Comment by Aseyhe on 23/09/2021 at 18:42 UTC*

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View submission: Why is the dark matter halo spherical?

Funny you ask that as I'm procrastinating the preparation of a seminar talk on a related topic! Prospectively, I think the two most powerful detection methods are

Pulsar timing[1], where you watch for perturbations in a pulsar's frequency. Passing dark matter objects could perturb the pulsar's motion, causing a tiny Doppler shift.

1: https://arxiv.org/abs/2012.09857

Transient distortions in strongly lensed images[2]. Distant galaxies can be magnified or distorted because an intermediate galaxy cluster gravitationally deflects the light, acting as a lens. The idea here is to look for image distortions caused by lens imperfections, which could indicate the presence of dark matter substructure inside the lensing cluster. These authors go farther and look at "caustic crossing" events, where an individual star's image crosses a part of the lens where the magnification becomes almost infinite (in practice maybe 100-1000fold magnification). Image distortions during these rare events offer the prospect to probe dark matter structure at very small scales.

2: https://arxiv.org/abs/1908.01773

Both of these methods could potentially probe dark matter structure at mass scales smaller than an earth mass. However, both are prospective. We don't actually know if dark matter structures exist at those scales. Their presence depends essentially on how cold the dark matter was in the distant past. If it was too warm, its thermal motion would have prevented extremely small structures from ever forming.

Currently, dark matter structure is only confirmed to exist down to about 10^7 to 10^8 solar mass scales. For comparison, the Milky Way with its dark matter halo weighs about 10^12 solar masses. However, 10^7 to 10^8 solar mass dark matter halos still accrete ordinary matter, so these scales are not truly dark. Structure at these scales is probed by

Gravitational lensing distortions[3] (without caustic crossings)

3: https://academic.oup.com/mnras/article/491/4/6077/5673494

Counting of Milky Way satellite galaxies

The Lyman-alpha forest[4]. This is a bit tricky. You look at light from a distant quasar. On its way to us, it crosses clouds of gas that absorb a particular frequency (associated with a particular atomic transition). However since the expansion of the Universe gradually redshifts this light, you see absorption (missing light) at a bunch of different frequencies. Each such frequency corresponds to a distance at which you can infer the presence of a cloud of gas.

4: https://arxiv.org/abs/1702.01764

Perturbations to stellar streams[5] (streams of stars that were tidally stripped from a cluster) due to passing dark matter objects.

5: https://arxiv.org/abs/1911.02662

and others, but these are what come to mind right now. You'll notice that two of these four methods are actually still just looking for ordinary matter that accreted onto dark structures. So far we have no evidence of any lower limit on the scales at which dark matter can clump, but we haven't truly started to probe invisible regimes yet.

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There's nothing here!