The Case Against Dark Matter | Hacker News Reader

The Case Against Dark Matter

quantamagazine.org

172 pointsbilifuduo53 comments

As a general principle I would advise a grain of salt whenever a physicist claims to have solved an outstanding astrophysical problem. Verlinde is a great physicist --- I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea. But someone with a background in pure physics often lacks the background in astronomy to know whether their ideas will work. (I remember hearing a visiting physicist present his ideas about stars producing axions, and having an astronomy professor point out that if he was right, Cepheid variables would not pulsate at the observed frequencies.)

The article mentions the Bullet Cluster as one observation that MOND has difficulty explaining. Another observation that I'm not sure how MOND would explain is the dramatic difference in dynamics between a globular cluster and a dwarf spheroidal galaxy. Globular clusters are observed to have virtually no dark matter, whereas some dwarf spheroidal galaxies have somewhere between 1000 to 10,000 times as much dark matter as ordinary matter [1]. These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies.

I'll admit that I have not looked very carefully at Verlinde's paper. But the evidence for dark matter is much more than galaxy rotation curves these days. A credible alternative to WIMPs has to explain more than that.

All that aside, from a theoretical perspective the work is very interesting because it provides a much-needed framework to understand why something like MOND could exist. But I don't think that any of the direct detection experiments should pack it in quite yet.

[1]: https://arxiv.org/abs/1510.03856

[2]: Well, sort of. Galaxy formation is still a big open problem, and globular cluster formation is probably even less well understood. But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter.

> Globular clusters are observed to have virtually no dark matter

That's a fascinating angle, first time I've read this.

> But with WIMP dark matter it's very plausible that different formation mechanisms would produce objects with different amounts of dark matter.

Could you expand a bit how WIMPs can explain that? Do dwarf spheroidals have a strong central black hole while globular clusters haven't? Is the galactic black hole needed to capture WIMPs from cosmic background, sort of like gas giants capture lots of moons?

Dwarf spheroidals would form in the process of galaxy/structure formation. In this process, it's really dark matter that drives growth, and ordinary matter is just along for the ride. In the early universe the dark matter is diffusely spread throughout the universe, with slight overdensities in various places. These then collapse under their own gravity and form sheets, then filaments, then finally fragment into different clusters. This would produce dark matter haloes of a variety of sizes, some of which would be dwarf spheroidals. I should note that there are problems with this picture --- the predicted distribution of dark matter halo masses does not match the observed distribution. In particular, standard galaxy formation theory predicts far more dwarf spheroidals than are actually observed.

In the case of globular clusters, the cluster would form independently of dark matter. Just as in an open cluster in the galaxy, you would perhaps have some sort of a gaseous cloud of ordinary matter (mostly hydrogen), which would collapse, fragment, and form stars. There are lots of open questions about how globular clusters form, though, so this is just a very vague sketch.

Thanks!

You make good points, just don't call it MOND, because it's not "THE MOND"

Maybe we'll only have the answers when JWST is launched

> I remember reading Verlinde's paper on entropic gravity when it came out and I'm still excited about that idea.

I remember reading this too. Did anything ever become of this theory?

> These two kinds of objects have comparable numbers of stars and are roughly the same size (though dwarf spheroidals are admittedly larger by about an order of magnitude). This discrepancy can be explained with cold WIMPs [2], but it's hard to understand how some MOND-like process would produce such incredibly different dynamics between the two kinds of galaxies.

I think the order of magnitude size difference would be important in the possible origin differences in a MOND-ian universe. If globular clusters formed in a higher-pressure environment (within a high-z galaxy or merger) they could be more compact and stay in the Newtonian regime. And if dwarf spheroidals formed in lower-pressure environments that might be the origin of larger sizes, putting them in the MOND acceleration regime. But, that's just speculation.

And to emphasize your statement that "the evidence for dark matter is much more than galaxy rotation curves these days", even MOND-ian models require some form of "dark" matter to explain cluster dynamics.

> Physicists do not like magic; when other cosmological observations seemed far easier to explain with dark matter than with MOND, they left the approach for dead.

But how is "dark matter" not magic?

To my untrained and pretty ignorant eye, dark matter looks like filler: the observation does not match the theory, and so, instead of looking for a better theory, we decided the observation was wrong and couldn't see what was there (and not just a little bit, but the greatest part of it).

Isn't that the definition of superstition? Invisible forces that explain events, in mysterious ways?

As long as there is no theory on dark matter it is not an explanation for anything. It's more a placeholder for a mechanism we don't understand. The best hypothesis so far is, that there is matter that doesn't interact electromagnetically. But this explanation shouldn't be confused with a theory that actually explains what is going on.

No, it's not a placeholder. There are the exact formulas that match what's observed.

https://medium.com/starts-with-a-bang/7-independent-pieces-o...

No other approaches (MOND included) are near, still.

MOND can at the moment match just one of the 7 different kinds of observations listed above. That's why the physicists aren't impressed.

And all the observations are not going away, even if MOND manages once to produce enough formulas to cover them or to be a better basis to explain the observation, see my other comment.

Where did you get this "no theory" claim?

"no theory" because we don't know what this matter really is. Up to now, explanations on the nature of the matter have problems in one way or another. High Energy Neutrinos are too light and not abundat enough. Axions were once a promising particle, that could explain dark matter phenomenology, but if axions were produced in stars, variable stars would oscillate in other frequencies. Supersymmetric partners are not ruled out but become increasingly difficult to explain, because the lightest susy-particle has not been found in the LHC experiments.

Yes, "no theory" is maybe not the best choice of words, lets call it "no verified theory".

By the way, I don't think MONDs are a good explanation either.

Massive sterile neutrinos, which are are part of a theory of explaining the very low neutrino masses, are also dark matter candidates.

It is already "verified," (as in, observations are there) it's that there are extensions of particle physics like WIMP for which these researchers would have liked to be able to fit them to the observations of the dark matter, and the conclusion is:

http://www.forbes.com/sites/startswithabang/2016/07/21/dark-...

"The null detection is incredible, with a fantastic slew of implications:

- Dark matter is most likely not made up, 100%, of the most commonly thought-of WIMP candidates.

- It is highly unlikely that whatever dark matter is, in light of the LUX results, will be produced at the LHC.

- And it is quite likely that dark matter lies outside of the standard mass range, either much lower (as with axions or sterile neutrinos) or much higher (as with WIMPzillas)."

(I'm not quoting it for you, but for possible others to get the idea what the topic is).

So it's that these popular supersymmetry models turned out that they don't fit to the experiments.

It's just problematic for those that expected too much of the supersymmetry approach.

It's good that some researchers try with MOND too -- sometimes there are some new results exactly there were they are less expected. But they have quite a lot to do to match what's easier to do otherwise.

> But how is "dark matter" not magic?

Because the theory made predictions that were then checked.

> instead of looking for a better theory, we decided the observation was wrong

Except they didn't. They looked for a better theory. In fact, they considered multiple different theories. And the dark matter theory was the one that fit the observations best.

"Because the theory made predictions that were then checked."

? What predictions and made and then tested by 'Dark Matter'?

Dark Matter was conceived of the other way around - it helps confirm predictions that don't actually work properly.

If Dark Matter theory made testable predictions, it would be accepted no? Where are these experiments?

It's widely accepted. And I'm also a sceptic. More recently scientists have started actually mapping out regions where dark matter is concentrated than - from gravitic perturbations - and have discovered galaxies which contain more dark matter than others, up to 99% as well as the mentioned globular clusters which contain very little dark matter.

"More recently scientists have started actually mapping out regions where dark matter is concentrated than - from gravitic perturbations "

This is not evidence of Dark Matter - it's really just quantifying and classifying the perturbations and anomalies derivations in gravity that we see.

Basically, more detail to the observation that 'galaxies are not rotating the way we think they should'.

That doesn't validate Dark Matter, it just more deeply quantifies the unknown.

The flogiston theory also made predictions that checked.

Dark matter is pretty well defined within a particular model, not magic. You can take lambda-CDM, a very simple model, run a simulation, and compare it to observations.

I can't really comment on why MOND is magic. That seems a bit unfair, it's just a phenomenological modification of F=ma, and can similarly be simulated and compared to observations.

Did the model predict dark matter, or was dark matter introduced to get the model closer to observed reality?

AFAIUI (layman, at best) it's a bit of both. There are multiple independent[1] lines of evidence, some of those led to the formation of the Dark Matter model, others were since discovered and just so happen to also conform to that model.

[1] Maybe that's really the big question here.

I've always had trouble seeing "dark matter" as anything more substantial than modern-day epicycles.

The quip I'm fond of is:

   Dark Matter is the Phlogiston
   of Contemporary Cosmology

That certainly is a common perspective from people who get their knowledge of dark matter exclusively from pop-science articles!

Dark matter is the oxygen of contemporary cosmology.

It's not as "magic" as you believe it to be.

The dark matter is at the moment is not only the simpler explanation (that is, you have formulas that can produce the results) of the observations (Occam razor etc) but the only one that explains different effects that are still fully unexplained (in the sense, there are no formulas that can even match anything) by MOND.

And since 2006 there are the actual photos of "dark mater effects":

http://www.forbes.com/sites/startswithabang/2016/09/19/dark-...

Now, how the MOND handles it?

"Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine."

Note "thinks" not that anybody managed.

The dark matter approach could be declared really problematic even before anybody manages if some observations simply can't be explained with the dark matter. But at the moment it's still not so.

And all the observations will remain:

https://medium.com/starts-with-a-bang/10-facts-everyone-shou...

"It’s not going away

You will not wake up one day and hear physicists declare it’s not there at all. (Well, you will, but those claims are rare, and those physicists are wrong.) The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either. Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter."

Then, not as the underlying principle, but as a simpler handling for all the observations up to now that look like "the invisible-matter-like effects there."

But the motivation behind dark matter is deeper than its gravitational observations. For instance, almost all particles physicists believe the standard model of particles is incomplete. No one is really sure exactly what we need to add to the standard model, but it turns out that if you add things to it in any sensible way, you almost always get things that look like they could explain dark matter.

So, it's more: particle physicists all believe the real theory has this aspect; when we look at gravity it looks like it had this aspect; therefore we should probably find a way to modify our theory so it has this aspect.

The major reason we haven't been able to go further than this is because we haven't been able to pin down exactly what these new particles are. Particle experiments are /hard/ (see:LHC), and it might take a long time.

"Invisible forces that explain events, in mysterious ways"

The cold dark matter component of the standard cosmology is not the first time anomalous momentum was attributed to invisible (as in, does not experience electromagnetism) matter.

Twenty-six or so years from proposal (which incidentally was about two years before the discovery of the neutron) to direct detection:

https://www.wikiwand.com/en/Neutrino#/History

Note that neutrinos are hot dark matter; hot because they move quickly compared to the speed of light.

While neutrinos are definitely part of the dark matter sector, they cannot be the sole component because their hotness would smear out galaxies. A colder neutrino would work very well to match all the observations associated with observed anomalous momentum that strongly suggests dark matter, and hunts are on for direct detections of a cold (implying high rest-mass) neutrino-like particle.

There are other options, of course, but the four key things are: there is non-ignorable anomalous momentum; matter generates momentum (in this case, through gravitation); there is no visible matter associated with the momentum in question (that's why it's anomalous); and we have discovered non-visible matter in the past while trying to explain other cases of anomalous momentum.

You really helped me understand the concept of dark matter in a new way. I appreciate that, thanks! Specifically: hot/cold dark matter and rest mass.

I am not a physicist, in fact, I am a complete dilettante, but can the transition from a power law to an inverse law simply indicate that on large scale the universe has less dimensionality? I mean, if we take a suitable definition of dimension, likely along the lines of Hausdorff dimension (https://en.wikipedia.org/wiki/Hausdorff_dimension) then we can see it as a number that defines how volume/surface changes with changing diamater of an observed "sphere" (I know I oversimplify things here). In other words, as we start to measure cosmic distances on a large enough scale, volumes/surfaces covered don't grow quadratically anymore but with a lower rate. This might be even in line with the assumption of a universe curved into itself, i.e. closed. This would change the gravity power law from having a constant 2 power to having a function as the power.

(Constant negative two power to having a function as the power.)

It depends on whether you imagine a fraction line there or not: https://wikimedia.org/api/rest_v1/media/math/render/svg/8c6e... ;)

The idea of fractal dimension being significant at galactic scale is intriguing and based on my totally naive reading of the article might resemble this insofar as the guy is arguing that gravity is an emergent property from quantum interactions that dramatically fall off when matter starts interfering with them at a certain point.

From the final paragraph:

“If somebody were to come to you and say, ‘The solar system doesn’t work on an inverse-square law, really it’s an inverse-cube law, but there’s dark matter that’s arranged just so that it always looks inverse-square,’ you would say that person is insane,” he said. “But that’s basically what we’re asking to be the case with dark matter here.”

Awesome point.

The McGaugh paper is really problematic for dark matter proponents. Amazing that that work wasn't done earlier.

They do state:

"They say the amount of dark matter in a galaxy’s halo would have precisely determined the amount of visible matter the galaxy ended up with when it formed".

It's not a far fetched idea, but it is problematic because we know of zero such interactions.

Rotation speed, rotation speed, rotation speed ... that's always been the poster child for dark matter.

But do we truly understand how galactic rotation speeds should vary with distance? A typical galaxy has a hundred billion stars in it. Given such a complex system, can we truly simplify the expected rotation speed to something like "distance squared from the galactic center"?

The "three-body problem" is difficult enough, do we trust our approximations to properly model the "100,000,000,000 solar system problem"?

There's a difference between "complex" and "has no closed-form solution".

It's actually a very simple calculation.

But fwiw, a lot of thought and tests have gone into showing that gravity works the same way with feather and bowling balls. With big masses and many little ones. If you have a reason to think it doesn't, discuss it and propose tests...

So is this really anything except an interesting way of deriving MOND? The article says

> Verlinde thinks his theory will be able to handle the Bullet cluster observations just fine.

but doesn't actually show it doing that. As that's a major differentiator between his system and MOND, shouldn't that have been looked at? Seems to easy to otherwise dismiss this as a weird way of supporting MOND

I'm gonna pull my hair out. I saw this:

>> The researchers analyzed a diverse set of 153 galaxies, and for each one they compared the rotation speed of visible matter at any given distance from the galaxy’s center with the amount of visible matter contained within that galactic radius.

That just smelled like they think Keplers law applies (which I've been arguing against for ages) so I googled "radial acceleration relation" and found this paper: https://arxiv.org/pdf/1609.05917v1.pdf The opening paragraph is this:

>> The missing mass problem in extragalactic systems is well established. The observed gravitational potential cannot be explained by the stars and gas. A classic ex- ample is that the rotation curves of disk galaxies become approximately at (V=constant) when they should be falling in a Keplerian fashion.

To say the gravitational pull at a given radius depends solely on the amount of matter inside that radius is utterly bogus. It is entirely dependent on the distribution of that matter. It is valid if the distribution is a uniform sphere (or a sphere of concentric layers each of uniform density) but that is not the distribution in a galaxy. Likewise, you can ignore all the matter at a larger radius IF it's distributed as a shell (or series of shells) of uniform density - but that is not the case when dealing with rings instead of shells. In my mind, this whole debate has wreaked of a misapplication of the Divergence Theorem from day one. If you thing galactic dynamics have anything to do with Kepler I have a nice bridge to sell you.

The use of Keplerian simply refers to the inverse square law, and if you think the inverse square law isn't involved then maybe I should take that bridge off your hands. You seriously think these guys don't know high school physics?

The inverse square law applies to 2-body systems ONLY. I do in fact think they know high school physics and are trying to misapply it. The use of that simple physics can sometimes be used in other systems as a special case of the divergence theorem (third year calculus) IF certain conditions are satisfied. I'm just saying they keep applying by missing a key point in the justification for doing so. Perhaps they're not actually using Kepler, but they sure do keep bringing up his name - and I am correct that it's not OK to use that.

As I sometime say - the only dark matter is between the astrophysicists ears. This assumes they're actually using Kepler as a reference for "expected" behavior - some of those guys are smart beyond my comprehension.

> One problem, said Brian Swingle of Harvard and Brandeis universities, who also works in holography, is that Verlinde lacks a concrete model universe like the ones researchers can construct in AdS space, giving him more wiggle room for making unproven speculations. “To be fair, we’ve gotten further by working in a more limited context, one which is less relevant for our own gravitational universe,” Swingle said, referring to work in AdS space. “We do need to address universes more like our own, so I hold out some hope that his new paper will provide some additional clues or ideas going forward.”

Woah. How can AdS model universes have less "wiggle room for making unproven speculations"? There is no AdS universe to observe or experiment in. How could speculations in such universes be "proven"? Or rather, disproved?

I think they're talking about mathematical proofs. AdS models are mathematically simpler than the physical universe containing matter, so it's easier to construct mathematical proofs.

OK. But proofs about mathematical objects are entirely distinct from testable hypotheses about the actual universe. So it seems odd to compare them. OTOH, there is much "theoretical physics" that's not much more grounded in reality than AdS models ;)

Xenon1T's result is much anticipated: http://www.metaculus.com/questions/199/will-the-xenon1t-expe...

Or MiHsC/quantised inertia: http://physicsfromtheedge.blogspot.de/2016/11/critique-of-ve...

Relevant recent xkcd comic: https://xkcd.com/1758/

I guess the state of the art is more complicated than the Department of Astrophysics in the comic admits.

The concept of "Dark Energy" is as nebulous as "Dark Matter"

Dark matter and dark energy are ad-hoc adjustments to explain discrepancies between observations and theory. The problem with such adjustments is that, being mostly unobserved, they end up with too many degrees of freedom (e.g. a distribution of invisible stuff where the density can be made "just right" to fit the observations), so they are quite hard to falsify. It's a bit like the "landscape" : when you construct a theory that can predict anything, you end up predicting nothing. In my opinion, we leave "science" as defined by Popper when we start doing that.

There is, however, an underlying assumption in all the existing theories of physics, which I see too rarely challenged, and which is easily falsified. It's the idea that the variable "x" in the theory is independent of the physical measurement process being used. In other words, the "x" in MOND is the same as the "x" in GTR and the "x" in quantum mechanics. The topology of a space-time defined by a solid rod of metal in the Pavillon de Sevres (pre-1960 definition of the metre) is the same as the topology of space-time defined by the movement of light in the vacuum (today's definition). And so on.

I believe that our current understanding of physics now has become so precise that we need to take into account subtle differences between this and that measurement process, and rewrite physics in the context of a "more general" relativity, where the laws of physics are to be described independently not just from, say, the state of movement, but more generally independently of the physical process being used. Just like GTR forced us to deal with quantities such as metrics, this more-general relativity would force us to deal with arbitrary transformations between two methods to measure the same thing.

A simple example is when we relate time as measured by the rotation of the earth (definition of "days"), the rotation of earth around the sun (definition of "years"), the decay of populations of C14 ("radiocarbon dating"), and our current cesium-decay based atomic clocks (the definition of "second"). The four laws are obviously correlated, they all measure time. But the "fit curve" between them is a distribution in itself, similar to the "metric field" in GTR.

I may be wrong, but I suspect that if we introduce this kind of complexity, the problem of "dark matter" will have not one, but multiple solutions, possibly different equations, depending on the measurement you are considering. This does not mean we should not look for equations that predict observations, but rather that we should be cautious when correlating measurements that correspond to different physical processes. It is not obvious to me, for example, that we should take for granted that we can safely deduce the distribution of masses from the distribution of luminosities (even if, I'll admit, I don't see any good alternative...)

In quantum mechanics, we do little else with renormalization. When you shift from one scale to another, you need to renormalize to fit the data, because the raw equations would otherwise yield to infinities or other abnormalities. "Shut up and calculate" is frustrating, but if we rephrase that as "observe the universe, it's the ultimate test", it really means the same thing, but is philosophically more grounded.

In the end, MOND may just be the shape of the relation between two measurements of mass at large scale, and Verlinde's paper one first step in understanding how this relationship emerges.

Or not. There are some interesting objections to "statistical emergence" of gravity, or "entropy-based gravity". One of them is how interference patterns are not destroyed by gravity, at scales where, if we were to trust Verlinde's approach, interactions along different paths would affect probabilities enough to be sensible in interference patterns. I've not done the calculations myself to validate if this objection is solid, though. Intuitively, it seems like there is a flaw (the effect of per-state populations on interference should be at most of the same order as the effect of gravitation itself, which is in the "barely observable" category), but I need to spend more time to understand both sides.

'and our current cesium-decay based atomic clocks (the definition of "second")'

Cs-beam and Cs-fountain frequency standards rely on the transitions between the principal microwave resonances of Cs-133 atoms in their ground state. You pop a gas of Cs-133 atoms prepared in one of the hyperfine states into a tunable microwave cavity coupled to a crystal oscillator in a feedback loop which maximizes the transitions of the Cs-133 atoms to the other hyperfine state. The longer the exposure time to the ~ 3.26 cm microwaves and the colder the gas, the better the signal with which to steer the microwave frequency towards the frequency in the SI second. Ideally you have a gas at 0 kelvins spending an infinitely long time in the microwave cavity, and your feedback loop does careful electronic division or multiplication of the tuned frequency to produce a round output signal, often at 1, 5 or 10 MHz exactly.

Decay has nothing to do with it. Cs-133 is stable.

I'm afraid I don't understand your commentary on General Relativity vs MOND at all, especially not the part about the '"fit curve" between them is a distribution in itself, similar to the "metric field" in GTR'.

The phrase "cesium-decay" was a rather inadequate shortcut. I was thinking of the excited state "decaying" back to the ground state, but you are right, it was not the right word.

The metric field in GTR is defined by the distribution of matter. So starting with GTR, to make another shortcut, we had to take the distribution of matter into account when "converting" from one set of coordinates (measurements) to another. In other words, the space of "transfer functions" from one set of measurements to another is already quite rich.

Similarly, when you transfer from one measurement of time to another, the "transfer function" is also quite rich. If you want to convert precisely from atomic clock to earth movements, you have to take into account interaction with many planets and other celestial bodies. So again, this transfer function is rather "arbitrary".

My point here is that the laws of physics are no less good or less precise when expressed in years than they are when expressed in oscillations of Cs-133 atoms. Some physical systems, e.g. atomic interactions, will be easier to describe relative to Cs-133. But others (e.g. meteorology) are expressed much more simply if you base them on celestial movements.

If the transfer function between, say, radiocarbon dating and cesium clocks can be practically anything, what tells us that the transfer function between light densities and matter densities can safely be assumed to be practically an identity?

And if it's not an identity, then why should the laws of gravity as expressed using light distributions match the laws of gravity as expressed using matter distribution? I think this is an oversimplification that we need to get rid of.

Hope I'm making more sense to you ;-)

I'm sorry, no, this is even less clear to me.

GR has coordinate freedom built into it; you can apply (almost) any set of coordinates (locally) on spacetime and calculate against them as you wish. The metric, however, is fixed by the geometry itself, and it is the metric that determines the interval in any set of coordinates. That's why you can write down e.g. the Minkowski metric in cartesian or spherical coordinates and come up with the same dS^2 -- the spacetime interval -- or as another example the Schwarzschild metric in Schwarzschild coordinates, in- or outgoing Eddington-Finkelstein coordinates, Kruskal-Szekeres coordinates, etc etc and come up with the same dS^2 even though the line element is written very differently in each case, and even though coordinate singularities appear in some systems of coordinates but not in others. (That they vanish in any valid coordinate system shows that they aren't physical singularities; the singularity in a Schwarzschild black hole that does not vanish under at least valid coordinate system is a strong argument that it is physical, although that remains a subject of research.)

One of the main features of GR is that it gives you a clear and mathematically provable mechanism for translating from one set of valid coordinates to another. That's one of the main reason it's called General : the coordinate systems need not be in uniform motion with respect to one another, and even more crucially, they do not even have to be in the same tangent space. There is nothing at all arbitrary about how one "transfers" from one system of coordinates to another: the Bianchi identities on the Einstein Field Equations give the degrees of freedom necessary for a gauge transformation.

The physicality of singularities in some exact solutions of the Einstein Field Equation (like Schwarzschild's as mentioned above) and the work of Kenneth G Wilson has led most general relativists to think of GR as an effective field theory (EFT) valid everywhere just outside of irremovable singularities. That means GR is accurate even inside the event horizons of black holes. Really only the singularities themselves are the problems with GR -- in particular it's the problem of its canonical quantization, and thus the driver for a theory of quantum gravity that works much closer to the irremovable singularities (or better still, makes them vanish).

But because GR is easy to consider an EFT, it is even easier to treat regions of spacetime that are not exactly flat as if they are exactly flat with some perturbations. So, in fact, I do not at all agree with you that "... you have to take into account interaction with many planets and other celestial bodies ..." since in a perturbative approach, their contributions can be removed as irrelevant. Even nearby celestial bodies can be removed as marginal. Here "marginal" and "irrelevant" are vocabulary right from Wilson, and translate very nicely into a Feynman diagram approach by counting loops of gravitons.

Or alternatively, I could technically agree that you have to take into account those bodies you mention, but since you do so simply by ignoring them explicitly on the grounds that their contributions to the metric provably do not matter for the local system under consideration, that contradicts your assertion in the paragraph "Similarly...".

I don't understand how your fourth last to second last paragraphs inter-relate. The first of those approaches an argument about coordinate freedom; the second I cannot decipher at all (it certainly isn't a General Relativity argument; I think you're looking to argue with the Standard Model instead, and that's defined on flat spacetime, and by the EFT argument above, flat spacetime applies in any radiocarbon dating experiment you choose to do on Earth -- and when you repeat the same experiment, the secular variation in the proximity of the the moon or sun or other planets will make not even a marginal difference). And your second last paragraph is I think a question about multimetric gravity -- we have strong evidence for the universal coupling of everything in the Standard Model to the single metric, and if you want to introduce a second metric for some sector of matter, you have to go beyond the Standard Model, and you have the problem of making the coupling just universal enough that you reproduce GR's matches with observation. There are ways of doing that, but most of them have the extra metrics decay extremely early in the history of the universe so that they do not produce ANY observables. Afshordi and Magueijo's recent paper is interesting because they believe they can produce an observable in spite of a decaying extra metric to which light couples. That comes at a cost though: you would have to introduce a zoo of gauge particles in order to make this work at all, which hardly makes the theory more simple than Cosmic Inflation.

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