Halo astrophysics on The Broken-Born program

The velocity dispersion σ

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

The velocity dispersion σ of a galaxy, a dark-matter halo or a galaxy cluster is the spread of the velocities of its stars or particles about their mean: the standard deviation of their velocity distribution along one direction, quoted in km/s. It measures how hot a system is in the dynamical sense. For a system held together by its own gravity and in equilibrium, the dispersion is set by the mass and the size, which is how the dark matter of a dwarf galaxy is weighed from the speeds of its stars along the line of sight. The values used in the Broken-Born program’s notes run from 3 km/s, for an ultra-faint dwarf galaxy, to 1000 km/s, for a galaxy cluster, and none of them is cited. In the program σ is the scale against which the velocity gate of conjugate matter is measured: where σ is below the gate velocity v*, as in dwarf galaxies, most pairs of dark-matter particles interact, and where it is above, as in the Milky Way and in clusters, only particles moving together do. σ also enters the Debt-3 calculation that killed velocity clumping, the occupation number that divides particle from wave dark matter, and the power law (v*/σ)⁴ of row 3, the island discriminant. The quantity is standard physics; this page explains it and lists where the program uses it.

Self-interacting dark matter

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

Self-interacting dark matter (SIDM) is dark matter whose particles scatter off one another through a force of their own, in addition to gravity. The scattering carries heat from the outer parts of a halo into its colder center, which turns the steep central density peak predicted for collisionless cold dark matter (CDM) into a flat core. How strongly the particles scatter, and how that strength changes with their relative speed, is the observable that the Broken-Born program’s velocity gate modifies. This page is the established background; the program’s own law is the gated scattering law σ_T(v).

The transfer cross-section σ_T

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

The transfer cross-section σ_T measures how strongly particles scatter off one another by counting each collision according to how much forward momentum it removes: σ_T = ∫(1 − cos θ) dσ, where θ is the angle through which the collision turns the particles’ relative motion. A grazing collision, which barely bends their paths, counts for almost nothing, and a head-on rebound counts most. For dark matter that scatters off itself, σ_T, rather than the plain count of collisions, is the standard measure of how fast scattering moves momentum and heat through a halo. Because the motions of stars and gas reveal the mass density of dark matter but not the mass of its particles, observations constrain σ_T per unit mass, σ_T/m, quoted in cm²/g. How σ_T changes with the speed of a collision depends on the force: it stays constant for a short-range contact force, and it falls steeply with speed for a long-range force, whose scattering is concentrated at small angles. This page covers the established quantity. The Broken-Born program writes its law for dark matter’s self-scattering in it: the gated scattering law σ_T(v) bends down at a knee near a gate velocity v*, with isotropic scattering below it, and the first three rows of the predictions table concern that scattering.

Dwarf galaxies: cores and ultra-faint dwarfs

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

Dwarf galaxies are small, faint galaxies, many of them satellites of the Milky Way, whose mass is mostly dark matter. In many of the brighter, classical dwarfs the dark matter’s central density is flatter than collisionless cold dark matter (CDM) predicts — the cusp–core problem — which a self-interaction of the dark matter, the wave nature of very light dark matter, or the gravitational stirring caused by gas that supernovae blow out of the center can explain. The ultra-faint dwarfs (UFDs) are fainter still, and they are the densest, oldest and most dark-matter-dominated systems known. In the Broken-Born program, the classical dwarfs fix the strength of conjugate matter’s self-scattering, and the ultra-faint dwarfs, Segue 1 among them, falsified Scenario A, one branch of the momentum-clumping test.

The Bullet Cluster

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

The Bullet Cluster is a pair of galaxy clusters seen shortly after the smaller one passed through the larger. A cluster has three components: its galaxies; hot, thin gas between them, which emits X-rays and outweighs the stars; and dark matter, which outweighs both. In the collision the two gas clouds struck each other and were slowed, while the galaxies passed through. The total mass, mapped by the way the clusters’ gravity bends light from more distant galaxies, moved on with the galaxies and not with the gas. The observation is widely cited as direct evidence that most of a cluster’s mass is a separate, nearly collisionless dark matter, and it limits how strongly dark matter can scatter off itself at the speeds of cluster collisions. The Broken-Born program calls its conjugate matter “Bullet-Cluster-safe”: it gravitates and bends light as ordinary matter does, and its self-interaction is switched off at cluster speeds by the velocity gate. The program’s documents quote no number for the limit and do not compare the gate with it.

Halo-substructure probes: streams and dynamical friction

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

Halo substructure is the small-scale lumpiness of a galaxy’s dark-matter halo, and two kinds of observation probe it. Cold stellar streams — long, thin trails of stars pulled off globular clusters or small galaxies by the Milky Way’s tides — are disturbed by any lump of dark matter that passes through them, which leaves gaps and kinks along the stream. Dynamical friction — the drag a massive body feels from the gravitational wake it raises in a sea of lighter particles — makes globular clusters sink toward the center of a dwarf galaxy, at a rate set by the dark matter’s distribution. The Broken-Born program’s lab notebook used both as tests of its founding image of dark matter clumping by velocity; both tests are withdrawn, because the Debt-3 calculations showed that velocity clumping is nowhere a required consequence of the program.