4 ms·

Moreover, locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. Nevertheless, our gravitational

by ISL 8y ago

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 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 https://arxiv.org/abs/1512.06165 We're trying.

thaumasiotes 8y ago
> locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?
- dTal 8y ago
  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. https://physics.stackexchange.com/questions/194107/is-dark-matter-expected-to-be-equally-distributed-in-our-solar-system/194137#194137 https://physics.stackexchange.com/questions/194107/is-dark-m... https://en.wikipedia.org/wiki/DAMA/NaI https://en.wikipedia.org/wiki/DAMA/NaI https://www.quantamagazine.org/trouble-detected-in-infamous-dark-matter-signal-20180412/ https://www.quantamagazine.org/trouble-detected-in-infamous-...
  - saagarjha 8y ago
    In general, though, shouldn’t we have dark matter “orbiting” massive bodies bound by their gravitational field?
    - raattgift 8y ago
      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. [1] https://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark-matter https://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark...
- raattgift 8y ago
  > 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.

by ISL 8y ago

thaumasiotes 8y ago

> locally, it is expected to be approximately uniform in density, which makes any gravitational interaction negligible. This seems like a weird thing to expect. What else has approximately uniform local density of distribution? Why would dark matter be different?

dTal 8y ago
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. https://physics.stackexchange.com/questions/194107/is-dark-matter-expected-to-be-equally-distributed-in-our-solar-system/194137#194137 https://physics.stackexchange.com/questions/194107/is-dark-m... https://en.wikipedia.org/wiki/DAMA/NaI https://en.wikipedia.org/wiki/DAMA/NaI https://www.quantamagazine.org/trouble-detected-in-infamous-dark-matter-signal-20180412/ https://www.quantamagazine.org/trouble-detected-in-infamous-...
- saagarjha 8y ago
  In general, though, shouldn’t we have dark matter “orbiting” massive bodies bound by their gravitational field?
  - raattgift 8y ago
    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. [1] https://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark-matter https://www.nasa.gov/feature/jpl/earth-might-have-hairy-dark...
raattgift 8y ago
> 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.