> 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?
> 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?
The thing you are missing is that the "missing mass" isn't observable on the scale of the solar system. It's missing on the scale of large scale galactic structures -- i.e. lots and lots of galaxies. When we look at those structures, we can't figure out how they got that way unless there is a whole bunch more mass than there looks to be. Apart from that, we know nothing.
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.
Okay, let's try to reformulate. What is the average density of dark matter in places we know there is dark matter? Then, assuming here were dark matter with that density in earth, how much that would be?
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:
But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind boggingly massive structures.
I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?
DM usually forms a halo as it orbits the center of mass of whatever it orbits (galaxy, globular cluster), and because it doesn't interact otherwise, it cannot slow down, cannot shed momentum, so it is likely mostly not a disk, but a big sphere, and probably a shell around galaxies.
That sounds a little like an additional dimension or alternative something truly “out there”. Ie maybe gravity’s force relies on some process that reverses at the macro scale.
I mean, If there were appreciable amounts of it (i.e. not on the scale of kilograms in or around the earth), so that it could have a measurable impact on planet orbits, we could gain information on its distribution within (or interaction with) the solar system. That's why this density estimate is interesting - it sort of determines what the smallest scale is at which we'd have a chance of observing interactions/structure with dark matter.
It's also observable on the scale of galaxies. Evidence for dark matter includes observed inconsistency between galactic rotation curver and total mass estimates. But yes, some of the earliest evidence (decades of it) was unexpected behavior of galactic clusters.
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.
Could it be antimatter? Ie are we sure through direct observation that antimatter reacts with light the same way matter does? I know antimatter has been made in laboratories, but plenty of phenomena act differently outside of the lab.
(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.)
I think that the properties of antimatter are pretty well established to be identical to that of matter. For instance, the light spectrum of antihydrogen is known to be exactly the same as ordinary hydrogen. I don't think many (any?) researchers are looking at antimatter as an option for dark matter.
(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.)
The problem with antimatter is that it very much interacts with normal matter, quite violently in fact. So if dark matter was antimatter we’d be bathed in a sea of radiation as it continually annihilated with ordinary matter, which would make it quite easy to detect ;)
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.
Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible.
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
Dark matter doesn't physically interact with itself or regular matter, so it doesn't "clump" the way regular matter does. A particle of dark matter will fall towards the Sun or Earth, but it doesn't stop when it gets there - it just carries right on through, with just as much energy as it had before. We expect there to be a dark matter "wind" passing through the solar system at galactic speeds, so it doesn't stick around.
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.
The galaxy's dark matter is largely rotating with the bulk of the visible galaxy. However, the solar system's peculiar motion through the dark matter (DM[a]) does perturb the DM, and some DM will entrain to solar system objects (mostly the sun) leading to small overdensities.
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[1]) 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.
What do you think of the possibility of 'dark sector' interactions?[1]
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?
> What do you think of the possibility of 'dark sector' interactions?
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.
This is exactly the comprehensive reply I was hoping for, thanks. I dug up a Carroll post on the arrow of time and the big bang for this thread which turned out to be from 2004, so I know the feeling.
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 [1] and Rob Reid's recent podcast with dark matter researcher Priya Natarajan [2].
Dark matter is a supersolid that fills 'empty' space, strongly interacts with ordinary matter and is displaced by ordinary matter. What is referred to geometrically as curved spacetime physically exists in nature as the state of displacement of the supersolid dark matter. The state of displacement of the supersolid dark matter is gravity.
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
Yeah, we get it. You believe in the ether. Perhaps you would prefer to call it "the firmament." The only people who believe this are certain fringe (read that pseudoscience) speculators who will never be taken seriously because they ignore the actual data, and have only a rudimentary understanding of the rigorously verified physics involved.
> "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.
No. You proved my point by demonstrating that you have no idea what they are talking about.
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.
> What else has approximately uniform local density
Vacuum.
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)
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).
Not within the solar system, but within the stellar neighborhood. If you construct a voronoi volume around the sun, then very roughly you'd expect 5 solar masses of dark matter in that volume. But that is a HUGE volume, on the order of tens of thousands of AU on a side. In the area we can directly observe (via orbits of bodies we can see), the fraction of dark matter comes out to like one part in a trillion or thereabouts.
We've already detected on type of dark matter: neutrinos. A single neutrino can travel through a light-year of solid lead (if such a thing existed) and only have a 50% chance of interacting. Every day something like 10^20 neutrinos pass through your body (most from the Sun) without doing a thing. So weakly interacting particles are not completely unknown, we already have examples of them. Dark matter is just some particle we haven't discovered yet (again unsurprising because we know our theory of particle physics is incomplete) which is even less interacting and possibly more massive than neutrinos.
We can detect dark matter via its mass, that's why we know it exists. We can see it speeding up galactic rotations and acting to gravitationally lens more distant galaxies and so forth. But we have yet to detect it on the small scale, especially at the individual particle level.
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.
You need to think about what types of interactions we normally perceive-they're all dependant on the electromagnetic force. DM does not appear to absorb or emit light, so it stands to reason that it also won't repel electromagnetically (ie touch things.)
As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.
"Think of the amount of mass required to generate a gravitational pressure needed to overcome the electromagnetic binding force between molecules inside the mass--the equilibrium occurs, basically, when an object in space becomes spherical. This happens at about 10^20 - 10^21kg. Divided by the mass of a proton implies you need about 10^47 atoms to generate the amount of gravitational pressure to break the electromagnetic strength between atoms." (1)
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)
But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth due to dark matter is calculated in e.g. kilograms. That's why I would like to know the expected density of the dark matter.
Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! No "clumping" even around e.g. Sun can be observed. The "hanging around" is on the level of the whole galaxies, and sometimes the dark matter even remains outside of the whole galaxies, being too slow to follow their gravitational interaction(!) That's the famous example of the "bullet cluster":
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).
> Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all!
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.
> Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all!
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.
> 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.
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.
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:
Indeed the mass of dark matter is why we think there is dark matter -- discrepancies such as you mention, but evident from astronomical observations. The problem for characterizing the particles is: Mass and what else? The what else is the thing people are trying to detect.
We don't have that good idea of what the earth consists of. I think much of those theories are based on counting backwards from knowing the mass, and if we have 5% dark mass orbiting Earth, it wouldn't make any material difference.
They are not distributed evenly throughout space. So if you measure more gravity in a region of space that you cannot see (because dark matter doesn’t interact with em), is it dark matter, or an error in your theory of gravity?
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?