Dark Matter: The Situation Has Changed
backreaction.blogspot.com
backreaction.blogspot.com
Regarding modified gravity theories, it's true that you can mimic some of the dark matter behaviour by the modification of gravity laws. Also if you can add some kind of global scalar field you can similarly mimic dark matter behaviour. The problem however is that the modified gravity theories (at this point) do not allow the same range of simulations that cold dark matter(CDM) simulations can do (for various reasons, such as many of those theories do not have a general relativity formulation). Because these theories can't make as many/as varied simulations as CDM ones, they are not as tested as regular CDM and much less predictive. Therefore based on that fact alone IMO these theories are less useful.
It is also true that if we continue for many years searching for the dark matter particle and will still be unable to find it, maybe we will need to refocus our attention on alternative gravity theories, but the problem is that so far the part of parameter space where we looked for the dark matter particle is still pretty small and there is no strong reason to think that dark matter should have been there.
Epicycles were successful and predictive. But they were later found to be the wrong way to understand the celestial motions.
The problem is these calculations with dark matter are just fitting observations with zillion parameters. It is busywork, yes sometimes it is useful, but it is not discovering new laws, just chugging along without having to challenge our concept of how star/galaxy motions really work.
And please don't BS about zillion of parameters. That's not the case. The dark matter only simulations don't have that many parameters (other than the basic cosmological ones) . When you start putting baryons in, yes that's where the parameters start appearing (probably of the order of a 100), but it's hardly surprising give the need to describe the complexity of star formation, gas dynamics, black hole accretion etc. The modified gravity theories don't need as many parameters because nobody has ever run any modified gravity simulation that is even close in complexity to what Cold Dark Matter simulations do.
What kind of initial condition about dark matter distribution are you assumming in your simulations?
Regarding initial conditions, I'll point you towards wikipedia. But basically the assumptions is that it's collisionless fluid with very small Gaussian density perturbations and scale invariant power spectrum. In total the specification of that model requires ~ 10 parameters.
If so, do these kinds of simulations with 10 parameter model predict galactical rotation curves in agreement with observation?
Or do we need more detailed model of dark matter distribution for the rotation curves?
Quote from from the page you've linked [1]:
> However, either the observations do not constitute a proof of the CDM models, or dynamic ingredients other than halo and disk density profiles are necessary to study the rotation of spirals.
[1] https://ned.ipac.caltech.edu/level5/March01/Battaner/node27....
One can also step up a scale level and consider that the distribution of peculiar velocities in galaxy clusters is consistent with plenty of dark matter inside galaxy clusters but outside galaxy halos. This DM is bound to be stirred up by the moving galaxies, even if only gravitationally, and such dynamical processes will also affect the density of gas and dust in the cores of galaxy clusters (the cores of which need not be occupied by galaxies) too.
We need to explain peculiar motion of galaxy-cluster-members, and member-galaxy-vs-whole-cluster lensing behaviour too, and astronomical surveys of galaxies and clusters continue to provide plenty of data to test various theoretical approaches. Those are what propose observable consequences of self-interaction (is there dark chemistry? do dark matter particles collide and annihilate?) or is dark matter warmer than cold dark matter (if it's a naturally warm gas, then it would have a hard time gathering in the cores of galaxies and clusters, much like a warm gas has trouble gathering on the floor of a laboratory compared to a very cold gas). Or does it couple non-gravitationally but very weakly to baryons or electrons, such that it undergoes a phase change as one goes from the very dense cores of galaxies and gas-rich clusters towards the sparser edges? If one proposes atmosphere-like dynamical processes at these scales, what are the observables one expects from astronomic spectroscopy?
Spirals are certainly interesting, but there are also elliptical galaxies where the outermost hydrogen gas is in arbitrary orbit in clouds of various sizes; unlike edge-on spirals, they don't show a advancing-side/receding-side red/blueshift dipole, and where the rotating elements are large enough to measure significant dipoles, they do not strongly correlate with oblateness. More intriguingly, the measurable red/blue shift velocities of (groups of) stars within such galaxies are mostly radial, which is very different from what's seen in spirals, where rotation dominates. That radial speed is different from what would be seen if the luminous elements (including that that is luminous in infrared, radio, and so on, or material that produces absorption lines rather than emission ones) were all that determined these orbits. But a cold dark matter halo explains -- or at least is in concord with -- those orbital velocity anomalies.
And as galaxy surveys continue, neat things like giant low-surface-brightness galaxies (and their substructures) will be found more often. Their properties are good stress-tests of dark matter theories and alternatives, in that they should generate predictions for observations which will arrive within the next few years. They are also probes of largest-scale structure formation, as so far they are not found in or near dense galaxy clusters. If we start discovering GLSBs in dense regions, that would be a challenge to largest-scale distribution of dark matter as the driver of the cosmic web, which is the best current explanation of the distribution of galaxy clusters that are not gravitationally-bound with one another.
Are you referring to the Friedmann equations? The equation of state? The half-dozen free parameters of the model itself?
What's your exact objection here? Because there isn't anything that I would quickly call a "lambda-cdm field" in the standard model of cosmology any more than there is a single "standard model field" in the standard model of particle physics.
"The field itself does" -- again, what exactly do you mean here? Are you even talking about a mathematical or even physical object?
So by analogy, you're saying that modern dark matter theories are so general that they a) could account for any conceivable observation, and b) fail to predict novel phenomena? I don't have the physics knowledge to evaluate that assertion, but it doesn't seem to be shared by the broader physics community.
The trouble with dark matter is that it's unimodal: It exists to fix a problem where our theory of gravity disagrees with observation, but at the same time the only way to measure dark matter is by observing gravitational effects.
The only true predictions made have been "if dark matter is actually x particle then ..." Or (weaker) "if dark matter has non-gravitational property X, then..."
And thus far all prediction of those sort have been refuted.
Regarding dark matter, I'm not comparing it to epicycles. The cold dark matter theory is very predictive and it has made many predictions that were confirmed. But in the same time, I am perfectly willing to accept other theory which will explain all current observations without dark matter. Just so far nobody came up with one.
To be fair, Fourier theory was not known at the time.
My point is, dark matter hypothesis is similar situation, we can bend it to explain very much, but there is no great revolutionary insight coming from it, we are adding tweaks, adding more independent quantities, like we did with epicycles. Perhaps there will be new Copernicus who will show a better point of view, a deeper understanding of the observations that lead people to introduce DM.
As I read it, and I think it was pretty straightforward, neither does she? She wants to keep it where it seems to be an appropriate model.
Rather the main case is the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that survived from baryogenesis). This requires there to be "dark matter" in the universe.
The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt, as Sabine points out). The real reason to believe in dark matter is Big Bang nucleosynthesis.
Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642
If the strongest argument for DM was fixing a particular model of nucleosynthesis, most people who care about this hypothesis would not care for it. In my opinion, extrapolating our standard theories back in time to "big bang" and drawing consequences such as "DM had to be created so that our model of nucleosynthesis can be preserved" is not a credible science.
But no, DM was introduced to explain observations of celestial motions. And it works. Too well, it is too accommodating to any observations to be a great insight of what is going on with celestial motions.
Rather Hossenfelder suggests that both dark matter and modified gravity may be necessary, with each being relevant in different situations due to some sort of phase change (?), noting in the comments Khoury's idea of superfluid dark matter (although I'm unclear as to whether that includes modified gravity?) as one example of such a theory.
Edit: Oops, misattributed & misdescribed superfluid dark matter
* MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations;
* Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly
* because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales)
* this can be analogized to a phase transition in solid state physics
* the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap)
therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools.
I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.
I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars.
https://duckduckgo.com/?q=phase+transition+neutron+star
> wave-particle duality
I don't see how that has anything to do with it.
The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism).
This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it.
But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory.
- --
[1] https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]
Bronze league players (lowest) know they're bad and that they don't understand the game.
Silver and Gold league players (2nd and 3rd lowest) think they're better and are really starting to get a grasp on how Starcraft operates
Platinum league players think they basically know everything and just need to work on a few minor issues
Diamond league players begins to realize there is so much more depth than they imagined, and those 'minor issues' are actually oceans of complexity and depth
Master league players fully comprehend the depths of their own ignorance and lack of skill, as they are now finally able to look at the professionals and grasp the massive gulf between themselves and the pros.
Reddit has bronze league understanding of physics, HN has platinum level understanding.
I wouldn’t really call it crackpot, however. It’s just incredibly misguided and ignorant. I’m sad to say even the rare technical threads are filled with people who should know better than to post rubbish they do, because it transfers onto those who know even less. I think a predominant reason for this is that if you call people out on their bullshit, you’re likely going to be downvoted so people tend not to bother with it anymore. HN has gone through a few cycles of actually useful commentators leaving the site (and writing up about it). The remains are less than stellar.
So much nonsense on here, same as any other forum!
BTW, why use a throwaway just to make a mildly controversial remark? The internet points aren't real. What's the point of anyone using consistent names at all if we use throwaways whenever we say something interesting?
* People cannot trace which dumb things I talked about in the past to use ad hominem on me. I felt disgusted when attacked by ad hominem. Without chilling effect of being dug from the past, I get more freedom in expressing opinion.
* To stay anonymous in a reasonable way, in the sense of preventing people from profiling username. Reading someone posting as 'anon314' or 'throwaway2021' would feel different from someone as 'hiimelonmusk'. It would leave no remark in the back of one's mind.
The first argument... well I just don't get it. Why do you care so much if some internet weirdos are looking through your history in order to win internet battles? Care about what you want, but it doesn't seem very healthy.
And what's worse is like you say... the comments are made with such certainty that unless you knew better, you'd think it was experts making authoritative claims on the subject.
There's a principle that says take a subject you're an expert in and see how poorly journalists explain it, then consider that they're likely just as poor at explaining subjects you're not an expert on.
Same principle likely applies to HN.
It makes you not just rely on particle or gravity field. People starting from scratch have to really explain themselves.
Dark matter, you have gravity forces with far less visible mass. If you want a dark matter particle, it has to absorb from the surroundings without changing its surroundings, while not radiating light.
Gravity, heat transfer through the changing of relative clocks works well enough.
So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies.
Link: https://www.space.com/19-galaxies-missing-dark-matter.html
Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.
Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter?
For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
They escape.
yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.
Dark matter looks/sounds hacky and broken, but in general matches real-world data better than the competing theories.
Dark Matter was first postulated because galaxy rotation curves strongly suggested if there was not unseen matter, galaxies would fly apart. But these observations of galaxies were each of a galaxy in isolation. So in essence, Dark Matter is a fudge to explain the observation that galaxies are not flying apart. Since, other observations that can't be explained have been lumped into Dark Matter. But it turns out, Dark Matter is unnecessary to explain galaxy rotation curves.[0] Galaxies are never isolated. They come in clusters and superclusters.
I couldn't find any non proof of concept papers that tested relativistic extensions of MOND for bullet-cluster-esq situations. Those proof of concepts tended to disagree greatly with reality, but thanks to the weird effects of (for instance) mass current models you'd expect to need a pretty accurate simulation to avoid compounding error. It certainly hasn't been ruled out.
That being said, while we haven't ruled out that some super weird interacting effect doesn't cause the apparent mass, we know that under a broad range of DM models the bullet cluster is normal. If you assume that apparent mass shows where actual mass is, pretty much all of the weird behavior goes away. It does suggest that so-called "hot" dark matter is unlikely, but that was indicated by cosmological surveys as well.
Galaxies are often isolated. They come in clusters, and also in groups, and in isolation (including isolated galaxies inside cosmic voids). The isolated galaxies have the same "dark matter" phenomena as galaxies in clusters (which, by the way, do not have perfectly circular orbits, just to start with).
This doesn't mean that you're wrong that the paper is wrong, but Dr. Hanson should absolutely not be our referent for certitude on the matter.
This reminds me of what happened with Sir Atiyah and his 'proof' on Riemann hypothesis few years ago. Sir Atiyah, despite of being a well-known mathematician (but in a different discipline), got absolute silence from researching community and experts when he proposed his proof on Riemann hypothesis and connection of fine structure constant of physics to mathematics.
If experts stay silent, if your work gets no follow-up, it's likely that what you said is wrong or has no value to people.
But the "dark matter" problem occurs for basically all galaxies, including things like elliptical and dwarf spheroidal galaxies that have no bulk rotation at all. (And also disk galaxies where a significant fraction of the stars and gas are counter-rotating.) The same applies to groups and clusters of galaxies.
So? Have you calculated the relativistic effects of those incredibly complex mass currents and come to the conclusion that GR is not a sufficient explanation for the motion?
I’m sure there’s a high risk that GR can’t explain everything, but it sure is frustrating that people tend to grasp for these adhoc hypotheses without exhausting GR first.
https://inspirehep.net/literature/690135
> Recently a new model of galactic gravitational field, based on ordinary General Relativity, has been proposed by Cooperstock and Tieu in which no exotic dark matter is needed to fit the observed rotation curve to a reasonable ordinary matter distribution. We argue that in this model the gravitational field is generated not only by the galaxy matter, but by a thin, singular disk as well. The model should therefore be considered unphysical.
Modified gravity is a formula that applies exactly at every point based on other mass, like Newtonian gravity F=G(m₁m₂)/r^2.
The density of dark matter varies throughout space and it isn't based on a simple formula, but all the complicated dynamics of the past.
In their basic forms, GR and the Standard Model are basically entirely different languages, as you said, one using a model of motion in a curved spacetime, and the other using a formalism of quantum fields. However this brings up a big question: why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
You can do quantum field theory in a curved spacetime, but that is more of a band-aid approach to the problem than anything and doesn't really help to unify the two approaches.
It could be that the answer is that two different Gods designed gravity and the other forces, they certainly seem that disparate. But as a Physicist is it quite hard to accept that gravity and the other interactions are fundamentally separate, we would like a full theory of all four of the interactions.
EDIT: I should point out that QFT is hardly very nice anyway. We don't have evidence for a specific ontology/interpretation for Quantum Mechanics yet, we don't know what Quantum Mechanics is or means. We may come closer in the years coming though.
> as a Physicist
So, while I could let your "basic forms" do some heavy lifting in your second sentence, I would like to draw your attention to:
> entirely different languages
First, let's start with the Hamiltonian formulation : https://en.wikipedia.org/wiki/Initial_value_formulation_(gen...
which leads us to
https://en.wikipedia.org/wiki/Canonical_quantum_gravity
cf.
https://en.wikipedia.org/wiki/Second_quantization
or if you're not looking deep into compact objects (astrophysically) or concerned with theoretical UV divergence problems,
Perturbative quantum gravity (quick lecture) https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...
vs
https://en.wikipedia.org/wiki/Perturbation_theory_(quantum_m...
Numerical methods can be even more similar: there's several approaches to gravitation on the lattice, for instance, that would be familiar to lattice QCD people.
Programmes to make use of objects used by HEP (Lie algebras, configuration/phase/state spaces) for strong gravity are not accidental.
> You can do quantum field theory in a curved spacetime
Birrell & Davies likes to stress "Curved Space" (as in the textbook's title). It's not a band-aid at all, it's a first approximation, up to the so-called one-loop level. There are higher-order approximations.
It's not the 1960s any more, and especially after the 1982 Nobel (the same year as Birrell & Davies was published), I think it's not super-controversial to argue that every good physical theory is an effective theory, even if the characteristic scale has not been determined.
> why are the electromagnetic, weak, and strong interactions described by a totally different formalism to gravity?
> certainly seem disparate
Teaching tradition!
Also fossilized successes of https://en.wikipedia.org/wiki/Abductive_reasoning
But to me a variational approach on the Ricci curvature tensor (following Pirani https://journals.aps.org/pr/abstract/10.1103/PhysRev.105.108... ) and on the Faraday electromagnetic tensor (Bondi & Pirani started this in http://www.theory.physics.ubc.ca/530-19/planewave-bondi.pdf ) are very similar, not very disparate. Indeed, you can see how one arrives at the spin-2 gauge boson for the former (symmetric rank-2 tensor) in the same way one arrived at the spin-1 gauge boson for the (totally antisymmetric rank-2) Faraday tensor.
But this is certainly not a successful approach to a "full theory of all four of the interactions", however it led to "just" an effective field theory that is good to shorter lengths than we likely will be able to probe any time soon.
Every point but one in the comment you replied to is as far as I know found in one or more standard graduate-level textbooks on gravitation (even "Teaching tradition!" is paraphrased from Kaiser's MTW preface). If you like I can direct you to them; they are all much more interesting than my HN comment(s).
(The exception is my point on lattice methods in numerical relativity, for which you will need a specialist graduate textbook like Baumgarte & Shapiro.)
By "the wider Physics community" I mean Physicists in general having a general sense that the problem has been solved, for example a solid state physicist has some idea that the Standard Model is our most accurate theory of the three interactions it covers, even if they know nothing about the Standard Model. Eventually a theory of Quantum Gravity will reach that kind of recognition if we have some kind of experimental evidence or strong theoretical arguments regarding it. It is a question of marketing and broadcasting
These two theories have very different mathematics and different concepts of state. Nobody has been able to connect them in a way that would be a "success" - a unified theory that explains both GR and QT in a consistent way.
Unfortunately not, quantum field theory doesn’t explain space time curvature. Nor does GR say anything about quantum effects.
For instance, a gravitational singularity should exhibit quantum mechanical properties, but neither theory covers this.
You need something to explain them, and the choice is either positing lots of invisible stuff, or modifying the laws of gravity. It seems reasonable to try both approaches, particularly since positing lots of invisible stuff has a pretty checkered history in science.
Then there is very interesting take on the equations in [1] that also explains the rotational curves and few more observed effects again without any extra particles or fields.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
[1] A static charge generates an electric field and a moving charge generates a magnetic field. There is a similar effect for gravity: the Einstein equations have a sort of symmetry to the Maxwell equations.
Note that what the paper refers to as "gravitomagnetism" is more commonly referred to as "frame-dragging": https://en.wikipedia.org/wiki/Frame-dragging
And once you look at it this way, it’s obvious why we have not made progress on the question what dark matter is for such a long time. There’re just the wrong people working on it. It’s not a problem you can solve with particle physics and general relativity. It a problem for condensed matter physics. That’s the physics of gases, fluids, and solids and so on."
See also Justin Khoury (2015) : A Dark Matter Superfluid, https://arxiv.org/abs/1507.03013 and Silvermann (2002): Dark Matter as a Cosmic Bose-Einstein Condensate and Possible Superfluid, https://link.springer.com/article/10.1023/A:1015934027224
I had an idea that maybe black holes have an additional near field component that becomes exposed once the singularity is formed, like there is a limit/floor to infinite density. Energy in the far field falls of at 1/r^2 but near field is 1/r.
This is directly contradicted by evidence specifically for the case of galactic-scale supermassive black holes, and broadly by observations of black holes.
We have directly observed accretion disks, which would look very different if the near field of a black hole was different from the far field. We have directly observed discrete objects falling into black holes with minimal accretion disks, ie with no other forces except for gravity, and have found that redshift evidence very strongly agrees with general relativity. There is just no way you are correct for most black holes. They behave the same close up as they do from far away.
If supermassive black holes acted differently then we would have seen evidence of your theory with Sagittarius A*. It's our galaxy's central supermassive black hole. We have measured the orbits of objects light years (cluster GCIRS 13E) away and light hours (star S2) away from its center. Gravity changes as expected.
A near-field linear effect is not sufficient to cause us to think that black holes are less massive than they actually are, which would be necessary for your theory to explain why stars in the near parts of galaxies don't orbit as quickly as we'd expect based on the outer stars. If gravity stopped increasing as quickly below a certain radius, we would have seen it.
No that's not right. It's the inverse. At far distances gravity is higher than expected for black holes compared to regular stars.
If it doesn't follow the square-cube law in the far field, it violates conservation of energy and creates a whole bunch of situations that don't make any sense. The farther away from something you are, the heavier it would appear to be. The event horizon of a black hole would shrink as you got closer, because light coming from far away would be pulled more over the longer distance. Neutron stars would appear to be black holes if you were far enough away.
Gauss' law for gravity says the total flux through any enclosing surface is proportional to the enclosed mass. If the gravitational potential decreases at 1/d and the area increases at 1/d^2 the mass must increase. That fact also makes it irrelevant if this is something that only happens for black holes- if you're far enough away, the enclosed mass will make it look like a black hole. That means that this effect would be apparent at measurable scales. Tungsten motes would look like miniscule black holes and asteroids would not emit light once you were far enough away.
The Triangulum galaxy is ~2.7 Mly away and ~2.6 kly wide. At 5 kly the rotation curve is totally unexplainable with conventional mass and gravity. If the strength of gravity falls off with 1/r past that distance, the flux enclosed by a 2.6 Mly sphere would be 1000x higher (d^2 area times 1/d flux density) and the apparent mass would be 1000x higher.
General relativity would have quite a few things to explain if galaxies caused more gravitational lensing the farther away they were despite not having any more stars. The fact that we are seeing significant, unignorable effects at the edges of galaxies means that those effects should be FAR larger at even fractions of the distances between galaxies.
EDIT: And looking at the non-mobile link I realize now it is a transcript. So that crispness in writing came out from essentially a lecture. Nice.
Just A doesn't work. Just B doesn't work. So Sabine suggests A and B.
Who dares to question Lambda-CDM? As a prior for A and B isn't that where the answer lies? If you aim to screw up your chances of an academic career in physics go and pursue that. Let me know how you get on.
https://scitechdaily.com/all-dark-matter-in-the-universe-cou...
And even our planet 9 is (unlikely but ...)
https://phys.org/news/2020-08-planet-primordial-black-hole.h...
A "large" particle would be extremely difficult to interact with. Think of something the size of a baseball. Normal matter would pass through it easily if it was so diffuse.
They would have to have a mass outside the range of our current accelerators or we would see them by their absence in collision traces.
I hope in my life time this question is solved.
Probably one of my favorite areas to follow - that and the quantization of gravity.
That wasn’t even so controversial at first because measuring the visible mass of distant galaxies is a hard problem.
Supermassive black holes accomplished two things: (i) they captured vast quantities of resources for future use; and (ii) they acted as gargantuan propulsion systems, thus allowing any captured matter (what we would refer to as a "galaxy") to be directed far, far, far away from any/all other captured matter.
And thus ended the great intergalactic war, which brought about the end of the homogeneous Universe, ensuring that no single civilization could ever start such a widespread conflict again. The distances would make it so.
Life was the big bang.
(I'll throw the /s and #notserious tags here; Poe's law and all that...)
If I’m trying to capture energy in the form of really hot things a black seems like a great way to do it. Hawking radiation takes eons.
There's a good discussion of this in his book "Fashion, Faith, and Fantasy in the New Physics of the Universe".
It isn't just light, as far as we know baryonic matter also passes through without interacting in any measurable way.
It has certain... resonances with mystic traditions such as Theosophy, and I could see that being unwelcome in some circles. To my taste it's perfect.
From the HN Guidelines [0]:
> Comments should get more thoughtful and substantive, not less, as a topic gets more divisive.
Ever since I first encountered G, I always wondered, "is gravity really constant? Is it emergent from 'the universe', or from matter?"
This eventually led to thinking about, "well maybe gravity has more or different properties than G".
As evidenced by the link to a typical comment from me above, these thoughts are always dismissed out of hand. For good reason. But now that a "legit" particle physicist is raising these questions, I hope to see some movement.
Based on all current observations, G is constant.
Did you read the article, however? What she's mulling over is the idea of different gravitational fields, and reconciling that with quantum theory along with "classic" Newtonian and Einsteinian physics.
And the distribution of it. Also on the matter outside their orbit. Galaxies are not spherical distributions of matter. Physicists have been misapplying the divergence theorem for years in this context.
I hope shes over simplifying here, but somehow I doubt it.
In other words dark matter must be there, according to the evidence, only because otherwise our math is wrong. It could just be that our math is wrong.
I always took dark matter to just refer to the observed phenomenon that doesn’t line up with the calculations, and that gap definitely exists.
> But more importantly, if you look at the mathematics, modified gravity and particle dark matter are actually very similar. Dark matter adds new particles, and modified gravity adds new fields. But because of quantum mechanics, fields are particles and particles are fields, so it’s the same thing really. The difference is the behavior of these fields or particles. It’s the behavior that changes from the scales of galaxies to clusters to filaments and the early universe. So what we need is a kind of phase transition that explains why and under which circumstances the behavior of these additional fields, or particles, changes, so that we need two different sets of equations.
A) We have a very good model of gravity, tested in a lot of systems (like the lab, planets orbits, ...)
B) We have some other systems (like galaxies, ...) were the model fail.
So the possibilities are:
1a) The model of gravity we have is wrong. [We know it's wrong in other cases and we need a Quantum Gravity theory, but this correction is necessary for other king of systems, like very small and with huge gravity fields, so it's probably not relevant.]
1b) The model is correct, but we are making bad approximations. [Other comments are about gravitomagnetism. I doubt that's the problem, but this is not my specialty.]
2) We have a bad understanding of galaxies and other systems where the current model of gravity appears to fail. Perhaps we are only considering the matter we can see. Perhaps there is more matter that is somewhat "invisible". We can guess where that "invisible" things are and see if we can fix the predictions of the gravity model. We only need a catchy name for this "invisible" thing because it may not be actually invisible [1]. What about "Dark Matter"?
3) As this article explain, perhaps both are correct and we need to fix gravity at a galactic scale and also find some part of the thing that are inside a galaxy.
What is the correct rational scientific behavior?
[1] They may be invisible from here using a telescope, but they many be visible if you are nearby. Nobody is sure. The names of the models are weird. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Weakly_interacting_massive_par... https://en.wikipedia.org/wiki/Robust_associations_of_massive...
However, as Sabine points out, it seems that really, at the mathematical level it's the same thing. Add a field, add a particle, both ways you're just adding some terms to the equations . Fields and particles are two sides of the same coin.
I guess MOND and dark matter aren't as different as I thought.
That said: dark matter is no more a “hack” than the discovery of Neptune or Alpha Centauri C: the existence of both were inferred from deviations from theoretically expected gravitational motion, and only later confirmed by direct observation. If Neptune did not exist it might have implied Newton’s equations of gravity were wrong - but it’s existence and correctly-predicted mass were instead a major validation of those laws. So I think the dark matter hypothesis and the subsequent research activity are fully justified areas of scientific inquiry.
[1] https://link.springer.com/article/10.1140/epjc/s10052-021-08...
[1] https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
In both cases, the assumption about another planet was a reasonable thing to investigate.
Dark matter is much worse, because we are not introducing single planet to explain another planet behaviour, but we are introducing new "stuff" that can be distributed in space largely arbitrarily and we have great amount of freedom to fit the observations. Add a little dark matter here, remove little dark matter there, now we can fit a bear.
(The "undetected mass" hypothesis also turned out to be successful in the cases of binary stars, including things like Sirius B, where the companion turned out to be a bizarre type of star never before seen or theorized.)
And, no, dark matter can't be distributed "arbitrarily": its distribution has to be consistent with gravity acting on an initially almost-uniform distribution with small cosmological perturbations (consistent with those seen in the cosmic microwave background).
That may be a restriction that some may operate with, but it is non-obvious why only models obeying this restriction are to be allowed. Some people do not care about cosmology and want to fit just rotation curves. And even if someone does care about cosmology, why would initial uniform distribution would be the only acceptable one?
Yes, there is arbitrary interpolating function there (so also a great degree of freedom to fit the observations), but the resulting modified law is supposed to be universal, valid for all points of space and all material particles. Dark matter hypothesis, on the other hand, just adds immense number of new obscure quantities(dark matter density at all points of space) that can be tuned to fit (almost) anything.
Such fields, if sufficiently reluctant to interact with normal matter and/or photons, would be exactly dark matter.
I "think" that dark matter observations are the effect of interference between different Everett branches. The smaller is a volume space, the less possible states it has, so gravity in the center of a galaxy would be less that in the borders. The borders of the galaxy are more entropic because more different states are possible, ergo the stars there experience interference with more Everett universes. Gravity being so weak, it would only show its effects in other branches at huge scales. In this "theory", it seems that dark matter effects would be less visible the further away in space (time) we look. No idea if that's the case.
Again, I have no idea what I'm talking about, so this is just for fun.
I think observations do not show such correlation between spacetime location of galaxies and dark matter/energy presence.
Dark matter effects should be less or not existing in the early universe.
Anyway, as I said, just idle speculation for fun.
How does the universe know it’s supposed to consider the positions on the entire circumference of a galaxy, and not just the “left” edge, or the left edge plus the center, or the right edge plus one quarter of another galaxy’s pinky finger, when deciding how strong gravity should be in the left edge? Only local information should be relevant, otherwise you’re back to assuming the universe respects our arbitrary boundaries.
You have a compressed gas, so, low entropy, then it starts expanding. The area where it started expanding has less possible states that the area around, but just because the area around is bigger, nothing to do with the relative position itself.
If the number of states are associated to gravity somehow, then gravity will be bigger the further you go from the center.
Because we are brainstorming in creative mode, I will add a bonus: if the number of possible states increase gravity, maybe, the older the universe gets, the bigger is that influence all around, accelerating its expansion.
I suppose that I will get my Nobel any day now ;-)
Not that this would affect California vs Rhode Island analogies, unless you could demonstrate that things in Rhode Island were a lot hotter...