[HN Gopher] Astronomers Just Solved the Mystery of the Universe'...
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       Astronomers Just Solved the Mystery of the Universe's Missing
       Matter
        
       Author : guerrilla
       Score  : 33 points
       Date   : 2025-06-17 18:44 UTC (4 hours ago)
        
 (HTM) web link (gizmodo.com)
 (TXT) w3m dump (gizmodo.com)
        
       | umeshunni wrote:
       | It's important to note that this isn't the same as Dark Matter.
        
         | dhosek wrote:
         | Indeed, the headline makes it sound like it is.
        
       | gwbas1c wrote:
       | I wonder if we'll have to revise our current measurements of
       | distances among stars and galaxies as a result?
        
       | blacksmith_tb wrote:
       | Perhaps some local astrophysicists can chime in on how the gas
       | could be characterized as "hot" - my naive assumption is that
       | could only be relative?
        
         | reliablereason wrote:
         | I want to hear what Sabine Hossenfelder says. I trust that she
         | will say her honest truth.
        
           | tux3 wrote:
           | The Youtube algorithm unfortunately had the same effect on
           | Sabine as it has on every Youtuber who depend on the platform
           | for income
           | 
           | Sabine has always been a little bit on the fringe of physics
           | (e.g. Superdeterminism has had a, let's call it, less than
           | mainstream appeal)
           | 
           | But now every other video is some complete crackpot nonsense
           | being given consideration for 5 minutes and, hastily debunked
           | in the last minute, and with a title like Could This New
           | Theory of Everything Solve Consciousness and Dark Energy?
           | 
           | Sabine's Youtube is a very different type of content than the
           | old BackReaction days.
        
             | jl6 wrote:
             | The modern version of History Channel shows with titles
             | like Ancient Nazi Alien Secrets Exposed.
        
               | qualeed wrote:
               | I'm curious what you mean by "modern", because History
               | has been showing garbage for closing in on 2 decades now
               | at least (Ancient Aliens is like 17 years old).
        
               | montag wrote:
               | Right, Sabine's channel is the modern version of that
               | "ancient" show.
        
             | bamboozled wrote:
             | It's sarcasm ...
        
         | hyperhello wrote:
         | Wouldn't that be trivially the average velocity of the
         | particles?
        
           | AnimalMuppet wrote:
           | Average velocity of the particles if there are enough of them
           | to collide frequently (and if you can factor out bulk
           | motion). But you can also look at average vibrational energy.
        
             | blacksmith_tb wrote:
             | So collisions would provide enough energy to call them hot,
             | or is that a term of art, like calling all non-hydrogen,
             | non-helium elements "metallic"?
        
         | thayne wrote:
         | So, temperature is basically a measure of the average kinetic
         | energy of particles in a substance. When you have an extremely
         | diffuse gas, as is the case between galaxies, the particles can
         | be moving very fast, but energy density is still low, because
         | there are so few particles. According to the abstract of the
         | paper, this gas is just 10^-3 particles/cm^3 or 1000 particles
         | per cubic meter. That is 5 orders of magnitude less than the
         | space between planets in our solar system.
         | 
         | So, yes, it is hot. But it also very, very sparse. According to
         | Wikipidia 10^5 to 10^7 K[1]. But there isn't very much of it.
         | 
         | As to why they are hot, from what I've been able to find, it is
         | at least partly due to gravitational potential energy being
         | converted to thermal energy, as it falls into filaments.
         | 
         | [1]:
         | https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic...
        
         | gus_massa wrote:
         | Assuming it's not a fabrication of the press release, it may be
         | jargon. Astrophysicists call "metal" everything that is not
         | Hydrogen or Helium, but Chemist disagree heavily.
         | 
         | In this case, the paper don't call it "hot" but it says that
         | 99.99% of the Hydrogen is ionized.
         | 
         | To ionize one Hydrogen you need 13.6eV. The average energy is
         | temperature*k_Boltzmann. So if the temperature is
         | 13.6eV/k_Boltzmann ~= 160000K then the 50% of the Hydrogen is
         | ionized and 50% not ionized.
         | 
         | To get only 0.01% not ionized you need to increase the
         | temperature, IIRC -log(0.01%)~=9 times.
         | 
         | So the temperature is ~1400000K. Unless I'm making an horrible
         | stupid mistake, I agree it's hot.
         | 
         | (I may be missing the 4.7eV of the dissociation of H2 molecules
         | into two H atoms, that would increase the temperature like a
         | 40%.)
        
       | andrewstuart wrote:
       | This sounds very certain, like it's accepted fact.
        
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       (page generated 2025-06-17 23:02 UTC)