# Question regarding What is the universe made of?

1. Dec 6, 2015

### Buzz Bloom

The link below is to an 12/2/15 article at phys.org titled:
What is the universe made of? Shedding light on the mystery of missing ordinary matter.
http://phys.org/news/2015-12-universe-mystery-ordinary.html
Here is a quote:
Numerical simulations made it possible to predict that the rest of this ordinary matter should be located in the large-scale structures that form the 'cosmic web' at temperatures between 100,000 and 10 million degrees.​

The article explains a lot about the missing baryonic matter, but does not explain why these intergalactic gas structures should have such large temperatures. Can someone offer an explanation about this?

Regards,
Buzz

2. Dec 6, 2015

### Chronos

The article is discussing intergalactic gas clouds that are not yet dense enough to form stars or galaxies. The temperature is largely the result of collisional heating due to gravity. The gas is quite diffuse so, not unlike the solar corona, a little enegy goes a long ways.

3. Dec 6, 2015

### Buzz Bloom

Hi @Chronos:

I can understand that as a body of gas contracts into a smaller volume because of internal gravitational effects, its temperature and pressure would rise. What still puzzles me is why such temperatures would not have dissipated itself through radiation before reaching an ionization temperature. I suppose that when the temperature raises sufficiently for the gas to become ionized, that the photons would be unable to escape continual interaction with the ions. But, in the earlier stages of collapse, before ionization, why didn't the increases in temperature dissipate through radiation well before the temperature reached the ionizing range?

Regards,
Buzz

4. Dec 6, 2015

5. Dec 6, 2015

### phyzguy

The majority of the intergalactic gas in the universe was ionized by energetic radiation from the first stars and quasars approximately 150 million years after the big bang. So the collisional heating that Chronos is referring to is acting on this ionized gas (plasma). This plasma does cool through radiation, but for very hot diffuse gas, the time for it to cool can be very long, longer than the lifetime of the universe.

6. Dec 6, 2015

7. Dec 6, 2015

### Staff: Mentor

This might be a silly question, but what makes single ($m^{-3}$) ionized atoms hot? The articles are mainly about why it's been detected and that it's called WHIM. How are the $10^7 K$ explained? The only hint I read was "This gas has been heated up by the cluster's gravitational pull and is now feeding its core." I can't match this with this extremely low density.

8. Dec 6, 2015

### Buzz Bloom

Hi phyzguy:

Thanks for your post. I much appreciate your very clear explanation. Can you site a reference that gives the math for this result?

I also looked at
Here is a quote that bothers me.
"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through."​
If the gas is easy to see through then presumably it is transparent to the photons that would dissipate the high temperature. Does that make sense?

Regards,
Buzz

9. Dec 6, 2015

### Buzz Bloom

Hi Chronos:

Regards,
Buzz

10. Dec 6, 2015

### phyzguy

The classic reference is "Radiative Processes in Astrophysics", by Rybicki and Lightman. Hot ionized gas like this cools primarily through thermal bremsstrahlung. A good set of online slides on this topic is at this site. Slide 3 gives the cooling time of the hot plasma, which is proportional to sqrt(T) / n. For galaxy clusters it can by 10 Gy or more. The gas is so diffuse that it is quite transparent, so the cooling photons leave easily. Realize that an ion traveling along at high speed will not radiate, it only radiates when it collides with another ion. And in hot, low density plasma, collisions are rare enough that it takes a very long for a given ion to lose much energy.

I'm not sure I understand your question. The ions gain energy as they fall into gravitational potential wells. They are hot because they are moving at high speeds. Velocities in galaxy clusters are typically 1000's of km/sec. Ions flying around at 1000 km/sec have a temperature of 10^7 - 10^8 K. The density is low, so the ions only collide rarely, but they do collide and share energy to maintain a roughly thermal distribution.

11. Dec 6, 2015

### Staff: Mentor

Yes, that was my question. Thank you.

12. Dec 7, 2015

### Buzz Bloom

Hi phyzguy:

Thanks for the reference
"Radiative Processes in Astrophysics", by Rybicki and Lightman.

Firefox doesn't like the link "at this site." It complains as follows:
This Connection is Untrusted
You have asked Firefox to connect securely to www.astro.rug.nl, but we can't confirm that your connection is secure.
Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified.​
I find that Firefox is more protective regarding security matters than other browsers.

Regards,
Buzz

13. Dec 7, 2015

### davenn

its linking directly to a PDF file .... my various anti virus etc progs gave no warnings
you should be OK to continue to the file

the maths in the paper is way over my head

Dave

14. Dec 7, 2015

### phyzguy

Oh, well. You get what you pay for. My copy of Safari didn't complain. I have downloaded the file and I doubt there is a security issue, but I make no guarantees. I uploaded a screenshot of the relevant pages

#### Attached Files:

• ###### Brems.png
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40.1 KB
Views:
30
15. Dec 7, 2015

### Buzz Bloom

Hi phyzguy:

Thanks for the post.

I did download the PDF file, and the math is over my head too. But it does show that someone did calculate the long time required to dissipate the high temperature.

Regards,
Buzz