How Is Dark Matter So Hot Yet Invisible?

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Discussion Overview

The discussion centers around the properties and detection of dark matter, particularly focusing on the high temperatures associated with baryonic matter and its implications for cosmological models. Participants explore the nature of dark matter, its visibility, and the challenges in estimating baryonic density in the universe.

Discussion Character

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

Main Points Raised

  • Some participants express surprise at the reported temperature of dark matter being around 1,000,000 °C and question why such hot matter remains undetected.
  • It is noted that the kinetic energy of individual particles in the cosmic web is high, but their diffuse nature means they do not have a significant impact on Earth.
  • One participant clarifies that at such high temperatures, the radiation emitted is primarily in the x-ray spectrum, which is not visible to the naked eye.
  • There is a discussion about the baryonic and non-baryonic components of dark matter, with some participants emphasizing that most dark matter is non-baryonic and its nature remains uncertain.
  • Questions are raised regarding the density estimates of baryonic matter and how they relate to historical assessments of baryon density in the universe.
  • Some participants mention that the current estimates suggest visible baryonic matter is about 0.4%, while total baryonic matter is around 4%, with implications for the "missing matter" problem.
  • Concerns are raised about the potential consequences of finding more baryonic matter than currently estimated, particularly in relation to the Big Bang model's viability.
  • There is a suggestion that the assessment of visible baryonic density may be influenced by cosmological constraints rather than purely observational data.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of dark matter, the implications of baryonic density estimates, and the relationship between these factors and the Big Bang model. No consensus is reached, and multiple competing perspectives are presented throughout the discussion.

Contextual Notes

Participants highlight limitations in current understanding, including the dependence on definitions of baryonic and non-baryonic matter, and the challenges in accurately measuring galactic mass. The discussion reflects ongoing uncertainties in cosmological models and the interpretation of observational data.

Mean-Hippy
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Hey ! So we found some of that dark matter, :smile: woohoo !
But honnestly, I thought their had been a misprint on the Chandra site when I read that they calculate the temperature of these webs of particles to be
1 000 000 °C :bugeye: ! ? ! ? ! ? ! How the #!*@¤ did we not see something that's so blazing hot before ? The SCIAM article talked about it being to hot to see ? Can someone explain that ? I gather this is a humongous reservoir of energy. Did we expect anything like this ?
 
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That is the kinetic energy of the individual particles. They are very diffuse. The solar wind particles bathe the Earth in temperatures exceeding 1,000,000 degrees [much hotter than the surface of the sun], yet we have not been incinerated. Why? Too few of them to be noticeable. Obvious questions often have obvious answers.
 
The SCIAM article talked about it being to hot to see ?

At this temperature the bulk of the radiation is in x-rays. There is very little at light frequencies - remember the sun's surface is about 6000 deg.

One point that should be made clear is that this matter is baryonic. The bulk of the dark matter (required by gravitational effect) is non-baryonic. Physicists are still trying to figure out what it is and how to detect it.
 
Is the estimate of the density of this gas independent of the standard BBN cosmological requirement of a total baryon density no larger than 4%?

The 'invisible' baryon density, in the form of ICM gas, Lyman alpha forest IGM gas, black holes and MACHOs etc. is known to be 10 times the 'visible' baryon density in the form of stars, HII regions etc.

Therefore the cosmological baryon density of stars etc. is now thought to be only 0.4%.

Yet earlier, in the 1970's, & 80's, the visible baryon density was generally thought to be about 2%. So why the modern reduction in that value?

Garth
 
So like how much more baryonic matter did they find ?
 
The last estimates I have seen (and they do change from time to time) is that visible baryonic matter is about 0.4% while total baryonic matter is about 4%. These hot gas clouds contribute to the part of the 4% that is not visible - I believe the estimate is about half.
 
Yes, the matter is normal baryonic matter that is just not very detectable, and has "gone missing" in previous mass estimates. It's still just a tiny portion of the mass that the Big Bang needs to stay viable.

Chandra Website said:
The Chandra X-ray spectrum of Mkn 421 provides strong evidence that a large fraction of the atoms and ions in the Universe are located in the cosmic web, and may point to the solution of the "missing matter" problem. The missing mass problem - not related to dark matter or dark energy - was discovered when various measurements gave astronomers a good estimate of the number of atoms and ions in the Universe 10 billion years ago. However, inventories of all the atoms and ions in stars and gas inside and outside of galaxies in the present era yields only about half as many as were present 10 billion years ago. Almost half the atoms and ions in the Universe had gone missing!

http://chandra.harvard.edu/photo/2005/mkn421/
 
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Thanks for the link
 
turbo-1 said:
Yes, the matter is normal baryonic matter that is just not very detectable, and has "gone missing" in previous mass estimates. It's still just a tiny portion of the mass that the Big Bang needs to stay viable.
The big bang model does not need more baryonic mass to remain viable. In fact, if very much more baryonic mass is found, it would falsify the big bang model.
 
  • #10
Chronos said:
The big bang model does not need more baryonic mass to remain viable. In fact, if very much more baryonic mass is found, it would falsify the big bang model.
Yes - the present assessment of visible baryonic matter is 0.4% and the invisible baryonic matter is 10x this or 4%. (% critical density) And this value is the maximum value allowed by BBN without upsetting either the helium or deuterium relative abundances.

Therefore any increase in the estimate of visible baryonic density might be catastrophic for the standard model.

My question is - has this already happened without it being generally acknowledged?

In other words are we sure that the assessment of visible baryonic density has not been driven by cosmological constraint rather than astrophysical observation?

How much visible mass is there in the Milky Way? How many ordinary stars are there, and what is their combined mass? In addition what is the mass audit of the HI ISM and HII regions?

Normally this has been answered by a survey of star fields together with an estimate of the Keplerian mass of the disc, however the latter is also strongly influenced by any interstellar DM and the effect of the galactic halo, that is of course assuming we know how to model that accurately.

If we rely of non-gravitational methods of assessing galactic mass, and we still have to use Kepler to assess the stellar mass function; nevertheless do we get an answer consistent with a cosmological average of 0.4%?

Garth
 
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  • #11
Chronos said:
The big bang model does not need more baryonic mass to remain viable. In fact, if very much more baryonic mass is found, it would falsify the big bang model.
I understand. I was trying to point out that the BB needs a lot more mass than sensed by Chandra (to provide the gravitational effects observed in galaxies and clusters). I was not aware that just a little more baryonic mass than Chandra found would upset the BB apple cart.

It puts the nature of dark matter in a new light if DM has to be non-baryonic for yet another reason: The old reason 1) we have never yet managed to sense any of it and now, 2) the amount of DM required would kill the BB model if indeed it is baryonic in nature.
 

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