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Dark matter nightmare: What if we are just using the wrong equations? (backreaction.blogspot.com)
262 points by lordnacho on Oct 17, 2019 | hide | past | favorite | 203 comments

I very much wish no one had used the terms "dark matter" and "dark energy", as those imply far more knowledge than we have. It implies to those unfamiliar that there is in fact some form of energy or matter that has been "sort of" identified, but whose properties are unknown.

That isn't the case at all. We have equations we use to predict observations, and in certain cases, those observations are not quite matching what we predicted. There are a lot of reasons why this can happen, and using the equations poorly is but one of them.

I really wish they'd just said we've found an inconsistency, between theory and observation, which is the actual case. It would, I think, better inform the public about the actual process of science, rather than making it sound like a new discovery has been made, which has not happened, clearly. Or perhaps the real discovery was the inconsistency, and that should be the framing of the problem. Describing it using sort of known words like 'energy' and 'matter' makes the general public think a new type of matter has been found, rather than what has actually been found which is a hole in our understanding.

There's inconsistencies, and then there's real inconsistencies.

Recall some predictions that were later confirmed: the top quark (18 years later) or the Higgs boson (48 years!). These theories were a jigsaw puzzle with a single piece-shaped hole. You could call "inconsistency!" and break apart the whole puzzle, or look behind the couch for the missing piece. (The piece was found!)

We also have real inconsistencies, such as between QM and GR. Not a missing piece but a yawning gap: no single observation can fill in that hole.

Dark matter is a "missing piece" problem. The dark matter hypothesis fills in the cosmological missing mass problem, galactic rotation curves, Bullet-cluster-style gravitational observations, the expectation of non-EM interacting particles, and others. The gap between prediction and observation is the size and shape of a single jigsaw piece; let's just find it behind the couch.

Dark energy is a "real inconsistency" problem. Predictions and observations are orders of magnitude apart. Well hell.

Here's an image explaining the "bullet-cluster" observation. It's a compelling piece of evidence.

Two galaxy clusters have collided. The hypothesis is that normal matter will be slowed down by the collision, while dark matter, not interacting (as strongly?) with itself or normal matter, passes through in the collision. We can determine the locations of the gas by X-ray telescope, and the supposed location of the dark matter (assuming it exists) with gravitational lensing.


(I am not a physicist, just had this explained by a physicist friend of mine).

Indeed, this is the observation most usually cited in support of the matter / particle interpretation of dark matter. It's very non trivial to come up with a decent theoretical extension to GR that explains it. Which is why, I think, that a lot of people feel more confident proposing a new dark matter particle, which does in fact explain it quite nicely.

Perhaps we're wrong in assuming that all of these observations on very different scales (galaxy rotation curves over galaxy cluster collisions to large scale structure) are tied to the same unknown (dark matter), but if you want to explain them all in a simple way, a particle still seems like the way to go.

Remember that elementary particles have been predicted and found many times over the past years, whereas modified theories of gravity haven't seen the same kind of success. Of course this is only anecdotal, and may not mean anything, but I think it's part of the reason for most people's preference for a particle explanation.

The final bullet point in that image contains the answer to the question of dark matter.

The counterargument given by one of the proponents of MOND (a non-dark-matter counterargument) is that the bullet cluster can be explained by a combination of slight modification of GR, and additionally by the presence of a significant (but not crazy) amount of cold regular matter. Cold regular matter can't be easily detected because it isn't emitting any light, but it is just regular matter.

I'm not actually familiar with the specifics so I don't know whether this is actually a good explanation, but it's at least worth discussing.

This is a good explanation, but I'm not so sure the missing piece is so clear in the case of dark matter (and the blog post explained this nicely).

Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

Maybe gravitational attraction is not r^2 at some non-obvious scale that we failed to measure (meters? mm? Casimir effect on cosmic dust? Solar system sized gravitational anomalies?)

Why relativity makes Mercury's orbit precess is very not obvious (I mean, the two-body problem is already hard without relativity). The bending of light is a more obvious effect for example.

Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle.

Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.

> Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

How do you propose for a modified equation to explain the bullet cluster? That is, we have multiple good observations that show situations where the distribution of mass differs substantially from the distribution of visible matter, and good theoretical explanations why this should happen in the case of dark matter. This is not adequately explained by modified gravitation or other such effects.

Disclaimer: I do think that the Bullet Cluster is good evidence for dark matter, but we can speculate a bit

One is that there's also a possibility that dark matter exists but it's only part of the story.

For the Bullet Cluster I wonder how reliable the estimation of gravitational mass is for cluster of objects, as I understand, it is the images of the galaxies behind it that are lensed by mass in front of it https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:...

For one heavy object in front of another, it's easy to see how lensing works and estimate masses from there.

But with clusters and "fuzzier" mass distributions I'm not so sure.

Thank you for posting this. I often read commenters who dismiss dark matter the way dark energy is dismissed without people addressing the bullet cluster style observations which in my layman view is the most compelling piece of evidence for dark matter

Is there a well defined boundary between QM and GR?

In most cases, only one of them maters and the other is negligible: at very small scales QM rules, while at very large scales only GR is important. But in special situations, such as black holes and big bangs, nothing can be neglected and there is no boundary. (AFAIU, IANAP)

What about in between small and large scales? Medium scales? Is there an in-between?

Yes, there is an in-between at which Newton laws hold. Too big for QM and too small for GR to make a difference. Fortunately, this is the scale in which we live.

Small point of clarification, the predictions of newtonian laws are a limit in GR. So, technically speaking, newtonian physics is still following GR for that energy regime.

So, QM at the small scale and GR everywhere else? Where is the line between QM and GR? Atomic level? Quark level?

QM has a classical limit too. At human scales, both QM and GR are at (very close to) their respective classical limits.

The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.

I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.

The plank scale is derived by saying "what length scale appears if we mash together the constants describing gravity and the constants describing QM". One definition of it is "what wavelength photons have enough energy to collapse on themselves and form a black hole". The only thing we know is that when an object is at that or smaller scale we need to consider both QM and GR. It is not "on the QM side of things", because making small wavelengths requires more energy, and more energy is more massive, and mass bends spacetime which requires GR.

and yet the universe is full of these objects - black holes, neutron stars, quasars, etc.

No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.

Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.

I don't think your description is correct. The situation is we have a reasonably exact theory that assumes that most of the matter in the universe is neutral in the sense of only interacting gravitationally. But that interaction can be calculated quite readily in certain circumstances - for example, we can observe the "dark matter halo" in galaxies through gravitational lensing.

This is the dominant theory, it's not in contradiction with observations, it's just, a little weird. Why things are like that is the mystery but that's it for dark matter.

Sure, this stuff is invisible. So is electricity but we deal with that.

(Dark Energy I don't enough to talk about).

> Sure, this stuff is invisible. So is electricity but we deal with that.

Good description but bad example :). Photons are literally excitations of the electromagnetic field. You could say that everything except electricity [1] is invisible. Neutrinos are a better example as a known form of dark matter.


[1] yes yes electricity is just one phenomenon of the electromagnetic field. I'm just differently wrong.

Many photons are invisible. The point is that not that dark matter is similar to other things in general, but that it's similar to a large list of things we take for granted in that you can't see it with the unaided eye and it would have sounded like woo a few hundred years ago.

Worse yet: electrons passing down into ground state will emit a photon. In essence, all we see ever is direct electrical activity.

That isn't a counter to the claim that electrical activity we don't see is something we take for granted that happens all the time around us.

I think it is a more than a little weird, as we find that over different scales our ability to predict loses its precision. We attribute this to an unknown something, that is sometimes referred to as "dark matter", but in reality is something we have not directly observed. Dark matter is essentially a placeholder for our ignorance, but with a little bit of assumption sauce thrown in.

> It would, I think, better inform the public about the actual process of science, rather than making it sound like a new discovery has been made, which has not happened, clearly.

I disagree with this -- dark matter and dark energy have been discovered in exactly the same sense e.g. quarks have been discovered. Like dark matter and dark energy, quarks cannot be observed directly. Rather, physicists have made observations that are most parsimoniously explained by the existence of a theoretical entity with certain characteristics we have named a "quark".

Similarly, physicists have made observations that are most parsimoniously explained by the existence of theoretical entities that behave like matter and energy but are otherwise difficult to observe (like quarks are difficult to observe directly).

The standard model provides a theoretical understanding of the nature of quarks -- but if further findings in particle physics disprove the standard model, then we'll have invalidated our understanding of the nature of quarks.

Similarly, if a future theory shows that dark matter and dark energy are not weakly interacting matter and energy then we'll have invalidated our understanding of the nature of dark matter and dark energy.

I disagree.

Dark matter is form of matter (matter is any substance that has rest mass) and it's dark (implying we can't see it and don't know what it is made of). Most hypotheses around the dark matter assume it's either new kinds of particles, or macroscopic objects.

Dark energy is also good name for hypothesis for unknown form of energy.

"Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter. It's quite possible the hypothesis is wrong.

The article, and the OPs aren't saying we shouldn't explore these hypotheses. They're just pointing out that they are, well, hypothetical. We should be cautious of talking about these things definitively as though we know they exist.

Hypothesis should be named correctly. If the hypothesis is it's matter, it should be called matter.

Primary evidence makes it look like it's matter even before any hypothesis.

Practically all hypotheses looking the evidence assume it is form of matter. Not calling it dark matter just because there is small change it would be something else would be misdirection.

> "Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter.

Are these not all cases of direct detection: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...

Direct detection would be detecting/trapping the dark matter particle(s). Those observations are indirect and it may turn out that there is another explanation other than dark matter such as we are using GR equations incorrectly as suggested in the post.

What kind of detecting/trapping? We have to be specific. If we hypothesize a kind of matter that does not interact electromagnetically (only gravitationally), then it's kind of nonsensical to say that only direct electromagnetic interactions with such matter will count to confirm its existence.

This is exactly why the hypothesis is weak. At the moment it postulates a particle that only ever manifests in such a way that it explains gravitational behaviour at a microscopic scale, in certain observations, and has no other detectable interactions or properties whatsoever. There’s no theory of how it forms, no theory of how it might be created or annihilated, no evidence for it in any particle physics experiments. We just have this one class of equations that produce unexpected results, so we put in an extra term and say were done. That’s just not good enough to say anything definitive about what it is.

Physicists know that’s not good enough, and that it’s a major problem. I hope we do make progress, do come up with further experimental evidence or ideas for experiments we can test. For dark matter I think that’s actually likely, but I’m not so convinced about dark energy, there the evidence is a lot thinner.

Well that is the crux isn't it? How do we directly detect gravitational particles?

Maybe we don't. The evidence for general relativity is all indirect--we can't directly detect space-time. But plenty of people seem to think it's a solid theory anyway.

There is a significant difference in that GR made numerous very testable claims about all sorts of phenomena, which would not be at all expected without GR to explain them.

It also depends what is consider direct detection, which I accept is a matter of opinion. For example I'd consider comparing the times on atomic clocks inside and outside a gravity well to be direct measurement of time dilation.

How do you distinguish direct as opposed to indirect? I mean, to me it directly observes something which is matter and non-luminous - and satisfies the technical definition of dark matter.

Rotation and velocity hint that something is amiss but doesn't directly measure dark matter. Gravitational lensing is a direct measurement of matter but doesn't tell us much about its composition. If using the most general definition of "dark matter", lensing would be a direct measurement. But if "dark matter" implies non-baryonic matter, the leading hypothesis, then gravitational lensing is indirect because it is hinting something is amiss just like the rotation and velocity evidence. Now if there was a predictive theory of non-baryonic matter and that predicted gravitational lensing with specific characteristics, that would be direct evidence. Just like GR predicted black holes and we searched and found them.

It doesn't appear to be dark, but transparent, AFAIU. For an object to be "dark", it has to be actually absorbing light, and I think dark matter doesn't actually absorb it, it just doesn't interact with it (except for gravitational lensing).

Or at least, that's what I've always understood. Not a physicist though.

I would say "dark" here is more like "unknown" than a color reference, while both obviously have a connection in how we experience.

> Dark matter is form of matter (matter is any substance that has rest mass)

There are many bosons that are massive (Z bosons for example) but are not considered matter. I think that normally fermions are considered matter.

Edit: s/barion/fermion/ bah, I know nothing.

Edit2: and of course gluons are not massive.

super pedantic and of course IANAP.

So, there is large quantity of matter permeating the universe with unknown properties? Doesn't that imply that our understanding, i.e., theories/models, are inconsistent with observation? All the the term 'dark matter' really implies is that there are observational anomalies caused by something that could be matter-like in its nature, but which we have not observed. It isn't necessarily at all like the matter we have observed, but could be matter-like. Or we could just be wrong in our models.

"Unknown form of energy" is not a hypothesis. It just means "we have a problem here; we don't have a solution, so we'll just rename the problem as a hypothesis". I think it's a clever sort of cheating (and you probably have to be very good at writing cosmology papers to get away with it).

Yes, interesting way to look at it. Calling it dark matter/energy also implies the issue is with the theory - we found something that the theory (or equations, as per the article) forgot to accurately account for.

It kind of implicitly throws out the notion that there could be an issue with the measurement process/tools.

Sometimes this type of unconscious framing directs all our minds down the same path making us miss an obvious detour to an insight.

> It kind of implicitly throws out the notion that there could be an issue with the measurement process/tools.

AFAIK this was the assumption early on, space dust, black holes/brown dwarfs (MACHOs), weakly interacting massive particles (WIMPs) and more were the leading theories. These are all normal (for physicists/astronomers) matter that just we couldn't see due to measurement limitations, so dark matter was an appropriate name. Only after all these theories were falsified we had to look at more exotic explanations.

Also obligatory xkcd: https://xkcd.com/1758/

WIMPs is still the leading theory right? The catch is that no candidate P has yet to be found and most models predicting them (Supersymmetry) have fallen out of fashion. But there are still promising and not yet completely discredited ones.

Yes, though there's a (minor, understood by everyone in the field) issue with the WIMP name: "Weakly Interacting" can lead to confusion that they must interact via the Weak force. They don't strictly need to: they could interact only gravitationally. That would make all the weak-force interaction experiments (big Xenon tanks & the like) fail, leaving only the gravitational evidence.

To me the concept of dark matter and dark energy reject issue with both the theory and measurements.

These concepts assume that both are correct, in which case the answer has to be matter and energy that is there but that we don't see.

The term 'dark matter' has something of an interesting history. Lord Kelvin (yes, that Kelvin) talked about 'dark bodies' in 1884 when discussing the missing matter of the galaxy. Henri Poincaré, writing in French, used the term 'matière obscure', which translates to 'dark matter' (kinda) while discussing Kelvin's work in 1906. In the late 1970's and early 80's, Vera Rubin, Kent Ford, and Ken Freeman gave the evidence that dark matter was a really real thing all over the universe and they used the term 'dark matter' in those papers to describe their measurements. In 1984, George Blumenthal, Sandy Faber, Joel Primack, and Martin Rees released a review paper on the recent findings of many groups that tied together Cold Dark Matter theory. I was a student of George, Sandy, and Joel's (in separate classes each) at UCSC.

Joel told us that 'dark matter' was a term used at the time and that coffee is a favorite beverage of many astronomers. As one of the local Santa Cruz coffee shops was serving a drink called 'double dark espresso' (caffeine levels fit only for a tired astronomer, I guess), he decided to name the ΛCDM theory after one of his favorite beverages, hence the 'Double Dark' Theory.

I think dark matter is a perfectly fine term. It's stuff that doesn't glow or interact with much of anything, but seems to just fall down. The term 'dark' is very much in keeping with the naming conventions of the post-war ear, where things are muted and humorously named (Big Bang, MAD, Charm quark, etc). The community has really embraced the term and, per at least Joel's theories, has remixed the term into coffee and other drinks, riffing and expanding the names to encompass other 'fun stuff'. I'd expect the names relating to various scotches to creep in soon (double cask, 15 year, etc), if my friends' predilections are any guide.

As a member of said general public I’m a little shocked by this revelation.

Thanks for the summary.

The inconsistencies are not named dark matter. But the leading explanation for the inconsistency is, quite abtly named: It's particles with a mass (=matter) which interact electromagnetically so weakly that they are non-visible (=dark).

Yes, the inconsistencies are not named thus, but the explanation for the inconsistencies are poorly communicated. Saying they are particles says too much, we don't yet know that there are other 'dark' particles either. We only know that our observations have an inconsistency, and we can 'explain' that inconsistency by positing that the observable/known matter is interacting with unknown/unobserved matter-like stuff. Giving this stuff a name like dark matter implies far more than we really know to the public.

I don't think so. The DM hypothesis is matter, not only matter-like, for whatever that is, which is dark. That's exactly what the hypothesis is. What do you think is implied which is not part of that hypothesis?

I understand one hypothesis is that it is some sort of matter, but the anomaly is that we can't fully account for why galaxies don't fly apart, given our current models and observables. One explanation for the anomaly is that there is stuff which interacts with observable matter in a matter-like way. That is, we don't know there is any new form of matter, rather, one of the conjectures is that the inconsistency we have observed can be explained by positing a new form of matter which interacts with known matter in a way we have not yet experienced or accounted for in previous models.

The point is, the real discovery is not the existence of any new form of matter, because we have not discovered a new form of matter, rather, the discovery is that we've found a very interesting point at which our theories are inconsistent with reality.

That's all more or less true. But what part of dark matter, the name of the conjecture, as you say, or the leading hypothesis, as I say, runs against this description? It does not imply that it's the only hypothesis, it does not imply that we discovered dark matter. Btw, stuff which interacts matter-like /is/ matter.

I've had a number of discussions with lay-folk who think we have in fact discovered a new/exotic form of matter. That is what gets lost. What we have discovered is that something went wrong in our predictions, which could be explained as a new form of matter (or more than one new form of matter), but, could also be explained by changing our models in any number of ways, etc. This is not at all obvious to the public.

I say "we" in the broadest sense of physicists, as I did study that for my grad/undergrad, but do not practice in the field (I now do the computering).

That may well be the case, but that is not caused by the term "dark matter", but by people not making enough of an effort to distinguish between leading hypothesis and discovery. That could happen with pretty much any name.

I'm pretty sure it wouldn't happen with the term Dark Hypothesis, or Dark Inconsistency. Or something cooler than those :^) . I think the words 'matter' and 'energy' are the confusion because they are already overloaded with meaning, even if not properly understood by the public, they still have some sort of idea of what those are.

It might be interesting if we had a standard, and publicly known, word or phrase used when talking about scientific results that find an inconsistency between reality and theory (not just slang). Like, "today physicists discovered a Theory Bug when applying Einstein's general relativity to galaxies" or some such. Something cooler than that though.

When we say inconsistency we normally mean a small difference; my understanding is that physics is missing 75%+ of the stuff of the universe that is required to make the sums add up. That's not an inconsistency, it's a dissonance.

It's only dissonance from the homocentric position of thinking we're the dominant form of matter in the universe. As it is all the parts of the universe relevant to us are tiny flecks of rounding error in the universe, now we seem to be finding the same with stars. Just like the familiar universe is mostly boring hydrogen and helium in stars, that's now being dwarfed by even more boring matter. It's like when we went from a geocentric view of the universe to a heliocentric one only to discover that the sun wasn't the center of the universe either.

A challenge to this line of thinking is the youth of the universe. Why is the universe ~14 billion years old, in the fleeting Stelliferous era, when we know that red dwarfs will last for ~10 trillion years?

Not sure it's a direct challenge though, it is an open question. Our interpretation of the age of the universe is based on matter interactions and measurements alone. It is an assumption that the big bang produced both matter and dark matter. What if only matter was produced in the big bang, and dark matter existed long before that?

The normal matter and the dark matter is distributed roughly the same way in space. For example, the dark matter associated with galaxies is in the same places as the galaxies. This strongly suggests that either the dark matter was produced more or less like normal matter in the Big Bang or that it somehow interacted very strongly with other species in the Big Bang.

I would be surprised if dark matter predated the Big Bang and survived it in and meaningful sense. I wonder if an theory in which dark matter is conserved during the very early universe is even consistent with observation.

That's very true, time is the one dimension that earth and maybe one day humanity make up a significant chunk of. But some possibilities are that we're one of the first intelligent species or the anthropic principal, were observing because we can and life later in the universe isn't possible.

I don't mind at all that matter isn't so important. I just mind that we can't account for most of the stuff that's out there. We have no knowledge of it; does that mean that science is a failure?

I agree, but for different reasons: I think the terms "dark matter" and "dark energy", like "black holes" and "wormholes", have assumed certain semantic qualities that are out of proportion to their physics meanings simply because they sound cool and edgy, like something from sci-fi.

The popularization of physics is a ultimately a significant source of funding for physics, along various vectors, and so terms like this are incredibly valuable to the field, regardless of how poorly descriptive they are.

In any case, physicists working in the field know what the terms mean: a huge bag of observations that are inconsistent with basic theories that we know to be incomplete.

Yeah, I don't think the physicists are confused about the terms, it's the use of the words in the popular lexicon that I think breeds confusion, and sometimes causes a missed opportunity for educating the public on how science works.

Right, but there wasn't a council meeting that voted to name these things. Names and notation arise organically. And the first thought of a physicist publishing a paper isn't (nor should it be), how will the general public misinterpret this?

This misconception can be explained in one sentence when somebody asks what exactly dark matter and energy actually are, and it doesn't really matter what people who don't care to ask think about the subject, because well, they don't care.

Yeah, I know. It's just a little frustrating, as I have had relatives be confused by this, thinking scientists have discovered new forms of matter, and have asked me questions about it - only to be disappointed.

I was disappointed with their disappointment mainly because to me the finding of so significant an inconsistency is magically interesting. To be fair, I have a physics degree, and one of the things I loved about learning this stuff was finding all the places that our understanding falls apart.

There’s definitely a very fine distinction between, “here are a bunch of observations that make it seem as if there is a bunch of weird matter that we can only detect by measuring its gravitational effects” and “there is literally a bunch of weird matter that we can only detect by measuring its gravitational effects”. One being a hypothesis to explain the other.

Agreed. But we have to keep in mind that this is how science works. For example, if were strict, we could not say "there is an exoplanet orbiting that star", we could only say "there are a bunch of observations that make it seem as if there is an exoplanet orbiting that star". And if you take this to the extreme, we do not know if the star is there. It is the same for laws: nobody has observed gravity or GR, just their effects. We can get philosophic and wonder if we are here...

At some point, we have to assume that observations correspond to an external reality or everything gets meta. While I agree that it may be convenient to make more clear what we know about dark matter, it is not easy to find the perfect name and, it seems to me, not a task for physicist, who just need a name for the terms in their equations.

I completely agree with you. But what if these terms we're giving names to are just flat out just nonsense in the equations? Just a term to assign to the error to make the numbers work out?

That is precisely the hypothesis of the article. It looks indeed like a plausible possibility. And a scary one, because we cannot just solve the equations with total precision. From my layman point of view, it looks like a very interesting but very difficult problem.

Yeah, to me having a more interesting discussion in a public forum (news shows, or other) would be about this sort of thing. Until we have dark matter in our hands or have reproduced this new weird matter in a lab, there still remains the possibility that we just got something wrong somewhere. Which to me is just as interesting. But exploring the possibilities and working through how theories could be amended vs amending our catalog of nature is the stuff I think our society could benefit from participating in.

'Dark Matter' is physicist code for 'gravitating stuff that we cannot see'.

'Dark Energy' is physicist code for 'anti-gravitating non-stuff that we cannot see'.

Interesting, the term dark is also problematic, first used when little is known about something. Later common usage shifts to something backwards or even non-reflective. See “dark ages” for another example.

“Dark matter” would be better described as “perfectly transparent and non-interacting gravitational mass”; and “dark energy” would be better described as “inherent tendency of empty space to expand”, but both are quite a mouthful.

Very interesting! Thanks for clearing up this misconception.

So what are all these "looking for dark matter" experiments actually doing?

They are looking for dark matter, obviously.

It is still the most parsimonious explanation that there is some kind of gravitationally interacting stuff out there that we just cannot detect with our current measurement methods. It is the simplest hypothesis that can explain all of the available observations without adding too many epicycles to our otherwise well tested theories.

Some experiments are literally looking for new types of matter that could play a role in interacting with observable/known matter than could potentially explain the overall observations. Some folks are also tinkering with the models/equations themselves to explore how to better model the universe. See Modified Newtonian Dynamics as an example.

The point the article is making is that we are inconsistent in our application of GR to large scale structures. Should we be surprised when the results we get are inconsistent?

That's like pushing new features rather than fixing bugs in software

So, yeah, I like this a lot. I have a degree in physics, but have been writing software for a very long time now. This resonates with me. Though in this case, the new feature may be dropped when it turns out it really doesn't work.

Fortunately, there are those who try to fix the bugs, see Modified Newtonian Dynamics as a Dark Matter alternative (as I mentioned elsewhere).

That comparison seems inadequate. Rather: that's like labeling bugs which you haven't found the origin of.

No, it is like triaging bugs by looking for the most plausible origins first.

Not saying anything about the dark matter, but dark energy is highly likely just the basic energy of the empty vacuum. The problem is the fact our equations don’t give the correct answer for it.

If this is a breakdown event like dark body or just a within system surprise event. In other words, Need a total new system or just refinement of observation or equitation?

>That isn't the case at all. We have equations we use to predict observations, and in certain cases, those observations are not quite matching what we predicted. There are a lot of reasons why this can happen, and using the equations poorly is but one of them.

This is absolutely correct. What is its clear (and as evidenced by some of the replies to your comment) is that many so-called "scientists" are filled with hubris and lacking intellectual curiosity. They believe (more or less) that we know how "things work" even though our commonly accepted formulas and equations about how "things work" don't add up. In their minds, its just a matter of discovering a still-hidden particle or detecting "dark matter" or "dark energy" that must exist, because they are certain that their understanding of the universe is how things must be. Unfortunately, this is the opposite of how science works. Real science is based on verifiable measurements and observations that can be replicated and verified. Its fine and dandy to come up with formulas and hypotheses based on incomplete data sets, based on how you think things might be, as long as you understand that you could very well be wrong - no matter how well a given hypothesis may seem to work for a selected, discrete set of measurements.

Yeah, basically that happens. I ran into this in school as well, as some professors accepted the current models as reality, which is what made me so sad in grad school when I realized how untrue this was.

I think of physics as a program. That program is built internally of the various models/equations, and that program takes a set of inputs, which are observables, and a time index. The output is the same set of observables, but with their values at the time index input. That is, physics essentially predicts (forward and backward) from a given set of observations, a new set of observations.

So to me, whenever the output observables do not accord with actual measurement, I first assume there must be something in the program that is wrong. Dark Matter is the equivalent of assuming that there must be another set of missing input observables into the physics program. It's possible, but to me the most likely first cause of the inconsistency is the program itself, as that is, as you said, how science works.

Unfortunately (as evidenced by the downvotes to the post you have replied to) many people have their entire careers and, more importantly, understanding of reality married to orthodoxy. In many ways this world view is like a religion. Dogma that has been accepted and internalized. Its sad and ironic that these people consider themselves to be scientist when, in actuality, their defense of dogma is the opposite of what science is all about.

I have this feeling as well, but now I realize that [citation needed]...

Yeah, maybe. The thing is, there are a lot of independent observations that match up really really well with the hypothesis, "there's a bunch of matter around that only interacts with us via gravity". If our models are off, they would have to be off in exactly the right way to affect our observations of gravitational forces but not really anything else. Which may very well be the case--or it may be the case that baryonic matter isn't really the end-all be-all of the universe and we're just a bunch of baryonic chauvinists for thinking so because that's what we're made of ;)

Obviously, nobody knows for sure yet. But I would caution most fellow laymen not to rely too heavily on the intuition that says, "dark matter? pffft!" or even, "but what about all of those observations that didn't match Newtonian dynamics?". Physicists (including the author here!) are generally smart enough to have thought of those objections themselves, which is why they've spent decades trying really hard to disprove the notion of "dark matter" by compiling all of these anomalous observations. Sometimes the universe is just unintuitive, at least to dumb apes like us.

I'm not saying you can't have a hunch that we're just doing the math wrong somehow; I just wouldn't be overly confident that's the case just yet.

Hossenfelder is refering to the ongoing discussion in the physicist community on this topic. Some of the giants of the field (Wald, Ellis) semi-regularly exchange papers on this.

It's not a hunch.

Green and Wald tried to proof that the inhomogeneities don't matter, others disagree that they proofed anything of the sort. It's a fairly robust debate, and I am not qualified to summarize the state of play. Here are some slides:


Edit: The rebuttal paper to Green-Wald from three years ago already has 100 citations... https://arxiv.org/abs/1505.07800

It's possible that the question will be settled in the next years through numerical simulations:


Edit 2: All of this is for dark energy, for dark matter I don't know if there is a similar discussion on the role of the post-newtonian approximation that is used there.

“Match up really really well” is not that impressive if you consider that you deduce the distribution of the dark matter from the observations. So it just means that in most cases you can find a distribution of in all other ways unobserved matter that explains your observation. That leaves you quite many degrees of freedom to play with.

Not exactly. Independent experiments that check completely different things find the same total distributions of masss and energy. So the mass-like and energy-like deviations from the observed mass and energy are always pretty much consistent.

They all observe the effects of gravitation, don't they? Whether it is deviations in lensing or the velocity and density distribution or visible matter, they all seem to have the same source, but it doesn't say anything more about the nature of the source of the deviation.

Exactly. The only thing that is really certain is that correction terms are required when fitting models to observations and these take on the role of mass and/or energy.

The usual working assumption is that if a well established theory indicates the existence of extra mass or energy, it usually exists in nature and can be found. This is essentially something that has its roots in particle physics: every violation of conservation of energy or momentum turned out to be caused by a hitherto unknown particle.

But we also see galaxies whose gravity is much weaker than we’d anticipate based on the objects we can see, impying they have less dark matter.

Are you saying that we observe galaxies whose gravity is weaker than we anticipated from the visible mass? Then that would be very interesting news to me, and something that you can't explain with dark matter.

If you are saying that we observe galaxies whose gravity is weaker than your models for the distribution of dark matter, then that is an argument against your models, not for them.

No, observations of galaxies that seem to have little dark matter are an argument for the existence of dark matter (over the competing theory of a modified gravitational interaction).

And here is why: If you have two galaxies with about the same size and about the same number of observed stars they should have about the same distribution of barionic matter. So gravity should work the same in them, no matter if that gravity is described by Newtonian theory (probably not), General Relativity (I would assume that, but who knows) or your modified theory of gravity (maybe, maybe not). Any difference between them can not be due to the theory of gravity. But it can easily come from different amounts of un-seen dark matter.

The point, as I understand it, is that we've found cases of galaxies that look like they should be about the same mass as each other based on the light they're putting out themselves, but gravitational lensing measurements indicate a significant mass discrepancy. That's what's hard to explain without "maybe there's a bunch of matter we can't see" - if it was more of a "maybe gravity just works differently at galactic scales" situation, that scenario would be impossible.

I understand the motivation for dark matter. My point is that as long we have a full density distribution to play with, we can explain almost anything with the model. And the start of this thread was me disagreeing with how much it actually proved that observations matched "really really well " with the model.

I don't see why the scenario "gravity works differently at large scales" couldn't explain the observations until we have some idea how gravity would work differently.

Don't forget that each of these observations rely on a quite large sets of assumptions about everything from how much luminosity from galaxies with a certain (normal) mass can vary to how we estimate distances to very far away objects. We don't have that many observations of e.g. gravitational lensing. And, if we realize we actually don't know how to calculate things correctly with GR, or our theory of GR is wrong, that will have implications for all of those assumptions.

The dark matter hypothesis might very well be right. It is a very reasonable guess. But so far I think the proofs for it have been overstated.

Nobody says it's proven. It's just the best what we have so far. Any theory that would work just as good and explain anything on top would be welcome.

And I can say myself that not everyone agrees with that discovery: http://www.astronomy.com/news/2019/09/astronomers-cant-agree...

Well, people still debate about the Bullet cluster. But if it's correct then it's actually a good (relatively) confirmation for some sort of dark matter. It two similar galaxies look the same but have dramatically different mass distribution then there's some mass we don't see yet.

I think you misread. Sabine is arguing that maybe dark matter is just the error term in our approximations of real matter, and pointing out that no one yet has actually taken the time to disprove that Occam's Razor theory.

I wouldn't presume to argue against her, because she's a physicist and I'm not.

I'm mostly making the case to the peanut gallery here to maintain a state of epistemic noncommittal. It's a very popular intuition to think that "dark matter" is just a weird consequence of modeling things improperly, kind of like how we used to think there was another planet between Mercury and the sun because that was the only way to make Mercury's orbit consistent with Newtonian dynamics. If we are modeling things improperly, I suspect it would have to be a pretty weird bug--and I think the one she's suggesting qualifies as such.

Yes, and she says it quite clearly:

> Now, what we do when we want to explain what a galaxy does, or a galaxy cluster, or even the whole universe, is not to plug the matter and energy of every single planet and star into the equations. This would be computationally unfeasible. Instead, we use an average of matter and energy, and use that as the source for gravity.

> Needless to say, taking an average on one side of the equation requires that you also take an average on the other side. But since the gravitational part is non-linear, this will not give you the same equations that we use for the solar system: The average of a function of a variable is not the same as the function of the average of the variable. We know it’s not. But whenever we use general relativity on large scales, we assume that this is the case.

> So, we know that strictly speaking the equations we use are wrong. The big question is, then, just how wrong are they?

[emphasis added]

It's not a "hunch". It's just how the math works.

average of 0-9 = 4.5

4.5^2 = 20.25

average of squares of 0-9 = 28.5

Thanks for this, that is the best summary I have seen yet in this thread.

It isn't a hunch, though. Physicists have known since the 1930s that the math is wrong because they are using averages in non-linear equations.

The problem is that no one knows how wrong.

> Physicists (including the author here!) are generally smart enough to have thought of those objections themselves

Yet we're no closer to a satisfactory answer. It's not that physics has some unresolved problems, it's that physics has a giant hole through its center.

Einstein got us closer than Newton, but his work is obviously incomplete. The dogma surrounding Einstein's work borders on religion.

But muh experiments!

How many times have Newtonian mechanics been proven? Lots and lots. And at one frame of reference (pun not intended) it's absolutely correct.

How is Einstein's work any different? It's not. Stop pretending only a chosen few can have meaningful ideas about how the universe works. You might need training to translate that idea into a workable theory, but not to generate the ideas themselves.

If I were you (collectively) I'd be taking ideas from everywhere. It's still your job to see if they make sense, so don't worry about the unwashed masses stealing your thunder.

It's amazing that every dark matter discussion goes the same way, with the same amnalogies to aether or epicycles, completely unlinked to the actual evidence. (Unlike the blog post, where Hossenfelder suggests a specific methodological problem, and that GR may in fact be completely correct. This is completely unlike anything people have suggested here in the comments.)

Dark energy is less well established, but based on the evidence we have, Occam's razor points strongly towards dark matter. Dark matter is a specific theory that makes predictions, and as far as we can tell these predictions are borne out. There was a competing hypothesis -- that GR itself is wrong and needs to be corrected. These were in competition, but the evidence has decisively broken in favor of dark matter. If it was just a question of fixing the equations, then you should be able to predict the apparent pattern of dark matter from the distribution of ordinary matter. For a while it looked like maybe you could, but more recent evidence has shown that you can't. (This is the reason why the Bullet Cluster comes up as an example so much -- the Bullet Cluster looks like what you would predict if galaxies have dark matter halos and two galaxies collided.)

People also misunderstand the incentives here. If you could write down a modified theory that could explain all of the evidence, you would be the greatest physicist since Einstein, maybe the greatest since Newton. You would achieve immortal glory as one of the central figures of the 21st century. So people try to write down that theory, but it turns out to be really hard to do.

Dark Matter is not a specific theory. Are you talking about WIMPs?

Some of the astronomical data is compatible with any kind of dark matter, though other observations (such as the Bullet Cluster) points at WIMPs.

I’m imagining this thread if it were particle physicists and cosmologists arguing about the difficulty of the traveling salesman.

I’m not convinced we hobbyist physicists have anything to add to this topic. ;)

I’m not convinced we hobbyist physicists have anything to add to this topic. ;)

On average, I'd say that's a safe premise to adopt. But I'd add two things to that:

1. The question "is that the point?" That is, cutting edge cosmological research isn't (AFAIK) generally done on HN. The "real experts" probably aren't coming here looking for new insights, and the rest of us idling around chewing the cud on this isn't really hurting anything.

2. That said, this "cud chewing" may be beneficial to the participants in the conversation, or perhaps - in a "long game" sense - to science at large. If somebody reading all this is inspired to take up physics as a career field, or just learns something that they turn around and use as metaphorical inspiration in some seemingly unrelated field, well... maybe that makes it all worthwhile.

Anyway, at worst this is entertainment, and arguably of a higher quality than watching the latest episode of the sit-com de jour.

For sure. If I didn’t totally agree with you I’d never read anything on HN. Entertainment indeed!

> us idling around chewing the cud on this isn't really hurting anything

Or... maybe it _is_ hurting?

Junk may displace quality. An HN page filled with low-quality comments may discourage people with expertise from contributing. "Eternal September" and "Someone on the internet is wrong!" being less engaging than informed (or even just not energetically misguided) discussion. I certainly have a predicate of "ok, there's no point in my commenting on this HN post; it's 'gone bad'; there's now an inverse correlation between people having a clue and their being likely to wade through all this and potentially see my comment".

There's also "You play like you practice", and this is negative training on ignoring the criticality of recognizing expertise and its limits.

A bar discussion with an expert can go like a seminar. Or if without, yet with experienced people, like a panel. But as the available expertise declines, things can rapidly collapse towards a stereotype of a DC cocktail party, with nonsense lapped up and shared due to an inability to judge expertise and its limits. You get bad reddit, with babbled nonsense undergoing memetic selection largely disconnected from reality. FOX News, and the NYTimes's deemphasis of the role of political and economic interests, also come to mind.

> chewing the cud [...] isn't really hurting anything

So perhaps, alternately, it might be viewed as a intellectual integrity fail? One with civilizational impact?

> Anyway, at worst this is entertainment,

So then... its like HN and US public discourse about trade wars, sanctions and shooting wars and their civilian casualties, regulatory capture and industrial policy, education, public health, ... and so on? All sort of a reality tv show? /end snark :)

Hmm, random thought... my fuzzy impression is that views on lies are bimodal, with camps of "white lies are necessary social lubricant; to think otherwise is to be immature and inconsiderate" and "all lies are toxic; to think otherwise is to be oblivious to unintended and broader impacts". I wonder if there's a similar divergence of views with respect to comment and thread intellectual hygiene?

Ideally, we would have more flexible discussion fora. Discussion preferences vary among people, topics, times of day, etc. Sometimes I enjoy reddit pun chains, sometimes I find them an annoying distraction. It'd be nice to be able to toggle seeing them, rather than everyone getting a similar view all the time. But for now... I think of HN discussion as "breaking down" as topics become distant from tech.

I have definitely observed physicists opining about machine learning and biology, especially biology in embarrassingly clueless ways.

Physicists are qualified to tackle every subject:


What have physicists done in the last 70 years...

I have no expertise at all in the matter, but my understanding is that the evidence for dark matter is overwhelming. In particular, this Reddit comment (which is mostly just a TL;DR of [2]) seems pretty convincing:


It seems to me a dark matter hypothesis is hardly extraordinary—that is, I don't see why anyone would have priors that particularly disfavoured it—and, if it were to exist, us not knowing much about it doesn't seem particularly surprising either.

[2] https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...

It’s mostly a question of accounting for intuition I think. Most people have the intuition that it’s very plausible for our model to be slightly off at larger scales than previously understood—and that it’s implausible for a bunch of weird shadowy invisible matter to somehow exist everywhere. It doesn’t help that the history of physics largely consists of our model being slightly off at larger—or smaller—scales than previously understood, and then replaced with a different model that can be approximated by the previous model but is more precise. Like generals preparing to fight the last war, people are prone to think we’ll solve these mysteries in the future the exact same way we solved them in the past.

There is no direct evidence for dark matter. It is a placeholder for observations that don’t match theory. There are many current experiments trying to detect proposed particles but so far none has been detected.

It's much more than a placeholder for observations that don't match theory.

There are a whole bunch of very different observations that don't match theory in different ways, if your theory doesn't include dark matter.

But if your theory includes dark matter, all those various experiments match theory pretty darn perfectly.

There's no a priori reason why something like dark matter shouldn't exist. It would be nice to directly detect it through scattering experiments on Earth, but there are a lot of possibilities for what dark matter is, and each would require a different type of experiment to detect.

Worst case, all couplings to the SM might be so small that it's undetectable on earth. A purely gravitational direct detection is probably out of reach for quite a while.

That would be pretty disappointing, but as far as I understand it, entirely possible.

There are three ways to detect dark matter:

1. scattering in a lab

2. annihilation products observed with a telescope

3. gravitational effects

We've seen 3, and we'll probably get a huge amount more detail on the distribution of dark matter through that method as time goes on. That would already be pretty convincing to me.

People have been looking for both 1 and 2 for a while, without any conclusive detection. But it's easy to write down theories of dark matter that are extremely difficult to detect with those methods.

At some point, we may have to accept that the only way to detect dark matter is method 3, through gravitational effects.

That's why I think it is so important to do precision tests of the Standard model here on earth. DM might show up as differences between theory and measurement. There are a couple of puzzles like that, g_muon-2, proton radius, ^8Be,... Maybe we get lucky.

Well, it's a placeholder, but a very specific kind of placeholder: it fits exactly everywhere. It's not like it's special-cased to one of the problems. It actually works in all of them.

The reason it "fits exactly everywhere" is that the only definition we have is that it's the exactly gap between observation and theory.

No, the definition is cold, gravitationally interacting matter. It just so happens that adding that one extra ingredient reconciles a whole number of different observations with theory. That didn't have to be the case. For example, if the dark matter is hot, structure formation doesn't match observations, or if dark matter is composed of massive, compact objects, we would observe a lot more microlensing events. But putting in just the right amount of cold dark matter makes everything we can test so far work. That could be a giant coincidence, but most cosmologists think it's because cold dark matter really exists.

yeah but you'd get consistent results whether is actual mass or a wrong value somewhere in a fundamental constant, the fact that the dark matter proportions comes out always in the same ballpark, alone, tells us not very much.

SciShow Space recently did an episode on Brown Dwarfs. They suggest that there might be so many brown dwarfs (which are hard to see), that they could account for a large portion of the "missing" mass.

And there's a theory that says there are small "primordial" black holes all over the place, which are also hard to detect individually, but together could also account for a large part of the missing mass.

Perhaps invisible brown dwarfs and invisible black holes are an easier pill to swallow than mysterious dark masses/energies?

This is the MACHO hypothesis, which seems to have fallen somewhat out of favor since we can measure this to some extent and we can't find anything close to the required mass [1]. Although it is likely there is unaccounted regular matter, I think it is considered unlikely that this will account for the 85% of "missing matter" that we need, not in its entirety and likely not for a significant part of that either. You will need a humongous amount of brown dwarfs to compete with the regular stars and known (supermassive) black holes that are already in the known 15%.

[1]: https://en.wikipedia.org/wiki/Massive_compact_halo_object

"A survey of gravitational lensing effects in the direction of the Small Magellanic Cloud and Large Magellanic Cloud did not detect the number and type of lensing events expected if brown dwarfs made up a significant fraction of dark matter."

The primordial black holes theory is much more of a theory than the dark matter. The corridor for their existence is getting smaller and smaller and they can’t really help establish the current large scale structure of the Universe (while cold dark matter can).

I think recently there was observation of galaxy with very little of dark matter. I think it makes the dark matter harder to explain as some sort of theory glitch not the real object.

That interpretation seems to be still up in the air: http://www.astronomy.com/news/2019/09/astronomers-cant-agree...

The classical multibody problem, Heisenberg already struggled with it in quantum mechanics.

And Hossenfelder really means the we are using the equations wrong. Not that are using the wrong equations.

This is a really important point, a lot of comments in this thread seem to think the article is related to proposals for new laws of physics.

And only CS can help with this problem. If you cannot average non-linear functions trivially, you certainly can approximate it and minimize the differences. Something like a simplex method. The problem is called "Nonlinear averaging".

e.g. http://www.eebweb.arizona.edu/Faculty/chesson/Nonlinear_aver... or http://faculty.cs.tamu.edu/schaefer/research/nonlinearSub.pd... with nice graphs

Can "using the wrong equations" explain gravitational lensing effects appearing apparently out of the void? The Bullet Cluster is a famous example referred to in this comment thread, but it's far from the only example.

As a non-physicist, observed dark matter gravitational lensing really points to "there's more gravity than we expect" rather than "gravity works differently than we thought" for me.

The Bullet Cluster observations are commonly misunderstood. There is indeed a missing baryonic mass problem for MOND but this is also a problem for ΛCDM too.

This seems very plausible as an explanation for dark matter but I'm confused as to whether the author proposes it as an explanation for dark energy as well.

Are there not galaxies on opposite sides of the universe, expanding away from each other too fast for them to ever interact?

The case is specifically being made about dark matter, and in particular about the galactic rotation curves that are the primary motivation for the dark matter hypothesis. Dark energy has a completely different basis that wouldn't be subject to this particular alternative hypothesis -- though the very general point of "maybe the approximations we're using are too simple" does still apply. Though really they apply to pretty much any open question.

I don't understand what the author means. Maybe someone can help me understand.

> Now, what we do when we want to explain what a galaxy does, or a galaxy cluster, or even the whole universe, is not to plug the matter and energy of every single planet and star into the equations. This would be computationally unfeasible. Instead, we use an average of matter and energy, and use that as the source for gravity.

Does this mean instead of individual objects we make our calculations with x units of mass and energy in every given patch of space and this may be an issue, or am I getting this wrong?

> Does this mean instead of individual objects we make our calculations with x units of mass and energy in every given patch of space and this may be an issue, or am I getting this wrong?

Pretty much yes, as we can only take observations like that.

More importantly, the GR equations are non-linear, so taking these 'average' observations and feeding them into our models may introduce significant error.

Or another way of saying that, it's possible that the model would give accurate predictions if you were able to feed it precise and complete information (the details of every single point mass) but not give accurate predictions if you feed it the sampled data we have (observations about whole galaxies, for example). People have discussed this elsewhere in this thread as the N-body problem.

It's also saying that even if we did have that level of detail, our numerical simulations aren't currently capable of calculating at that fine a level of detail.

I'm one of those students who questioned it.

Never got a satisfactory answer.

For example the gravity between two objects depends on the relative speed between them (since gravity is proportional to energy, not rest mass). Are we calculating that for each and every sun orbiting in a galaxy?

It's a lot of interactions, each sun interacting with all the rest.

It gets worse - gravity is also proportional to potential energy. So you need to include how much energy you would have if every sun fell into every other sun, not just their rotation.

I just can't shake the feeling that dark matter is just this unaccounted for energy in a galaxy.

Except this is trivially testable because it should produce defined gravitational lensing effects which follow baryonic matter, rather then interactions like the bullet cluster where gravitational lensing has detached from the observable matter.

> For example the gravity between two objects depends on the relative speed between them (since gravity is proportional to energy, not rest mass). Are we calculating that for each and every sun orbiting in a galaxy?

Disk stars in the Milky Way only have velocities on the order of a few hundred km/s. The relativistic contribution to gravity from such velocity is negligible.

I believe yours is the standard response that the author of the article was referring to, and taking issue with.

Will it be feasible in the near future to run these equations, or is it a millions of years on a million supercomputers type of question?

Based on my very limited knowledge, there is no way to calculate these equations exactly for the entire universe, the way they are currently formulated.

Perhaps more importantly, we don't even have complete information, but only measurements that are necessarily 'averages'.

We don't even have a formulation of this model that allows us to understand the 'error' in these measurements, where here error is about making future predictions based off those measurements.

To use a perhaps tenuous analogy, say we have a model that describes exactly how air molecules interact with each other. We take measurements of parcels of the atmosphere and are able to describe what the molecules within are doing - pressure, temperature, humidity, etc. If we want to describe what happens to the entire atmosphere, however, we need a different model that describes how the parcels of air interact - cold fronts, wind patterns, rainfall etc. We have continuity between these two models, because as far as we can tell the way these parcels of air interact is consistent with the way individual molecules interact, but the actual models are very different to each other.

With general relativity we have a model that works incredibly well for understanding how a small number of things interact with each other, and we assume that it describes the interactions of every object in the universe. We currently use the exact same model to talk about parcels of objects (eg galaxies) in just the same way as we talk about individual objects (atoms? stars?), and we don't know if that is reasonable or not. It's obviously reasonable at a small scale, as when we model things like our solar system we are accurate to an extremely high level of precision but it's not obvious that this continues to hold as we scale up.

When trying to work out a weather forecast we don't simulate every single molecule, but instead large parcels of air (at a surprisingly fine level of detail, it must be said, but still relatively huge). It's not even reasonable for us to measure every single molecule, even if we had the computing power to simulate it.

Even so, it is far more tractable to simulate every molecule of air in our atmosphere than to simulate GR for every body in the universe, or even every galaxy!

We need a model for the interaction of large collections of objects in the universe, and at the moment the best we have is GR. What we don't have is a way of measuring how accurate that is, or even a way of formulating the model that would be consistent when applied to the kinds of collections of objects we're capable of measuring.

It's possible that dark energy and dark matter are real things we can't (currently) directly observe, or they could be artefacts that arise when you try and apply GR to 'averages' of large collections of objects, but right now we don't have a way to quantify how accurate those models are in order to rule out the possibility one way or the other.

How interesting, thanks

That sounds somewhat plausible to me because it fulfills the criterion of something that would mess up our observations of gravitational force but nothing else. Is the n-body problem really so intractable we can't even handwave our way to an approximate answer that accounts for this stuff?

Everyone has thought "maybe it's just a bad gravity equation?" at some point. It's a fun idea, but it's contrary to observation.


This always gets trotted out, but it isn't ironclad proof of anything. Everything about the Bullet Cluster is conjecture based on secondary observations. They're not measuring "mass", they're measuring light, from which they're inferring mass. The galaxy collision is an unusual case, being at relatively high velocity, so the models may not even cover it well.

They're also assuming that the majority of the mass is in the gas, not the stars or invisible brown dwarvf stars or even black holes.

It's layers of assumptions a mile deep being used as irrefutable proof.

Oof, a gravitational lensing based result at 8 sigma... so much for this fun little theory

>> gravitational lensing based result

What's wrong with basing your results on gravitational lensing? Is it related somehow to the complexity of inferring on how much the images are wrapped by gravitational lensing?

nothing, I'm talking about OP's theory. A gravitational lensing based result is particularly strong due to it being model independent. OP's theory cannot account for lensing inferred CoM differing from the baryonic CoM in the bullet cluster.

I'm not sure what you mean. The n-body problem has no analytical solution in general. You have to simulate. The author is suggesting that assumptions you make (such as representing the earth moon system as a point mass at their CoM) in order to make simulation feasible might lead to a catastrophic divergence from reality over time.

This is all a consequence of n-body dynamics being chaotic [1]


I don't know a lot about physics, but I do know a little bit about computer science, and it seems to me that--barring some crazy breakthrough in computation--any simulation accurate enough to overcome the chaotic nature of this problem may very well be fundamentally impossible. That in itself would be a pretty fascinating result.

The problem isn't the fidelity of the simulation persay, it's that chaotic systems are quite literally infinitely sensitive to initial conditions.

Sure; the possible result that the physical universe is fundamentally uncomputable is the result I find interesting.

That is _not_ the result though. This has nothing to do with computability. It's about the behavior of a certain pervasive class of dynamical systems.

Okay, I’m missing something then.

Meh. The axiom of choice is well documented.

Each initial set of conditions would create a unquie system.

Look at Banach–Tarski paradox

What does this have to do with the axiom of choice?

You implied the issue is the infinite set of initial conditions.

All systems are based off the initial set of conditions. The axiom of choice.

The issue is the degrees of freedom introduced with more dimensions...

N-body we can do, for example Illustris Project. To address the concern of this post, one would need non-linear general relativity simulation. That's more challenging.

I’m so tired of this blog and its author that just tells we’re wrong without really leading anythere.

...providing a nice echo chamber for people who aren't ready to understand even what's already experimentally established (and it's much more than the echo chamber believes).

The author published a book with the same "we're wrong" narration, so she has certain motivation to repeat herself.

The post was intended to educate the general public. Physicists already know about this problem.

Most pop science, even the ones featuring physicists, present speculative ideas as if they were well established facts. (Hello string theory)

Finally, criticism of a theory does not require presenting an alternate theory. She presents an argument. If you don’t agree with her conclusion then you need to rebut her argument.

I understand, but she has the same point about essentially every modern idea in physics.

anythere is a great word. inspiring. i might pinch it for a computer game name.

The hubble constant is where the equations breakdown.

Remember why he even added it in the first place.

It was to create a model of the universe that is frozen. It's creating a reference frame.

Depending on which reference frame we use, we get different numbers.

CMB vs Candles

In the most recent development it’s been debunked. The problem was with the Hubble telescope measurements of the candles. Using another satellite and another class of candles they got to the measurement that was very close to CMB.

idk the plank data is still very iffy. Some even say it still requires new physics. Do you have some sauce where it's been "debunked"

Can computational horsepower be thrown at this problem? To put some bounds on how wrong the equations are? We have enormous capacity these days with racks of GPUs and 64 core EPYCs and so on.

Cosmological simulations already take up month of run time on worlds largest supercomputers. (astro-)Physicists were the first users of large computations, in many cases inventing the computers along side with the physical theory the were trying to compute. Computer people like you who think that physicists are too stupid to make use of GPUs are as bad as physicists that think they are qualified to comment on biology or chemistry.

Physics isn't about equations. It's about models of the universe that suggest a certain underlying "working" of the system. If our models are not predicting observations....it just means we still haven't figured out how things really work.

Not surprising given we still can't even explain the double slit experiment wwithout resorting to paradox and contradiction.

For all we know telepathy, magic, reincarnation and wormholes all might be true, we just haven't advanced our understanding of the universe and reality to that point yet.

>> Dark matter filaments. Computer simulation.

To be honest, that looks just like a very old (ca. 1990's) noise filter from 3d Studio Max (where it would be used to make smoke, fog, but also very realistically looking veins, etc).

("Filter" is not the right term. Sorry, I haven't touched Max for ages.)

Doesn't that describe almost any unsolved problem in physics?

The specific point is that we are using linear averages (over large collections of objects, such as galaxies) to model a non-linear effects (General Relativity interactions between individual objects).

It may be accurate to use those averages, or it may not be if for example the chaotic nature of N-body problems causes reality to diverge significantly from the simpler model; we don't know how large the error actually is and currently don't have a way to measure it.

It's the aether, stupid ;) If anything they should look into Maxwells original 20 equations.

But surely, we already _know_ that we'll never be able to model the universe without a law of intelligence.

For instance, if you were an alien scientist on the other side of the galaxy measuring the atmospheric composition of Earth, there's no way that you could make a prediction to match the result without also modelling emerging properties of intelligence. We know that this is true, just as we know that intelligence and computation are physical properties of the universe - so why isn't there a serious branch of physics which tries to take this into account?

So I'm no expert. My instinct is that these two dark stuffs are (as the OP said) "placeholders" - stuffs that fall out of the equations, but which we cannot find, or even describe. If you want me to believe in stuff, then you need to be able to point at it (at least in some kind of metaphorical sense).

I have a similar opinion about "cosmic inflation", as it happens; it appears to be a solution to a problem with the equations.

In both cases, it looks to me very much as if someone has described the problem, and then announced that their problem-description is actually a solution. Nice work, if you can get it.

You know harry potter also deals with the same question in this chapter

Dark arts?

Possibly referring to http://www.hpmor.com/

To my entirely untrained eye, dark matter and dark energy always looked wrong. Like a hacky way to patch our incorrect models. Not at all unlike Aether [0] later rendered unnecessary by Special Relativity, or the Cosmological Constant [1] later rendered unnecessary by, precisely, dark energy.

I find it very arrogant to believe that our current model is the model. New physics are discovered when the model is found to be broken at the edges, and a new model emerges whose explaining power is a superset of that of the previous model.

I guess we're coming to the end of the "normal physics" stage of Kuhn's cycle of scientific progress [2]. I wonder whether the current incentive structure for science will let us progress to the next phase, or trap us for unnecessarily long in the current one. I hope it's the former.

[0] https://en.wikipedia.org/wiki/Luminiferous_aether

[1] https://en.wikipedia.org/wiki/Cosmological_constant

[2] https://en.wikipedia.org/wiki/The_Structure_of_Scientific_Re...

> I find it very arrogant to believe that our current model is _the_ model.

I think it would be arrogant, but I've never met anyone who studies physics and believes this (though I wouldn't be surprised if some exist). It's not arrogant, however, to be dogmatic when faced with ideas that are incompatible with established models. It's unreasonable to claim someone must be using a broken model, because dark matter and dark energy looks wrong.

Scientific models are good when they are predictive, and powerful when they are simple.

It's incredibly rare for a model-in-use to be wrong - we would never use something that has no predictive power - but every day scientific models are tested, refined, expanded, and simplified in order to make them either more predictive (better accuracy) or more powerful (describe more things).

Sometimes this does manifest as being 'broken at the edges' but not always. Sometimes a model is just fuzzy, and improvements add resolution to every prediction.

I generally agree with all of your points.

> Scientific models are good when they are predictive, and powerful when they are simple. [...] every day scientific models are tested, refined, expanded, and simplified [...]

It seems to me that dark matter goes directly against this. The current model of gravity can't explain certain observations, so we come up with a form of matter and energy that we can't observe in any way, and doesn't interact with anything, except with precisely the one thing we need for our observations to match the theory. A theory that hasn't even been reconciled with quantum mechanics yet!

Smells super fishy to me. But as I say, my knowledge of physics and astrophysics is super basic, and I wouldn't call myself "someone who studies physics" in any meaningful way. I understand your feeling that it's unreasonable for me to criticise the status quo because dark energy looks wrong, as I have no idea about the underlying math.

> so we come up with a form of matter and energy

The current top comment by superqd[0] probably deals with this best. Maybe there is a literal dark energy and dark matter, but the thing that is manifested is a 'correction' term in the way we use these equations, and it so happens that using those correction terms makes these equations more accurate.

I'd almost go so far as to say the uncorrected equations are a model that is broken at the edges, and dark energy and dark matter are simply a quantification of how broken they are (with the added benefit that adding them to the equations makes the model better).

[0] https://news.ycombinator.com/item?id=21279200

That makes a lot of sense, thanks!

I'm sort the opposite (re dark matter and dark energy looking wrong)- although I agree that too many scientific theories are inherently arrogant. I mean, yes we certainly could be wrong and they could be a hacky solution. But...

I look at some of the more fundamental discoveries of the past, especially things like radiation, electricity, molecular structure, etc, and wonder what it must have been like to not have a basic understanding of those principles. "Those people were such primitive idiots, they didn't even understand that water is h2o!" Or something like that. Then I look at stuff like dark matter, and it all of sudden makes sense. We're idiots, too. That's what it feels like.

Einstein never really got comfortable with the real world implications of relativity, outside of the math. And of course he straight up hated quantum probability.

It's really intriguing to me that of the the fundamental forces, we've been able to do so little to manipulate gravity. Hell, we can barely even measure it precisely. The most we've done with it is figure out how to sling satellites with flybys. I'd say we're Ben Franklin in the lighting storm, but I don't know that we're even at that level of understanding. He at least had a testable idea.

> To my entirely untrained eye, dark matter and dark energy always looked wrong.

Wouldn't things like "gravity", "light", "magnetism", "electricity" look wrong if you weren't experiencing it daily ?

Science is moving all the time, see "dark matter" as a placeholder word for something we don't understand, it might be one things, or multiple things, or nothing and we just got something wrong before, it doesn't matter, what matters is that we're observing something we can't fully explain and we're trying to find a solution to the problem. Most high level science look "wrong", there is no intuition for these kind of things, that's why the scientific method was developed after all.

Dark matter sounds super intuitive as for me.

Let's assume we observe a system of planets and we have a good theory to describe its behavior. Then with better tools and more observation we found an orbit deviation. Patch it by assuming additional yet unobserved planet? Or patch it with much more sophisticated mathematics?

Dark matter explains both the gravitational pull of the galaxies and the orbits of the stars in it and the large-scale structure of the Universe – we live in the LambdaCDM universe. Dark energy is the energy of the vacuum space itself. We can’t yet find a theory to calculate it without measuring correctly, but that’s the best explanation.

Aether was more a question of esthetics. It didn’t really help to explain anything and was quite easy to debunk. Dark matter and dark energy are quite different. It might not be the endgame, but these are the best theories we have so far and they work pretty damn good.

The Plasma Physics & Electric Universe models considers "dark matter" to be plasma in dark mode & utilizes equations used in Electrical Engineering & other Electrodynamic/Plasma models.


Relativity (time dilation) & > 3 physical dimensional mathematics is rejected in favor of physical experimentation & interpreting physical phenomena through the lens of plasma behavior (dark & glow & arc modes, discharge, z-pinch, Birkeland Currents, etc). The EU model has successfully predicted surprising phenomona of asteroids/comets & astrogeology and makes bold interstellar predictions. Luminaries, deviating from the standard model, include Hannes Alfvén, Anthony Peratt, Ralph Jurgens, Immanuel Velikovsky, Kristian Birkeland, Nikoli Tesla, David Talbott, Donald Scott.

Recent physical experimental success include the SAFIRE project. "The SAFIRE PROJECT reactor generates energy densities analogous to the Sun's ...in a laboratory on Earth".


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