Which is what Chornos said when he first brought it up. Most of it is unaccounted for.
But it’s almost certainly what Chronos meant when he said some of it had been discovered.
The majority of NBDM is what everyone wants to find. It has different properties than neutrinos. For one, it doesn’t zip off at nearly the speed of light. Instead it hangs around in galactic-sized clumps and changes the rotation rates of stars and other effects. MOND, by the way, was invented to account for those rotation rates, but didn’t account for other things like gravitational lensing. At least not without tweeks.
Yes, I was referring to neutrinos, which undoubtedly exist, and which undoubtedly make up some portion of the dark matter. No, it’s not a very large portion of it, but it’s some. And given that it’s clearly possible for particles to exist that don’t interact with electromagnetism, why shouldn’t there be more of them?
Yes, that’s why I mentioned it, so people might re-read his earlier posts.
You must have missed the first part of his post. You are made of dark matter. So is everything you’ve ever touched including every bottle ever made and the contents therein. Every bottle of anything is a bottle (or whatever) of actual dark matter.
But don’t I absorb and emit electromagnetic radiation? When burned, do I not produce light? If you prick me… the blood is red, right?
I could be entirely misinformed about this, (not the first time.) but I don’t believe that there is a specific lower limit to the speed of a neutrino. There is some pretty convincing evidence that the creation of neutrinos generally ocurred at extremely high energies, but as space has inflated, and time has passed even those formed in the first moments of the universe are much less energetic. So much so that detection of them is likely to be problematic for years, or longer.
Some researcher has an experiment planned, but he needs 100 grams of tritium to do it. Sadly, there are some other things that you could do with 100 grams of tritium that makes the folks who have all the tritium nervous about giving it to him. (six month old information at best.)
So, I would need someone fairly well versed in particle physics, and cosmology to explain the upper and lower energy limits for processes producing neutrinos, and the likely history of events like that before I would feel entirely enthusiastic about dismissing the possibility of a significant fraction of the non luminous mass of the universe might be neutrinos of very low energy.
Dark Matter. It seems to have mass, and it doesn’t emit light. A bunch of neutrinos with a local relative velocity of a few hundred KPH fits that bill.
I find it entirely unremarkable that the mass budget of the universe includes a bunch of different non radiating stuff. Not as exciting as previously unknown Niftytrons wrapped in new physics, but hey, there might be some of them, too.
Tris
The actual largest thing ever identified is a very large empty place. Not surprisingly, it was hard to find.
We think we know quite a lot about the early days of the Universe, and if the majority of non-baryonic dark matter was just neutrinos slowed to more reasonable speeds by space dilation, I would think it would be a quite major theory of dark matter, but I’ve never seen it seriously discussed before. I myself have thought “why can’t they just be slow neutrinos?” but I never really looked too hard into it and assumed that people smarter than me had ruled them out since I was a lot less knowledgeable about astronomy than people who have studied it most of their lives.
One can in principle have slow neutrinos, since they aren’t completely massless (well, at least, two out of the three masses aren’t zero). But slow neutrinos have also been ruled out for the bulk of the dark matter (though I don’t know the precise details of those measurements).
I think high-mass sterile neutrinos are still a possibility, but one that I’d frankly consider less likely than SUSYs, or some other sort of particle that doesn’t play any part at all in our current frameworks and hence is unpredicted (a “who ordered that?” particle).
The thing is, that even if Dark matter is slow neutrinos, or some other particle that has yet to be discovered, it still doesn’t explain why these galaxies don’t have it while every other galaxy seems to be 85% made up of it.
Yup. Like I said, we really have no clue on that one. At least, not yet: I’m sure that this result will prompt a flurry of other papers, explaining and/or refuting it.
Everything I know about physics, after say, Newton, comes from reading into peer reviewed papers until I can no longer understand what I am reading. Over the years that has come distressingly shorter distances into the articles. I still read quite a bit, but published papers are more and more behind pay walls I cannot afford to buy into. That means I have to piece it together from the ranting of those who don’t agree. (They are much more willing to peek out from behind pay sites.)
So, on neutrinos my questions, and the general answers I got were:
Was the early universe, before the events that caused it to become transparent to EM radiation transparent to neutrinos? Maybe seems to be the most popular answer among published sources.
Could neutrinos have moved “beyond” other particles during the inflationary period before transparency. Got another maybe, or two, but mostly probably not. Got no particular explanation of the probably.
No one seems to want to speculate on the effects of photons, which remain at c, on the slow neutrinos. Couldn’t understand what little I got on neutrino/photon interactions.
One source gave a proportion of the mass of the early universe which was due to neutrinos (before they slowed down) of 10% The explanation was entirely mathematical, and well beyond my understanding.
My question about whether the change in velocity due to inflation represented a loss of mass was never answered. (For photons, the decrease in frequency does represent a lower energy value per photon, I had difficulty following the reasoning for that not being a violation of conservation of energy.)
All this arises, of course from having an unfortunate knowledge/understanding quotion. But I keep reading, through the despair.
Tris
Perhaps some time later in creation, God will say,”Let there be understanding."
You can’t apply the law of conservation of energy to the Universe as a whole.
And if you go back far enough, the Universe was opaque even to neutrinos, and it’s actually not much further back than you have to go for it to be opaque to photons. Gravitational waves, on the other hand, could potentially probe much further back.
And the obvious question, was the universe ever opaque to gravitational waves? Does that have anything to do with the initiation of the inflationary period? Is there a fundamental property of answers that causes receipt of one to engender more than one additional question?
This learning thing is more complicated than I thought.
Thanks Chronos. Ignorance fought and created in a single post.
Tris
I got nothin’. (evidence is that I have rather a lot of it, though.)
PBS Space Time just added a video on this topic: No Dark Matter = Proof of Dark Matter? - YouTube
If you go early enough, then yes, the Universe was opaque to gravitational waves, too, but that gets you back to the first fraction of a second.
And the current best guess is that there was no start to the inflationary era: Most of the cosmos as a whole at any given time is under inflationary conditions. This is unstable, and so every so often a patch stops inflating, but inflation expands so rapidly that it still always remains mostly inflationary. What might be described as “the start of our Universe”, then, was just our particular patch stopping inflating.
So a different type of ‘many worlds’? One that doesn’t depend on quantum probabilities and branching?
I wouldn’t use the “many worlds” term for it, because that’s pretty firmly associated with the interpretation of quantum mechanics. But yes, it’s yet another version of a multiverse (I can count at least four things in physics that could be described that way, in completely different ways).
Let’s see:
The ‘usual’ many worlds and the inflationary multiverse. Two.
How about the possible paths taken by a particle in Brownian motion (or any other continuous but nondifferentiable curve). Or, any physical system characterized by ‘chaotic’ behaviour.
More seriously, I doubt I’m alone in wondering which four things you were thinking of. They were . . .?
Brownian motion can be regarded as a stochastic process, that is, its value at any point in time is a random variable, like the outcome of a roll of a die.
Note that a classical chaotic system might well be modelled as a deterministic dynamical system, with no randomness, and conversely there is nothing necessarily chaotic or non-linear about something like Brownian motion.
Note also that, philosophically, the word “multiverse” just means universe (ie the whole world), only that one is emphasizing absence of order or predictability. In that sense, both your examples are good illustrations of the fact that we live in a multiverse.