Dark Matter, Dwarf Galaxies, and the current standard model of cosmology.

The current standard model of cosmology (SMoC) requires The Dual Dwarf Galaxy Theorem to be true according to which two types of dwarf galaxies must exist: primordial dark-matter (DM) dominated (type A) dwarf galaxies, and tidal-dwarf and ram-pressure-dwarf (type B) galaxies void of DM. Type A dwarfs surround the host approximately spherically, while type B dwarfs are typically correlated in phase-space. Type B dwarfs must exist in any cosmological theory in which galaxies interact. Only one type of dwarf galaxy is observed to exist on the baryonic Tully-Fisher plot and in the radius-mass plane.

Pavel Kroupa remains unconvinced that Dark Matter exists. He cites observational evidence from dwarf galaxies. His Dual Dwarf Galaxy Theorem basically says that we should see two kinds of dwarf galaxies: Dark Matter dominated which formed in the early universe, and non-Dark Matter dominated dwarf galaxies that formed from tidal or ram pressure interactions later in time. Since Dark Matter doesn’t respond to electromagnetic forces, galaxies passing through each other will only affect Dark Matter via gravitational force, not via interactions with normal, visible matter. Hence the dwarfs formed from matter interactions–the ram pressure type–will be deficient in Dark Matter. It isn’t clear to me why dwarfs formed by tidal interactions would not include Dark Matter. After all, Dark Matter does respond to gravity, and tidal forces are differential gravitational forces. Anyway, his conclusion is that we don’t see two classes of dwarf galaxies, we know for a fact that ram-pressure and tidally formed dwarf galaxies exist, so there are no Dark Matter dominated primordial dwarfs, hence there is no Dark Matter.

I haven’t thought this through enough to do anything but observe that 1) I don’t understand why tidal dwarfs would be Dark Matter deficient, 2) I don’t understand how he distinguishes between primordial dwarfs and dwarfs formed by later galactic interactions. I’m simply observing that there are objections to the Dark Matter explanations, and that Kroupa has identified a potential for dwarf galaxies to be a Dark Matter probe. I think it is a sound and clever idea, and Dark Matter proponents need to have an explanation for this.

I’ll end by noting that if Kroupa is correct, and dwarf galaxies offer evidence against the existence of Dark Matter, we will need to revise our understanding of General Relativity and spacetime. Something is causing the appearance of excess mass on galactic scales, and that something could well be departures from pure GR at astronomical distances. This proposal isn’t new, but there has been little evidence in favor of it (or proof that it must be wrong) due to the difficulty in doing experiments where the distances involved are galactic scale and up.

Particle physicists haven’t produced any DM candidates to date, but we haven’t looked in the right energy ranges for all that long. Discovery of a DM candidate particle, say the lightest supersymmetric matter partner, would really help resolve the doubts about the existence of DM.

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