12

What is the smallest scale at which we have detected "dark matter" (more kinetic energy than would be predicted)?

If you don't mind indulging a follow-up: Would its detection on smaller scales be beneficial at all to advancing our understanding?

(aside: I base this question purely on the notion that our instruments have surely improved a bit since the 1930s.)

Steve Clay
  • 269
  • 1
  • 5
  • 1
    The smallest possible scale of course, not yet measured though, would be to detect DM as particles, i.e. microscopically. Though, to know, what is the smallest macroscopical effect, is interesting. Some researches speculate about DM as a heating mechanism inside stars, which would be a rather small scale. Yet, some other researches looked at DM scattering/accumulation by neutron stars, which would give even smaller scale. – Alexey Bobrick Dec 13 '13 at 14:45
  • Does the motion within our solar system match what's expected by general relativity? If DM is 95% of the mass, surely it would have an effect locally we could measure, right? – Steve Clay Dec 13 '13 at 20:14
  • 3
    the motion of things in our solar system is an effect of many things (GR, gravity from the sun, planets, moons, tidal interactions, etc), but not of dark matter. The reason for that is that the DM has very small density comparing to stellar systems. It has average density comparable to that of the galaxy on galactic scales. Therefore, one cubic parsec contains roughly one solar mass of stellar matter, but also several times solar mass of dark matter. However, stellar mass is concentrated in regions, comparable to solar system, while DM is distributed evenly. – Alexey Bobrick Dec 13 '13 at 22:50
  • 3
    Therefore, in the sphere enclosing the solar system there is only $10^{-15}M_\odot$ of dark matter or so, so it is negligible for dynamics. Note also, that DM is not 95% of mass, but rather five times barionic mass and 25% of the total energy of the Universe. The rest is due to dark energy. – Alexey Bobrick Dec 13 '13 at 22:53

2 Answers2

4

It looks like Segue 2, found by the Keck Observatory, is a candidate for the lightest galaxy, but I don't know if its the smallest one.

Steve Clay
  • 269
  • 1
  • 5
4

Searches for dark matter particles in particle detectors have been unsuccessful thus far, so we must rely on astronomical observations.

What is the smallest scale at which we have detected "dark matter" (more kinetic energy than would be predicted)?

Currently, the smallest clumps of dark matter that have been observed were found by Hubble 2 years ago using lensed light from distant quasars. These clumps are between $10^{4}$ to $10^{5}$ times less massive than the dark matter halo of the Milky Way. Many of these groupings apparently are too small to even contain galaxies and are thus cannot be found via traditional methods. These clumps were deduced by the distortions they imbue on the lensed images of the background quasars.

If you don't mind indulging a follow-up: Would its detection on smaller scales be beneficial at all to advancing our understanding?

EDIT: This can inform us about galactic formation and evolution, and the relationship between the mass of galaxies and their corresponding dark matter, however these topics are very uncertain. One thing that was learned from this, is that the warm dark matter hypothesis (relevant specifically for dark matter in small galaxies) can be ruled out since these dark matter clumps would require a temperature lower than what is required by the warm dark matter hypothesis. This is the "strongest evidence yet for the presence of small clumps of cold dark matter." Also, in principle, properties of the possible particle nature of cold dark matter can be inferred from the number of these small clumps in a galaxy, but this is also uncertain and requires more work. This is the first time (I think) that dark matter was detected without using stars as tracers for the dark matter, so hopefully this could be done again in the future with new detectors and expand on these findings.

Science paper here;

Daddy Kropotkin
  • 4,523
  • 1
  • 10
  • 33