Hydrogen line observation frequency range and space Hydrogen speed?

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Discussion Overview

The discussion revolves around the observation of the hydrogen line, particularly the 21 cm line, and its implications for understanding the speed of hydrogen in space relative to Earth. Participants explore the effects of redshift, including cosmological and gravitational components, and the challenges in defining speed in an expanding universe.

Discussion Character

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

Main Points Raised

  • Some participants note that the observed frequency range of the hydrogen line on Earth, from 200 MHz to 15 MHz, incorporates redshift effects.
  • There is a question about how to define the speed of hydrogen in space, with some arguing that it is relative to Earth, while others challenge this notion.
  • Participants discuss the breakdown of observed frequency shifts into cosmological redshift, gravitational blueshift, and kinematic Doppler effects, emphasizing that this breakdown depends on the choice of reference frame.
  • One participant asserts that the "speed" of distant hydrogen gas clouds relative to Earth is not well-defined due to the complexities of redshift.
  • Another participant raises the point that for hydrogen clouds within our galaxy, the total effect of redshift would be small, suggesting a different context than for cosmological distances.
  • There is a discussion about the implications of the 21 cm line for measuring hydrogen speeds, with some participants expressing confusion about the relationship between redshift and actual motion through space.
  • One participant mentions that local group objects are not affected by universal expansion, leading to a discussion about the nature of redshift in different contexts.
  • The role of temperature in the spin state of neutral hydrogen and its relation to the 21 cm line is also brought up, with references to processes occurring in stars and star formation.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the definition of speed in relation to redshift and the implications of the hydrogen line observations. There is no consensus on how to interpret the redshift effects or the speed of hydrogen in space.

Contextual Notes

Participants note that the definitions of "at rest" and the breakdown of redshift effects are not invariant and depend on the chosen reference frame. Additionally, the discussion highlights the complexities involved in measuring velocities in the context of an expanding universe.

  • #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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