The Reason We Haven’t Directly Detected Dark Matter

Photons couple with charged particles due to their spin, this interaction can be seen through Compton scattering. https://en.m.wikipedia.org/wiki/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 I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations [1] 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 [2]) 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.

[1] https://en.wikipedia.org/wiki/Vera_Rubin [2] https://einstein.stanford.edu/TECH/technology1.html

> 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

http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht...

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).

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.

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.