> Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right.
I look at dark matter/energy as a measurement of what we don't know - it is not necessarily matter or energy (although it could be). It is an effect or the sum of effects that we are not aware of. It's almost like the predictions that Mendeleev made with the periodic table.
Personally, I don't like dark matter because my "hack" alarm bells start ringing. That being said, I also don't know what I don't know.
> nobody has been able to come up with anything like a coherent modification of gravitation that explains the observations.
There have been promising "non-matter" dark matter hypotheses[1], though these hypotheses seem to be quickly relegated to the disinterested bucket. In that way, it wouldn't hurt if slightly fewer eyes were looking at particle dark matter and slightly more at "non-matter" dark matter.
That said, I think you're wrong about alternate hypotheses being quickly dismissed. Alternatives are regularly proposed and evaluated. Many are eventually found to be lacking in the face of the evidence collected, but some remain viable candidates (for example: https://arstechnica.com/science/2017/01/new-ideas-on-gravity...).
Dark matter is called matter because it has mass, i.e. it experiences gravitational attraction to itself and to other forms of matter.
Dark energy is called energy because it acts like radiation, i.e. it has negative pressure.
> Personally, I don't like dark matter because my "hack" alarm bells start ringing. That being said, I also don't know what I don't know.
Your second sentence is far closer to the mark than your first. Dark matter was indeed seen as a hack in the 1930s when it was first proposed, but there is now a large and remarkably consistent body of observations that show its presence. Here's one lay-friendly blog post that briefly describes some of the evidence: https://medium.com/starts-with-a-bang/why-dark-matter-9252bf...
> In that way, it wouldn't hurt if slightly fewer eyes were looking at particle dark matter and slightly more at "non-matter" dark matter.
It's kind of like quantum mechanics at this point. The only reason there aren't more eyes looking at alternatives to dark matter is because we already did that over the last few decades, and the evidence in favor of dark matter kind of became ridiculously overwhelming.
If you want to float an alternative to dark matter at this point, you have to, at the very least, come up with a theory that explains all the things that dark matter explains. (Presumably you would also come up with something that dark matter doesn't explain that your theory does, but one thing at a time.)
> Dark energy is called energy because it acts like radiation, i.e. it has negative pressure.
I was going to post something similar, but tempered a little, since some participants in this discussion are skeptical of the existence of these things. I think the dark energy statement is fine, but would weaken the dark matter one to say, e.g., it's called matter because we can observe effects (lensing, rotation curves, etc.) which can be explained by the presence of matter.
In this way, dark matter and dark energy definitely exist, in the same way that centrifugal and coriolis forces definitely exist: they explain concrete observations, and by squinting at the formulae we see that the terms we've introduced have units of mass (dark matter), energy (dark energy) and force (centrifugal and centripetal).
The effects explained by centrifugal and coriolis forces can also be explained in a different way, by taking into account the rotation of our frame of reference. Articles like this one show that the jury's still out on whether the effects of dark matter and dark energy can be explained in a different way.
AFAIK dark matter is quite widely accepted as a fundamental explanation, rather than a "fictitious" one like centrifugal/coriolis forces, while it's common to treat dark energy as a "placeholder" at the moment: it may be a form of energy which permeates space, or it may be an effect of something else (e.g. some unexpected behaviour of spacetime, like discretisation or some other change of metric).
While we're still some way from understanding dark energy, I have no problem with models of dark matter (although SUSY's looking less likely every day); after all, why should we demand that all matter interact elecromagnetically?
On the other hand, when theorising about what dark energy might be, I think it's fine if that explanation encompasses the effects of dark matter too. Just because we discovered the dark matter effects first, it doesn't mean they must be explained first.
There are multiple "contenders" for dark matter, none of them actually explain observations and measurements.
Dark Energy is also in that same category especially in it's purest form as a "universal constant" that was added then removed then added back again to GR when we were sure if the universe is static, expanding at a constant pace or expanding under acceleration.
As far as MOND goes the biggest "curiosity" people have with it is that it works at all, because it shouldn't, you shouldn't be able to just fudge so many factors and have a theory that works, is stable, and predictive (even tho it has various issues especially with regards to cosmology like star and cluster formations).
Which might hint that there is actually something there. That gravity might not work in the same manner over distances, that maybe there is more than one gravity (there are hints that quantum gravity and what we call gravity might be different things already).
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.
Correct me if I'm wrong, which is quite likely, but isn't it one of the qualities ascribed to dark matter that it doesn't interact with 'ordinary' matter or photons in any way other than gravity, and therefore can't be directly observed?
Or to turn the question around, is the dark matter hypothesis actually falsifiable?
Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but it's an example of something that would interact weakly.
And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Or if someone finally comes up with a change to the known laws of physics which explains the observations better than dark matter does, that would do it.
Remember, nothing is truly "directly observed." All we can do to observe stuff is to see how it affects other stuff. Whether it's with photons or with gravity, it's all ultimately indirect.
Whats to stop us from "adding epicycles" when that happens?
Two dark matter particles headed directly at each other would pass through each other and be only slightly deflected by their gravitational interaction.
Based on observed dimming events while monitoring many stars, we have ruled out brown dwarfs as more than 20% of the total. Other possibilities still remain, but none have shown evidence of being sufficient to explain dark matter. However we have not ruled out the possibility of it being something that we haven't thought of yet.
The spacetime curvature, mass/energy, and gravity have quite a few interesting interactions as in gravity as a force, vs gravity as a feature of spacetime, vs potential other forms of gravity. We aren't even sure at this point if a curved spacetime without mass/energy that would generate that curve, would actually produce gravity or not since we aren't able to observe this nor do we have a complete theory of gravity.
A hypothesis that there's local unexplained curvature...Well that's really what dark matter is all about. What's curving it?
(for posterity) My understanding is that spacetime being flat is akin to it being the surface of the universe's curves. Explain, expand, or correct me if that's wrong; I'm certainly no physicist.
Except space isn't perfectly smooth. It does curve, and that curvature is gravity. That's what general relativity is all about. If you want to postulate some way for the universe to curve absent observable mass, go ahead: you're doing nothing but restating the dark matter problem in a (minimally) different way.
The de Sitter vacuum is an exact solution to the Einstein Field Equations of General Relativity that does exactly that.
Our expanding universe will very likely closely approximate de Sitter space in the extreme far future.
There are many other vacuum solutions; it's normal in General Relativity to think of curvature as being a background laid down by hand (ideally approximating nature) and then perturbing that background with matter or probing it with test particles.
> Except space isn't perfectly smooth.
General Relativity is defined on a smooth (infinitely differentiable) manifold.
This is very important! https://en.wikipedia.org/wiki/Smoothness
There can be arbitrary curvature in the manifold, however, and unfortunately with the Einstein Field Equations you can pretty easily produce manifolds with curvature singularities, and those are manifestly not smooth (and that is one driver for research aiming to extend General Relativity).
But to the first order, your experience of gravitation near the surface of the Earth emerges from the very weak manifold curvature the planet and its microscopic components and their motions source. The nearest strong curvature is a couple of kiloparsecs away.
But then again there are multiple contenders for dark matter.
If anything, the Vulcan hypothesis was not bold enough. It was a much greater leap of faith to assume that Newton's mechanics was just a low energy approximation, hence the twist in Mercury's orbit.
Anyway, we don't really know what this whole dark matter business is, but for now the prevailing hypothesis is the dark-matter-as-a-new-type-of-particle. Of course, you're welcome to pursue a different line of thought, and let the truest hypothesis win. This is how science works.
Instead of postulating endless world-splitting or whatever.
That's a nice example of a prediction being wrong, inventing an as-yet-unobserved chunk of matter to fix it, and that invention then being correct.
Of course, there's a counter-example in the inner solar system in the form of Vulcan. It was invented to explain Mercury's orbit, but the solution ended up being Relativity. So it's not always the right answer.
If you want examples involving entirely new classes of matter, I believe neutrons and neutrinos would fit the bill. Both were invented to explain discrepancies in theories well before they were directly observed.
Remember that coming up with a new type of matter is a form of fixing the model. The model is, essentially, "what does the universe actually look like?" If adding a new type of matter gets you closer, then that's fine. If changing the equations works better, then that's fine too.
Remember that dark matter isn't just some arbitrary fudge factor that gets inserted into the observations to make the numbers come out right. It is a comprehensive theory that allows for observations and predictions. No, nobody knows just what it actually is, but there's a great deal of evidence that there is a lot of "stuff" in the universe which interacts gravitationally but not in other ways.
I see this sentiment a lot, and I'm not sure that dark matter skeptics understand just how much supporting evidence there is for dark matter, or just how difficult it is to "fix Newton." People have been trying real hard for a long time, and nobody has been able to come up with anything like a coherent modification of gravitation that explains the observations.