I What was expansion speed at the time of the CMB?

AI Thread Summary
The discussion centers on the expansion speed of the universe 370,000 years after the Big Bang, specifically the movement of points in space. It clarifies that there is no single "speed" of expansion, as it varies with distance due to Hubble's law. Participants emphasize that while the Hubble parameter can be determined for that time, it does not equate to a specific speed. The conversation highlights the complexity of measuring recession velocities in cosmology. Understanding these nuances is crucial for accurately interpreting cosmic expansion.
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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