Astronomers Just Solved the Mystery of the Universe's Missing Matter | Hacker News Reader

Astronomers Just Solved the Mystery of the Universe's Missing Matter

gizmodo.com

39 pointsguerrilla22 comments

It's important to note that this isn't the same as Dark Matter.

Indeed, the headline makes it sound like it is.

I remember this being news two years ago [1]. What changed between then and now? Seems like dejavu reading about the same missing matter being found again.

[1] https://www.astronomy.com/science/half-the-matter-in-the-cos...

Perhaps I'm missing something critical.

As I understand it: when astronomers are looking at things a very long distance (measured in lightyears) away, they are looking at how things were that number of millions/billion(s) years ago.

Based on my possible misunderstanding, shouldn't any such claims be made on the basis of how things were and with no indication as to how things may have changed since?

The universe... what a concept. You know, the universe is a little bit like the human hand. For example, you have Growmann's Center right here and then you have undiscovered worlds and and Sector 8 and up here is Tittleman's crest so you can kinda picture it's a little bit like a leaf or, well, it's not a bowl. The universe is beautiful. Put all the stars, the hundreds of stars that there are in the universe into a bag and put the inverse into a bag and all of a sudden they become, well, when i was a child there was thought to be 9 planets but there are now ninety planets you know the ultimate fate of the universe is so dark and mysterious that it generates butterflies in my stomach and that goes to tickles in my spine and that creates goose pimples and then that penetrates my mind and then the whole big bang explos - BWOOO BWOO BWOO BWOOO BWOOOOOOOOOOOOOO

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?

I want to hear what Sabine Hossenfelder says. I trust that she will say her honest truth.

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.

The modern version of History Channel shows with titles like Ancient Nazi Alien Secrets Exposed.

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).

Right, Sabine's channel is the modern version of that "ancient" show.

It’s sarcasm …

Wouldn’t that be trivially the average velocity of the particles?

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.

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"?

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...

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%.)

I wonder if we'll have to revise our current measurements of distances among stars and galaxies as a result?

This sounds very certain, like it’s accepted fact.

Word. Scientific consensus isn’t announced in headlines.

> By measuring this decrease in speed

How do you measure the speed of a radio signal going away from you?

Red shift.

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