I Hydrogen line observation frequency range and space Hydrogen speed?

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The discussion centers on the observation of the hydrogen line frequency range, which spans from 200 MHz to 15 MHz on Earth, influenced by cosmological redshift. Participants debate the calculation of hydrogen speed in space, noting that defining "at rest" is complex due to varying reference points, particularly between local and distant hydrogen clouds. The conversation highlights that the observed redshift is primarily cosmological rather than Doppler, complicating the determination of hydrogen's velocity. There is confusion regarding the relationship between temperature, Doppler shifts, and the 21 cm line, with clarifications that thermal velocities do not equate to redshift measurements. Ultimately, the thread emphasizes the challenges in accurately measuring hydrogen speeds due to the interplay of multiple redshift factors and the need for precise definitions in astrophysical contexts.
  • #31
Jaaanosik said:
why we do not measure 'fast' hydrogen atoms.
Because the hydrogen atoms being measured are in gas clouds in our own galaxy, whose velocities relative to Earth are not going to be very fast when compared with the speed of light.
 
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  • #32
PeterDonis said:
But until post #23, you did not tell us what "hydrogen atoms" you were even talking about. Not until post #23 did you actually give us a reference that showed specific measurements of particular hydrogen gas clouds--and then they turn out to be hydrogen gas clouds in our own galaxy, whose measured power spectrum, as shown in the "previous post" on physicsopenlab (linked to at the top of the one you referenced), peaks at about 1420 MHz, exactly as expected for the 21 cm hydrogen line with no cosmological redshift. Yet in your OP, you talked about frequencies ranging from 200 MHz down to 15 MHz, which cannot possibly be from the measurements discussed in the physicsopenlab article.
My understanding was the measurements in the 200MHz-15MHz as per wiki page include Doppler effect in combination with the other factors.
If that's not the case then I stand corrected.
The title: "Hydrogen line observation frequency range and space Hydrogen speed?
points towards the question trying to resolve the speed of hydrogen in space.
 
  • #33
Jaaanosik said:
the measurements in the 200MHz-15MHz as per wiki page
Have nothing to do with the actual measurements you referenced in post #23.
 
  • #34
Jaaanosik said:
The title: "Hydrogen line observation frequency range and space Hydrogen speed?
points towards the question trying to resolve the speed of hydrogen in space.
And at this point that question has been sufficiently answered, and enough time has been spent on it. Thread closed.
 

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