What was expansion speed at the time of the CMB?

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    Cmb Expansion Speed
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Discussion Overview

The discussion revolves around the expansion speed of the universe at the time of the Cosmic Microwave Background (CMB), specifically 370,000 years after the Big Bang. Participants explore the distinction between historical expansion speeds and the current Hubble parameter.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant seeks to understand the specific speed at which points in space were moving apart at 370,000 years after the Big Bang, rather than the current Hubble parameter.
  • Another participant questions the focus on the specific time frame of 370,000 years post-Big Bang instead of the current Hubble parameter.
  • Some participants argue that the concept of a single speed at which points in space are moving apart is problematic, as it depends on the distance between those points.
  • It is noted that Hubble's law indicates that recession speed is proportional to distance, suggesting that only the Hubble parameter or specific recession speeds at given distances can be provided, rather than a singular recession velocity.

Areas of Agreement / Disagreement

Participants express disagreement regarding the notion of a singular expansion speed, with some emphasizing the dependence on distance and others focusing on the Hubble parameter. The discussion remains unresolved regarding the specific speed at the time of the CMB.

Contextual Notes

The discussion highlights limitations in defining expansion speed without specifying distances, as well as the dependence on the Hubble parameter for meaningful interpretation of recession velocities.

sjordannc
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To be clear, I'm looking for the speed at which points in space were moving apart from one another in the universe as it existed 370000 years after the big bang, not the Hubble parameter (expansion rate) derived from our current distance within our current time.

Thanks in advance!
 
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Why are you interested in specific 370000 year after bigbang and not current Hubble parameter ?
 
sjordannc said:
the speed at which points in space were moving apart from one another
There is no such thing. The "speed" depends on how far apart the points are.

If you want to know the Hubble parameter at the time you give, that is a single meaningful number, but it's not a speed.
 
sjordannc said:
I'm looking for the speed at which points in space were moving apart from one another
The problem with this is that Hubble's law says that the recession speed is proportional to distance. So we can tell you the Hubble parameter back then, or we can tell you the recession speed of points a specific distance apart, but there is no single recession velocity. Not then and not now.
 

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