About half of the 5% of regular (ie baryonic) Matter was missing from usual astronomical observations. Some hypothesized it took the form of warm-hot intergalactic gas. This could in principle be detected by looking at the dispersion (ie slightly varying transit speeds of waves) through this gas of fast radio burst from distant galaxies, and we just found like five of these FRBs that showed dispersions that matched the predicted amount of baryonic warm-hot gas. Problem solved.
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.
American publishers, YouTube and newspapers all suffer from the same affliction of rewarding their content creators for how long they can keep their audience engaged, nudging the content creators toward bury the lede.
So that means we'll need a sort of magnetic snow-plow-like device for intergalactic travel to shovel all that hot plasma out of the way. Maybe it's easier to convert a whole planet into a starship to take advantage of its magnetic field...
/s
I thought the visual explanation of the Lyman-alpha forest, showing how the distribution of shift in the absorption lines give us a precise map of neutral matter density along the track of Quasar transmissions, was particularly good.
This is a pretty good video -- not overhyped and not filled with frenetic video clips mashed together from across the internet, and also not overly focused on a Youtuber's overly-bubbly personality seeking self-adulation and clicks...
Veritasium is a great channel for such content. He has a degree in online pedagogy, IIRC. So he's particularly good at teaching complex concepts through such a medium. Having a degree in physics helps him too.
I just watched a bunch of his videos -- how interesting! Thanks for the recommendation.
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.
Veritasium videos are pretty darn high quality. Fun to watch and learn from, sometimes mindblowingly. Definitely recommend subscribing. One of my favorites is Derek's video on compliant mechanisms: https://www.youtube.com/watch?v=97t7Xj_iBv0
The WHIM is actually extremely non-dense: 1-10 particles per cubic meter. It turns out that matter can be extremely hot without being dense at all, as long as there's nothing to cool the matter off, as is the case in the space between galaxies.
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.
> Matter is easy to see when it emits light (like stars)
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.
Blackbody radiation describes an electromagnetic field in thermal equilibrium. Typically, we associate this radiation with a particular source, or boundary; but the blackbody spectrum itself is a Maxwell fact, not a matter fact. Real systems have many degrees of freedom, some matter, some Maxwell. In this particular example, the Maxwell degrees of freedom have a temperature of 3K, while the matter degrees of freedom have a much higher temperature.
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.
> If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame.
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?
You ask a very good fundemental question. For things like planets in orbit (under constant acceleration) there really is radiating gravitational waves which carry energy and in principle the orbits must decay over time. Even in particle accelerators the electrons moving in a circle emit radiation which slows them down and must be compensated for. So why don't atoms give off a ton of radiation quickly while the electron decays and crashes into the nucleus?
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).
Considering the particles are charged (plasma, right?), and that moving a charged particle through an electromagnetic field causes acceleration and emission of photons (bremsstrahlung), does that mean that WHIM is going to be more energetic where it didn't have a chance to encounter electromagnetic fields?
For some reason this (along with piannucci's explaination) really helped it click for me - while the gas has a very high blackbody spectrum temperature, emitting hard radiation when it's particles collide at high speed, it effectively has very little surface area from which to emit that blackbody radiation.
The lack of density is the cause of confounding behavior.
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.
I assumed it was some sort of giant intergalactic "clouds"/bands? If this is the case how it remains localized enough given the particle velocities over (assuming) billions of years? There has to be some sort of containing field but it can't be its own gravity.
Or is WIMP basically uniformly distributed, high energy component of interstellar medium?
Note, you've both used WIMP here, but that's another thing (weakly interacting massive particle -- a candidate theory for dark matter). You want the acronym WHIM, for warm hot interstellar medium.
I believe the gravity keeps it from diffusing out too much. The high temperature creates a pressure that keeps it from collapsing into balls. Not an expert though!
Well it was said this is plasma with densities of like 10 particles per m3 so there is hardly any pressure.
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.
At a few nuclei per cubic meter, the particles can hardly be interacting at all, even though, being ions, they interact without actually getting very close to one another, unlike particles in a gas.
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.
> 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.
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?
I think heat energy distributes itself in a system so that each particle carries as much energy as each other, whatever its mass. Molecules can carry energy in spinning and vibrational modes that electrons can't, which means the electrons have to go even faster, for any temperature. (Electrons have constant "spin" that varies only by direction.)
Not a physicist, but "hot" just means high average kinetic energy ("moving really fast"). So imagine a single particle moving really fast (even close to the speed of light if you want)—why that mere fact cause it to emit any kind of radiation? That would require losing energy for no reason, but from its perspective, it's already perfectly stationary. Losing energy just because you (an observer) happen to be moving with high speed in the opposite direction wouldn't really make sense.
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.)
See others' answers about the equilibrium required for BB radiation -- maybe also the density of this matter is not high enough to be a normal BB radiator, or "thermalized"?
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.
By the way, this got me thinking for a long time without knowing the answer:
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.
Hawking radiation is not blackbody radiation. It is caused by pair production, where pairs of particles are produced at random by quantum fluctuations near the event horizon. One of the pair of particles falls into the black hole and the other escapes as Hawking radiation.
