The age of the universe vs. the speed of light

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Discussion Overview

The discussion revolves around the concept of the universe's expansion, particularly in relation to the speed of light and the phenomenon of cosmic inflation. Participants explore how matter could have traveled vast distances shortly after the Big Bang, questioning the implications of the speed of light as a limit in this context.

Discussion Character

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

Main Points Raised

  • One participant questions how matter could travel thousands of light years in just ten minutes after the Big Bang, given the speed of light limit.
  • Another participant introduces the concept of "inflation," suggesting that space itself expanded at a rate that carried matter with it.
  • It is noted that while nothing can travel through space faster than light, the expansion of space can result in distances between objects increasing at rates that appear superluminal.
  • Several participants discuss the "raisin bread" analogy to illustrate how expansion can lead to relative speeds exceeding the speed of light.
  • Questions arise about what the universe is expanding into, with some participants asserting that it is not expanding into anything.
  • There is a debate about how to measure or define an object's speed relative to space, with differing views on the implications of expansion on observed velocities.
  • One participant references a paper that addresses misconceptions about the expansion of space, including the nature of recession velocities and inflation.
  • Another participant challenges the clarity of earlier explanations, suggesting that they may lead to misunderstandings about the nature of expansion and velocity.

Areas of Agreement / Disagreement

Participants express differing views on the nature of cosmic expansion, the implications of the speed of light, and how to conceptualize velocities in an expanding universe. There is no consensus on several key points, particularly regarding the definitions and measurements of speed in relation to space.

Contextual Notes

Limitations in understanding arise from the complexity of defining speed relative to an expanding space, as well as the nuances of how recession velocities are interpreted in cosmology. Some participants suggest that further resources or tutorials could clarify these concepts.

  • #31
Drakkith said:
So, do you disagree with the paper in the FAQ which I linked a few paragraphs from above? And enough with the attitude. It's doing you no good.

Ok Drakkith, I will try it once more, if you are willing to understand then fine, if not then, well, what can I do. Motion in expanding universe is inertial (in the limit I explained earlier). Nobody in the right mind disputes that. Certainly Charles Lineweaver is not disputing that. Do you think that you can get rid of inertia, make it non-existent, just because of your coordinates choice?

The problem that I am trying to point out in our discussion is interpretation of what expanding space really means. So, yes I am disagreeing very much with statements like:

The velocity in Hubble’s law is a recession velocity caused by the expansion
of space, not a motion through space.

I think that words should be chosen with much more care, because they are easily causing misconceptions.

Very same authors (Charles H. Lineweaver and Tamara M. Davis) in summary of their "Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects" paper conclude:

We have shown that the unaccelerated
expansion of the universe has no effect on whether an
untethered galaxy approaches or recedes from us. In a
decelerating universe the galaxy approaches us, while in
an accelerating universe the galaxy recedes from us.
 
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  • #32
Calimero said:
The problem that I am trying to point out in our discussion is interpretation of what expanding space really means.

Ok. So are you going to explain what it means then?

I think that words should be chosen with much more care, because they are easily causing misconceptions.

Maybe. All I know is that a lot of people who are much more knowledgeable on the subject than I am are saying the exact same thing I'm saying. You're going to have to do more than simply tell me I'm wrong to convince me.
 
  • #33
Could someone clarify this:

General relativity was specifically derived
to be able to predict motion when global inertial frames were not available. Galaxies
that are receding from us superluminally are at rest locally (their peculiar velocity,
vpec = 0) and motion in their local inertial frames remains well described by special
relativity. They are in no sense catching up with photons (vpec = c). Rather, the
galaxies and the photons are both receding from us at recession velocities greater than
the speed of light.

From here: http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

Does this mean that expansion is pushing both the light and the galaxy away from us? Or does GR not say what is causing this, only that it's happening or what?
 
  • #34
Drakkith said:
Ok. So are you going to explain what it means then?


I'll try. It is a consequence of a FRW metric. Metric is defined that way. Observers are at rest, and space between them is expanding or contracting. There is no limit on what rate space can expand. As metric evolves through time, between the two adjacent points in space there will always appear more points of space - space is expanding. Now the most important part - metric is evolving according to the presence of matter in it. Not the other way around. So, one could claim that space is expanding due to the fact that matter is receding. Hope that helps.
 
  • #35
Calimero said:
So, one could claim that space is expanding due to the fact that matter is receding. Hope that helps.

I don't see how your explanation is any different than what has already been said.
 
  • #36
As to whether or not galaxies are moving through space or space is expanding and carrying them apart, GR is indifferent. Keep in mind that GR solutions are 4D and multiple spatial foliations are possible. If you choose the spatial hypersfcs of homogeniety and isotropy as "space" in FRW cosmology, then the co-moving galaxies are not moving through "space" by definition. If you choose some other spatial hypersfc as "space," then the galaxies are moving through "space" by definition.

When it comes to telling dynamic stories with GR solutions, there are always many options.
 
  • #37
Drakkith said:
To the best of our knowledge it is not expanding into anything.

I must admit I love these answers, always people hear them for the first time, it makes them (even me) look like a question mark:rolleyes:
 

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