Two Epochs of Baryonic Dark Matter, With Free-Floating Super-Puff-like Planets as the Second and Present Epoch

Surely the simplest and most intuitive DM candidate is a primordial concentration of hydrogen and helium, and surely the most intuitive explanation for the radial acceleration relation (RAR) is the interconversion of luminous and dark versions of baryonic matter, dependent on the local radial acceleration.

Quasar microlensing studies find a remote population of 10 Earth-mass gravitational lenses, but Galactic microlensing studies scanning stars for variability preclude the same population above a threshold column density. Planetary-mass Bonner-Ebert spheres (self-gravitating gas globules) satisfy both criteria but are too short lived to be a viable DM candidate; however, hybrid objects between gas globules and mini-Neptunes in density and stability have yet to be considered. The following is an abstract of a conceptual study for a hybrid candidate designated, "ultra puff planet DM", which is a free-floating ultra version of stellar-system super-puff planets. This study also addresses the early universe problem of baryonic DM by early sequestration of baryon-photon fluid from the Hubble flow.

Two Epochs of Baryonic Dark Matter

Baryonic dark matter (DM) has an early universe problem. It appears to require a 6-fold increased baryon-to-photon ratio, which is incompatible with Big Bang nucleosynthesis (BBN) and with the first acoustic peak in the cosmic microwave background (CMB) power spectrum. But the baryonic DM version presented here requires a canonical baryon-to-photon ratio, with sequestration of 5/6 of the baryon-photon fluid from the Hubble flow prior to recombination, resulting in globally canonical conditions at BBN and locally canonical conditions at recombination. This conceptual baryonic DM hypothesis proposes 2 epochs of baryonic DM straddling recombination, with the first epoch composed of proto dwarf galaxies (PDGs) prior to recombination and the second epoch composed of free-floating super-puff-like planets following recombination.

In the first epoch, 5/6 of the baryon-photon fluid was sequestered from the Hubble flow by primordial black holes (PBHs), and the associated primordial photons were sub-sequestered behind PBH event horizons, with sequestration running to completion by about the epoch of matter-radiation equality. In this scenario, rotating (Kerr) PBHs caused frame dragging of baryon-photon fluid in the PBH ergosphere that collimated the energetic and super-abundant primordial photons. Tangential kicks from collimated photons levitated fermions on a photon sea, preventing fermions from reaching the event horizon, while promoting photon accretion. The levitated fermions were magnetically channeled to the PBH poles and ejected in polar jets, forming photon-depleted baryonic halos around PBHs, converting PBHs into PDGs. Accretion of primordial photons in their early energetic state swelled PBHs to supermassive black hole (SMBH) proportions, and in turn, these early SMBHs provided the horsepower to sequester 5/6 of the baryon-photon fluid by the epoch of matter-radiation equality. This very-early formation of SMBHs appears to require a small degree of new physics. Roy Kerr proposes that singularities don't physically exist in black holes, raising the possibility that photons may continue to undergo cosmic redshift after crossing black hole event horizons. This suggests that very-early SMBHs effectively underwent 'black hole redshift', wherein their photon-bloated early mass decreased asymptotically over time toward their fermionic mass.

The second and present epoch of baryonic DM emerged within PDGs following recombination, forming 'ultra puff' planet DM, similar to stellar-system super-puff planets, but with still-lower density and ultra-low metallicity. Gravitational lensing of quasars detect variability by an apparently large population of objects with a mass of ∼10 M⊕. By contrast, gravitational lensing of stars in Galactic studies excludes the possibility of planets or planetary-mass black holes, "but objects that have a peak column-density Σ0<∼105 g cm−2 do not automatically violate the Galactic constraints because they’re not strong gravitational lenses in that context". (Tuntsov, Lewis & Walker 2023) Photon decoupling at recombination released the external photon pressure on PDGs, causing expansive cooling that presumably triggered 'secondary recombination' within PDGs; however, secondary recombination was not accompanied by photon decoupling, since the associated primordial photons had already been sub-sequestered within PBHs. Cooling of ionized gas can lead to rapid, progressive thermal fragmentation, designated "shattering" for its rapidity, which ceases at a characteristic length scale of ~ 0.1 pc/n (where n is the number density of the gas), which is the scale at which fragmented cloudlets reach thermal equilibrium with their surroundings (McCourt et al. 2016). Significantly, the mass of these shattered cloudlets is ~ 5.7 M⊕, which is within a factor of 2 of the estimated mass of quasar lenses (∼10 M⊕). Thermal fragmentation and Jeans instability required stellar metallicity to provide infrared cooling, which was provided by the very first stars, Population III (Pop III) stars, at least some of which must have formed and expired prior to secondary recombination in densified PDG halos. Jeans instability of planetary-mass cloudlets composed of atomic hydrogen was promoted by the external pressure of the surrounding plasma, collapsing cloudlets to form ultra puff planet DM. Ultra puffs were presumably accretionary magnets, competing with stellar-mass Jeans instability for the residual gas, transforming gaseous PDGs into modern gas-free 'primordial dwarf galaxies', namely, dwarf spheroidal galaxies (dSphs), ultra faint dwarf galaxies (UFDs) and dwarf elliptical galaxies (dEs). Then condensation and sedimentation of stellar metallicity formed icy-rocky cores, rendering ultra puffs nearly transparent. Over time, gravitational accretion of primordial dwarf galaxies formed large, secondary, accretional galaxies, such as spiral and elliptical galaxies.

Sequestration of 5/6 of the baryon-photon fluid at about the epoch of matter-radiation equality created a sub-canonical sound horizon at recombination, since less expansion was required to reach the canonical conditions of recombination with sequestration. And a sub-canonical sound horizon may explain the tension in the Hubble constant, resolving the discrepancy in favor of the local distance ladder.

Baryonic DM provides the simplest and most intuitive explanation of the Tully-Fisher relation and the associated radial acceleration relation (RAR), which is the direct interconversion of luminous and dark versions of baryonic matter, based on the local radial acceleration. Ultra puffs are presumably surrounded by greatly extended ionospheres, ionized by cosmic rays, UV radiation and strong interstellar wind, which are buffeted into long trailing tails. If the tails of the ionospheres overflow their Hill spheres, then ultra puffs will undergo evaporative mass loss, where their Hill spheres are determined by the local galactic radial acceleration. An evaporating ultra puff will progressively increase in metallicity, since its metallicity is sequestered in its icy-rocky core, and increasing metallicity will densify the ultra puff until the tail of its ionosphere no longer exceeds its Hill sphere, shutting down further evaporation, providing the regulating mechanism for the RAR. On the opposite side of the ledger, LM can be transformed to ultra puff DM through gas accretion or possibly by forming new young ultra puffs in the circumgalactic medium by a formation mechanism quite similar to that which formed primordial ultra puffs.

McCourt, Michael; Oh, S. Peng; O'Leary, Ryan M.; Madigan, Ann-Marie, (2016), A Characteristic Scale for Cold Gas, 2018 MNRAS.473.5407M

Tuntsov, Artem V.; Lewis, Geraint F.; Walker, Mark A., (2023), Free-floating “planets” in the macrolensed quasar Q2237+0305, MNRAS 000, 1–18 (2023)
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