# Rms speed of H

1. Oct 28, 2005

### Jonny_trigonometry

Are there any accepted values of an rms speed of hydrogen atoms in the observable universe?

Can you use a similar method as the maxwell speed distribution of gases, only centered on the center of mass of the observable universe (within velocity space)?

2. Oct 28, 2005

### Garth

That depends on the temperature Vrms ~ √T.

As the temperature of hydrogen in the universe varies from 2.7oK to ~ 107oK in stellar cores there is no easy answer to your question.

Furthermore, how do you define the "centre of mass of the observable universe" in an unbounded universe?

Garth

3. Oct 30, 2005

### Jonny_trigonometry

um, i guess you have to put it at our solar system or something. Since you can see about as far in all directions. We are the center of our observable universe, but not the center of the universe... Since mass is pretty much evenly distributed over this scale, then it's kind of like a big shpere, and we're at the center. I also mean, taking into account the hubble constant too, that hydrogen far enough away is moving at the speed of light away from us (or more due to space expansion), so thats where the edge of the observable universe is. You know what i mean by now.

so if you take the average temperature of hydrogen, calculate the rms speed from that and account for the rms speed from the hubble constant (as a functin of distance from us) integrated over the observable universe would that be an acceptable extimate?

Last edited: Oct 30, 2005
4. Oct 31, 2005

### Garth

Well, the solar system is moving relative to the Surface of Last Scattering of the CMB at ~ 10-3c and the Earth is going around the Sun. There are large masses, such as the Galaxy itself, the Local Group, the Virgo Cluster and the Great Attractor 'nearby' and so perhaps we do not live on a very good "centre of mass of the observable universe".

But my second question is:"What are you trying to do? Why do you want to know the 'rms speed of hydrogen atoms in the observable universe'?"

Garth

5. Oct 31, 2005

### Jonny_trigonometry

well, I learned in QM that hydrogen is the most abundant element in the universe, and it should absorb all the light from distant sources, but it doesn't, and the reason is because it's mostly ionized. I was wondering if it has an overall energy high enough to lead to ionization, like in a plasma. Basically, I was wondering why most of the hydrogen is ionized. Where are all the electrons?

6. Oct 31, 2005

### Garth

Ionised hydrogen absorbs/emits radiation, non-ionised hydrogen is largely transparent except in the UV - Lyman Alpha - and radio - 21 cms. Even then the densities of the ISM and IGM are so low that the optical depth is small except over very long line-of sight.

Garth

7. Oct 31, 2005

### Jonny_trigonometry

za? I knew that guy was kinda fishy. How'd he get to be a professor teaching intermediate QM? He seems kinda too deterministic to be in QM anyway, being that it fundamentally deals with probabilities. I personally think that em waves can transfer momentum to a free electron, or proton, and they don't need to be bound together in a hydrogen atom form.

So you're saying that a single proton will absorb and emit radiation, and a proton and electron who together make hydrogen will not do so as much? I think that makes sense, because H can only absorb and emit certain distinct energies, but a proton can be affected by any energy of em.

It's wierd though, how did he convince the entire class that radiation isn't absorbed by ionized hydrogen?

8. Nov 1, 2005

### SpaceTiger

Staff Emeritus
The hydrogen is ionized by the light of stars and quasars throughout the universe. Once the hydrogen is ionized, the electrons are no longer bound to the hydrogen atom and can roam free in the intergalactic medium. They will, sooner or later, recombine with a proton to form another hydrogen atom, but this too can then be ionized.

9. Nov 1, 2005

### SpaceTiger

Staff Emeritus
That's correct. There are many forms of absorption/scattering that occur in the interstellar and intergalactic medium, including scattering off of free electrons and protons.

Free protons don't usually contribute much to the radiation or scattering we see because their mass is much larger than that of the electrons. Why does this matter? You can get a crude understanding of this classically. More massive particles are more difficult to accelerate (by Newton's second law) and the intensity of radiation is dependent upon a particle's charge and acceleration. Since the proton and electron have equal magnitude of charge, the dominant source of radiation and absorption will come from the much less massive particle -- the electron.

This is basically correct, but you also need to consider how strong the radiation/absorption processes are. Despite its limited energy range, neutral hydrogen is a much better absorber/emitter than a free proton.

He shouldn't have said that, but it is true that the intergalactic medium would absorb much more if it were neutral.

Last edited: Nov 1, 2005
10. Nov 1, 2005

### Jonny_trigonometry

Thanks a lot ST, this clears things up well enough. So he was kinda right, and kinda not right.

So, there is enough em energy in the vacuum per unit volume to ionize most of the hydrogen atoms? Does that mean that there must be more plasma than any other phase of matter out there?

Would you be willing to critique a paper I've been working on concerning a semi-classical view of the H atom that would help explain the ionization based on its rms speed too, not just em energy in the vacuum? I'll private message you the link to the paper if you're interested, it's 5 pages.

11. Nov 1, 2005

### SpaceTiger

Staff Emeritus
Yes, most of the hydrogen atoms in the universe are ionized. The intergalactic and intracluster media, which make up most of the baryonic mass in the universe, are indeed plasmas. It's important to distinguish this from dark matter, which is actually much more abundant (by mass) than hydrogen.

Certainly, but I can't promise I speedy response. :tongue2: