I' ve read that the CMB could be emitted by intergalactic medium. What about it?
I've heard that there are invisible pink unicorns in my garden. What about that? I think you can see that this isn't a particularly fruitful way to start a discussion. The fact that you have heard something doesn't make it true. Do you have any evidence? That would be the way to start a discussion.
Provivding a source would be a better way too...
I think he might be asking whether anyone else has encountered this idea. I have not but I am not a cosmologist.⚛
I vaguely remember hearing ideas like what he says, but it was a long time ago. I think those ideas went by the board when the spectrum was plotted (perfect fit to a thermal black body curve) and then the temperature variations were mapped.
Originally there was some controversy---Wikipedia indicates it although its article is not necessarily guaranteed authoritative. Still it gives an idea:
The interpretation of the cosmic microwave background was a controversial issue in the 1960s with some proponents of the steady state theory arguing that the microwave background was the result of scattered starlight from distant galaxies...
...However, during the 1970s the consensus was established that the cosmic microwave background is a remnant of the big bang. This was largely because new measurements at a range of frequencies showed that the spectrum was a thermal, black body spectrum, a result that the steady state model was unable to reproduce.
WikiP has a nice picture of the spectrum fitting a blackbody curve. And lots of other stuff:
OMG I remember now reading an article by Irving Segal back around 1974 (it must have been). As late as 1974 there were diehard supporters of the steady state idea, and a big problem for them was how to explain CMB radiation. So they came up with various ideas. None of it was very convincing though. Maybe the problem was that you can tell something about the stuff in intergalactic space by how light from distant galaxies is absorbed, and I think basically there just isn't enough stuff to have an effective temperature and radiate all that microwave power. I'm just an interested person though, not a cosmologist, so I'd have to defer to anybody who can give a more coherent explanation.
I refer to the radiation from intergalactic medium. A body at that temperature must have a black body spectrum, and it is a 3 K matter spectrum, like the dust among the galaxies. Is the dust emission quite powerful to emitt that radiation in the microwaves?
marcus's guess was no:
Yes, but the sum of all the photons that arrive from everywhere to the intergalactic medium could warm it to the emission in the microwaves. Or not?
Well, that would mean that there would be more background radiation coming from directions where there are more galaxies, right? But I thought the radiation was basically even in every direction.⚛
But the intergalactic medium is isotropic on large scale, like the distribution of galaxies. And there are anyway anisotropies in the CMB, isn' t it?
This is a potentially interesting thread, inasmuch as it involves learning some of what is known about the Intergalactic medium (The IGM is the title of the thread.)
A fair amount seems to be known. Temperature in the millions Kelvin. Not 2.7 kelvin! Not uniform static structure---but wispy cobweb dynamically collapsing structures. It's something that astrophysicists study. It is not the uniform evenly distributed thing all at the same equilibrium temp, that you might initially think. Because parts of the medium are falling into other parts, gravitational energy released by falling and collision is a major energy input to the IGM. I gather it does much more heating than, for example starlight.
With widely different temperatures in the millions K, the Intergalactic Medium is not a good candidate to be the origin of the CMB. To put it mildly
I have to stress not being expert in this. I don't know whom in our immediate PF community we could call on---maybe Chroot knows.
Ignition's initial idea that the CMB could be radiation from the IGM is not the issue. What's interesting is the general nature of the IGM. We should pool what we know, and hope someone with more to say will show up.
For starters, even though WikiP is not a guaranteed authoritative source, I'll give the Wikilink. It's often useful:
Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter. The density of the Universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to conditions in vast voids that have much lower density than the Universe's average.
Surrounding and stretching between galaxies, there is a rarefied plasma that is thought to possess a cosmic filamentary structure and that is slightly denser than the average density in the Universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen, i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the Universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the Universe in rich clusters of galaxies.
The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of 105 K to 107 K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half the atomic matter in the universe might exist in this warm-hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108 K and above.
Paraphrasing Wikipedia, for the moment, in lack of any better source:
The reason you can't just point a bolometer (a radiation thermometer) at the sky and get readings on order of a million kelvin (widely differing in different patches of sky) is that the IGM is so rarified. The IGM radiation must be negligible compared with the CMB---the light coming to us from the hot (3000 K) matter in the early universe.
So for practical purposes the actual temperature of the sky is the CMB temperature, that classic blackbody curve that you get from a 3000 K blackbody shifted by stretching wavelengths about 1100-fold, so it becomes 2.7 K blackbody.
The 3000 K matter in the early universe has the properties of equilibrium and uniformity that we need to explain the uniformity of the observed CMB, whereas the IGM matter does not have the required properties. It is all uneven and wispy and patchy and the wrong temperature.
So it cannot be possible because the space among the clusters is mostly vacuum, isn't it?
But is the structure on large scale similiar to a net?
Thanks for clearly answering.
Thanks for broaching the topic! I would like to know more about the IGM. I hope someone will find some sources directly in the scientific literature. I would like to know how they map the IGM. I remember seeing some pictures, but I don't remember the details.
Do you know how to use the search tool at Spires, or at Arxiv? Keywords?
I feel pretty out of my depth here and I'm not having any luck quickly skimming the cited papers in this Wikipedia article but it does seem to agree with what Ignition was saying in this quote - it claims that there's a region of the sky where there's both a CMB anisotropy / "cold spot", and a large-scale structural void.
Thanks for the discussion of IGM, marcus.⚛
One thing to note, if there was a medium dense enough to be a decent radiation source, we'd detect it gravitationally and via the blocking of light from distant galaxies, would we not?
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