Is the Universe's Expansion Truly Accelerating Based on Redshift Observations?

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Discussion Overview

The discussion centers on the question of whether the universe's expansion is truly accelerating based on observations of redshift in distant galaxies. Participants explore the implications of redshift, the nature of cosmic expansion, and the constancy of the speed of light, with a focus on theoretical and observational aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the interpretation of redshift, suggesting that observing older galaxies with higher redshift might imply that the universe was expanding faster in the past, potentially indicating a slowing expansion.
  • Another participant asserts confidence in the acceleration of the universe's expansion, suggesting that this question has been addressed in previous discussions.
  • A participant explains that in an accelerating universe, light from distant supernovae appears fainter due to the longer time taken for light to travel, compared to a decelerating universe.
  • Some participants express curiosity about whether the speed of light has always been constant or if it is influenced by the universe's expansion, with one noting that current evidence supports the constancy of the speed of light, albeit with some uncertainty.
  • Concerns are raised regarding the implications of high redshift values, with one participant noting that such values suggest galaxies would need to be moving faster than light, leading to confusion about the nature of cosmic redshift.
  • Another participant discusses the relativistic Doppler effect and clarifies that cosmic redshift is not simply a matter of relative speed but is related to the expansion of space itself.

Areas of Agreement / Disagreement

Participants express differing views on the implications of redshift and the nature of cosmic expansion. There is no consensus on whether the universe's expansion is accelerating or what the implications of redshift truly indicate.

Contextual Notes

Some participants highlight the complexity of interpreting redshift and its relation to cosmic expansion, noting that assumptions about speed and distance may not be straightforward. The discussion reflects ongoing uncertainties and varying interpretations of observational data.

Vidar T
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Can we really say that the universe is accelerating its expansion because we find that galaxies further away have more redshift? We know that we are looking back in time when we look at the stars, and when we look at the galaxies furthest away from us, we are seeing them as they were a long time ago. Accually, the older the galaxy the more redshift it has. Dont this mean that things are slowing down and that everything was moveing faster in the beginning?
 
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This question is asked here with great regularity. Yes, we can be sure. I suggest a forum search for discussions.
 
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phinds said:
This question is asked here with great regularity. Yes, we can be sure. I suggest a forum search for discussions.
Thanks, i will check.
 
Say you observe a supernova at a redshift of z=1, i.e. a supernova that occurred when the universe was half its current size. In an accelerating universe, expansion in the past was slower than today and so, in comparison with a decelerating universe, the universe took longer to expand from half its size to its present size. Because it takes longer, light travels farther and hence supernovae appear fainter at a given redshift in an accelerating universe than in a non-accelerating universe.
 
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Thanx:-) I would also like to ask if we are sure that the speed of light has allways been the same, or if it is somehow affected by the expansion of the universe? I will search the treads for an answer, but i apritiate any good explanation to this:-)
 
Vidar T said:
Thanx:-) I would also like to ask if we are sure that the speed of light has allways been the same, or if it is somehow affected by the expansion of the universe? I will search the treads for an answer, but i apritiate any good explanation to this:-)

We are as sure as we can be at this time that the speed of light is constant. That's not to say that we are 100%, absolutely sure with no possibility that we could be wrong, only that the available evidence is overwhelmingly in favor of the speed of light being a constant.
 
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Well considering the numbers of redshift is going into the 8,s The galaxies would have had to be moving 8 times the speed of light.
I've been wrapping my head around this all day lol.
 
Gaz said:
Well considering the numbers of redshift is going into the 8,s The galaxies would have had to be moving 8 times the speed of light.
I've been wrapping my head around this all day lol.

There's a nice formula for the relativistic doppler effect. It is not as simple as just z itself or z+1 but you can look it up.

Google "relativistic doppler" and see what you get.

I get
https://en.wikipedia.org/wiki/Relativistic_Doppler_effect#Motion_along_the_line_of_sight
9e9e466ab27412c6833ac49d0b74894a.png
 
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So if z = 8 (e.g. the example you mentioned) then 1+z = 9
that z+1 is usually a more useful number than z itself, it is the actual ratio of wavelengths, or frequencies

And then you can solve for β which is the radial speed expressed as a fraction of the speed of light---namely β = v/c

(1+β)/(1-β) = 81

80/82

But nobody is saying that the galaxies are moving that fast thru the flat non-expanding Minkowski space of special relativity . The cosmo redshift is not a doppler effect of some relative speed at this or that time.

It is the cumulative effect of how much the distance grew while the light was traveling. A GR thing not an SR thing.
z+1 = distance now/distance then
distances grow at changing speeds throughout the whole time so there is no simple way to analyze in doppler terms based on this or that speed. Wavelengths are stretched by the same factor as distances.

I think you know that, but say it anyway in case anyone reading does not.
 
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