Talk:2035: Dark Matter Candidates

Explain xkcd: It's 'cause you're dumb.
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"thin patina of grime covering the whole universe" is a reference to the "International prototype kilogram" and the necessity to keep it dust-free to preserve its reference status. 11:14, 20 August 2018 (UTC)

I think it's just referring to how your room or furniture can get super dirty and completely covered in dust, but you don't really notice it getting dirty because it happens so gradually. But once you actually get around to cleaning your room and you remove all the dust you realize how insanely filthy your room was, now that you can compare it to clean. Since there hasn't been a massive universe cleaning within human history, we wouldn't really be able to tell if the universe was coated in dirt because we wouldn't remember what it looks like clean. Yosho27 (talk) 12:53, 20 August 2018 (UTC)
I concur, my thought upon reading the "thin patina of grime" was when I helped a friend power wash his back deck and we realized it was far more dirty than we thought; as the newly washed sections stood out in stark contrast to the grimy parts. 19:29, 20 August 2018 (UTC)

One can only hope that the solution for dealing with space cows involves space cowboys. 20:40, 20 August 2018 (UTC)

109 kg - 1033 kg black holes

Not sure if it's a mistake by Randall or he has something other in mind. But most of his black holes are far too lightweight:

  • 109 kg is a million tons, the Great Pyramid of Giza wights six times of that
  • 6x1024 kg Earth
  • 2x1030 kg Sun
  • 1031 kg smallest known stellar black hole
  • 1040 kg the real big black holes with a diameter in the size of our solar system

Everything except the Buzzkill is below a single solar mass. --Dgbrt (talk) 16:24, 20 August 2018 (UTC)

The theoretical lower limit for black hole mass is the planck mass (22 µg), although such micro black holes would evaporate very quickly under standard models. However, larger black holes were excluded fairly early by gravitational lensing searches ('buzzkill' cases), so smaller black holes had to be considered separately as dark matter candidates. --Quantum7 (talk) 20:40, 20 August 2018 (UTC)
You misunderstand my point: Those not discovered smaller black holes would need an explanation how they did form but more important here how they could be ruled out as Randall states. A nano black hole at 1010 kg disproved by gamma rays? What's Randall's point? He was more accurate in the past. --Dgbrt (talk) 22:18, 20 August 2018 (UTC)
Axon pun?

My first thought upon reading 'axion' was that it was a pun on axon. Neurons have typical membrane potentials in the mV range, which lines up nicely with the meV energy of axions. Coincidence? --Quantum7 (talk) 20:44, 20 August 2018 (UTC)

An axion is a suggested subatomic particle. I'm not a biologist but if one meV is enough energy to trigger an axon our biology wouldn't work that smoothly. --Dgbrt (talk) 22:18, 20 August 2018 (UTC)
It's mV (electrical potential), not meV (energy/mass). It's a stretch, but Randall's included more distant puns before in XKCD. Source for action potential strength: [1] --Quantum7 (talk) 23:15, 20 August 2018 (UTC)

While this comic is about Dark Matter, does the explanation really need to include a justification on why Dark Matter really exists as a "substance" instead of being some error in our understanding of gravity? It seems a little excessive and unnecessary to me. Ianrbibtitlht (talk) 21:46, 20 August 2018 (UTC)

I'm with you but this comic is about that "substance" like most astronomers are. This always reminds me to aether - also a famous "substance" in space more than hundred years ago which nobody could explain. --Dgbrt (talk) 22:32, 20 August 2018 (UTC)
Thanks for the laugh - my thoughts exactly! In fact, part of me wonders if Randall is actually making fun of the whole idea that there's a dark matter particle at all, since there's such a wide range of possible sizes for such a particle. His humor can be so subtle at times that someone may not realize when they're actually the butt of his joke. Ianrbibtitlht (talk) 23:55, 20 August 2018 (UTC)
Actually, the observings that dark matter doesn't seem to be at same places as normal matter is countering the idea that it's because of error in out understanding of gravity. Like, not completely disproving it, but making it less likely. -- Hkmaly (talk) 04:15, 21 August 2018 (UTC)
Excessive? Maybe. But the first responses to you indicate that people who have presumably even read the explanation as to why dark matter really exists don't understand why we expect that dark matter really exists. (Sure, modified gravity theories were a reasonable alternative hypothesis fifteen years ago, but that was before we'd made multiple independent observations that the gravitational effects are decoupled from the presence of visible matter, and thus cannot simply be gravity working differently at galactic mass scales than General Relativity predicts.)

Furthermore, while space cowboys were mentioned earlier in the discussion, I suspect Randall included space cows in the chart specifically as a reference to the movie Space Cowboys. Also, I think the point about Neutron Star Data ruling out black holes in that mass range is because you can't have both of them with the same mass, since the current theory is that they both form from a star collapse, but at different masses. You're always going to get one or the other from that size star, and since we find neutron stars in that range, we can't have black holes there too. Ianrbibtitlht (talk) 21:59, 20 August 2018 (UTC)

The mass of neutron stars is well understood. A smaller star ends at a white dwarf and the big ones produce a black hole. Nonetheless our sun will end up into a white dwarf and the others require higher masses as in the buzzkill range at the graph. --Dgbrt (talk) 22:32, 20 August 2018 (UTC)
My point exactly - we now know quite a bit about the mass needed and process required to form a neutron star, making it unlikely the same mass would be able to form a black hole. I think that's what Randall meant in that part of the chart, but that's not what the explanation states. (Unfortunately, I've reached the point where I no longer want to argue with other editors over correct explanations.) Ianrbibtitlht (talk) 23:55, 20 August 2018 (UTC)
The Mysterious Eight Ball

How many of you remember the 8 Ball as a funny toy that you would ask questions and then turn over to receive an answer. Could that be the joke referred to in the 8 ball as a possible source of mysterious dark matter? --ProfKrueger (talk) 00:41, 21 August 2018 (UTC)

Explain xkcd: It's 'cause you're being physics-nerd-sniped! Ianrbibtitlht (talk) 00:09, 21 August 2018 (UTC)

Sorry to be picky, but I'm having trouble with "a star which was nearly in line with the sun appeared closer to the sun than usual." Doesn't a distant star's apparent position move away from the sun compared to the direct path? The light ray we see has been bent toward us, so it appears further away than an unaffected ray would, no? 03:31, 21 August 2018 (UTC)