Astronomers solve the missing baryon problem
astronomy.com
astronomy.com
Similarly, I am frustrated that videos come up as top hits for simple things. While a picture can say a thousand words, the point is communication, not verbosity.
I wonder what his special techniques are to keep viewer engagement and help explanation, and whether he's consciously choosing to do them.
For example, I notice that unlike most Youtubers who'll record the narrated portion purely from their desk (showing their face) perfectly still and nicely lit, he will even handhold his camera and move around a room. Or when interviewing someone, hold a camera himself showing the interviewed person, and yet have another still camera filming him and his other camera and the interviewee. Or standing in front of a wall projector screen video and talking (despite the projected image being mediocre quality) when most people would insert that as a original video source clip and narrate over it.
And yet while this might usually be distracting, for some reason with the quality of the narration, this makes for a nice break from usual. Or makes it seem more "authentic" and engaging.
Interesting!
Videos are excellent for content that should be shown. For descriptive content like this, a video is a strictly inferior mode of transmission.
It has to be rather dense to be that hot for so long, no? Yet it's not some sort of stars?
Matter is easy to see when it emits light (like stars) or absorbs light (like uncharged matter). But charged plasma does neither, so it's hard to detect. Fortunately, the WHIM has a prism-like effect, where it smears out light travelling through it, so we can detect it.
That's the part I don't get; if the intergalactic medium is that hot, shouldn't it be emitting huge amounts of blackbody radiation? Supposedly ionization interferes with that, but even black holes emit blackbody radiation (hawking radiation), so that seems like a insuffient explaination.
Clearly, then, the matter is not in equilibrium with the electromagnetic field. How can this be?
Equilibration is a process that proceeds through a large number of interactions that each exchange energy between two thermal systems (or subsystems). If energy transfer is prevented, for instance by the conservation of mass and momentum, or because no post- states of an appropriate energy are available for one subsystem to evolve into, then the systems may remain far from equilibrium relative to each other.
I’d have to do some homework to verify this, but I think a correct statement is that free massive charged particles don’t gain or lose energy in net from passing radiation; and conversely, they cannot radiate. This is guaranteed by Lorentz symmetry, btw: if you examine a million-kelvin electron in a frame of reference moving alongside it, you’d agree there’s no way it can be radiating: there’s a frame in which it is stationary, and stationary charges don’t radiate. If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame.
Only during collisions between charged particles is there a possibility for the matter and Maxwell degrees of freedom to exchange energy: and these are very rare in tenuous plasma. (We would call these “three-body interactions” because the photon counts as one.)
[edit to add] Now one might object that charged particles are always interacting, because their fields fall off like 1/r^2; they don’t just end. Indeed! But one of the effects of being in a plasma is that charges are “screened”. You can think of that as meaning the net effect of many-body interactions can be described as altering the “effective” field of each particle to fall off much faster, like e^-r.
Maybe this is the answer to a question I've been harboring in the back of my head since high school: if moving charges radiate, they should lose energy, and thus eventually disappear. Almost nothing ever is stationary, so how come all the charges haven't radiated away?
A quick googling now (I don't know why I never bothered to do that before) pointed me to https://physics.stackexchange.com/questions/65339/how-and-wh..., which would suggest that it's not movement but acceleration that makes charges radiate. But again, almost nothing ever is stationary, nor is it ever in truly uniform motion, so how come all the charges haven't radiated away?
The answer is on that scale it no longer makes sense to think of an electron as a tiny planet orbiting another tiny planet. The rules of quantum mechanics apply, and quantum mechanics says the orbits are quantized and there are no tinier orbits to decay into.
So the full answer is that there only seems to be a paradox because your notion of when things should and shouldn't radiate is an approximate truth which only holds for "normal" cases (neither microscopic nor cosmological).
Anyway, the density of this matter could be so low that the amount of energy being emitted/received here per unit area (e.g. per square degree of sky) is too low to be detected by equipment. Also note that the radiation is getting redshifted at every distance, so the signal would be further "smeared" and weakened compared to if it were all at a narrow frequency range.
Can single atoms undergo BB radiation? I would think no, but would love to know when it transitions to yes. Articles on BB radiation kind of gloss over this part, only discussing the bulk phenomenon.
A normal human perspective on 'hot' or 'cold' relates to collections of matter at the pressure and density that we live within. There is ample material (mass) and opportunity for energy exchange (conductivity), so if something is relatively hot or cold there is a large difference in potential energy and much opportunity to 'correct' that difference by equalizing energy.
The WIMP gas they describe would be __highly energetic__ (hot), but also extremely sparse (vacuum). For any given atomic element present it's rather hot. Yet the actual energy density within a given cubic meter of space, relative to some idealized vacuum, is nearly zero compared to the dense muggy energy in a cubic meter of even frigid polar arctic night air.
Or is WIMP basically uniformly distributed, high energy component of interstellar medium?
We know what kind of gravity it takes to keep particles this hot together: this is how we have stars. But it's hard to see how gravity can keep an object this vast, diffuse and energetic together.
Moreover, the "million-degree" figure is actively misleading. Temperature is really a measure of average particle speed, in a reference frame where they have zero average velocity. (This is why rocket exhaust as it exits the bell is said to have low temperature and pressure -- the particles are moving very fast, but in one direction.)
In the intergalactic medium, if the atoms are fully ionized, the electrons are the ones moving like bats out of hell. The atoms, mostly hydrogen, are at least 1836 times more massive ("but graceful for their size"), so going much, much slower. The average of the two motions defines the temperature. The electrons' paths bend when they get close to another electron or ionized atom, and emit synchrotron radiation that is just at the right energy to be absorbed by another electron not far away.
Since the density is so low, blackbody radiation that escapes must be very dim. Orbital x-ray telescopes might see a fog if they are sensitive enough, but the photons it picks up probably originate in much nearer interactions.
Anyway that is my guess.
Isn't it the case though that thermal energy is not the same as inertial energy of motion? I.e. that "hot" atoms can be essentially "vibrating" in place much faster and more violently, and not necessarily moving through space?
The conclusion being that blackbody radiation isn't just due to temperature, but also interactions between particles. (Which explains why it's a topic under statistical mechanics. It's an emergent property from large numbers of particles.)
This reads like something from XKCD!