Why do galaxies drift apart in the expanding universe?

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Discussion Overview

The discussion centers around the phenomenon of galaxies drifting apart in the context of an expanding universe. Participants explore concepts related to redshift and blueshift, the reference points needed for spectral analysis, and the implications of time as a dimension in the expansion of space-time.

Discussion Character

  • Exploratory
  • Conceptual clarification

Main Points Raised

  • One participant questions the necessity of a reference point for determining redshift and blueshift, suggesting that knowing the composition of a galaxy might be essential for accurate spectral analysis.
  • Another participant explains that the shift is determined by analyzing spectral lines of elements, particularly Hydrogen, which have specific spacings that serve as reference points.
  • A participant expresses curiosity about the implications of time as a dimension in the expansion of space-time, proposing the idea of space-time expanding as time progresses.
  • A later reply acknowledges a misunderstanding regarding the concept of space-time coordinates but does not elaborate further on the implications of this realization.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the necessity of reference points for spectral analysis or the implications of time in the expansion of space-time. Multiple viewpoints remain regarding these concepts.

Contextual Notes

Some assumptions about the nature of spectral lines and their role in determining redshift and blueshift are not fully explored. The discussion also touches on the relationship between time and space-time expansion without resolving the complexities involved.

Hexorg
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Hello, everyone.
This is my first post in this forums. I wanted to discuss a few concepts after reading Stephen Hawkin's "The Grand Design" book, and a search engine suggested your forums :)

Particularly, I want to talk about the fact that all the galaxies drift "apart" from each other. I understand how blue-shift and red-shift techniques work, but wouldn't you need to know some other factor as a reference point? In other words, when we observe the star's light, giving of a wavelength, say 580nm (I know starts emit all kinds of wavelength, but just use one for simplicity), how do we know that it's supposed to be smaller then 580nm (blue-shifted) or bigger then 580nm (red-shifted)? We'd have to know the compound of the galaxy to tell the actual spectrum that we'd see from it, wouldn't we?

But setting that aside, and working from the point that universe is expanding. Let's say that space-time = tXYZ (time, and 3 major dimensions). As lim(t) [tex]\rightarrow[/tex] [tex]\infty[/tex], lim(tXYZ) [tex]\rightarrow[/tex] [tex]\infty[/tex] too, so wouldn't space-time expand just because the time is "running" forward?
 
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The shift is observed by analyzing the lines of the spectrum. Each element (particularly Hydrogen) has a set of lines which are specifically spaced. The spacing tells us what element is responsible and the shift is determined by the shift from the lines for that element from a non-moving source (such as the sun or in a lab on earth).
 
Oh, ok I saw those lines before. So the pattern is set?! I see thanks :D

That leaves the last part of the post though, what you guys think about the tXYZ expanding as t increases?
 
Hexorg said:
Oh, ok I saw those lines before. So the pattern is set?! I see thanks :D

That leaves the last part of the post though, what you guys think about the tXYZ expanding as t increases?

(t,x,y,z) is just a coordinate point in spacetime. If you increase t, it means you move from one spacetime location to another spacetime location with a different t coordinate value.
 
Oh... whops... concept fail.
Thanks :)
 

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