Universe expansion and hubble's law

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Discussion Overview

The discussion revolves around the concepts of universe expansion and Hubble's law, particularly focusing on the implications of red-shift observations and the relationship between distance and the observed velocities of galaxies. Participants explore the effects of light travel time on our understanding of galaxy velocities and the expansion rate of the universe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the further a galaxy is, the faster it appears to be moving away, which relates to the concept of an expanding universe.
  • There is a suggestion that observations of distant galaxies reflect their states from the past, raising questions about how this affects our understanding of current expansion rates.
  • One participant proposes that if the universe's expansion were slowing down, closer galaxies might appear to be moving slower than more distant ones, which could be a consequence of the time-shift effect.
  • Another participant emphasizes that the relationship between red-shift and distance is crucial for understanding the expansion of the universe, and that variations in this ratio indicate changes in the expansion rate over time.
  • Questions are raised about whether the observed red-shift could differ from the current state of the galaxy due to the finite speed of light, suggesting a need for further mathematical exploration.
  • A participant mentions a "cosmos calculator" as a tool to explore the relationship between red-shift and recession speeds at different times, referencing the Friedman equations as a basis for these calculations.
  • There is curiosity about the speeds at which galaxies are receding from each other, with a clarification that this speed is due to the expansion of space rather than actual velocity.
  • One participant expresses a concern about a potential logical flaw in their understanding, indicating a broader uncertainty about cosmology and relativity.

Areas of Agreement / Disagreement

Participants express a range of viewpoints, with some agreeing on the implications of red-shift and distance while others raise questions about the assumptions and calculations involved. The discussion remains unresolved regarding the exact nature of the relationship between observed red-shift and current galaxy velocities.

Contextual Notes

Participants acknowledge limitations in their understanding of cosmological models and the implications of light travel time on observations. There is an indication that further mathematical exploration is needed to clarify these concepts.

papa lazarou
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I've been reading about the red-shift, the universe expanding, ect... And about the observation that the further a galaxy is from us, the faster it is moving away from us, accounting for an ever expanding universe.

I'm also curious about the fact that the further the galaxy, the further 'in the past' the observation of the state of the galaxy is.

Say a galaxy cluster is 500 million light years away from us, it is only telling us what the rate of expansion of the universe (in a broad sense) was 500 million years ago.

I'm sure that is accounted for when deducing the current rate of expansion of the universe, but I was wondering if the fact that galaxies far away move faster and accelerate faster is only because we observe them as they were closer to the origin of the universe.

So in term of a universe where the expansion is slowing down, it would make sense to see a closer, more 'up to date' galaxy be moving and expanding slower than a distant, 'younger' galaxy.

Or is that 'time-shift' effect due to observation and the speed of light insignificant compared to the actual 'red-shift' observed?
 
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It is not just the fact that the further a galaxy is, the greater its recession, that leads us to the conclusion that the universe will continue to expand. We would see that even if the universe's expansion was slowing. That conclusion is based on the exact ratio of red-shift to distance and how it changes over distances.

If we saw that this ratio did not change with distance, we could conclude that the rate at which the galaxy expands does not change over time. But we do not see this; what we see is that the red-shift/distance ratio varies with distance, which indicates that the universe expansion rate has sped up over time.
 
thanks! :)

That conclusion is based on the exact ratio of red-shift to distance and how it changes over distances.

But does it changes over time? The red-shift we observe was some time in the past. Would it be fair to say that the red-shift of that galaxy 'now' could be different? Obviously we cannot observe that, since the speed of light is finite.

Maybe I need to read a bit more about the actual maths.
 
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I see what you are asking.
A galaxy that is 2 billion light years away exhibits a velocity that was happening 2 billion years ago and one that is 13 billion light years away is exhibiting the velocity it was traveling 13 billion years ago. We won't know what their present velocity is for another 13 billion years.

Has this been taken into account in the calculations and how?
 
papa lazarou said:
thanks! :)
But does it changes over time? The red-shift we observe was some time in the past. Would it be fair to say that the red-shift of that galaxy 'now' could be different? Obviously we cannot observe that, since the speed of light is finite.

Maybe I need to read a bit more about the actual maths.

Papa,
for some hands-on experience with the conventional cosmo model, try this.google "cosmos calculator"

You first need to prime it by typing in the correct values of 3 parameters
Put in .27 for "matter fraction"
Put in .73 for "cosmological constant"
Put in 71 for Hubble.

Then you can put in any redshift and find out the recession speed THEN and the recession speed NOW!

this is the kind of thing I see you asking about. What the standard model says about the recession rate back then when it emitted the light, and the recession rate now on the day that we receive the light with our telescope.

All the numbers come from a simple math model called the Friedman equations. You can google Friedman or Friedmann with two Ns and get that to look at but it can actually help more to play around hands-on. The friedman model cosmos is built into the calculator.

there are a bunch of these calculators, all give you basically the same answers but some may give a lot more, with fancier terminology. Compare for example when you google
"wright calculator" or "cosmo calculator" (cosmo without the final S). You get more frills.
The first one I told you about is simpler to read because it is barebones. Doesn't give all the extras.

So why don't you google "cosmos calculator" and give it a try? Ask questions if you run into any difficulty or want to discuss the results.
 
Cool. So it is taken into account.

Are galaxies really traveling that close to the speed of light from each other? That's amazing.

"*speed due to the expansion of space, not actual velocity.

ah.
 
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There was another logic flaw that was troubling me on the same subject, but seems to stem from my lack of understanding of cosmology and relativity.

I'm reading http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" first!
 
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