We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology.
Also, dark matter doesn't interact much with regular matter, which makes the search even harder.
This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass?
Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?
Yeah, but the only interaction is gravity. And nothing that clumps these particles together (such as the electro-magnetic force for ordinary matter).
> Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system?
The additional mass can be measured, this is why we suspect the particles are there in the first place :)
Yes, in interstellar scale. How much there should be dark matter within solar system? 1 gram? 1 kilogram? I mean, if there was supposed to be 5/6 parts of mass of dark matter within solar system, it would quote obviously be somehow observable?
People speculate that maybe there is some weird particle that we can't see in any other way than through looking at the mass of these large (really, really, really ridiculously large) structures. But is there any of it in the solar system? In our galaxy? Nobody knows. Does it hang out between galaxies? Between galaxy clusters? Does it even really exist? Nobody knows.
You're just jumping way, way, way too far down the way ;-) Literally, dark matter could be anything -- even a misunderstanding about how the universe works. That's what's interesting about it.
I am just trying trying to get my head around how sparse the dark matter actually is, wherever it actually exists. Would we have one kilogram of dark matter to observe?
Damn. Should have googled in the first place. Here is an estimate of dark matter density in solar system:
Volume of earth is around 10^27 cm3 so that makes the dark matter mass within earth around 600 grams. Admittably that is a bit difficult to measure...
Anyway, what we expect to see here is what's typical at our distance from Sagittarius A[star], and distance from the galactic plane.
Also, I gotta say that the galactic dark matter distribution reminds me a lot of Vinge's "Slow Zone" ;) A Fire Upon the Deep came out in 1992. I wonder whether he had dark matter in mind. He never used the term, as I recall.
(I’m pretty sure we’re sure dark matter isn’t antimatter, but just the same, worth the thought when 5/6th of the universe is unexplainable.)
(Also, I always thought of antimatter as an abstract thing that gets made in the lab and that was cool ... But we use it for practical things every day. For instance, PET scanners in medicine produce positrons and watch the annihilations inside your body. Astoundingly cool to me.)
Short answer: 10^-18 as much as the mass of the sun, 1 proton-mass per 3 cubic centimeters. Not enough to be detected gravitationally - it just gets swamped.
Nevertheless, our gravitational experiments can say things about the properties of dark matter (it generally obeys the Equivalence Principle): https://arxiv.org/abs/1207.2442
Furthermore, for certain classes of ultra-light dark matter, gravitational and spin-coupled searches can have something to say, e.g.: https://arxiv.org/abs/1512.06165
This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?
But! There may be seasonal variations in the amount of dark matter reaching Earth due to a solar "lensing" effect. Attempts have been made to find this signal, and an annual modulation has been found, but the debate as to its cause is ongoing.
However, remember that within the orbit of Neptune there is only about ten Phobos-masses worth of dark matter, or barely more than Jupiter's small moons Lysithea (disc. 1938) or Sinope (disc. 1914, until 2000 the outermost known moon of Jupiter).
Moreover, Jupiter can't really gravitationally entrain anything beyond 0.35 astronomical units away from it (otherwise the sun dominates), and gas (whether dark matter or electrically neutral atoms or light molecules[b]) is too low-mass to be drawn into a orbit around Jupiter with such a small radius.
There is likely a small overdensity of DM within the sun, but that's really a focusing of dark matter gas through gravitational lensing rather than dark matter gas staying trapped within the sun. It may help understanding if you hold the sun stationary and blow a wind of dark matter gas past (and through) it -- the gravitation of the sun pinches some of the gas inwards. Since on timescales of small numbers of years the sun has roughly constant velocity against the gas (or the wind blows with constant strength from a constant direction), the pinched wind is at a constant location and constant density deep within the sun.
Whether any of the less-massive bodies of the solar system have overdensities within them (or possibly tails) depends on the mass of dark matter particles, and right now that's not well-enough constrained to answer with any confidence.
[a] here I mean specifically cold dark matter (from the standard cosmology) rather than neutrinos. Solar neutrinos and (relativistic) neutrinos from far away sources are "hot" and so run away from the galaxy too quickly to account for much of its non-visible mass; cosmic neutrinos (the neutrino analogue of the cosmic microwave background) are cold and dark, but individually they're too low-mass to form galaxy or even galaxy-cluster size halos. The total mass of the cosmic neutrino background is also small.
[b] of course, ionization of neutral atoms and gas molecules is pretty likely in the solar system, and Jupiter has an enormous magnetotail. Dark matter doesn't feel magnetism (and isn't ionized by UV or X-rays), otherwise it wouldn't be dark. So while gases can be drawn around Jupiter electromagnetically, dark matter cannot.
The idea that dark matter might consist of a class of self-interacting particles, and that we might be embedded in a universe full of hidden phenomena as rich as the ordinary-matter phenomena that are visible to us (e.g. dark 'planets', dark 'stars', dark 'galaxies', or something very different) was always intriguing to me, but it seems that a consensus is emerging, based on observations of large-scale distribution, that dark matter is dominated by a single type of particle incapable of self-interaction.
Is it still possible that some fraction of the dark matter in the universe is self-interacting, capable of 'clumping' and exhibiting physics similar to ordinary condensed matter, or are all the indications now pointing strongly towards a single non self-interacting particle?
It remains a possibility. It does not seem to be required by observation, though. Worse, if you move away from parsimonious non-interacting quantum field theories to more complex models, you have to suppress a lot of symmetries that inevitably produce observables which are not seen. Most people working with general relativity just use non-interacting scalar fields, but specific ideas about dark matter have to consider the ins and outs of gauge theory (e.g. does DM only feel gravitons and Higgs or does it also feel one or more of the other non-photon gauge bosons? if it feels the weak force, what goes on at electroweak scales? and so on...). The microscopic details of the microscopic alternatives within the broad family of QFT dark matter get hairy quickly, and there's very little astrophysical evidence to prefer one over the other (people favour axions or sterile neutrinos for reasons from within particle physics, and are looking for such things to complete their extensions to the standard model, they have to be very weakly interacting for particle-physics-in-laboratories reasons, but oh by the way as a side effect dark matter could be wholly or at least partially these proposed standard-model-problem-slaying particles).
> [what if we propose dark photons, dark atoms, etc.?]
One problem you run into is that if you can form composite dark particles analogous to atoms, or dark molecules, what prevents them from forming larger structures that collapse gravitationally? Likewise, if you can emit dark photons, you're removing momentum-energy from a particle in an orbit, and you would then expect the particle to fall into a closer orbit. Again, how do you prevent gravitational collapse? You might fix that by feeding back (squash DM together in galactic cores, release enormous "dark shine" dark-photon-analogues which then kick the massive DM particles into wider orbits, but it's like balancing a pencil on its tip; this is called DMAF (dark matter annihilation feedback), and is speculative. On the other hand baryon-flow feedback is a thing in solving e.g. the core/cusp density problem in particle dark matter, and that's a lot less speculative, because we know things like galactic jets are practically mandatory.
You're generally stuck with appealing to rareness, which is in conflict with Copernican principles which work really remarkably well in cosmology (and astrophysics too), or slowing down dark chemistry so much that it basically doesn't have to enter into equations anyway.
Carroll blogged about this a decade (!) ago (how to feel old: remember reading his cosmic variance blogpostings and making the discovery, pardon the pun, of how many years it's been since he stopped blogging there...) here : http://www.preposterousuniverse.com/blog/2008/10/29/dark-pho...
In astrophysics instead of using base-ten for enumerating interesting things in the sky, the counting system goes roughly: forbidden-everywhere, unique, mandatory-everywhere. If you introduce dark matter stars, you would expect there to be so many of them that you could not miss the Einstein lenses they generate. (Similar to MACHO hunting). Dark matter galaxies, being much more massive, would be even harder to miss. You will struggle to find a deep-field image that isn't filled with background galaxies (or clusters) lensing even more background ones. If there are dark galaxies, surely they would be in the foreground of some of the visible galaxies -- otherwise what prevents that?
Finally, we do have some gravitational structuring of dark matter; the standard description of structure formation requires it, and it's hard to get the late-time structures we see without dark matter filaments.
There's undoubtedly some meat on the Structure Formation wikipedia page and the things it links to, but a briefer and simpler overview is in the last three paragraphs at http://sci.esa.int/planck/51560-the-history-of-structure-for...
If there are any other non-experts like me this far down this reply chain who are interested in dark sector speculation, in addition to raattgift's excellent links I'd recommend the Wikipedia page on the Lightest Supersymmetric Particle  and Rob Reid's recent podcast with dark matter researcher Priya Natarajan .
The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.
Displaced supersolid dark matter is curved spacetime
> "the empty vacuum of space … is filled with 'stuff' ... The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether."
Laughlin’s ‘stuff’ is the smoothly distributed, strongly interacting, supersolid dark matter that fills ‘empty’ space and is displaced by ordinary matter.
Einstein: Ether and Relativity
> "According to the general theory of relativity space without ether is unthinkable"
Einstein’s ether is the supersolid dark matter that fills ‘empty’ space and is displaced by ordinary matter.
You could educate yourself. Go study quantum mechanics and the Heisenberg uncertainty principle. Learn about particle fields, vacuum fluctuations, virtual particle pairs, and the Casimir effect. Find out what this "boiling sea of vacuum energy" actually is, and why one might call it ether, and in what sense that would be true.
Cool-phase neutral atomic gas in interstellar settings.
Cool-phase neutral molecular gas ditto.
What breaks the uniformity of the latter two is mainly electromagnetic interactions. UV or X-rays will ionize them, and the freed electrons will cause secondary ionizations. This is the main pathway for heating neutral interstellar gases. Subsequent cooling is by photon emission. This drives dust-grain-forming chemistry; these grains are more dense than gas, and so have different gravitational observables as well as different emission/absorption characteristics. Very roughly, the dust grains can collide and stick to one another chemically, leading to further density non-uniformities.
Cold dark matter doesn't feel UV or X-Rays or it wouldn't be dark, and if it is collisionless (as in the standard model of cosmology) then there is no dark chemistry that can locally densify dark matter.
> What else has approximately uniform local density of distribution?
Really, anything that approaches an ideal classical gas. "Local" is an important qualifier.
A cubic centimetre of taken from near the middle of a small jar of water inside your household refrigerator or a cm^3 of gas taken from near the middle of a helium-filled balloon.
The middle, because the density differs at the boundary of the material in the container. Likewise, the density changes sharply at the edges dark matter clouds, but is fairly uniform in large volumes far from the boundary.
The overdensity in the jar's contents compared to the contents of the fridge overall and the underdensity of helium in a balloon in a room at sea level compared to the whole room are very roughly analogous to overensities and underdensities of dark matter at scales much larger than solar systems.
> How much [dark matter] [is] in the solar system ?
A lot, because the solar system is a huge volume and dark-matter fills it fairly uniformly (there is a small overdensity inside the sun, and the planets will also cause small departures from essential uniformity).
However, it's extremely sparse, so there isn't much in any small fraction of the solar system.
Compare that with the planets: they are extremely dense, but do not fill more than the tiniest fraction of the whole volume of the solar system. However, even so, even Phobos and Deimos have much more mass than all the dark matter inside Mars's orbit (see a couple paragraphs below).
~ one third of a million proton-masses for every cubic metre
~ 6 * 10^-22 kg for every cubic metre (Earth's density is 524 kg/m^3)
~ 0.65 kg / earth volume
Of course the total mass scales with volume.
Inside Neptune's orbit there is about 10^17 kg of dark matter; that's about ten Phobos-masses, or about the mass of 253 Mathilde (a carbonaceous intermediate-belt asteroid).
The volume of the galaxy is enormous, and with dark mater filling all of it roughly uniformly, the mass of all the visible matter is dwarfed -- there is an awful lot of space between star systems.
> somehow observable
It's not moving anywhere close to relativistically compared to the Earth's surface, and it doesn't feel electromagnetism. If we compare two other neutral particles, we have no practical ability to detect non-relativistic neutrinos (we can only spot a microscopic fraction of relativistic neutrinos from known sources) and have trouble spotting thermal neutrons (again, we generally need a known source that is "loud" with them, and additionally the collision momenta will still be larger than most collisions with solar system dark matter -- neutrons spit out of nuclear reactions are much faster than Earth's orbital motion through the extremely sparse dark matter the inner solar system sweeps through, and neutron beams used experimentally are generally a lot denser than ~ 3 neutron-masses per cubic centimetre).
Measuring the interactions between galaxies, and things like gravitational lensing are providing evidence of dark matter’s existence.
Keep in mind that one of the key differences between dark matter and atomic matter within a galaxy, for example, is that dark matter has a nearly uniform density over very large volumes (many light years across) whereas atomic matter has enormous density variations, with huge expanses of near vacuum punctuated by ultra dense stars, planets, neutron stars, etc. Within our own Solar System the amount of dark matter inside a spherical volume that would extend out to Neptune's orbit is only as much as a comparatively small asteroid. As you scale out to larger and larger scales the fact that the density of dark matter is relentless causes the mass enclosed inside a volume to start to match (and ultimately exceed at the largest scales) the mass of stars, nebulae, planets, etc.
Richard Feynman would tell you that this immense difference is "what's keeping you from not falling through the floor down to the Earth center." Or why the apple hanging on the "few atoms" of its stalk is not falling from the tree, when the summary gravitation of all atoms of the whole Earth are pulling it down.
Or, again to compare such big numbers, there are "only" 10^86 atoms in the whole Universe observable to us! (2)
That's how weakly the dark matter interacts with anything else. And it obviously doesn't even interact strong enough to "fall" to the center of the galaxy. Otherwise it would be there, but it remains to the outside of even where the "normal" matter is (mostly the stars and black holes, the "supermassive black hole" in the center is only at most 1e-5 of the estimated total mass of our Galaxy).
There’s “just a little more” of the dark matter in our Solar system but way below the levels it could affect the measurements we can perform.
If it interacts so weakly I don't understand why clumps around anything. In fact if it only interacted via gravity I would expect it to start slowly accelerating toward normal matter, spend a short time near it, then speed away as it slowed down again. If the path is an eclipse this would mean it spends most of its time away from the matter rather than near. It would be almost as if gravity caused normal mater to repel dark matter.
Would I be right in thinking that also puts severe limits on how much it interacts with itself? Because my intuition would be, if you loose normal matter into a gravity well, it will clump, even if it doesn't interact with the source of the gravity.
Am I inferring correctly?
Now I have a little of Newton for you: look at our Solar system: you see the planets, and even more interesting, all the small asteroids circling around the Sun? Can you answer why don't they all fall to the Sun, but move in the orbits?
The way the gravitation works was not "intuitive" before Newton, 300 years ago, and now it's obviously still so for many non-professional readers.
What's actually happening, according to the dark matter model, and the dark matter actually more easily fits much more of our cosmological observations than anything else, is that there is a lot of dark matter but it is simply much more "spread" around the volume of the galaxies. And just like all the visible stuff of the whole galaxy doesn't fall to the galaxy center (like the planets don't fall to the Sun!), the dark matter remains "around" the galaxies, where more of dark matter is "outside" (as in "in the outer regions of it") than in the "inside" of the galaxy (and in the case of the "Bullet Cluster", that I've mentioned in some other comment, dark matter is obviously lagging all the movement of non-dark matter! (1)). That dark matter that is in the inside of the galaxies actually initially "clumped" somewhat, but that "somewhat" is, according to our estimates, significantly below what we are able to measure, when we're interested in the gravitational effect on the Solar system.
1) https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul... and https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...
Sweet of you to bring me Newton. Always a welcome gift.
But I think you misunderstand where my question is pitched.
The solar system is rather clumped, you see. A little Aristotle for you. :)
In all seriousness, the more dark matter is around, the less it can have mutual interactions, before it would clump, surely? Assuming such forces exist, there is a nonzero probability that two particles of dark matter will approach close enough that non-gravity forces will be significant, And they will no longer act under an ideal Newtonian gravity. Dust clouds coalesce into suns, given time. Can dark matter have its own dark-only version of the electromagnetic force? Or is that ruled out by the lack of clumping? Or is there just too little of it to make a conclusion on that? My question was entirely consistent with ol' Issac.
My "feeling" is, it's not "intuitive enough" to "guess" any answer without a lot of computing:
We think we're quite sure in our observations, so that helps, but to be able to claim how the simple laws can exactly produce what we see, we have to do a lot of work. Just like what Newton figured out was not provable before without all the computations:
So there is two possibilities:
- There is no dark matter and the general relativity is wrong.
- There is dark matter and the general relativity is correct.
And maybe a combination of both: there is something we don't detect but the theory is also wrong.
However, no attempt at formulating a theory that propose a different long-range gravity than 1/distance² (and still reduces to the known behavior in the short range) has held up to observations.
(And with "short" I mean "on the scales inside our solar system", not meters).
The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.
Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.
The analogy isn't perfect though. I don't think it's quite right that cosmologists believe we are living on the surface of an expanding hypersphere; that would imply that the expansion of the universe had a real spatial center somewhere in a large extra dimension, just as the inflating surface of a balloon has a real spatial center in the balloon's three dimensional interior, inaccessible to observers that can only probe the surface.
That the universe has a real, albeit extra-dimensional spatial center isn't a mainstream idea, but there are theorists exploring the possibility that the universe exists on the surface of a brane in a higher dimensional 'bulk', and that the big bang resulted from a collision between branes in that higher dimensional space .
There might be some utility in thinking of the universe as an inflating hypersphere whose radius corresponds to time, rather than to an additional spatial dimension. In that analogy, the center of the hypersphere (or balloon) would represent a point in time, rather than a point in higher-dimensional space. The surface of the hypersphere (corresponding to the space of our universe) would appear to expand the further an observer was from the temporal 'center', which would be equivalent to the big bang. There is a consensus among cosmologists that the big bang appears to be a special point in time, if not in space.
To me, it's like 3d canon shot flying down a 4d barrel.
Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.
Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.
It’s all really fascinating.
Youtube video that explains this in depth: https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s
Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: https://youtu.be/3HYw6vPR9qU
Of course, it is definitely interesting to think about :)
I guess for me the analogy is to the sphere in flatland being perceived as a circle - we can see evidence of dark matter but so far we can’t really detect or measure it directly. I wonder if the gravitational effect is the only clue that we’ll find inside “normal” spacetime.
The most exciting thing about that is the idea that the hunt for dark matter might eventually lead to an understanding of the “rest of the universe” where we would otherwise never have a hint of its existence.
Expert: I am an expert on Dark Matter.
Me: What is it?
Expert: I have no idea but I think it exists.
Me: Ok, you must be really smart.
My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields.
Expert on Oak Island knows all the rumors and theories but not where the treasure is.
Expert flat-earther knows all the wrong facts.
Expert politician might know .0001% due to the vastness of government.
Experts on religion know the experts in other religions are wrong.
Expert MLB hitters fail more than succeed.
I guess it gives me hope I may one day be an expert at something.
It is very difficult for a poser to fool actual experts. It is a lot easier for a poser to fool non-experts. Also it's often more lucrative.
Experts may disagree with one another without either party risking be considered non-expert by her or his peers.
Experts, also, having a decent overview of the literature (which includes observational and experimental results) will generally have a decent understanding of what they don't know. It is very rare for a poser to admit what he or she does not know. It's also fairly common for non-experts, who are not experts in some other discipline, to have no idea the amount of knowledge experts have been exposed to during the course of developing their expertise.
> No doubt they are very smart
Really, it's more that actual experts are excpetionally well-read in their narrow little disciplines, and along the way have lots of practice writing as well. The key to expertise in theoretical physics is actually reading theory (and the results of tests thereof). A lot. Writing enough will probably result in a dissertation. And, perhaps, getting a paper published, writing a book or a chapter of a textbook, and so on.
The other thing you have right is that experts can be wrong. But being wrong usually offers up an opportunity for more writing that other experts are likely to get around to reading. So being wrong can increase expertise, as one learns from others' mistakes and how they were caught, and as one thinks about how to avoid similar mistakes in the future (which offers up another opportunity for writing... and so on and so forth).
> I guess it gives me hope I may one day be an expert at something.
Noone becomes an expert without effort.
The great hitters from decades ago did not have access to video footage of themselves, their rivals, and opposing pitchers, for example. So while some of them were exceptionally skilled at hitting baseballs (and not just home runs) (and also running bases and being competent in field positions usually) they were not really experts for want of a body of rigorous literature.
Conversely, one can of course have sub-major-league skills but enormous expertise -- there are batting coaches and sports science academics after all, and even popular analysts. And sports skills decay with age.
Nobel-prize-winning scientists can get senile dementia too; sadly that not just wrecks their skills, it also wrecks their expertise as they forget much of what they've read and studied.
Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assume it holds at all scales.
Look at tiny water droplets. They don't behave at all like large bodies of water.
General relativity being wrong would be waaaay weirder.
If I'm not wrong, Newton's laws can still be held to be true in some cases even inside GR?
Some where in the past, some one could come up with a new model which could show us new truths, and yet could contain GR to be true in some cases.
In GEB, Hofstadter show us part of the ways so see intelligence emerging from noise and garbage, is to understand when you design a system, you start with axioms, truths and rules that are true in a system, could be total garbage in another.
Models are what we use to understand things, That's not how things are.
However, when this happens, it will be one of the greatest physics breakthroughs of all time.
Because of this, any scientist needs very strong, compelling reasons for trying understand their results in a way that holds GR as somehow broken. The vast majority of of experiments don't have good justification for not treating GR as a given truth.
It only got empirical confirmation recently, it should still be open to disconfirmation.
I always wonder about the interplay between a gravity well and dark energy specifically at the galactic boundary. If dark energy is in a sense "anti-gravity", would it not be stronger outside of a gravity well and weaker within? Would there be an interference pattern ( wave or valleys ) between dark-energy and gravity? Isn't a rotated wave just a spiral? Could the difference in rotational velocity at the edge be dark energy "pushing" the matter away to make up for gravity's weakness at the boundary?
The cosmological constant is taken to be a geometrical phenomenon rather than some dynamical field.
We can within the limits of observational accuracy use a "swiss-cheese" cosmology model. We start with a Friedmann-Lemaître-Walker-Robertson (FLRW) background, which is an expanding spacetime with a uniform dust that dilutes away uniformly with expansion. The expanding spacetime's metric is Robertson-Walker, which is an exact solution of the Einstein Field Equations (EFEs) of General Relativity. The dust particles are galaxy clusters, which are gravitationally bound, and are manifestly not expanding in the same way, so they cannot have the same metric. Around each particle, we cut out a "hole" in the background and replace it with a collapsing spacetime metric, like Schwarzschild or Lemaître-Tolman, which are two other exact solutions of the EFEs.
We can use Israel junctions to stitch together a pair of metrics like Robertson-Walker and Lemaître-Tolman, and while it's annoying procedurally, it produces good results. Indeed, a simpler case is the Einstein-Strauss swiss cheese, which served as a practical cosmological model until the late 1980s, when the evidence began piling up for the presence of a small positive cosmological constant.
In swiss cheese models, the cosmological constant vanishes in the inner metric (the "holes") and is only non-zero in the outer metric in which the holes are embedded. Since dark energy is simply the representation of the cosmological constant given a particular slicing of the universe into things-in-space+time rather than spacetime-filling fields, this means that mathematically there is no dark energy in galaxy clusters and other gravitationally collapsing "holes" in the otherwise expanding universe.
This seems shocking ("why isn't there dark energy everywhere?" seems to demand a mechanism rather than just a statement of geometry) but it's testable, and so far there is no evidence for the metric expansion of space within the solar system, or within galaxy clusters.
If we find out that space does expand near and in galaxies, then we have a variety of ways to go beyond the swiss-cheese approach, and we might explore a couple of them anyway since we now have powerful computers and do not have to lean on exact or analytical solutions of the Einstein Field Equations. This is the research field of "inhomogeneous cosmology".
Alternatively, we can stop making holes and instead treat the cosmological constant as a (location-dependent) dynamical field that is weaker near matter except in the early universe (when matter is all squashed close together). Various proposals along those lines like "quintessence" ("quint" because such a field automatically produces a fifth force) have been written about. Evidence strongly constrains a fifth fundamental force of nature, however, so this approach seems much more speculative than either simply accepting that the cosmological constant is geometry and the universe is swiss-chese-like geometrically, or pursuing a much more complicated "real" metric rather than starting with a simple, exact, analytical metric and perturbing it where that's important (e.g. because of how the matter in large structures might be laid out in ways that are hard to be considered pointlike or axisymmetric when viewed from large distances).
> gravity well
Not a useful concept and definitely not an object in General Relativity.
Kinda, but it's important to understand what that means. When gravitation squashes matter into a denser shape you can think of it as creating pressure on and in the matter. The metric expansion of space is not preventing the gravitational collapse of galaxy clusters: they're still shrinking and the pressure inside them is still increasing. Galaxy clusters are essentially bubbles floating in a sea that is getting larger around them. By contrast, the pressure in the "sea" is reducing over time, proportional to the value of the cosmological constant. Since we can treat pressure as a component of the stress-energy tensor, the "matter" or "sources" side of the EFEs, we can treat increasing pressure inside collapsing stars as a source of gravitation (an IMPORTANT source when stars collapse into white dwars, neutron stars or black holes -- pressure dominates the other masses and energies as a gravitational source in those cases). Likewise the increasingly negative pressure in the regions outside galaxies source can be treated as a source of gravitation, but really this is just a special way of looking at the fact that galaxy clusters are separating from one another without any motion-distortion (shear, for example) being evident in our images of the galaxy clusters at increasing distances. There is obvious shear within collapsing galaxy clusters and their internal components -- galaxies and objects near the centres of clusters are more stretched radially than those further from the centres of clusters, and the shearing strength depends on the overall mass of the cluster. There is no mass-dependency on cosmological redshift from receding galaxy clusters; individual galaxy clusters are not stretched towards us at different redshifts.
> interference pattern between dark-energy and gravity
Well, between dark-energy and collapsing matter, yes. The closest concept to your idea of an interference pattern is the presence of holes in the swiss cheese. If superclusters are gravitationally bound and form long filament structures that collapse collectively (rather than there being a line-up of individually roughly-spherically-collapsing galaxy clusters, with the individual clusters not moving towards each other over time) then the geometry would not be quite so swiss-cheese like, or at least not everywhere.
You are free to do handstands and other contortions to describe this in terms of waves-and-interference. You'd probably use perturbation theory, where you throw away the holes and complicate the background metric or the matter fields. When you do that you're engaging in the study of inhomogeneous cosmology or quintessence-like dynamical dark energy. Those are decent search-engine terms if you want to do a quick survey of those fields of research. They aren't popular because the standard cosmology with swiss cheese matches observations to extremely high precision while being much easier to work with than the other two approaches.
> ... at the galactic boundary ...
The Israel junction is described in Chapter 21 of one of the standard textbooks, _Gravitation_ by Misner, Thorne & Wheeler ("MTW"). In brief, there is an infinitesimally thin shell drawn as a boundary around the collapsing spacetime arranged carefully so that an internal time coordinate matches the external time coordinate, which is the scale factor (or lookback time) in the standard model of cosmology. There's a mathematical matching of values of the electromagnetic fields and other fields of the standard model of particle physics on either side of that thin shell.
In reality, the boundary around real galaxy clusters is not that sharp; the edge is a fuzzy end to the sparse gas and dust one finds at the outer limits of galaxy clusters' gravitational influence, so it ends kinda like Earth's atmosphere. It's mostly gone at 100km up, but not enough that satellites and spacecraft much higher don't have to deal with tiny drag from stray molecules and atoms. But in practice, above 100km the residue of atmosphere doesn't enter into equations, and in practice far from the centres of galaxy clusters the residue of gas doesn't enter into equations either.
But if you had much much much more powerful computers and software than we have today, you could in principle do numerical relativity that accounts for all that, and would be "fuzzing out" the Israel junction procedure most likely. (Or, again, you could go right in and study and account for inhomogeneities right down to photons travelling between galaxy clusters, wheeeee! But where do you cut it off? A stroke of lightning on Earth around eight million years ago flashed some light in a direction that caused a tiny fraction of the flash to exit our galaxy cluster. Should we subtract that out from our galaxy cluster's position in the expanding universe? We're also near edge than the centre of our cluster, so the direction of the flash is relevant to how much of it exited, and when it exited. And so on and so on and so on. At some point, the light in question is still contributing to the gravitational collapse of our galaxy cluster; at another point, it's removed some of the galaxy cluster's stress energy from the region in which everything in it is gravitationally bound. We just choose an arbitrary point and say "there's the crossing-over". (We also would ignore the flash because it is such a tiny fraction of the total stress-energy of the galaxy cluster).
As with almost all physical models at some point you have to say "I can only be so precise in modelling and in matching the model to nature" and hope that precision keeps improving over time.
It's when you add in all the other problems that a little tweak to Newton's won't be enough.
On an interstellar scale, maybe this could create the lensing effects and explain some other phenomena. Is it unreasonable that 5/6 of the mass of the universe is like the stuff that makes up planets, but isn't lit up as brightly as a star?
For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore.
It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"?
Looks like it's time for me to revisit Philosophy of Science...
But, ultimately, it’s important to not become skeptical to the point of tearing down progress. It’s one thing to keep an open mind and consider other options. It’s all too easy for bad actors to sell snake oil as a product of the unknown or unexplainable - or even just attack and destroy progress.
I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.
Using techniques such as deep learning for program synthesis. https://www.microsoft.com/en-us/research/blog/deep-learning-...
You could feed a spreadsheet into and then it would iterate for awhile, converging toward most accurate equations describing the system. Or in my case, diverging from what I was sure was the solution. It was a hit and miss with most data (honestly, an eyeball and a few braincells did better with some data sets), but all the same I used it to lazily approximate position equations for some programs I was writing.
The desktop program was called Eureqa which then got ported to the cloud for obvious capacity increases, and relabeled into a company/product called Nutonian. Most recently bought by "DataRobot" and it's now being sold for sales optimizations. Because science.
Theories of modified gravity can fix those individually, but as far as I'm aware, no alternative has been shown to be viable once taken in combination.
I think their may be a small number of languishing models that aren't totally disproven but don't look very promising. But I'm not sure.
Astronomers are scientists, they don't make shit up just out of boredom, they only believe theories when they've withstood rigorous testing through observational evidence.
It's important to understand the history of dark matter / "missing matter". It's a multi-decade history that originally started out with a small amount of intriguing but seemingly persistent observational data that couldn't be explained easily (namely that when you measure how much galaxies weigh by looking at how fast the stars are orbiting that figure differs a lot from the weight you get by calculating up the contribution from all the stars and gas and dust and whatnot that we can see (the "light" matter)). In response a huge number of different "theories" (more properly hypothesis) were brought up to try to explain the mystery, while new ways of studying the problem were thought up as well.
And over the many years after the initial evidence came to light (in the '70s) considerably more evidence came to light from a wide diversity of observations. I won't list them here but I'll point out that the wikipedia page on dark matter has a good run down. Anyway, the fascinating twist to the story here is that as this evidence came to light it started eliminating various hypotheses about this missing matter until finally only one was left: the current theory of dark matter. It's not just a crazy idea, it's a crazy idea that fits all of the evidence when nothing else did.
So don't look at the WIMP dark matter theory as though it's just some half-baked idea, it's a hard fought veteran of numerous campaigns to kill it, but it just keeps going because by all the evidence it seems to be the only theory that explains reality.
As to specifically the "modified gravity" theories that compete with dark matter, they have a hard time explaining several observed phenomena, especially things like the famed "Bullet Cluster". There are a few examples where we can observe collisions between galaxy clusters and through different techniques we can map out the distribution of stars and of gas and of mass in the collision. What we observe in the Bullet Cluster as well as some others is that the gas is in a completely different place than the stars (because stars in galaxies mostly pass through one another whereas gas clouds squish together) and the center of mass of the stars of the galaxies is separated from the center of the non-visible mass of the galaxies. This absolutely cannot be explained by any of the modified gravity theories.
tl;dr no alternative models for gravity can sufficiently explain all of the data, but general relativity and "dark matter being particles" does, so that's what we're going with.
It's indirect proof. The indirect proof is so overwhelming strong that any rational person will concluded with reasonable certainty that dark matter is real.
Directly detecting a dark matter particle will give us more data than "dark matter is real".
Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.
Not always: https://www.nature.com/articles/nature25767
If I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations  that some galaxies were rotating too darned fast to hold together based on the ordinary matter we could see. Gotta be something invisible generating more gravitational force.
If so, couldn't we expect larger masses to attract more dark matter than lesser masses? And mightn't very high resolution measurements of those masses' gravitational forces disclose a discrepancy attributable to more dark matter clustering around a larger mass?
I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3), the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and very different masses. Park them out in space - maybe in an orbit inclined 90° to the ecliptic so there's some time when they're far away from the complicating effects of planetary masses.
Then release test objects with accurately known masses (think the silicon spheres made for Gravity B Probe's gyroscopes ) near each of the two spheres. Minimum approach speeds would be given by the assumption of only ordinary matter in the spheres, no dark matter present. If there is dark matter, and if it accumulates according to gravitational interactions, the test mass approach speeds should be greater than calculated from ordinary matter gravitational force. Also, the larger mass should attract more dark matter and exhibit a greater deviation from ordinary matter force.
Depending on the local density of dark matter, one might see the results change over time with differential accumulation of dark matter around the two spheres.
Do we have the measurement capabilities to do something like this? I recall LIGO measures distances four orders of magnitude less than the width of a proton.
As to your own experimental design, using large spheres of matter, it's completely non-workable. As mentioned above, dark matter particles have velocities in the range of orbital speeds around the galaxy, which is 100s of km/s. So most of the dark matter particles in the vicinity of Earth are going to simply pass through your spheres without stopping, and certainly without increasing their mass.
Additionally, you seem to be confused about the scales of densities here. Dark matter is distributed in a somewhat uniform density on interstellar scales (there are density gradients across the galaxy however). Near the Sun the density of dark matter is about 0.0025 solar masses per cubic light-year, or about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter passing through objects around Earth, mass wise. There's only maybe 6 kg within the entire volume of the Earth at any given time, for example.
If string theory is true, gravity does leak into the multiverse, and we survive the Great Filter, then someday we may be able to use gravity to communicate universes outside of our own.
That's the rather underwhelming conclusion. And it's wrong insofar the size argument is not certain, but depends trivially on the distance of observation. Otherwise I'd like to know what magical number we are talking about.
I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.
Full theory can be found here: http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...
For example the guy who tried to modify or extend gravitational theories to account for it.
Maybe this was a fringe idea or has lost momentum.
The dark matter problem rings as yet another mass-related anomaly we stumbled upon previously but on a nuclear scale.
"Mass defect" , as appropriately named, is observed on a nuclear scale (whole nucleus mass is less than mass of nucleons that the nucleus is composed of). It is explained with binding energy required to keep the individual nucleons of a nucleus together.
That is an 'invisible' mass that gets subtracted from the 'free' mass of nuclear particles as measured.
Any possibility to project such analogy onto a galactic scale?
There should be a similar effect in a galaxy, because when the stars/gas/whatever get closer they have a small gravitational binding energy. I didn't do the calculations, but I'm sure it is so tiny (relatively) that you can safely ignore it.
For comparison, in the Helium nuclei the difference is only 0.8%, in a galaxy it is (I think) abysmally smaller because the gravity is smaller than the strong force and the stars are much far away than the nucleons. But the dark matter is much bigger than the normal mater, something like x5 or x6. So it's not possible to explain the dark matter with a something that has a tiny effect.
Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans?
If you disagree with the consensus, then offer some alternative more convincing (and more useful for civil and interesting discussion) or dispute the article rather than than just writing it all off as "fake science."
For whatever reason, dark matter seems to repeatedly rub people the wrong way today, more so than any other scientific concept I can think of. I actually wonder if it's something psychological -- if the universe is only 1/6 normal matter, it makes us feel even more insignificant than we already do, in the vast, seemingly infinite universe? Or maybe it's just the name, sounding too much out of fictional Star Trek.
Special and General Relativity - "it's all relative man"
Quantum Mechanics- "spooky action at a distance"
Dark Matter - "if you can't see it, maybe it's just not there"
Flat Earth - "you can't prove it's round"
There are more complex versions of this like the interesting, but debunked Tao of Physics and various cranks with new theories of everything (or perpetual motion), but they aren't accessible or interesting to the masses because they don't fit a convenient narrative.
Most big science and engineering takes a lot of work, understanding, and time to be accepted or have a big impact, but we're always looking for the new News.
I think that it is because it is so hard to understand. You can't see it or touch it but it is supposed to make up the majority of mater in the universe. So what is it then? For a lot of people that's a hard thing to get and it would be easier if the answer was something we understood like the scientists being wrong.
In that way it isn't any different than creationists or flat-earthers. Of course dark matter "deniers" don't have the same religious convictions of creationists.
Can you support that "of course"? I'm not so sure that there is no correlation. E.g. it is known that the same persons who tried to convince us that the "smoking is not dangerous" try to convince us that the "global warming doesn't exist, or at best it's beneficial." There are definitely the circles that immediately welcome and use any way to raise the doubt in the relevance of the majority of the scientific claims. It is intentional, it is supported by a lot of money, and it comes not only from one political direction. It is complex, it's not only a single ideology or a single religious group, but there are multiple correlations.
See the books: "The War on Science" by Shawn Otto and "Merchants of Doubt" by Oreskes and Conway.
Dark matter still holds more in common with the divine than the practical, for now. We've developed and carried out a slew of extremely clever experiments to try to affirm its existence, yet each and every experiment has returned a resounding negative. This is one of the biggest problems with the gulf between experimental and theoretical physics that's been rapidly expanding over the past several decades.
Getting po'd at the media for not being able to explain stuff that scientists don't understand, might as well yell at a jellyfish.
"Wow, look how insignificant it makes us feel, how insightful", or, "Just imagine how much is hidden from us every day!"
The simple problem is that DM is often defined as "a form of matter", when it could be multiple different forms. It's just the semantics that irks me. And then, if they will find maybe a new particle to account for at least some of the motion we couldn't explain, I will still be on the fence, because its not clear at all how to distinguish such from a virtual particle. That's nevertheless just a matter of semantics. The actual theory is probably way over my head. Therefore, doubt about the prediction for dark matter is just as much provoked by curiosity as for the proposed explanations. Ultimately, most explanations would be, in a mundane sense, turn out rather boring. So the question is, what's the deeper insight, as it goes for cosmology, what's the significance for life in general? There's a lot we don't know? I didn't even know that!
And its tough to imagine the consequences of something you are absolutely not able to detect, you could be flying towards with 250 km/s. Its kind of scary- imagine you hit something like that and it causes earth quakes or a shake up of the solar system. Just think if that missing gas giant was actually a Blob of DM cycling the solar system.
And, well - the situation seems similar to this over focused on the problem situations you sometimes have in coding. Everyone has committed to a very narrowed down problem solution, that is just not working out, but instead of stepping back, a thousand angles are tried to solve the problem in the narrow scope.
Many here, just want to help.
Which, given the Enlightenment as a project of everyone capable, against a Elite fighting for dogmatic ignorance - is a good thing in my book.
The article is missing in my opinion a confidence rating for every observation- how often this has been tested in experiments or observed in space. Otherwise it was good.
If it was capable of doing this, we would observe it doing the same to light on its way toward us - gravitational lensing in other words. If your scenario was possible, we would've definitely made observations to that extent, but we haven't.
For what it's worth - neutrinos fit your description right here. There are a trillion of them passing through your hand every second, essentially without interacting at all (from https://what-if.xkcd.com/73/).
> but instead of stepping back, a thousand angles are tried to solve the problem in the narrow scope.
What makes you think nobody has stepped back? Tons of people have stepped back and proposed lots and lots of alternatives. Dark matter is the only one that can explain all the phenomena mentioned in the article. It is the result of a lot of stepping back and failing even more in the other directions.
Because, while interesting to a great many scientists, the practical implications of this knowledge are centuries away. Defining dark matter, understanding the backbone of our universe, isn't going to cure cancer. It isn't going to fix global warming. It isn't going to get us to Mars. So when people read of massive experiments throwing unending brainpower and money into the DM hunt, it is natural for them to react as they do. Astronomers give us pictures of far away places that satisfy our natural need to explore. DM hunters stare at numbers and statistics, generating papers and messy diagrams. They aren't fighting an uphill PR battle.
There are also a not-small number of people for whom the DM hunt represents a challenge to their fundamental beliefs. Talk of colliding galaxies billions of light-years away conflicts with the young-earth model that is part of their daily lives. Rather than criticize on that basis and appear ignorant, they lash out on other grounds.
The high-power dilution refrigerator that ADMX uses is of the same sort that the quantum-computing industry needs more and more of. Indeed, the students and staff being trained by ADMX are finding homes in both academia and across industry.
The high-sensitivity detector technology developed for WIMP searches have alternative use in nuclear non-proliferation monitoring. Improved detector ideas may continue to rattle down into medical imaging in the long run, improving some combination of sensitivity and dose.
The real prize, however, is what happens when the nature of dark matter is understood. It is a long-game play, but the technological implications might be on par with subjects like electricity, nuclear physics, quantum mechanics, etc. We won't know until we get there.
Any new detector should be described in terms of the new technologies it will require and how those new technologies will be used elsewhere. That gives it value regardless of whether it detects anything or not.
I suspect that practical outcomes and technologies arising from understanding dark matter will be huge.
Also, I think you're missing the point of astronomy. "astronomers" today are generally astrophysicists or planetary scientists, and they are studying fundamental, mostly invisible, processes and substances.
I doubt it. More than 50 years after discovery of neutrinos, we are yet to find any practical outcomes or technologies.
Moreover, even if the dark matter itself doesn't itself lead directly to to new tech, it is very likely that subsequent discoveries will.
Same for experimental planes to first commercial flights.
So it might take centuries, but it also might not.
Dark Matter isn’t a thing it’s a place holder it has many candidates some more probable than others it might be that there is only one class of dark matter it might be that there are multiple classes of dark matter.
It might be that all the dark matter will be new forms of matter it might be that some or all of it will end up being baryonic matter.
It also might be that we won’t find it at all in which case we might need to say that our understanding of gravity and the geometry of space time isn’t nearly as representative of reality as we thought maybe dark matter is gravity leaking in from other universes maybe it’s the affect of additional dimensions of space time maybe quantum gravity causes some wired emerging phenomenon that affects the geometry of space time that we attribute to missing mass.
There isn’t a consensus amongst scientists on anything related to dark matter other than based on observations we are missing a truckload of mass in the universe.
And currently people are trying to explain it by looking for new forms of matter, normal matter that is very cold and so we can’t see it, and even attempting at modifying the theories of gravity to account for the observations we see with new relativistic and classical modified gravity models.
So it has to be made of particles which have no electric charge. The only such longlived particle we know of is the neutrino, but these guys travel at nearly the speed of light, so can't clump together to form the concentrated blobs of dark matter we observe from gravitational effects.
So dark matter cannot be explained by the particle we know about.
It doesn't matter that a neutron is overall charge neutral, because it's the elementary particles that make up the neutron, that participate in interactions.
This is why, you don't fall through the earth, despite being made of charge natural atoms, because the surrounding electrons of our atoms repel those of the atoms making up the ground, counterbalancing the gravity pulling us down.
Photons couple with charged particles due to their spin, this interaction can be seen through Compton scattering.
However we have for example Neutrinos which are weakly interactive even more so than the photon and are not baryons (the real reason why neutrinos are not a baryonic DM candidate ;)).
A neutrino will not interact with a photon at all since photons do not have an interaction through W and Z bosons (both W, Z and Photons are electroweeak gauge bosons) and neutrinos are not charged.
There are a lot of reasons why you wouldn’t fall through the earth electromagnetism is just one of them, however even non charged baryonic matter can be “solid” and resist gravity for example neutrons will resist gravity through degeneracy it’s all a question of which type of interactions are possible.
There is much more to particle interactions than charge.
As far as photons interacting with neutrons, neutrons while being neutral in charge have a magnetic diepole which is why the photon can couple with them.
You can make baryonic matter that is massive and does not interact well with photons, the LHC has been making some new baryonic matter but none of it is a good candidate for dark matter so far.
But overall yes currently it looks like baryonic matter at least the one of the standard model without any extensions produces isn’t a good candidate for DM.
If you drop some neutrons on the ground, they will go through the ground some short distance until they interact with something, most likely via the weak interaction.
Photons do scatter off neutrons due to various effects, but the cross-section is really very low. I haven't checked the numbers, but I suspect that it's more than low enough that this type of scattering does not rule out neutrons as dark matter. (Plenty of other things do rule out neutrons as dark matter.)
The reason you don't fall through the ground even though you have almost no net charge has nothing to do with quarks.
Think the main reason neutrons are ruled out as DM because free neutrons decay, they have a
half-life of ~10 minutes. So if DM was neutron, nearly all of it would have decayed into protons, electrons & neutrinos, the former two of which we would definitely be able to see.
PS Protons are also baryons and are stable.
Why couldn't it be short-lived?
We need to observe these guys being produced in a particle collider to definitively know what dark matter is made of.
It’s quite possible that we will find WIMPs in a particle accelerator but then be stuck with the fact that at best they’ll only account for 3% of the dark matter we observe and then it’s again back to the drawing board.
It can’t be. It’s already known that 5/6 ths of all the matter in the observable Universe can’t be baryonic.
> There isn’t a consensus amongst scientists
Only for completely useless definitions of the “concensus.”
No it's not known, it's highly unlikely that the "final" dark matter candidate will be completely made up of baryonic matter but it's not been exclusively proven.
Only Siths deal in absolutes.
>Only for completely useless definitions of the “concensus.”
Do you have a more useful definition of consensus for this matter? if so please share it.
That "highly" in "highly unlikely" is the key here, that highly is "immensely huge" e.g.:
"While future observations will determine the strength of the constraints from Eri II, existing data from Eri II and from the sample of compact ultra-faint dwarfs appear sufficient to rule out dark matter composed exclusively of MACHOs for all masses above ~10e−7 mass of the Sun."
And if your definition of the "consensus" is 100.0000% then I do consider it useless.
If you use Merriam-Webster's: "the judgment arrived at by most of those concerned" then yes, there is a consensus because "most" begins for me at "more than 60%." And I accept only those that are actually "in the field" and not those that studied something completely different but "have a strong opinion." Sorry, that's automatically useless. I know personally the Ph.Ds who believe nonsense as soon as they are out of the area they have researched personally. It's simply a https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect as in "I was successful while studying X my whole life and therefore I believe I can 'understand' Y better than those who studied Y their whole life." And I'm talking here only about the natural sciences.
And please don't bring silly Star Wars references here, they don't help and this particular one is basically offensive. HN isn't reddit.
Addendum: Science (and more generally, specialization) requires that we trust that that the experts in the areas outside of our own chosen fields do in fact know what they are doing. It's become less fashionable to do so lately (one politician in the UK famously said a few years back that "the people of this country have had enough of experts"), but that is to our cost - the extreme end of this is flat-earth conspiracy.
This contrasts to most physics I read about. Physicists seem to make a big effort to reconcile all their measurements and if they contradict they admit that and say "we don't know yet".
So physics and other hard sciences suffer from being put together with a lot of other bad science in the public.
The public doesn't understand science, but it understands corporate PR even less.
The corporate PR industry has no plans to change this.
One of the biggest and most destructive wins for corporate PR has been the steady erosion of scientific credibility with FUD and character assassination, combined with science-for-hire commercial shilling. This has done huge damage to the credibility of tentative research, and made it very hard to get away with saying "We just don't know yet. So we need to keep looking, because looking where we don't have answers yet is our job."
But... not only does the public not understand how corporate PR operates. The scientific community often doesn't either.
There's a presumption of good faith in scientific debate. No matter how personal the to-and-fro gets, there's an assumption that everyone is doing the same job for recognisably similar reasons.
That presumption is wholly wrong in any situation with significant political and/or financial consequences outside of science.
Note that I am not saying it is the case here, but simply pointing out that the "experts" authority can also be challenged.
Furthermore, this happens rarely, but notably that someone with no serious "expert" qualifications, they produce revolutionary work (Ramanujan, etc.)
Science works best when you are skeptical of claims, test alternatives, and find evidence that contradict the alternative hypothesis. If you want to discuss controversial science you need to anticipate objections, show the work has been done to test the alternatives, and then leave the reader with the sense that your hypothesis is undeniable (because they can't think up any more objections.) This absurdly high burden is what makes science both hard and effective.
For dark matter a large number of the initial objections have been tested thoroughly, but since they weren't covered people unfamiliar with the subject (and I don't claim to be an expert) will naturally dredge up their own objections.
I strongly disagree.
Principled scientific skepticism from practicing scientist and expert is welcome and can contribute.
Layman scientific skepticism in hard sciences like physics is worthless and even harmful. No matter how skeptical a layman who reads popular science magazines is, they don't contribute. Only thing we can do is to learn. Our judgment and opinion is worthless.
People want to understand, contribute and discuss, but even if we have the education to understand the issues, we don't have the time or interest really think it trough. Everything valuable flows top down from experts to the layman.
Generic examples taken from the history like "Even gravity we thought was fully solved until Einstein doubted that we understood it fully" are just using different words to express tautologies. They don't provide information to the issues at hand.
Demanding that people believe in a theory that so far has not been supported by the experimental data is more like religion than science.
But unless we apply the principle of charity to fields that may lie outside our areas of expertise, then discussion won't move beyond the typical middlebrow dismissals HN seems to be fond of.
My personal supposition is that most people don't know very much if anything about most subjects, and tech people tend to know a bit more than that quite low average (though still not anywhere near expert level) in an above average number of subjects, and that "novice polymath" status amplifies the Dunning-Kruger effect. Of course it's entirely possible that demonstrating the Dunning-Kruger effect by even thinking we tend to have an above average exposure to an above average number of fields.
ETA: you also get experts in one subdiscipline getting another subdiscipline terribly wrong, analogous to experts in programming language design making crazy statements about wide area networking.
and even if someone figures it out i probably won't understand it :/
It feels unintuitive to think about “dark matter” that can’t be directly observed outside of its indirect gravitational effects, but that carries with it an implicit assumption that we should be able to directly observe all the matter in the universe, with the further implication that all the matter in the universe obeys the four fundamental forces that our matter does. Except we already know of other particles that don’t quite do that, like neutrinos, which we can just barely detect directly.
If you take at face value the observation that most observed mass in the universe (via the effects of gravity) is dark, then it appears that our fundamental understanding of reality only describes a minority of the universe and that what’s truly special isn’t the “dark matter”, but rather the “anthropic matter” that we can more easily observe. The dark matter may simply be multiple entirely alien classes of matter with their own fundamental forces, stuff that doesn’t interact with our matter at all outside of gravity and contains, perhaps, observers of its own trying to figure out what the hell we are and whether we exist, since we don’t respond to their fundamental forces.
Left-handed neutrinos interact mainly by the weak force and also by gravity. We have a lot of knowledge about them. You can create a neutrino ray and a neutrino detector, and (if wired properly) measure their speed. We know that there are three types of neutrinos (or perhaps there is a fourth neutrino). We know that they can change midflight from a kind of neutrino to another kind of neutrino. The change takes some time, so you can create a neutrino ray of one kind and a neutrino detector of the other kind and measure how many of the neutrinos changed midflight. So the neutrinos are well understood and there are many labs that can measure them.
(Right-handed neutrinos are more difficult to measure because they interact only by gravity. Some people think that they don't even exist. Some people think that they may be part of the dark mater. I think this is still not settled. Anyway, all the neutrinos we know are left-handed neutrinos.)
People should be able to recognize they haven't put the effort to learn the basics of what they're talking about and that maybe there are very good reasons for all this they can't be aware of because of it and should ask about... and start from there.
Dark matter and Lamda-CDM have some fundamental problems.
Dark matter and Lambda-CDM have fundamental problems too.
One problem with dark matter is that there have been an increasingly large number of quite clever experiments to try to detect various concepts of it, WIMPs in particular. And each and every one has been a negative. This should, in and of itself, begin to cast down on the hypothesis even lacking a better alternative.
Only if your search was exhaustive: Scooping up a spoonful of ocean is not enough to conclude fish do not exist...
You can leave carrots everywhere, but perhaps it doesn't like carrots. Or maybe you didn't spot it eating those carrots. Maybe you left them in the wrong spot, or at the wrong time. Maybe you need bigger carrots. Maybe you need smaller carrots. Maybe it likes cooked carrots instead of raw carrots. Maybe you need a better camera. Maybe the reason you camera didn't catch it is because you weren't using a flash, or maybe you need to setup an infrared camera, maybe you need to play a Gaelic greeting on loop, etc, etc. You can compile a literally infinite number of ideas to try to detect our dear deer.
Instead with non falsifiable theories, all you can do is approach them from an abundance of failures. You've tried carrots, you've tried infrared, you've tried ..., and it's still not coming out. Each failure makes it more and more likely that our deer simply does not exist. Of course this does not mean it definitively does not, but nothing will ever mean that.
Dark matter isn't like this, though. It's more like an invisible deer that's been eating your grass and shitting on your lawn. At some point you have to say "I might not be able to see it, but the only thing that eats grass and leaves deer shit is ... a deer."
We have the problem that the observable mass of a galaxy is not sufficient to explain the motion of its constituents. As gravity is a pretty well validated theory, it can all be solved if we postulate some matter that we can't see which is still gravitationally affecting those things we can see. Dark matter, in this respect, is no more than the simplest explanation of what's happening via a single item that, if it were true, would explain things.
If you think you have a deer you haven't seen so far eating your lawn, you can put out infrared cameras and all sorts of equipment - and just maybe you might find that someone down the road has been messing with you and is mowing it at midnight. But equally, you might see grass disappear and see shit appear from mid air, and come to the conclusion whatever's doing it is invisible.
Ultimately, it's very conceivable that someone might disprove dark matter. All they have to do is posit some other candidate (eg a modification to gravity), and then predict a result that would follow from that. So far, all the other candidates haven't resulted in a hit, but the field is open for the next great idea to come up with one.
And on the topic of those test failures, every single test we make to detect dark matter, which everything we hypothesize about it indicates we should be able to measure, says it's not there. This should not be a problem except for the fact that we've performed alot of experiments at this point to try to detect dark matter of all sorts, with increasingly high precision. And they've all returned a strong negative. With Bayesian logic it's almost surprising we haven't had a false positive yet, given all the failures.
I really do think the geocentric model has a lot in common here. Prior to Newton's discovery of strong mathematical indicators for gravitational mechanics, heliocentricism had long been proposed. And it was in many ways a much more simple explanation for many phenomena than geocentricism, which over the centuries became ever more convoluted and complex. The 'petal weaving' geocentric astronomic models are enough to boggle the mind even today. But one of the big problems was not even the church, it that there was an immense amount of academic work based around a geocentric model. As but one example astrology, which was considered a scholarly and academic pursuit in the past, was based heavily on the assumptions of a geocentric universe. For instance Mercury starting to 'go backwards' (which was required to glue together a geocentric astronomic view) is still a part of astrological 'theory' today.
Alternatives to dark matter look increasingly more likely with every failure of experiment designed to test it. And these alternatives need substantial refinement, but I think really the biggest issue with them is that if dark matter is not correct it would basically destroy more than a century of astronomical and cosmological work. Nearly everything we know about the cosmos is predicated on the assumption of dark matter. If it turns out dark matter is not the issue, we're looking at setting astronomical and cosmological work in many areas back a century. It would be, by fair, the biggest failure in modern science. So there is reason that alternatives to dark matter are held to a higher standard than dark matter itself, beyond simplicity - which dark matter no longer really is.
I don't think this is anything to do with a higher standard. It's more like Occam's razor. If there's an alternative explanation that will upend our whole understanding of gravity and cosmology, have at it. But such an explanation must come with something testable that shows it's true.
Here's an example of an alternative explanation, "a wizard did it." But that's not science, because there's no way to prove it. As a result, in the absence of a testable hypothesis, you have to go along with the one that does the least violence to all currently existing knowledge, surely?
I.e. what I'm saying is that it isn't on the Dark matter theory to prove itself. We already have observable gravitational issues that need something weird to be true. Dark matter is just the simplest explanation anyone can come up with that fits the facts. It's on any other theory to either throw out less of already-established science by being an even simpler explanation, or to prove it's justified in throwing out more by showing experimental evidence.
Or, to put it another way. I agree that dark matter isn't a simple theory. But there isn't currently a simpler one on the market.
I think you'll see a lot of the issues with alternatives such as MOND are a 'lack of elegance', yet we've happily accepted that the universe on a very small scale is far from elegant, so it seems peculiar to insist that on the very large scale it must behave with elegance. Another issue is that MOND generally also has a requirement of some dark matter, but the critical difference is it's just that - matter (including baryonic) that cannot yet be observed, not the increasingly exotic material that dark matter has morphed into. And it also only requires a small fraction of the amount that current dark matter does.
I tend to agree that dark matter is the most probable explanation. But, at the same time I think the resource allocation for dark matter as opposed to everything else is probably not the one we would choose if we were being completely impartial.
So unless a better model that works equally well comes along, I see no reason to discard the dark matter hypothesis just yet: The universe has no obligation to be filled with a type of matter detectable with the means of today...
Postulating unobservable things is still not a good way to do science. The one good thing of the dark matters theories was that physicists were clear in that it's not a done model, just a map of what's wrong with our understanding. This seems to have changed lately.
When hypotheses are not falsifiable, they just tend to grow ever more elaborate rather than die. Dark matter started off as literally that - a place holder for bodies that did not significantly radiate. It's slowly changed into becoming the vast majority of all matter in the universe, existing practically everywhere, it can't be baryonic, it interacts with nothing except itself and gravity, there must exist near entirely dark matter galaxies, and so forth. So we're left with this ever evolving hypothesis that can't ever really be falsified and which every test, increasingly clever and precise, we create to detect it -- says it's not there.
And it seems that the zeitgeist is starting to increasingly treat dark matter as a fact whose proof is little more than an unscheduled formality. This seems illogical given that we've done more than ever to try to show it exists, and every result is saying - nope. And again this does not mean it does not exist, but each failure makes it more likely that this is the case, and there is an alternative explanation.
Have you met scientists? Or more accurately, PIs? Unfortunately, our system of being awarded grants and academic advancement does not necessarily reward the intelligent, those with the best scientific judgement, and rapidly burns out those who would in the ideal make the best scientists.
We're riding on the back on a turtle
We're the center of the universe
All things are made from earth, air, water, fire
Space is filled with ether
Dark Matter "feels" like it's going to be yet another of those things that will be added to the list of things scientists used to believe.
I fully acknowledge they aren't stupid and that I am not remotely knowledgeable about any of it but go read the transcript from this Nova episode about actual scientists predictions about what we'd find on Venus
The scientists in 1995 we're not stupid either but their predictions certainly sound stupid in 2018.
a) dark matter does not interact with measurable matter
b) proof of dark matter's existence derived by the measurable, relative, interaction of mass and energy
Regular matter interacts via the four forces: strong interaction, weak interaction, electromagnetism and gravity.
Strong interaction: binds fundamental particles together to make atoms. In every day terms, it’s what makes mass, mass.
Weak interaction and electromagnetism: causes radioactive decay and EM radiation (photons). In every day terms, it’s how we get light, radio, cell phones, etc. Also nuclear power.
Gravity: attraction of mass across great distances. It’s how we stick to the earth and why the earth circles the sun.
There are particles that don’t act through forces but are affected by them:
Photons don’t have mass so don’t cause gravity but are affected by it.
Neutrinos don’t have an electromagnetism effect so they can’t be seen at all only measured in very rare weak interactions.
Dark matter: No strong interaction so not a part of atoms. No EM interaction so we can’t see it blocking light. No weak interaction that we have observed. However they do cause gravity, so we can see that. All of the proofs of existence are via gravity and relate only to mass — we can’t observe any other properties.
However, good news: several possible dark matter particles have been proposed, all of which interact very very slightly non-gravitationally as well. Practically all such proposals start with a particle physicist trying to repair some problems in the standard model of particle physics. When such proposed particles are decent candidates for cold dark matter, astrophysicists and physical cosmologists take note.
One family of candidates are the WIMPs, which feel the weak force, and so can produce a recoil reaction in atomic nuclei, and we can spot such recoils produced by neutrinos sourced by the sun or nuclear reactors. Galactic dark matter doesn't have a "bright spot" like the sun or the Super Kamiokande reactor, so distinguishing recoils from Brownian motion is tricky, since a WIMP may enter a recoil-detector from any direction. The density of WIMPs (if they exist) is much lower than the neutrinos streaming out of SK reactor or the sun, so there will be fewer recoils in the first place. WIMPs are generally found in various attempts to explain chirality in the standard model.
Another family of particle-physics-problem-solving dark matter candidates are the axions which feel both the strong and weak forces, and axions can be smashed up into photons (or formed from photons) in a very strong magnetic field.
There are several much less popular hypothesized particles that can be detected in principle because they feel one of the non-gravitational fundamental forces. This does not mean it is easy to detect them, though: whatever the microscopic makeup of dark matter, it is very sparse inside the solar system, and galactic dark matter reaches Earth with relatively low momentum, so even when it does interact with ordinary matter on Earth, it won't produce a large reaction.
> orbiting with crazy speeds around our planet
Galactic dark matter particles must move with the rotation of the galaxy, and for the most part so does our whole solar system, so the speeds will be slow, and in particular not at all relativistic. Also, because dark matter forms a dust where the individual bits of dust have extremely low mass, they will not be drawn into orbit around the Earth. The dark matter particles' orbits around the centre of the galaxy will be very slightly perturbed by the Earth, though.
They do have momentum though, and their momentum flux is encoded in the stress-energy tensor, which up to constant factors forms the right-hand-side (the "source" or "matter" side) of the Einstein Field Equations of General Relativity.
So photons are a source of curvature, and thus to say that they "don't cause gravity" is wrong.
This has been known since the late 1920s, and was made very clear in some correspondence between Einstein and Bohr on the topic of "Einstein's box" (box of light).
See e.g. the following subsection (and the figure it refers to) https://en.wikipedia.org/wiki/Bohr–Einstein_debates#Einstein...
A more extreme case is https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
In flat spacetime, we can alternatively start by considering the special-relativistic dispersion equation, E^2 = (mc^2)^2 + (pc)^2, m being intrinsic mass and p being momentum. Usually you see this as E = mc^2, taking square roots and considering the centre of momentum to be fixed. When you let a beam of light (or even a photon) travel across a set of coordinates, rather than keeping it fixed at some coordinate (e.g. the origin), p is nonzero, even though m is always zero. Since (pc)^2 is positive, so is E^2, so even though light is massless, it has (frame-dependent) energy. Indeed, being more formal, one says that light has momentum-energy. A further relationship E = hf, h being Planck's constant and f being the frequency of the beam of light (or just a photon), also underlines this: E = hf = pc^2, so the momentum of light relates to it's frequency, or alternatively it's wavelength (as f = c / lambda, where lambda is the wavelength). Light's frequency is observer-dependent because of relativistic doppler effects or equivalently light's wavelength is observer-dependent because of relativistic length contraction. (And this should not be surprising as even in high school physics you will have learned that kinetic energy is a frame-dependent dependent quantity. Relativistic kinetic energy is E.)
When we add gentle curvature and use suitable coordinates, E is simply promoted into the time_time component of the stress-energy tensor. (Gory details if you look up "comma-goes-to-semicolon rule", which you can find discussed here https://ned.ipac.caltech.edu/level5/March01/Carroll3/Carroll... or in most decent textbooks on General Relativity. Carroll prefers to call it the energy-momentum tensor instead of the stress-energy tensor; they're the same thing.)
Since any nonzero component of the stress-energy tensor serves as a source of curvature, then light must generate curvature.
You were half-right though: photons do indeed respond to curvature.
Just because we can't see it or measure it doesn't mean there's nothing there. Fascinating to get some insight into how scientists concluded such a thing exists.
Do you mean we should switch to using another theory, which reproduces all existing results, also correctly predicts some currently-unexplained observation or experiment, and has also correctly predicted previously-unobserved results (i.e. isn't just a "fudge factor")? If so, I would love to know what that theory is, since I certainly didn't encounter any such thing in the dark matter module of my masters degree (the options were basically MOND or (various possible forms of) dark matter, and the former is certainly much more of a "fudge factor").
If by "put to rest" you mean we just throw away our current theories without having a better replacement, then I don't even understand what that would mean. Would we ground all rocket launches since we don't completely understand gravity?
I think the problem here is that maybe there is something else we don't understand. We also did not understand the propagation of light in a vacuum in the 19th century, and invented a hidden medium called the "aether" to explain it. I think there is some fundamental thing we are missing, and we're inventing dark matter to explain it.
In seriousness, we can observe the effect, and even map the distribution of the stuff.
No, we can measure the dissonance between our best theories and reality. And make a conjecture about 'stuff'.
Give me a recent and well supported study indicating that dark matter may be something other than matter, and I'll happily read it.
My understanding is that the existence of dark matter is uncontroversial among physicists, and that the best bet is some sort of WIMP.
But it feels very strange to accept something that:
- is a particle
- doesn't interact with other normal particles (except through gravitation)
- doesn't interact with particles of its own kind
Maybe dark matter interacts with things via the weak force, but even less than neutrinos- not quite zero, but just small enough that we can't measure it, or can't measure it yet. Hopefully, they do interact a little bit by some other mechanism otherwise they'll be very hard to detect- I believe this is what current attempts to find dark matter are relying on.
It doesn't really work. Overwhelmingly the evidence points to mass, which is gravitationally affected by other mass, and nothing else.
I know the mind rebels against the notion of invisible matter, but is it really so much more implausible than the invisible spookiness of gravity to begin with? If the scientific evidence points that way, your monkey-brain intuitions do not provide a reliable veto.
Of course MOND can't really explain the third peak of the cosmic microwave background radiation, so it isn't perfect either. It is also phenomenological, with no underlying physical theory at the moment. Still, it's surprising that it does work at all.
I should also mention that the bullet cluster, which is touted as proving dark matter, causes issues for dark matter as well as MOND. The velocities involved in the collision are higher than can be explained by the current dark matter models. MOND kind of sucks at dealing with clusters, as well.
MOND is a lot better than LCDM at explaining galaxy dynamics, LCDM is a lot better than MOND at explaining the cosmic background radiation. Both aren't that great at dealing with clusters (but you can also make dark matter work with enough fiddling).
But it's very very hard to come up with plausible models for such modified gravity. There are lots of people trying, and their ideas tend to break all sorts of other things we know. For example (IIRC) lots of candidates gave a slightly different speed for gravitational waves, and were ruled out by recent detections where we also saw X-rays from the same event.
Some form of only-detectable-via-gravity matter fits the math. Coming up with provable alternative math would be the greatest scientific breakthrough of the past century...or possibly of all time.
I’m sure people are working on it (along with quantum gravity). So either it’s wrong or it’s dark matter.
Physics is wrong on many fundamental levels, but everytime physicists find fundamental errors, they propose another entirely theoretical layer of complexity, with the "benfit" that no one is able to practically refute it.
What's your alternative to the scientific method exactly?
Physics, when practiced according to the principles of the scientific method, is never wrong (or right). Physical laws try to explain the world around us using mathematical models. Those models can be tested over a certain domain of experimental parameters D. A physicist, when speaking accurately, would never say that a physical law L is correct. They would say that L correctly describes our experiments on domain D. New or more accurate measurements can expand or shrink D, and in the worst case D can become empty. This constant refining is the essence of physics and the scientific method.
Arguing that physics is wrong implies a lack of understanding of what physics actually is.
You can't disprove a philosophy though, you can only try to understand different philosophical models next to each other.
The Einsteinian philosophy represents materialism and a world without meaning. It was problematic right from the start when mental gymnastics was needed to explain how energy can move through nothing, or how nothing (space) can have properties.
Funny that you mention experimental evidence, as most of physics is basically theoretical nowadays. (That's why it's called theoretical physics, dark matter included)
It was disproven based on experimentation, no "philosophy" involved.
>The Einsteinian philosophy represents materialism and a world without meaning
There is no "Einsteinian philosophy," nor does anything in Einstein's theories relate to "meaning" or any lack thereof, in a philosophical sense. Whether you want to believe in aether, or God, or that the Machine Empire built the universe as a VR simulation, E=MC^2 remains true. It can be tested, has been tested has been proven true.
And its probably worth mentioning that the same is true for aether theory, because it also was not a philosophy, but a scientific theory (which was, as mentioned earlier, disproved by experimentation.) The universe is no more or less meaningful or materialistic either way.
>It was problematic right from the start when mental gymnastics was needed to explain how energy can move through nothing.
On the contrary, the mental gymnastics were needed to continue supporting aether theory after experiments and observations continually failed to produce any evidence of it, and the properties aether would need to have to conform to the current cutting edge of science started to become ludicrous.
I still agree with Tesla that empty space can't have any properties though. Things that don't appear logical, probably aren't.
At the time that Galileo proved that objects fall at the same rate regardless of their mass, the prevailing and more intuitive theory was that heavier objects fell faster. Miasma theory was far more intuitive and "logical" to people than "tiny invisible monsters." Newton's theories of gravitation alone couldn't account for the orbit of Mercury... but the illogical theory of relativity could.
Tesla was an uncontested genius, but genius isn't omniscience. Empty space does have properties (notwithstanding that the aether would have been one of them) like warping under gravity and vacuum energy. Relativity, quantum mechanics, dark matter and dark energy are counterintuitive, sometimes profoundly so, but the universe isn't obliged to conform to human intuition.
All that we can say is that, as far as we know, based on observation and experimentation, the universe is not only stranger than we suppose, but still stranger than we can suppose. And that the luminiferous aether isn't a thing (although the Higgs field is probably close enough...)