First test of rival to Einstein’s gravity kills off dark matter | Hacker News Reader

First test of rival to Einstein’s gravity kills off dark matter

newscientist.com

137 pointsedran85 comments

Folks, the observation doesn't test Verlinde's idea, this tests a very general class of models, such as the well-known MOND scheme.

> Milgrom, however, supports the work. He also points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study.

(Also: http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-is... )

This data cannot distinguish between Verlinde and other explanations for MOND-like behavior. We've long known that MOND seems to fit a subset of all observational data better the dark matter, in particular the Tully–Fisher relation

https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation

However, for the rest of the data (e.g., the CMB, large-scale structure, the bullet cluster), MOND is generally considered to give a worse fit than dark matter, or to be silent (because it's not obvious how to extend MOND to a complete cosmological theory). That's why experts think the totality of evidence supports dark matter.

Nothing has changed.

Seems like it's pretty easy to make dark matter a good fit, because you can just adjust the presumed distribution of dark matter to fit the data.

Nope, this is only an issue on galactic scales and below. On supergalactic scales, where most of our data comes from, the DM distribution is determined by theory. That includes all the stuff I mentioned: LSS, CMB, bullet cluster, etc.

So, my impression from popular articles is that physicists don't actually know what dark matter is, and have a fair number of candidate ideas. According to the theory that makes good predictions, what is dark matter made of?

Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species.

In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electron, these numbers are known to high precision. For DM, they could lie anywhere in a range over many orders of magnitude (although they are generally bounded from above, because we have not seen evidence of strong interactions).

At supergalactic scales, the predictions of the DM model are not very sensitive to it's mass or couplings. On galactic scales, various nonlinear processes in normal matter (such a supernovea) and possible DM interaction effects become important. Since we don't know those things, the theory has free parameters.

Folks definitely consider other DM models, e.g. bosons, but these are only considered to be favored over MOND insofar as they agree with the same observational data. In other words, the set of all theories of DM which produce the same observational predictions forms an "equivalence class"; and it's the equivalence class, not a particular choice of DM mass and couplings, that is favored over MOND. This is basically what people mean when they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the standard model of cosmology.

https://en.wikipedia.org/wiki/Lambda-CDM_model

In my intro cosmology class, we modeled dark matter as a perfect fluid with some classical density field that interacted with everything else until some very short time after the Big Bang and then stopped interacting except by gravity. We set the equation of state to match that of normal matter: if you put 1 gram of dark matter in a box and slowly expand the box to double its volume, you still have 1 gram.

What makes fermionic dark matter be preferred over (massive) bosonic dark matter? I'm not sure why it should make any difference.

> I'm not sure why it should make any difference.

It doesn't (modulo theoretical prejudices against bosons). I was just picking something concrete and popular so I could talk about how it's behavior becomes uncertain on small scales.

It does seem unfair to fault one theory for making up terms but to ok the invention of dark matter and its placement wherever needed. Perhaps dark matter will be like vulcan: a big chunk of mass invented and placed so as to confirm an outdated model. The new model described a new force, mooting the need for the mass.

It's faulting one theory for making up terms to fit one source of data and then those same terms failing on 6+ other experiments. Dark matter's predictions are consistent across totally different experiments with fewer sets of parameters.

people who just make up dark matter distributions are morons, obviously you use a model derived from the theory.

Well, I wouldn't focus on the MOND part of Verlinde's recent work; the crucial things are that it's emergent gravity, that it's strings underneath, and that he proposes a long range entanglement between strings from which he argues the CDM-like generator of the metric emerges (or, if you like, from which the MOND-like kinematics emerge).

From a couple of days ago :

https://news.ycombinator.com/item?id=13176483

The topic of the article is the experimental data, which does not distinguish between Verlinde's model from many other alternatives to DM. You may think the story Verlinde tells gives new a-priori reason to favor this sort of theory, but it doesn't have much to do with the experimental data being discussed.

Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean.

The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but I don't see what it has to do with MOND. Indeed, in Brouwer et al., I can't even see a reference to MOND.

Only in the newscientist article is there a scattering of the abbreviation "MOND", and the article goes so far as to (entirely unjustifiably) say, "[Verlinde's Emergent Gravity] also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom".

Instead Verlinde's work builds on Ted Jacobson's work from twenty years ago (arXiv:1505.04753) (cf. Verlinde's first paragraph in section 5) and it is no surprise that discussions directly with Jacobson are disclosed in the acknowledgments section.

Sure Milgrom is also further down in the acknowledgments section and MOND is in the references in force, but consider Verlinde's, "We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation." (top of Verlinde's page 39). Further down, "although we derived the same relation as modified Newtonian dynamics, the physics is very different". (emphasis mine)

Indeed, if you look at my link in the comment you replied to, you'll see in the first paragraph that his unremovable background is standard GR dS.

Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories. The key point though is that Verlinde's is more fundamental and MOND-like and FLRW-like results can emerge from it at appropriate length scales (and \Lambda is simply built in).

But that's the point of emergent theories; you can derive the emergent theory from the more fundamental one, but you can't derive the fundamental theory from the emergent one. And in Verlinde's work, the fundamental theory, for better or worse (fwiw, I think worse) is strings in dS, and what emerges is General Relativity as an effective field theory with some IR corrections.

Finally, what I think is most interesting about Verlinde's paper is that string people usually avoid dS. "That's interesting and not outrageously wrong" is not saying that it is anything close to a complete theory (it does not even seem wholly self-consistent), or that the paper is especially well structured. At best you could say that since I am open-minded about the possibility that our current fundamental theories emerge from something more fundamental, but that's hardly an a priori reason to favour Verlinde's EG, or anyone else's. I don't even know where you got that from; maybe instead of "You may think" you meant "Someone might think".

> Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but I don't see what it has to do with MOND. Indeed, in Brouwer et al., I can't even see a reference to MOND.

Yea, I'm well aware of all this. Brouwer et al. are jumping on the entropic gravity media bandwagon. However, as stated in the New Scientist article, and as I quoted in my original top-level comment you replied to, "Milgrom...points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study."

So you really seem to be missing my point. I am well aware that entropic gravity tells a different and more complicated story than MOND proponents. But within the actual physical system being observed, they make indistinguishable predictions. Therefore, this experiment by Brouwer et al. does not actually support entropic gravity over MOND.

> Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories

It's not a matter of conflation, it's that in many regimes (including the OP's) they have the very similar phenomenology. See, e.g., "Entropic-gravity derivation of MOND"

https://arxiv.org/abs/1201.4160

> So you really seem to be missing my point

Yes, I was. I got it now, thanks for the clarification; I think we're in closer agreement than I thought.

Essentially where I think we now differ is in the motivation of Brouwer et al. -- they don't seem interested in contrasting with other approaches as much as showing that Verlinde's EG (or at least the claim at the start of their section 4) survives contact with observational data. Additionally, I think we differ on whether that's enough to make the paper worth writing in the first place.

(Where we probably most strongly agree is that there is a lot of breathless hype from Leiden, Amsterdam and Groningen lately.)

I like Ethan Siegel's explanation of the Verlinde's paper much more than the article with the "kills off" in the title:

http://www.forbes.com/sites/startswithabang/2016/11/12/ask-e...

"The hope is, with the right assumptions, a full theory of gravity, That's the big hope, and what Verlinde is working towards. (Others are also working towards it independently.) This paper is an update on how it's going. So, how's it going?

"There are some successes given very specific assumptions, but there are a lot of problems. The largest problem, quite simply, is that one needs to make a multitude of seemingly arbitrary "interpretation" decisions to wind up with something other than nonsense. For example: the full motivation for this approach is based in anti-de Sitter space (or space with a negative cosmological constant), but our Universe is observed to have a positive cosmological constant (i.e., de Sitter space), and the mathematics of the two spaces have very different properties. For another, you need the entropy to obey a strict area-based law to get the Einstein equations out, but you don't get a cosmological horizon out if you do. (And our Universe has one.) And finally, if you make all the assumptions you need to in order to get the gravitational acceleration for galaxies out, you destroy all of General Relativity's successes on larger-than-galaxy scales. (Verlinde, on pp. 39-40, makes the argument that it could succeed, but the observations of colliding galaxy clusters completely undermine his line of thought.)"

There's even more discussion in the Ethan's article, which is much more balanced.

And another take on Verlinde's paper by Sabine Hossenfelder:

http://backreaction.blogspot.co.at/2016/12/can-dark-energy-a...

"General Relativity is a rigorously tested theory with many achievements. To do any better than general relativity is hard, and thus for any new theory of gravity the most important thing is to have a controlled limit in which General Relativity is reproduced to good precision. How this might work in Verlinde’s approach isn’t clear to me because he doesn’t even attempt to deal with the general case. He starts right away with cosmology."

"I'd say he takes inspiration from models that are best understood in AdS." but "he starts with de-Sitter space which means he assumes dark energy. It doesn't make sense to say that this is a source of dark energy, it's the same thing, period."

> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.”

I find this rudeness and pedantry in academia infuriating. There's no justification for this level of nastiness -- not in private, public, peer review, or science as a whole. Seeking out the unknown and creative explanations thereof is the hallmark of good science; check your ego at the door. I know nothing about him... but in my book, Lubos Motl can go pound sand.

Motl is notorious for this sort of thing. He once called me a "category 5 loon" because something I said in a talk was wrong. But the reason it was wrong is because I was advancing it as a straw-man precisely to show it was wrong.

http://motls.blogspot.com/2012/10/evading-quantum-mechanics-...

That sounds like a great HN handle.

I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk.

Also read the comments, where he says:

I am watching the talk (final minutes now) and except for the history, meaning of interpretations etc., he actually understands QM more correctly than many people employed as physicists...

(The audacity! Someone that actually watches the thing they'll be bitching about!)

[0]: http://motls.blogspot.co.uk/2012/10/evading-quantum-mechanic...

> I encourage you to not stop reading after he called you a loon. He dedicates a few paragraphs to explain what he feels is wrong with your talk.

What makes you think I stopped reading? How do you think I knew that he was attacking a straw man (and hence making a fool of himself)?

> read the comments

Well, that's nice. I feel vindicated. But that doesn't change the fact that he published a totally unwarranted ad hominem attack directed at me based on incomplete information and never retracted it, let alone apologized for it. (Sorry, but a comment doesn't count. Most people don't read the comments.) So I stand by my position that his behavior was unprofessional to say the least.

> What makes you think I stopped reading?

Well his complaints about the talk are after the "loon" bit, and you didn't say anything to the effect of, "well he called me a loon but did explain his position", so I figured.

I explained it as clearly as I could: he called me a loon because I said something that was wrong. And he was correct: the thing he claims is wrong is indeed wrong.

What he failed to notice (or at least failed to mention) was that the wrong comment was preceded by me saying, "I am about to tell you something that is wrong because it makes an interesting and illuminating puzzle to figure out why it is wrong." And it was followed by me saying, "Obviously what I just told you can't be right (because if it were I would at the very least have won the Nobel prize in physics), so there must be a flaw in the reasoning. But where is it? Well, it's here..."

Lubos Motl hasn't been in academia for almost a decade. Almost everyone inside and outside it finds him obnoxious.

> I find this rudeness and pedantry in academia infuriating.

So you'd rather people quietly knife you in review? Because that's what happens if you penalize people for public rudeness. Personally, I'd rather have a loud critique to my face than a polite one behind my back or, even worse, no engagement at all.

I don't need your politeness or friendship when I'm putting forth some new theory. I need accurate, engaged criticism and the number of people who will do that is vanishingly small. And a lot of the ones who will do that have social issues almost by definition.

Accurate, engaged criticism is great, that stuff belongs everywhere.

"I wouldn’t okay this wrong piece of work as an undergraduate term paper" is not accurate, engaged criticism, it's mean-spiritedness under a veneer of toughness.

It's easy and fun to be rude and insulting, and when you can get away with it because of the subculture you're writing within, why not, right? Because being a dick is being a dick no matter who or where you are, that's why, and being a dick doesn't advance anything beyond your own momentary joy.

Every industry, every field, everywhere, will be better off once people get past being dicks just for fun, and give accurate, engaged criticism while maintaining civility.

Every industry, every field, everywhere, will be better off once people get past being dicks just for fun, and give accurate, engaged criticism while maintaining civility.

I disagree. The blunt people (who come off as "dicks" to the outsiders) are like a community's immune system. They protect the community against the sort of people who take accurate engaged criticism, make one or two changes to fix specific issues, and then resubmit their work, wasting the community's time and resources, driving off the people who are making good-faith efforts to help the community. They're not being dicks "just for fun". They're protecting the time and attention of other community members by saying bluntly what many other people are often thinking.

Communities that have too many of these people become closed and ossified. Communities that have none of these people evaporate, as the core contributors just walk away and find better things to do than deal with people who are more interested in getting fame and attention for themselves than making good-faith efforts to advance the project of the community. The trick is to find a good balance.

At least the quote pulled out here isn't useful critique, it's just insulting. I think that's the distinction being drawn here.

There's the constructive rude:

> Mauro, SHUT THE FUCK UP! ...To make matters worse, commit f0ed2ce840b3 is clearly total and utter CRAP even if it didn't break applications. ENOENT is not a valid error return from an ioctl. Never has been, never will be. ENOENT means "No such file and directory", and is for path operations. ioctl's are done on files that have already been opened, there's no way in hell that ENOENT would ever be valid.

https://lkml.org/lkml/2012/12/23/75

And then there's this:

> It's a Category 5 loon. Popular sources are full of anti-Copenhagen crackpot pseudoscience and of exactly this kind of bullshit – claims that the proper Copenhagen quantum mechanics implies superluminal or acausal effects (for the latter, see some largely confused fresh text by Nude Socialist about entangled photons in graves, London, and Beijing: this sort of stuff gets produced every minute, it seems) – but you may still find people who think that the presentation should be even more anti-Copenhagen.

http://motls.blogspot.com.au/2012/10/evading-quantum-mechani...

That's pretty much what academia told Motl.

He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is.

Characterizing him as an example of any sort of trend in academia is both uninformed and deeply insulting.

> He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is.

It's hard to understand why New Scientist decided to quote him then.

They certainly seem to be presenting him as someone worth paying attention to.

> It's hard to understand why New Scientist decided to quote him then.

Is it harder than to understand that they publish an article with the false title and false interpretations?

As others have mentioned, Lubos Motl is no longer in academia. He seemed to have spent more time criticizing others (and not just their works) than on actual physics. His appointment at Harvard was not reviewed.

I'm frankly surprised New Scientist bothered to quote him.

Eh - I meant "not renewed"

> I find this rudeness and pedantry in academia infuriating.

vs all those "I just don't believe in being fake and sugar coating things" folks in every other industry? Not much unique to academia on that front.

Like food critics, some have taken the art of shitting on things to absurd heights.

"I find this theory highly disagreeable, it contradicts findings by X and Y et al."

Compared with:

"I once asked my dog for a theory on the nature of the universe, whereupon which he vomited violently on the floor. This explanation has more truth to it than the one presented in this paper, which I can only presume was written by students who, in their youth, were savagely beaten upside the head by their parents much to the detriment of their intellect."

He's engaging in a power fantasy that has a fellow scientist as a student. I'll bet this guy is a lot of fun to have as a professor.

For me, more interesting "gossip-like" detail that Motl enviously mentions is the "money" Verlinde received. Wikipedia has the details:

https://en.wikipedia.org/wiki/Erik_Verlinde

"In June 2011, the Netherlands Organisation for Scientific Research (NWO) awarded Verlinde the Spinoza Prize, the highest award available to Dutch scientists including a 2.5 million euro grant for future research.[14] The committee cited his work on the Verlinde formula, the Witten–Dijkgraaf–Verlinde–Verlinde equations, the Cardy-Verlinde formula and entropic gravity as the major achievements leading to the award."

By the way, my impression is that reading Motl is not a pleasant experience for anybody but himself.

He's not exactly a crank, but he's not a working physicist either, afaik.

I'm being pedantic, but I downvoted you because you started with the obvious observation "Motl is really really rude" (yeah, he's a real piece of work, check out the rest of his blog for stuff much worse), and then wrote "I find this rudeness and pedantry in academia infuriating.", which isn't supported by the observation that that one guy is rude. (But the rest of your comment is fine.) Individual people can be really rude with or without it being a cultural problem. There's no need to bring academia up like that—that's a lot of people, decent people, and most of them are really quite nice people and good scientists.

I'd be kind of bummed to lose dark matter as a concept. I want to hold onto the idea that it's just as rich as our matter, with its own quantum chromodynamics and an equivalent to the electromagnetic force, leading to a "shadow" universe with their own planets, life, etc. that exists side by side us, trying to figure out why their gravity measurements are just slightly off...

You can also have the idea that there are one-way interactions between the normal-matter universe and the dark-matter universe.

It isn't too difficult to formulate a mathematical system where two subsets are completely identical when considered in isolation, but when interacting, one subset operates as if the other doesn't exist, while the other is easily perturbed by any values in the mirror subset. I'm not sure if you can do it without at least one dimensional basis unit that squares to zero, though.

Imagine, if you will, that there are dark-matter people. We have no evidence whatsoever that they exist, any they are constantly annoyed by all the real-matter people that wander between their dark-couches and dark-televisions, and the real-matter bicyclists that pedal through their dark-bedrooms at the worst possible times.

Wouldn't there be some sort of energy exchange for this to happen? Wouldn't we notice that there's an unexplained loss of energy whenever we do anything?

Or did I just invent a crappy layman-author's explanation of where inertia comes from?

Conservation of energy/mass is a rule for the known universe. If we knew all the rules for the dark part of the universe, we wouldn't even be having this discussion.

Anyway, the way the math works out, it is impossible for a known-universe person to even measure the effects the known-universe has on the dark-universe. The numbers can't come back to the known-universe without an additional input from it, and they would be explainable with another hypothesis that did not include a dark-universe. It way be that all effects we observe to be random are actually perfectly predictable to a dark-universe observer.

IANAP, but I think the most widely accepted view on dark matter is that it is made of "dark" particles that cannot lose energy by radiation. In other words, it cannot condense and form dark stars or planets. When two lumps of dark matter collide they just pass through each other.

Sorry for ruining your dream. :/

Yeah, I googled around after my comment, and that explained it pretty well - it apparently also explains why most of the dark matter is in halos around galaxies, since it can't slow down enough to collapse into galaxies themselves, and is on mostly highly elliptical orbits, spending most of the time away from whatever it's orbiting.

I can still dream, though.

I wouldn't be so worried yet. Verlinde's theory is quite fresh and it will take time to conclusively rule out Standard Cosmological model. As of yet, no one has properly worked out entropic gravity equivalent of various gravitational effects that arise in General Relativity such as gravitational lensing, a part of which was used in the paper discussed in the article.

Never the less, Verlinde's ideas are more interesting and reasonable than other proposed modification of gravity, but that's just my opinion.

I had this thought as well. Time to start subscribing more heavily to a viable multiverse theory, at least to support my tastes in science fiction novels anyway.

Sounds like Stranger Things to me.

Please change the submission title. It is downright misleading. This test did not "kill off" dark matter. To do so would imply overwhelming evidence against the concept of dark matter as a whole. Instead, this theory, which eliminates the need for dark matter in certain situations, has passed its first test. The sentence "... test ... kills off dark matter" is simply false.

Typical New Scientist really. I went to a lecture and Q&A with Alain Aspect when he visited my department in 2007 and he had a bit of a rant about how they had fundamentally misunderstood his work. They published an article claiming his Bell experiment could be used to send messages back in time.

Moreover, it's not even a complete "theory" in the sense that it's still not known if it can even repeat the results of the General Relativity.

https://news.ycombinator.com/item?id=13197595

It's not complete for other reasons, several of which are detailed in Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by design. Indeed, Verlinde starts with standard dS as his unremovable background on which he puts strings. What in GR we would consider perturbations of dS and IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the strings and how they form long-distance entanglements.

However, it's not outrageously wrong, and it is noteworthy that an accomplished string theorist has decided to tackle gravity in a universe like ours rather than leaning on AdS/CFT arguments.

Thanks for the correction and clarification.

For others who, like me, aren't in the field, the "EFT" mentioned is explained, for example, here:

https://arxiv.org/abs/gr-qc/0311082

"The problems of quantizing gravity within the experimentally accessible situations are similar to those which arise in a host of other non-gravitational applications throughout physics. As such, the size of quantum corrections can be safely estimated and are extremely small. The theoretical framework which allows this quantification is the formalism of effective field theories."

dS is "de Sitter space": https://en.wikipedia.org/wiki/De_Sitter_space

I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."

> I'd also appreciate a link to the explanation of "What in GR we would consider perturbations of dS and IR corrections."

Here's a quick overview of perturbation theory then an example of how one might apply it in GR.

https://www.wikiwand.com/en/Perturbation_theory

http://theory.physics.helsinki.fi/~inf/Lectures/Lecture2.pdf

IR is "infrared", low-energy physics, as opposed to UV, "ultraviolet", high-energy physics. In this context, UV means strong gravity where quantum effects are expected to be important. Galaxy-galaxy gravitational lensing (which is the subject of the Brouwer et al. paper that's the subject of the newscientist article linked at the top) is purely a weak gravity problem[1], so is in the IR limit of General Relativity. If you want different results from standard General Relativity you can add corrections by hand in a variety of ways; in the galaxy-galaxy lensing case they would be in the infrared.

[1] strong gravity is hidden behind the event horizons of the black holes of the galaxies and, arguendo, if we could see the strong gravity near the black hole centres, it would not be relevant by virtue of being drowned out by all the stars, gas, dust and other matter.

ETA: This is nice and terse, too: https://www.wikiwand.com/en/Non-exact_solutions_in_general_r...

Physicist here. Please do not put too much stock in this paper.

Some of the folks on this paper are good scientists, but this appears to be more about looking for funding than good science.

Their paper on the Arxiv is about building some math to make gravity look like dark matter. But this has been done 1000 times before with no success. They have no physical basis for their math; there is no reality behind it. I do not expect this to get them the Nobel Prize.

'Kills off dark matter" is probably overstating the case, but I love this theory because it ties together a lot of big ideas.

Verlinde's paper is here: https://arxiv.org/abs/1611.02269

What about the Bullet Cluster[0]? How does his idea address that?

His idea deserves some study but a title will "Kills off dark matter" is very infuriating

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

If you read the Wikipedia article, it points out what MOND advocates say about the Bullet Cluster: they try to handwave it away. It's likely that Verlinde's theory needs a similar solution. Unfortunately for handwaving, over time, the number of weird dark-matter-related observations is growing.

I think the study that this article is about looks at lensing data in particular which should explain the bullet cluster, as well, no?

I remember reading about this as an undergraduate and thinking how attractive his ideas were -- I think Verlinde's ideas have been around for some time. Back then it seemed like few people were giving these ideas thought.

I hope more people decide to explore this avenue. It really is a fascinating take on gravity.

You may find the top answer to this SE question helpful (asked 5 years ago, though): http://physics.stackexchange.com/questions/6561/are-modified.... Especially I think the following comment is spot on:

"Galaxy rotation curves are not very good tests for GR itself, since there are so many parameters about mass distribution in a galaxy which are simply not nearly as precisely understood as GR itself"

That says MOND is a non-theory, so now I'm even more confused since this theory "builds on" MOND.

The New Scientist article writer is who is confusing you.

Verlinde does not build on MOND. An explanation and some extracts from his paper are in an earlier comment of mine:

https://news.ycombinator.com/item?id=13198573

(specifically at the paragraph half-way along in the comment, starting with "Sure Milgrom", if you want to zoom right in.)

Now I'm confused. What is a non-theory? O.o

Theories by definition, may not be correct. It's still a theory.

How does one become a non-theorist? I suppose by non-theorizing.

One way is to propose something that's mathematically equivalent to existing theory. If you have no testable predictions then you're not really doing science. You're doing philosophy.

I'm not sure if that's the objection here. They might also be calling it too vague to be a theory, or something else.

> Now, a team of astronomers studying the distribution of matter in more than 30,000 galaxies say their observations can be explained by an alternative theory that does away with dark matter. If this “modified gravity” is correct, it would up-end hundreds of years of fundamental physics.

No, it would reshape the "top" of these theories. The base would remain the same.

Can someone explain in layman's terms what really is dark matter and why there's so much obsession around it? If we can't observe it and we are only relying on inferences to explain it's effects, that seems like pretty weak set of legs.

Obviously I'm missing the academic discipline to appreciate this topic but nevertheless, I'm curious about our understanding of the very fabric and mechanisms of our reality and the universe.

So basically there are four forces: weak and strong nuclear forces; gravity and EM.

EM is mediated by photons and visible matter is constantly absorbing and emitting photons.

Some particles don't though. Neutrinos for instance. They're dark, they don't interact with light.

So when observation showed that there needs to be more matter laying around than we could see some enterprising young turks said "hey, maybe there's a shit ton of dark matter" and that makes the math come out right.

the obsession is mostly because people who aren't astronomers seem to think "dark" is a metaphor and go "aha now that no one cares about the oxford comma this will be my new opinion"

Up for debate is what kind of particles make up the dark matter and how much mass is dark vs just poor measurements and/or some tweaks to gravity, but unless some one manages to disprove neutrinos: dark matter exists.

"Dark Matter" is a phrase we use to describe a hole in our understanding of the universe. To put it simply, when we look out into space, at very large scales like galaxies, everything is just too heavy. It appears as if there is a LOT more matter out there than we can see. Things are orbiting faster than they should be, like galaxies are heavier than makes good sense..

Maybe there is some matter out there that is "dark", so we just can't see it. That would be a solution. Or maybe the laws of gravity we wrote down are bad at large scales. No one knows. Trust no one who tells you they know the answer: if they did they would have a Nobel Prize.

Where "Dark" literally means: does not interact in any way with light

The title overstates the results of research work. From the article: "Now, a team of astronomers studying the distribution of matter in more than 30,000 galaxies say their observations can be explained by an alternative theory that does away with dark matter."

Suggest "Astronomers studying galaxy mass distribution say their observations can be explained without dark matter."

What would be the next test? Clusters or that dark matter dwarf galaxy causing ripples in our Milky Way?

The "next test" would be to look at more galaxies. They did not actually try to apply their theory too many real world observations. The goal of this paper is to get funding to try and apply their new theory to a huge number of galactic measurements. Until they do THAT, no one in the physics community is going to listen to them.

  “But then if you mathematically factor in the fact that
  Verlinde’s prediction doesn’t have any free parameters,
  whereas the dark matter prediction does, then you find
  Verlinde’s model is actually performing slightly better.”

What? How does this work?

From the discussion section of Verlinde's paper[1]:

> We have shown that the emergent laws of gravity, when one takes into account the volume law contribution to the entropy, start to deviate from the familiar gravitational laws precisely in those situations where the observations tell us they do. We have only made use of the natural constants of nature, and provided reasonably straightforward arguments and calculations to derive the scales and the behavior of the observed phenomena. [..]

> In our view this undercuts the common assumption that the laws of gravity should stay as they are, and hence it removes the rationale of the dark matter hypothesis. Once there is a conceptual reason for a new phase of the gravitational force, which is governed by different laws, and this is combined with a confirmation of its quantitative behavior, the weight of the evidence tips in the other direction.

[1]: https://arxiv.org/abs/1611.02269

That's just Occam's razor, though. The phrasing made it sound like they had a quantitative argument for why fewer free parameters made it "perform" better.

Say that you want to estimate what is the chance that observation 1 fits theory A just by accident (the lower this chance is the higher your confidence can be that theory A is "real").

If theory A has parameters you can adjust, it becomes easier for an observation to fit the theory just by accident (because you can adjust the free parameter).

This is a fairly flawed description, but it should be a good enough ELI5.

When you have free parameters, you are actually opening the door to a whole family of predictions. You're saying that of this entire family of models, you'll choose the one that best fits the data. Without free parameters, you have to come in with a fully specified model to begin with.

Loosely speaking, with an incorrect family of models, there's a lot of wiggle room to find one that still looks quite a lot like your data, while that freedom is not there when making a single specific prediction. There are semi-rigorous ways to take that into account while comparing models https://methodology.psu.edu/node/504

principle of parsimony

The central equation in this paper is 4.40. It doesn't seem to account for some galaxies recently discovered that are almost entirely dark matter.

Totally OT: I read "Einsteins graffitti" and could watch my own brain trying to make sense of the sentence. And fail.

Juuuust gonna leave this here...

http://xkcd.com/1758/

This is different because it's an experiment for this theory.

It's not different because this experiment can't discriminate between Verlinde's ideas and long-known alternatives to dark matter.

https://news.ycombinator.com/item?id=13197214

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