It turns out that the mass of neutrinos produced so far adds up to as much mass as all the rest of particulate matter. I guess neutrinos must be distributed about evenly throughout the universe. If we could not ever detect neutrinos, that would be awkward.
I have not heard of a mechanism by which these dark matter particles can cool and condense to clumps to seed galaxies. By contrast, baryonic matter gets to emit photons to give up kinetic energy.
So don't discount adding currently undetected things as explanations for surprising measurements. It's been extremely fruitful.
I don't really even object to inventing five times the mass of the known universe.
It just feels, as I said, un-parsimonious. It feels quite a lot like inventing God to patch a "missing link" in your evolutionary succession. Maybe you'll find God there somewhere, but it doesn't seem like the first thing to try.
So, maybe dark matter really will turn out to be an ordinary axion or something detectable only if you manage to squint just right, and God just loved those so much that almost everything is them, and we are all made out of just leftover scraps. Maybe there are dozens of elementary axions, with relationships and exciting interactions we can never figure out because we can't touch them in any way but gravitationally.
But I still want to know how they cool.
Dark matter isn’t just adding a fudge/“God” parameter to the equations to make the numbers work. The “stuff” that’s thought to be missing is described with very precise properties… the fact that it’s “dark” means something, the fact that it’s “matter” means something.
The evidence for dark matter is pretty solid.
As this [1] article states, while giving 5 independent reasons scientists think dark matter exists, "No other idea explains even two of these".
Wikipedia lists eleven different places it shows up.
The research literature has more. And AFAIK there is no other unified (or even close to unified) explanation for all these observational pieces of evidence.
[1] https://medium.com/starts-with-a-bang/five-reasons-we-think-...
(Lovely typo.)
I don't believe that's true? The energy density in neutrinos today is < 0.5% (baryonic matter is ten times that.)
That means space also has to decay. And unless you want space to become non-existent far away from matter, the rate of decay must also fall off somehow as you get further away from matter.
I wonder if you could get such a space creation/decay process to match a curve that's 1/r2 at short distances and 1/r at long distances...
It's because particles are exceptional things, not the normal, calm field they come from. If you base your theory on those exceptional thing it will be non representative or outright wrong.
That's why quantum physics is giving us weird results, because particles are exceptions, not the common state of the field.
If it doesn't depart too much from the analogy, what is the lake (water below the surface, under the field) and what is above the field?
Are we supposed to assume that the water below the surface is inside the field, and above the surface is outside of the field?
What is the non-exceptional form of the particle, or thing that makes up the particle? I guess this is the essence of your analogy, and your point is that we've spent so much time trying to identify this thing as a particle, we can't yet answer this question?
Or have I now dissolved the analogy by taking it too literally?
More than what it is, the question is what properties does it have. Anyway the object of interest is the surface.
What is the non-exceptional form of the particle, or thing that makes up the particle?
The surface.
Was trying to understand what was inside (water) and outside (air) the field to help me start thinking about what the field (surface) might be.
The narrative of MOND-the-meme is totally wrong but extremely compelling. That's enough to give it outsized fitness in its niche, and then that general compellingness (which comes from being really good at tricking us) is misinterpreted as scientific merit. Even by those who say they're interested in truth, not stories.
It's an excellent test-case for critical thinking because everything about it as a meme should set off alarm bells, but often it doesn't.
MOND the physical hypothesis is perfectly good science, whether it turns out to match reality or not. My issue is with MOND the meme, the complex of beliefs among non-physicists (like me) that MOND explains "dark matter"[0] without matter which is dark, whose fitness isn't really based on the success of MOND-the-model.
I say MOND-the-meme is pseudoskeptical because it a) manufactures disbelief by misrepresenting a subject and then b) provides a "skeptical" alternative in order to appeal to skeptics which c) does not itself stand up to skepticism because (a) was a strawman and (b) was motivated reasoning. It survives because it makes you feel smart, not because it's sound or valid or the MOND hypothesis is true.
Accepting MOND-the-meme would still be problematic even if the MOND hypothesis was overwhelmingly confirmed tomorrow.
[0] Taking full advantage of the ambiguity between "dark matter" as the name for observations, one hypothesis explaining them, and an overall cosmological model.
> despite not being a successful physical theory.
I think the jury is still out on that, and, like I said, it has actually made predictions which is more than what you can say about LCDM, which as far as I can tell has only made post-hoc rationalizations.
Narrowing the meaning of "success" can lead (me, at least) into a philosophical morass about what really are predictions, what is parsimonious, how do we science, why even is anything. Sorry, maybe it's not fair, but I don't really want to go there.
Rereading my first comment, I'm not happy with the tone. I wrote it quickly on a phone and didn't get it right. I was honestly just making observations without judgement, but it doesn't sound that way.
It's an easy way to be "skeptical" without really knowing anything about the relative success of the two theories. Because MOND is an underdog theory, you can feel like a skeptic questioning dogma by supporting it... even if you don't really know much about the merits.
It seems a little flippant to denounce it as a ridiculous "meme" idea. It's healthy to have different scientists pursue different theories. And valuable insight can be gleaned even when taking the time to _disprove_ a distant possibility.
I'm criticizing MOND-the-meme, I guess, but also admiring. I don't like it, but it's very good in its niche.
Like a shark. Stay over there please... but wow.
The answer is most likely MOND + dark matter.
Here[0] is a toy model experiment to demonstrate that neither Dark Matter nor MOND are necessary to explain spiral galaxy rotation curves, which coincidentally is the original purpose Dark Matter was postulated to explain. Dark Matter has since been fingered to explain other phenomena since, but it is not commonly known or accepted that galaxy rotation curves no longer needs Dark Matter (or MOND, for that matter).
Tl;dr Dark matter model simulation can't explain bullet cluster, and MOND can.
> Either way, the Bullet Cluster remained a stunningly unlikely event to happen in the theory of particle dark matter. It was, in contrast, easy to accommodate in theories of modified gravity, in which collisions with high relative velocity occur much more frequently.
it introduces more parameters to fit its arbitrary conception about how gravity is supposed to scale. even worse, since basic MOND disagrees with a ton of basic evidence since it just doesn't work except for the few problems it was created for, the actual MOND derivatives that do try to make it work introduce even more parameters to fit.
MOND has made successful predictions:. The EFE, for example (there are others). LCDM has not, as far as I can tell -- please correct me if I'm wrong [0]
> work except for the few problems it was created for
That's actually a poor heuristic. Suppose we hadn't discovered relativity yet (but had galactic rotation curves, which only depend on good telescopes and newton, and Doppler effect), and we observed mercury's precession. Would you then use precession as an argument for dark matter on the basis that there's a dark matter entity in our solar system pulling mercury around? I would think that would not be a good idea.
[0] ok I looked it up and actually LCDM predicted no galaxies with redshift greater than 7... And thanks to jwst we now know there are galaxies with much much greater redshift, a feature explicitly predicted by MOND.
I also assume this has been accounted for though
Also, just as a note, I'm using the term "mass" for what used to be called "rest mass" (as opposed to "relativistic mass"). Even if their "rest mass" is truly exactly 0, and their speed is exactly c, they still have energy and momentum, and thus they have mass in general relativity, but that is a different discussion.
There is an advantage to arranging dyson spheres closer together in order to reduce latency between nodes.
If your dyson spheres were optimized enough, could absorb all known spectrums and recycle them. Wouldn’t that appear completely dark save for gravity?
I mean, isn’t it possible entire galaxies have been converted in this way? Seems like it could be a much simpler explanation within the confines of current understanding.
But I’m sure this has been thought about and accounted for.
A really cool outcome of statistical mechanics and thermodynamics is that what you suggest is impossible. The dyson sphere can be as amazing as you want, using every single trickery permitted by physics, but at some point it will enter thermal equilibrium with its host start (or pipe that heat somewhere else where you would still be able to see its thermal radiation). Inherently, if they are optimized to be good absorbers, they will have to also be thermal emitters. There is no such thing as "recycle all spectrum".
Tangentially related to the fact that a "perfectly black body", i.e. a perfect absorber, is also the perfect emitter of thermal radiation of its own.
Couldn’t one build a dyson sphere around a blackhole? Wouldn’t the blackhole have properties of a Bose–Einstein condensate given the matter at the center cannot vibrate?
It would be the coldest temperature possible. Heat moves downhill, you deposit heat into a singularity and have a shell around it.
That aside, at even a scale of a single dyson sphere, something tells me when a civilization has the power to bend spacetime, the rules of possibilities begin to change. I think it is unwise to operate with so much certainty in the face of possibilities.
And the Milky way would have to be un-dyson sphered, because if it were, we would be able to notice these spheres close up. e.g. if dark matter is 5x regular matter, we would expect there to be 5 Dyson spheres of stars the size of the alpha centauri system all within 4ly of Earth. This would be easy to detect through occlusion. Also, gravitational interactions between sphered stars and unsphered stars would be easily observable in our own galaxy. So for this to work, at minimum, the whole observable universe must by Dyson sphered, except for our own galaxy.
(Reference https://www.destroyallsoftware.com/talks/wat)
Now he can go back to stealth meta sarcasm talks.
However: "The most serious problem facing Milgrom's law is that it cannot eliminate the need for dark matter in all astrophysical systems: galaxy clusters show a residual mass discrepancy even when analyzed using MOND. The fact that some form of unseen mass must exist in these systems detracts from the adequacy of MOND as a solution to the missing mass problem, although the amount of extra mass required is a fifth that of a Newtonian analysis, and there is no requirement that the missing mass be non-baryonic." [0]
[0] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics