> Personally, I wouldn't be surprised that when we get the real theory of quantum gravity, if we ever do, that a lot of these "oh, look at this solution for general relativity!" obscurities go away
In particular, I've thought dark matter is in this category of "bad theory rounding error" since I learned about general relativity and quantum mechanics in high school.
Nah, dark matter is the real deal. It's got way more evidence for it, and even the alternatives like MOND (which is something closer to a layman would argue as "maybe we understand the theory wrong") do need dark matter to explain some measurements.
Dark matter isn't really a theory, its the name of an observation. Not sure how it can be any more or less legit. The name doesn't try to explain what's happening. Its just describing the weirdness of the observations.
One thing that annoys me is the common description of dark matter as a single coherent theory of extra matter that can't be detected. Even wikipedia makes this mistake at the very beginning. But there are a lot of theories that try to explain it. None of which actually work yet. Once a theory that stands up is formed then the name will change.
Dark matter is not merely an observation. It is a class of theories to explain that observation. Specifically, if your explanation for the underlying observation is of the form "there is some matter-like stuff with mass that is causing the observed effects through standard G.R. gravity. We haven't otherwised observed it because X", then you have a dark matter theory. Typically X involves week to non-existant interactions through other forces.
If your explanation for the observation involves saying "General Relativity is wrong", or positing the modification or addition of some other force, it may account for the observation behind dark matter; it may even be correct; but it is not a dark matter theory.
Assuming some dark matter theory is proven correct, we will probably stop calling it dark matter when it is discovered. Although we still call nuclearly bound protons and neutrons with electrically bound electrons an "atom", even though we have since disproven the hypothesis that such objects are atomic.
If the answer ends up not being a dark matter theory, we will probably just call it "gravity".
No its not a hypothetical. The measurements are there. Something unexpected is happening and nobody knows for certain what.
The problem is someone picked the misleading name "Dark Matter" and then pop science, and actual scientists talking to laymen did the massive disservice of wording the explanation in such a way as to make it sound like there is a real theory that proposes there is some matter that interacts with gravity yet is invisible and undetectable in literally every other way called dark matter.
I only learned the distinction in college when another student pointed out that dark matter as described was unscientific because it was an unfalsifiable, not to mention generally useless anyways. That led to patient explanation that it wasn't a hypothesis or theory and it was just what they called the observations. Would have been nice if anyone had bothered to start with that.
There are a lot of hypothesis about what it really is. Neutrinos, WIMPS, SIMPS (heh), MOND etc etc.
Personally I lean towards the got gravity wrong bit although not any specific explanation. I'm not a physicist but history and logic would suggest that while relativity is a pretty good explanation its far from complete. And I see no reason why forces necessarily have to work the same on the small scale and grand.
Dark matter first gained notoriety when hypothesized to explain observations of galaxy rotation curves. Since the late 1930's, there have been a number of observations unrelated to galaxy rotation curves that couldn't be explained with contemporary science, so... it's dark matter! A lot that can't be explained got shoved into the dark matter hypothesis.
Once enough things that couldn't be explained were lumped into dark matter, it became a part of the science paradigm. Once you understand paradigms, you'll understand why it seems like all of science is convinced that dark matter exists. But this is not actually the case, it is merely that dark matter has not been disproven. The luminiferous ether, another famous hypothesis, was once also embedded in the paradigm of science, and it only took one experiment to prove it did not exist to dislodge it. Dark matter is going to stick around until disproven, that is the nature of paradigms.
As it turns out, dark matter is absolutely unnecessary to explain galaxy rotation curves, which can be simply explained and demonstrated accurately with a toy model without needing anything remotely like dark matter nor even Modified Newtonian Dynamics, but just the ordinary Newtonian physics. I suspect as time goes on, more and more of the stuff unrelated to each other that dark matter is employed to explain will be explained without the need for dark matter.
There are no direct observations of dark matter, none whatsoever. And by definition, there never will be. It can't be seen! What there is instead are observations that don't have immediate explanations, so dark matter is hypothesized to explain it. That makes dark matter a hypothesis.
Can you expand a little bit on how the galaxy rotation curves were explained from standard Newtonian physics? From some Googling it appears that dark matter is still the dominant favourite theory, followed by some kind of MOND.
> dark matter is still the dominant favourite theory, followed by some kind of MOND.
Yes, this is a precise problem with scientific paradigms!
> Can you expand a little bit on how the galaxy rotation curves were explained from standard Newtonian physics?
Maybe I exaggerated a bit as with these scales surely Relativity is involved. No, I think it's just tidal forces, no need for dark matter. Fundamentally, I think, it was unrecognized human error that became foundational, massively built upon, such that even though the original pivotal need for dark matter is eliminated, dark matter is still successful for other models.
Galaxy rotation curves were originally observed in a galaxy in isolation, without consideration of nearby galaxies, creating the illusion of gravitational isolation. When accounting for nearby galaxies tidal effects, rotation curves are neatly explained. [1]
No, dark matter is not a set of observations, but instead it's a hypothetical form of matter, invisible and weakly interacting except with gravity, employed to explain a set of observations.
I don't think the weak interaction is required? Theres both theories for dark matter that include the weak force and that don't iirc?
The only thing we know for sure is that we can define a field of masses that corrects for the difference between the observed behaviour and predicted behaviour of the cosmos vis a vis general relativity.
"Weakly interacting " does not mean the Weak Nuclear Force specifically. Dark matter only interacts with ordinary matter through gravity and does not interact through (or is vanishingly weakly interacting through) electro-magnetism, strong nuclear or weak nuclear forces.
The problem with dark matter is that it is a bit too easy to predict where it is. That's why so many people are looking for galaxies without dark matter. We sometimes find them, but I am not sure if there is an instance where we have confirmed that it is not a fluke of observation.
The reason why it is a problem is that if you can reliably predict where dark matter is, then you can turn these predictions into equations and get away with dark matter as a form of matter. That's the idea behind modified gravity.
"We just don't understand gravity well enough" is one of the top approaches physicists try to solve the mystery about dark matter, but so far it doesn't seem overly likely. Especially galaxies without dark matter [1] are hard to explain without labeling the majority of dark matter as some kind of particle or field.
I think they mean the reverse: since dark matter gives a possible solution to some of the unexpected observations resulting from the current theories, so if the theories later change, they are hoping the dark matter/energy terms will vanish. I likely have about the same physics knowledge as them, so I don’t speak from authority. But dark energy is not just a small fudge factor, so current theories seem unlikely to change by that much. As mentioned in the article, we have some measurements now also consistent with the existence of dark energy being pervasive, not just the appearance of it in theories.
Because I was taught at a young age to assume everyone is fallible and question assumptions, and it seemed an awful lot like a rounding error to me at the time.
As people have mentioned, there is apparently now more substantial support for it, but back then it was posited based on equations only, and seemed a lot more like trying to explain away a theory not fully matching reality
Um, as I understand it, dark matter was not posited by equations but by how observations differed from the the equations. And essentially, this is still the state of things. To wit, the standard model doesn't match observation unless there is this "dark matter" out there. We have no idea what it is. It's just what we call a particular type of ignorance. Current work is either a) assume that the theory is correct and that "dark matter" actually exists so they try to figure out what it is or b) assume that current theory is wrong and so try to come up with a better theory. Or c) a combination.
I recall that back in the day my thought was that "Or our model of gravity is just wrong and what we're observing is correct (as in, visible matter is causing this behavior), we just don't understand how gravity works at massive scales because there are variables we aren't taking into account" but honestly it's been so long I forget all the basic info on this.
> Because I was taught at a young age to assume everyone is fallible and question assumptions
It is generally a good idea to have some familiarity (or better still, expertise) with a topic and knowing what the actual assumptions are before questioning them.
I was very surprised that you had that kind of expertise in high school. For eg, did you know about the electromagnetic field theory and Maxwell's introduction/invention of displacement current in high school? Or reimannian geometry?
In particular, I've thought dark matter is in this category of "bad theory rounding error" since I learned about general relativity and quantum mechanics in high school.