Shouldn't there be a big lump of dark matter at the center of every planet, moon, and star?
If we were to hollow out an asteroid, would there still be a blob of intangible dark matter inside of it?
If we were to hollow out an asteroid, would there still be a blob of intangible dark matter inside of it?
Matter forms big balls because it collides with other matter and turns kinetic energy into heat. If dark matter is weakly interacting, it won't collide, so after falling into a star it will just shoot right back out the other side. It might form a stable orbit, but it also might just fly away and fall through some other gravity well.
You can probably expect some bulk gravitational effects (e.g. Dark matter might tend to cluster along large matter distributions like galaxies), but in general it wouldn't be amenable to staying in one place.
Dark matter may be some field or interaction of existing fields that we don't understand yet.
What you are saying is the exact same thing.
Particles aren't exactly the same idea because they don't explain the coupling between fields, or things like 'virtual particles' which don't "count" as real particles but have important real behaviors of the field itself.
The latter two are awfully hard to distinguish from relativistic approaches to MOND; indeed you could quantize most of the approaches in Chapter 7 of https://arxiv.org/abs/1112.3960 .
More generally, you can put a scalar or other degree of freedom on either side of the usual write-down of the Einstein Field Equations - on the R side it's "modified gravity", but on the T side it's "modified matter". There's no strong reason to choose one side over the other (Einstein and Schrödinger had a couple of letters to each other about "which side" as early as 1918![1]).
Either way you're adding a field to General Relativity -- and you'd see that in a Langrangian formulation like an expansion of the Einstein-Hilbert action[2]. There is no real reason that you couldn't quantize a field on either side of G = T (consider how g is quantized in perturbative quantum gravity, for example), and if they couple at all non-gravitationally to standard model particles, you'd very much want to.
Finally, historically, particle CDM was strongly motivated by QFT considerations in the first place, with the expectation that they would be lightest MSSM sparticles or mass-explaining sterile neutrinos on the WIMP hand, or alternatively strong-CP resolving axions. So your "lens" is standard, so I agree with your parent comment.
Lastly, you wrote essentially the same comment several days ago https://news.ycombinator.com/item?id=13600085 .
[1] https://arxiv.org/abs/1211.6338
[2] e.g. S = \int \left[ 1/2 R + \mathcal{L}_{baryons} + \mathcal{L}_{photons} + \mathcal{L}_{neutrinos} + \mathcal{L}_{CDM} + ... \right] \sqrt{-g} {d}^{4}x.
.. on the same topic regarding dark matter. Is there a "no duplicate comments" rule? Topics are often cyclic on HN, I don't see why an answer has to change for the same recurring topic.
I'm afraid I don't know much about CDM or sparticles or Ricci scalars.
My comment isn't intended to be a self-contained proof of my inclination here. It is simply intended as a "this is my feeling or belief on this matter absent any further exploration " e.g. I am not an expert.
I agree my statement doesn't contain much information, again it's just an uneducated feeling/guess.
So sure, I get that there's more to the story regarding the nuance between particles and fields than I hinted at. Your clarification of the differences shows that.
I also suspect you have a deeper understanding on the relationship between the two perspectives than most.. so comments like mine aren't as such addressed to folks with your background but rather to folks who don't have such a background and seem to omit the "field picture" in lay discussions.
Wherever you have a quantum field, you have particles, at least for some set of observers in a relativistic quantum field theory. You were right that there are lots of subtleties in a complicated QFT like an extension of the Standard Model that includes CDM, but this generic property of relativistic QFTs is behind the term "particle dark matter".
CDM is "cold dark matter". Matter because it's a source of gravitation; cold because it moves very slowly compared to the speed of light (otherwise it would run away from galaxies rather than hanging around keeping them heavy); dark because it doesn't feel electromagnetism.
From the perspective of a relativist, CDM need not be particle dark matter. However, we can treat General Relativity as a field theory and can even quantize it (with some caveats) and the result gives us particles (gravitons in perturbative quantum gravity). New degrees of freedom that are "gravitational" are probably only "gravitational" in terms of the strength of interactions with other particles (i.e., very weak) but will still be representable as e.g. a gauge boson.
In a quantum field theory like the Standard Model, the fields' local contents are invariant under a set of transformations. When a configuration (like a proton) looks the same under various transformations, there is a symmetry at work preserving that invariant "look", much like when you rotate a circle around its centre in the Euclidean plane, it looks the same whatever angle you turn it.
The Standard Model has a number of parameters whose values are only known empirically, mostly by experiment, with little clue about why the values are the way they are. A common approach to reducing the number of such parameters involves expanding the symmetry group. Supersymmetry is one such approach, and creates a bunch of "superpartner" particles -- sparticles -- for each of the particles in the Standard Model. In minimalist supersymmetry theories, the lowest-mass sparticle tends to have the characteristics of Cold Dark Matter listed above. Other sparticles appear and can (via a new symmetry) explain apparent problems in the Standard Model.
The Ricci scalar is a value at each point in spacetime that describes the curviness of a shell of all points at the same distance from that point or equivalently the internal volume of such a shell. When the scalar increases at a point, the surface of the ball is locally flatter or equivalently the internal volume is smaller. If you move a ball through a region of spacetime where the Ricci scalar takes on various values, an observer at the centre of the shell will think the shell is more or less cramped as the value changes up and down, while a small observer standing on the outside of the shell will see the "ground" curve away (or the horizon move closer, equivalently) as the Ricci scalar decreases. If you seriously increase the Ricci scalar inside the Earth you would still measure a diameter of about 12 700 km, but from eye-level you could see to the other side of your continent. Likewise, if you seriously decrease the Ricci curvature below zero, you'd measure the same diameter of 12 700 km, but from eye-level your next door neighbour's house would be over the horizon.
The Ricci scalar appears on the gravity side of the Einstein Field Equations along with other quantities describing how lengths, durations and angles are calculated at each point.