The forum discussion centers on the observation of the hydrogen 21 cm line and its implications for measuring hydrogen speed in space. Participants clarify that the observed frequency range of 200 MHz to 15 MHz is due to cosmological redshift, not just intergalactic hydrogen. The discussion emphasizes that defining "speed" relative to Earth is problematic due to the complexities of redshift components: cosmological, gravitational, and Doppler effects. Ultimately, the speed of distant hydrogen gas clouds cannot be accurately defined without a clear reference frame.
PREREQUISITESAstronomers, astrophysicists, and students interested in cosmology, particularly those studying hydrogen's role in the universe and the complexities of measuring cosmic phenomena.
From Hydrogen line in cosmology
Speed relative to what?Jaaanosik said:
The Earth, as per the quote from the link, that's where the measurement is being done.PeterDonis said:
Then the answer is that the "speed" of a distant hydrogen gas cloud relative to Earth in an expanding universe is not well-defined.Jaaanosik said:
Then you are assuming a particular breakdown of the total effect, which is what is actually observed, into three components--but as @Ibix told you, that breakdown depends on your choice of what "at rest" means--and that's not the only thing it depends on. There is no invariant way of making such a breakdown.Jaaanosik said:
Then the reference you gave in the OP is irrelevant, since that reference, at least the part that you quoted, is talking about clouds of hydrogen gas at cosmological distances, not in our own galaxy. For hydrogen gas clouds in our own galaxy, the total effect would be very small.Jaaanosik said:
That would depend on your choice of coordinates. There isn't an absolute answer.Jaaanosik said:
And, as has already been explained to you, there is no invariant answer to that question.Jaaanosik said:
As a simple calculation will show you, the base frequency of the 21 cm hydrogen line is about 1400 MHz. That is the frequency observed in the lab. So 200 MHz is already redshifted by a factor of about 7 (more precisely, ##1 + z = 7##), and 15 MHz is redshifted by a factor of almost 100. So those frequencies are not just for "intergalactic hydrogen", they are for hydrogen that is far enough away to have cosmological redshifts in that range.Jaaanosik said:
See above. And I shouldn't have had to tell you what I said above. You should have checked yourself to see what the base frequency of the 21 cm hydrogen line is. Anyone with "I" level knowledge of this subject should be able to do that.Jaaanosik said:
You have already been told that there is no invariant answer to your "speed" question. For hydrogen gas clouds close enough that cosmological redshifts are not an issue, such as in our own galaxy, where we can come up with reasonable definitions of "relative speed", no, we don't see relative speeds anywhere even close to that. You should be able to figure that out too by looking up data on the orbital speeds of stars around the center of the galaxy.Jaaanosik said:
Understood, and I knew the 21cm line measurement has multiple factors.Mordred said:
Thanks!Mordred said:
Thank you, the article helps but it also says this:Mordred said:
For distant hydrogen clouds that have a significant cosmological redshift, as you have already been told multiple times now, there is no invariant way to separate out a "Doppler contribution".Jaaanosik said:
Which obviously has no cosmological redshift relative to the Earth.Jaaanosik said:
Thermal velocity has nothing to do with Doppler shift, or indeed with any redshift or blueshift at all.Jaaanosik said:
Um, ionized hydrogen is free protons and electrons. (Or, rarely, free deuterons and electrons.)Mordred said:
Yes, measurements are in Kelvins, yet somehow it represents Doppler shift.PeterDonis said:
Is this the reason you actually started this thread? Because if so, you could have (and should have) given this particular reference and asked the question you just asked in your OP of the thread. Which would have saved us more than a day of trying to figure out what your actual question is.Jaaanosik said:
Where on the physicsopenlab page does it say anything about temperature?Jaaanosik said:
Says who? It looks like your "temperature to frequency calculator" is on a different web page that you have not provided a link to. Please provide a link.Jaaanosik said:
PeterDonis said:
The reason for the OP is to determine why we do not measure 'fast' hydrogen atoms.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.Jaaanosik said:
The "Signal" in degrees K in this graph appears to actually represent received power. The "previous post" says "The y axis shows the equivalent temperature of the signal in degrees °K." This temperature itself cannot correspond to a frequency, since frequency is shown on the X axis, as given in the first graph in the "previous post"--and then, as the "previous post" explains, the frequency is converted to velocity by assuming that the "base" frequency is the 1420 MHz expected frequency of the 21 cm hydrogen line, and that all shifts in frequency are due to Doppler shift. So we have a spectrum of received power vs. frequency, which is then used to infer Doppler shifts and hence relative velocities. The articles do not explain how received power is converted into a temperature, which seems to me to be a significant omission; but the "previous post" uses the term "power spectrum" explicitly, indicating that the "signal" is received power.Jaaanosik said: