the exact same velocity no matter where they are in the universe. --Percy
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slevesque Member (Idle past 4662 days) Posts: 1456 Joined: |
I think NoNukes and I agree with Cavediver that as observed from Earth, But you said, In reference to my comment: ''I took this as meaning from our point of view, if we observe 4 objects, the two far ones will move away from each other faster then the two closer ones from each other.''
Along a single line of sight this would be true, trivially true in fact. It wouldn't be possible for it to be any other way. Nonukes replied:
It isn't true at all. Assuming that the expansion is isotropic, then space between the pair of points expands at a rate proportional to the current separation of the pairs of points. The distance from us to the pair of points is not relevant. And Cavediver replied:
And actually it is, observationally, but opposite to what has been suggested. Distant pairs of objects will be observed to expand away from each other more slowly than similarly separated pairs that are closer, as the more distant pair are being more red-shifted. Each successively contradict each other.
two distant objects at radial distances x1 < x2 will have a smaller measured radial Δv than two closer objects of the same radial separation. But this seems to contradict what you said in post no12, which I took as meaning that farther objects would have a greater radial Δv (and, I think Nonukes interpreted your no12 as meaning this as well)
Ignoring local motion, two objects at separation x from each other will be retreating from one another at the exact same velocity no matter where they are in the universe. Which is exactly what I said in message no3: ''Objects farther away move faster away from us then closer objects, but not from each other.''
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
Hi Slevesque,
Here's the original statement/response in your Message 3:
slevesque writes: Jefferinoopolis writes: My understanding of this is that things aren't actually moving away from each other but space is actually expanding. We know this because objects that are farther away are moving away from each other faster than closer objects. I don't think the 'from each other' part is correct. Objects farther away move faster away from us then closer objects, but not from each other. You said the "from each other" part was incorrect. I can see now that you meant, "They *are* moving away from each other, but not faster than they're retreating from us." It seemed at the time that you were saying, "They're not moving away from each other." That's why I responded that on large enough scales everything is moving away from everything else. And of course if you ignore local motion, everything is moving away from everything else regardless of distance. One reason you might think you see a contradiction is that you were thinking of objects along a single line of sight, in other words, that are lined up radially when looking outward from the Earth, but you never said that. Two distant objects, one further away than the other, could easily be retreating from each other faster than they're retreating from us if they were in opposite directions when looking outward from the Earth. What you were saying is only true if the distant objects are lined up in the same direction. Regarding the NonUkes "It isn't true at all" comment in his Message 13 reply to me, my reply looks wrong because it has to be interpreted in the context of what you said earlier in your Message 3 about the "from each other" part being incorrect. Cavediver's Message 14 is also correct where he notes that the actual observed separation velocities will be slower for the far pair than the near pair because of relativistic effects. Once relativistic effects are taken into account then this anomaly goes away.
When quoting you can use the form [qs=NoNukes], and then you won't have to type things like "Nonukes replied". --Percy
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NoNukes Inactive Member |
slevesque writes: ''Objects farther away move faster away from us then closer objects, but not from each other'' It seemed to me that there was some anbiguity about where we were disagreeing. In addition to the discussion about expansion effects, there was also the mention of Andromeda. At the time I made my comment, I was pretty sure you and I were saying the same thing and that your comment re Andromeda was tongue-in-cheek. I accept that Cavediver and Percy's current statements regarding radialy motion are correct, as were yours and mine to the extent that we were talking about actual separation distances. It also seems likely that any apparent disagreement between your position on rates of expansion and Percy's were miscommunications.
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thingamabob Junior Member (Idle past 2638 days) Posts: 23 From: New Jerusalem Joined: |
ME.....A.....B
If the space between ME and A = 500' and the space between A and B = 500'. Then the space between ME and B = 1000'. The space between each expands by 500'. Now the space between ME and A = 1000' and the space between A and B = 1000'. Then the space between ME and B =2000'. Space has expanded twice as fast between ME and B as it has between Me and A. Space is not expanding inside the Milky Way. Space is not expanding inside Andromeda. The two are getting closer together so space is not expanding between them. If expansion is true can somebody explain to me just what is expanding? thing
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slevesque Member (Idle past 4662 days) Posts: 1456 Joined: |
It seemed to me that there was some anbiguity about where we were disagreeing. In addition to the discussion about expansion effects, there was also the mention of Andromeda. At the time I made my comment, I was pretty sure you and I were saying the same thing and that your comment re Andromeda was tongue-in-cheek. I accept that Cavediver and Percy's current statements regarding radialy motion are correct, as were yours and mine to the extent that we were talking about actual separation distances. It also seems likely that any apparent disagreement between your position on rates of expansion and Percy's were miscommunications. You understood this perfectly. I misunderstood the OP, and in turn Percy misunderstood what I was trying to say, and you and cavediver apperently misunderstood what he first said. But in the end, we're all saying the same thing it turns out. The thing is that I study Math and physics, so this could have been avoided if Percy had assumed I was probably not wrong about a basic aspect of expanding space, and would have searched for an alternative interpretation of what I said
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
thingmabob writes: Space is not expanding inside the Milky Way.Space is not expanding inside Andromeda. Space is expanding everywhere throughout the universe, but the Milky Way and Andromeda galaxies are close enough together that their gravitational attraction is more than sufficient to overcome any retreat due to the expansion of space. There's the old example of two ants on a rubber band. If you stretch the rubber band the two ants get further apart. But if you first tie the two ants together with a thread (analogous to gravity) then their separation distance will not increase as the rubber band is stretched. Objects sufficiently far apart have so little net gravitational attraction (this would be the attraction resulting from adding up all the gravitational pulls from all other objects in the universe, which for distant objects will tend to average out and be a net zero) that the expansion of space is sufficient to carry them further apart. --Percy
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Drevmar Junior Member (Idle past 4784 days) Posts: 24 From: Spokane, WA, USA Joined: |
What happens to this line of thought if we discover a galaxy that is say 12 billion light years away that is headed directly towards us (the Milky Way) or directly to where we (the Milky Way) are/is headed?
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
I am not myself aware of any mechanism that would produce so large a local velocity.
--Percy
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Drevmar Junior Member (Idle past 4784 days) Posts: 24 From: Spokane, WA, USA Joined: |
Well, the reason I ask is because I have a hunch that the universe is actually infinite and that the galaxies (and other things) are not moving around in any direction that is pre-disposed by the big bang. I'm not saying it didn't happen but wondering what the notion of thought would be if we were to find a galaxy doing this.
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Percy Member Posts: 22480 From: New Hampshire Joined: Member Rating: 4.8 |
Drevmar writes: Well, the reason I ask is because I have a hunch that the universe is actually infinite and that the galaxies (and other things) are not moving around in any direction that is pre-disposed by the big bang. The big bang is not responsible for the expansion of the universe. There's something more fundamental going on, something that played a significant role in both the big bang and the accelerating expansion of the universe.
I'm not saying it didn't happen but wondering what the notion of thought would be if we were to find a galaxy doing this. I suppose that initially possibilities would be explored within existing theory, such as a serendipitous sequence of this galaxy being whipped around other galaxies in a manner similar to the way we accelerate spacecraft by whipping them around planets. If none of those pan out then and it turned out to be inexplicable within any current paradigm of cosmology then we'd have to seek modifications to current theory, or even replacement. It would be an exciting time within science. --Percy
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Taq Member Posts: 10045 Joined: Member Rating: 5.3 |
Well, the reason I ask is because I have a hunch that the universe is actually infinite and that the galaxies (and other things) are not moving around in any direction that is pre-disposed by the big bang. Then you are going to have to explain the type Ia supernovae data set: Gemini's Nod-&-Shuffle Provides Critical Deep Spectroscopic Data for Supernova Legacy Survey | Gemini Observatory And the COBE and WMAP cosmic microwave background results: COBE to WMAP Sky Animation Both of these data sets demonstrate expansion (type Ia supernovae) and the big bang (cosmic microwave background).
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slevesque Member (Idle past 4662 days) Posts: 1456 Joined: |
I'm pretty sure a galaxy 12 billion light years from us would have to be moving faster then the speed of light to be coming at us faster then expansion would be seperating us
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cavediver Member (Idle past 3665 days) Posts: 4129 From: UK Joined: |
I'm pretty sure a galaxy 12 billion light years from us would have to be moving faster then the speed of light to be coming at us faster then expansion would be seperating us Close but not quite. Even so, I think if we saw this we'd be re-writing most of cosmology...
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