Because it’s an extremely thin gas. It has no large clumps (such as the Sun) that would concentrate its gravitational pull.
To understand how thin the dark matter gas is, consider how thin the baryonic matter would be if it were evenly spread out. Then multiply by about 4.5 to get the DM density.
The space in between the stars in the galaxy is not a perfect vacuum. It’s very close though, with an average of about 1 to 2 atoms per cubic centimeter. (This is called the Interstellar Medium (ISM) and the atoms are mostly hydrogen, a fair amount of helium, plus minor amounts of heavier elements.) Now if you took all the clumpy bits, stars, planets, asteroids, etc., broke them down into their constituent atoms, and spread that mass out evenly, it would roughly double the density of the ISM. Let’s say to 4 atoms/cc. Multiply by 4.5 and you get less than 20 atoms/cc. That’s still a very very high grade of vacuum by our standards. Dark matter isn’t made out of atoms, of course, but it’s about that density.
I’m still not getting why it only shows effects on such large scales. I get its very thinly spread out but because gravity falls off with the square of the distance it doesn’t make sense to me that this extra mass effects things on a galaxy wide scale but not on a local scale.
I wrote a long post and decided it was mostly a digression. Here’s the short version …
The key thing is that the ability for baryonic matter to form solids gives us all a mistaken notion of the density of space. Despite having that big massive sun in the middle, our solar system and neighborhood out to where the nearest stars’ gravity is stronger than the suns’ is very un-dense. All the matter there divided by all the space comes out to very, very, very close to zero.
Near the galactic core that’s much less true. Each star’s sphere of gravitational influence is much smaller before you run into the next star’s area of influence. So even if the mass in that star system is comparable to ours, the volume is a tiny, tiny percentage of ours. So the density is vastly, vastly greater.
That greater density is why the dark matter (“DM”) clumps there. And in turn, as the DM clumps there, it pulls in more conventional baryonic matter (“CBM”).
There absolutely is DM all throughout you and throughout our solar system. And it’s more prevalent in and near the Sun than in or near Pluto. It’s just much more prevalent than even that near the galactic core. Around here the influence is so slight it can’t be teased out from the influence of all the other forces. At least not yet.
The hard thing to wrap your head around is that the density of DM can be ginourmous and still not produce anything tangible / visible to humans.
If Baryonic matter didn’t have the propensity to clump due to forces other than gravity, it would also still be spread as a very diffuse vacuum. The trick seems to be that gravity gets you a big win over inter-galactic densities of matter, (by a few hundred times) but only to those few particles per cc. Where Baryonic matter wins is that if two bits collide, they will stick together, and slowly form dust. Dark matter won’t. The particles will pass like ships in the night. The Baryonic dust can further aggregate, and eventually start its own little gravitational accretions. Dark matter can’t. So it stays diffuse.
Because it’s a gas with no significant local density fluctuations, the pull that any particle experiences from it will be the same in all directions, at least locally. So the gravity from the local DM cancels itself out. It’s only when you get to large scales, i.e. there’s a lot more in the center of the galaxy than outside the galaxy, that it makes a significant difference.
Of course. And that may have happened in the very early Universe, when everything was lots denser. After all, something must have created the massive black holes that most galaxies have in their center. But note that it would have been a combination of DM and baryonic matter that formed these holes. It’s not like the two were segregated at that time.
Another way of thinking about it is how matter gets captured into an elliptical orbit. Suppose there were a particle zipping by the Earth. If it’s falling from infinity it can’t get captured in an elliptical orbit because it’s going faster than escape velocity a perigee. That is, unless it gets slowed by friction from all the matter in and around the Earth. A proton falling to the Earth from infinity might bounce off another proton when it gets close to Earth, and that’s likely to capture the proton. A rock falling from space that hits the Earth doesn’t zip through the Earth, it stops. And so the Earth accretes normal matter.
But dark matter wouldn’t do this. Even if the falling dark matter goes through the Earth’s core, it just zips out the other side without slowing down. No friction, as far as we know, maybe if it hits a nucleus exactly there’d be an interaction, like a neutrino would have.
Anyway, imagine that dark matter particle. It doesn’t get captured by the Earth. But even if it gets ejected from Earth, maybe it gets captured by the Sun. The Sun has a lot higher escape velocity. The point is, a cloud of objects like the Solar System is more likely to capture a dark matter particle than a single object out in space. So we do think our Solar System should have more dark matter density than interstellar space. But even so, not that much more, not enough that we can detect it gravitationally.
But if that dark matter escapes the Sun, what happens next? It could get captured by the next star, or whatever. Particles falling into the galaxy from infinity have a much better chance of being captured by the galaxy than particles falling into solar systems or falling into individual planets. The hyperbolic orbit has to be perturbed in such a way as to convert the hyperbolic orbit into an elliptical orbit. For normal matter that perturbation is friction from electromagnetic forces. For dark matter it would have to be random gravitational forces–pass near to several stars, and instead of zipping back out into intergalactic space it’s now in an elliptical orbit around the galactic center. A galaxy is “opaque” enough gravitationally to trap dark matter, while a solar system mostly isn’t.
It is my understanding that even clouds of baryonic matter don’t much really like to clump a whole lot either. That is, when you move past the hand-waving argument that some areas of a cloud are denser than others and condense to form stars and planets, and dig down to the nitty-gritty of how it all actually happens, astrophysicists need to resort to several mechanisms to explain it all. And it is a rather slow process, too.
Yeah. Seems that the overall story of the universe is that we live in that tiny bit of it where even just a slight propensity to do something versus not doing so is what matters. Given enough time it adds up, or enough stuff to start with, or both.
The Earth can be increasing in mass due to dark matter, but not by enough to notice. The Earth has not had enough time to capture enough mass of dark matter to make a large enough effect on the mass of the Earth relative to the precision with which we can measure the mass of the Earth.
The one thing dark matter really has going for it is a huge number of unrelated pieces of observational evidence. None of these alternative theories can readily describe the sheer number of different astrophysical and cosmological handles we have on dark matter.
Dark matter can interact weakly and can clump in gravitating objects. There are experimental searches for dark matter that specifically look for evidence of dark matter annihilations in the Sun or the Earth since there should be a clump of it in those places if dark matter interacts at all. And if the system has had time to reach equilibrium (influx rate = annihilation rate), then you know exactly how many annihilations per unit time you are looking for. The signature would be something like excess high-energy neutrinos streaming from the Sun.
The amazing evidence for GR need only be a secondary (albeit compelling) argument. Modifying gravity no longer can fit all the evidence for dark matter. If you modify gravity to explain some of the evidence, you still need something else (at which point you may as well just use that something else if it works for all pieces of evidence).
The solar system is zipping around the galactic center and is thus hammering through the dark matter halo at that sort of speed. The Earth is thus hammering through at a speed that varies with the season. At one point during Earth’s orbit the Earth’s speed around the Sun adds to the speed of the solar system as a whole to yield a relatively high net speed through the halo. Six months later the Earth is moving against the solar system’s direction and thus the net speed is reduced. This cyclic change in the apparent dark matter velocity (sometimes calles the dark matter “wind” or “WIMP wind” is +/-6% and it changes expected dark matter interaction rates a bit in detectors. This cyclic behavior can thus be searched for as a feature in potential dark matter signals.
The dark matter itself is also moving around, so there’s a wide range of dark matter particle velocities at Earth when everything is folded together. And the predictions on these velocity ranges are fraught with uncertainty, which complicates (but does not prevent) detection attempts.
It can interact via other forces besides gravity and electromagnetism, and can thus clump. One popular direction of thought in the dark matter community is that dark matter self-interaction can lead to structures like discs. You could also have multiple species of dark matter forming independent discs. While these aren’t necessary features of the dark matter universe according to observations yet, it alters how you think about detecting dark matter.
There are no via dark matter candidates in the Standard Model. Any WIMP candidate that works would decidedly be physics beyond the scope of the Standard Model.
This is a good point. For dark matter the ether analogy doesn’t work well (since dark matter has lots of evidence already in the can, however indirect that evidence may be), but the point applies to other more tentative research today.
The observations would need to be firmed up a good bit, but as you note one explanation of the AMS observations is a neutralino. The LHC hasn’t seen evidence yet, but it has only just started running at increased energies where this neutralino could be seen. Some folks are certainly excited by the possibility. Others point to the difficulty in understanding astrophysical backgrounds that could mimic an AMS-like signal, particularly in the plane of the galaxy and near the galactic core.
Write to your congressperson! Funding makes science happen. (This isn’t a joke response )
Well, strictly speaking, neutrinos are one component of the dark matter. Current evidence is that they’re a very insignificant component, but they’re still there.
I think, though, that the original statement should be read as “consistent with the Standard Model”, not “part of the Standard Model”. There are all sorts of extensions to the Standard Model which are consistent with the parts we know.
Well, maybe, but I’m not sure what it means semantically to say that a new species is “consistent with the Standard Model” when it doesn’t exist in the Standard Model. In any case, the facts are clear: you need something that the Standard Model doesn’t provide.
It means that you don’t have to scrap or majorly alter the existing parts of the standard model to fit WIMPs in, it just requires an extension to what we know not a complete rethinking.
A lot of physics at the cosmological level or the quantum level consist of equations that seem to accurately describe some observed phenomenon, even if the can’t make heads or tails of what “is actually there”, and we can only describe some observed behavior. It’s common to “explain” this by simply giving it a name. Oh, we’ll call it “dark matter”. We don’t really know what that could possibly be, but it seems like a fitting description for some phenomenon we observed and described.
Cecil had a column some time ago in which he made use of this same rhetorical trick, to infer the existence of something simply by giving a name to some observed phenomenon:
However, whilst we give it a name, we also use the understanding to make predictions. If we can make predictions that hold up we have the beginnings of a theory. If the understanding makes predictions about things that shouldn’t happen, and these things that shouldn’t happen are testable, then we have a real scientific theory. It still might be wrong, or incomplete. But it is useful, and not just a way of labelling something.
Right now there are a lot of well established theories that are pointing in the direction of dark matter being real. And there are a few competing new theories about what it might be. And these all make predictions - albeit difficult to test predictions. There are also theories that have fallen by the wayside because they made predictions (or required contradictions of existing knowledge) that didn’t hold up.
Santa Claus as a theory isn’t all that testable. It isn’t clear it is even possible to define it in any form that makes any sort of prediction. Maybe it is - it could be fun to try, and see whether there is a theory of Santa Claus that is actually scientific. (Note, if you keep having to add special case clauses to the theory, it isn’t a scientific theory. There are going to be a lot of those for Santa Clause if one isn’t careful.)
“A lot of physics at …” is not the choice of words I would have made. “A few items at the very frontiers of understanding, at the edges to the vast swaths of well understood terrain…” seems closer to the truth. I also wouldn’t have put dark matter in the category of “equations with a name”. The observations (plural) of galaxy rotations, gravitational weak lensing, CMB anisotropies, hydrogen spectral lines, large scale structure, ratios of light isotopes, galaxy cluster velocites, supernovae distances, baryon acoustic oscillations, and cluster collisions all point to a consistent picture of dark matter, its many potential properties, its quantity, and its distribution. Apologies for the string of jargon in the preceding sentence. The point is to convey the sheer number of consistent pieces of observational evidence for dark matter, even though one that happens to be absent is direct detection of dark matter particles is an Earth-bound experiment. But the lack of such detection so far is itself an observation that is consistent with all the rest.
