D8 Dark Matter

Topic

The central element of this model is the dark matter inhabiting galaxy clusters: it emits no light of its own, yet it deflects background light passing through it and drags the cluster's hot gas along with it.

The first direct evidence of this dark matter comes from weak gravitational lensing, which measures the statistical distortion of background galaxy shapes caused by a cluster's dark mass; this distortion allows for the reconstruction of a map showing how that invisible mass is distributed. The most iconic example is a collision between two clusters in which the hot gas—visible in X-rays—lagged behind, while the halos of dark matter (visible only through lensing) passed through the collision without slowing down at all; this serves as direct proof that dark matter does not interact with itself beyond the force of gravity.

The second method, by contrast, uses the temperature of the cluster's hot gas—also measured in X-rays—to infer the cluster's total mass via hydrostatic equilibrium, thereby revealing what fraction of that total mass is invisible.

The most remarkable feature of the X-ray instrument enabling this second method is its grazing-incidence optics, polished to a nanometric scale; this is necessary because X-ray photons reflect only at angles nearly parallel to the mirror surface, rather than head-on like visible light. This design achieves an X-ray angular resolution finer than that of many optical telescopes. Thus, from the galaxy cluster dark matter with which this text begins to the X-ray angular resolution with which it concludes—taking in weak gravitational lensing and hot gas hydrostatic equilibrium along the way—dark matter causes a single invisible mass to reveal itself in two independent ways: through how it deflects surrounding light and through how it steadfastly refuses to slow down the gas passing through it.

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