Dark-sector candidates on The Broken-Born program

Relic abundance mechanisms

Mon, 01 Jan 0001 00:00:00 +0000

A relic abundance is the amount of a particle species left over from the hot early universe, and a relic-abundance mechanism is the process that sets that amount. In thermal freeze-out, particles annihilate in equilibrium with the hot plasma until the expansion of the universe outruns their annihilation, after which their number per unit volume only dilutes; the weaker the annihilation, the more survive. Other mechanisms set the amount differently: by an excess of particles over antiparticles, as for the baryons of ordinary matter; by slow production that never reaches equilibrium (freeze-in); or, in a sector whose particles can only annihilate among themselves, by number-changing self-interactions that turn rest mass into heat (cannibal dark matter). The program uses this standard physics to argue that conjugate matter χ cannot get its abundance from freeze-out, and the round-3 review used it to compute the relics of the retired soliton candidate’s hidden sector.

Thermal freeze-out

Mon, 01 Jan 0001 00:00:00 +0000

Thermal freeze-out is the standard account of how many particles of a stable species are left over from the hot early universe when the species was once in equilibrium with the plasma. While the plasma is hot, the particles are made and annihilated in pairs at equal rates. Once the temperature falls below their mass, their equilibrium number falls exponentially; when annihilation becomes too slow to keep pace with the expansion of the universe, particles stop meeting partners, their number per unit entropy stops changing, and the species has frozen out. Typically this happens when the temperature has fallen to about a twentieth of the mass. The weaker the annihilation, the more particles survive: the relic density is roughly inversely proportional to the annihilation cross-section. The program meets freeze-out in three places. The lab notebook argues that particle χ, connected to ordinary matter only by gravity, cannot get its abundance this way, which leaves the ratio of dark to ordinary matter as an open problem, OP7. The standard expectation against which row 7 predicts a permanent null in direct detection is a freeze-out relic coupled to ordinary matter. And the relics of the hidden sector of the retired soliton candidate are freeze-out relics, computed with the standard formula and recomputed in round 3 of blind review: in the sector’s minimal version the heavy W′± would outweigh the observed dark matter 10¹⁶–10¹⁷ times, a result that survives the candidate, and the mass window of its light fermion ψ was wrong at both edges.

The χ-particle candidate

Mon, 01 Jan 0001 00:00:00 +0000

The χ-particle candidate is the proposal that dark matter consists of particles of conjugate matter χ: matter in the momentum-polarized representation, the mirror image of ordinary, position-polarized matter. χ gravitates exactly like ordinary matter, so it falls, orbits and bends light like standard cold dark matter, but no gauge force of ordinary matter acts on it. Its own self-interaction works only between particles with nearly equal velocities — the velocity gate — which predicts strong self-scattering in slow dwarf galaxies, none in fast galaxy clusters, and a knee in between. It is one of the program’s two remaining dark-matter candidates; its stability is assumed rather than derived, and its abundance is unexplained.

Bigravity dark matter

Mon, 01 Jan 0001 00:00:00 +0000

Bigravity dark matter is the proposal, from the literature on bimetric gravity, that dark matter is the massive spin-2 particle of a theory with two interacting metrics. In ghost-free bimetric gravity the two metrics combine into a massless graviton, which carries ordinary gravity, and a massive spin-2 field; Babichev and collaborators proposed in 2016 that this heavy field makes up the dark matter. The program’s momentum graviton X is a massive spin-2 field of the same bimetric kind, so this literature is the lineage of the program’s X-condensate candidate. The program cites it only for that lineage, from model memory, and has not yet checked the source.

The X-condensate candidate

Mon, 01 Jan 0001 00:00:00 +0000

The X-condensate candidate is the proposal that dark matter is a coherent, spatially uniform oscillation of X, the program’s momentum graviton: the massive spin-2 field that the broken reciprocity adds to gravity. If it holds, dark matter and the hidden half of the geometry are one field rather than two separate things. The idea was logged during the Debt-1 work as a direction deliberately left unopened, became one of three dark-matter candidates in version 2.0 of the checkpoint, and has been one of two since the third was retired. The record holds its profile and its history, not a calculation; like the χ-particle candidate, its abundance is unexplained.

Open: polarization-genesis and the 5.4 : 1 ratio (OP7)

Mon, 01 Jan 0001 00:00:00 +0000

Polarization-genesis is the program’s name for a mechanism it does not yet have: the process that would set how much dark matter the early universe made compared with ordinary matter, whose densities stand in the observed ratio Ω_χ : Ω_b = 5.4 : 1. For particle χ the lab notebook argues that the standard route is closed: a light sector that interacts strongly with itself and touches ordinary matter only through gravity cannot get its abundance from thermal freeze-out, so the abundance has to come from an asymmetry, as the abundance of ordinary baryons does. The notebook sketched a mechanism in which heavy fields decay into both kinds of matter, with the split set by an angle of the vacuum’s polarization, and called it “a shape, not a mechanism”. The problem is open and applies to both remaining dark-matter candidates.