Contents › Part 13

Part 13 — Frontiers and Open Problems

1. What We Observe but Do Not Understand
   • The mass and mixing puzzles
   • The hierarchy problem
   • Dark matter
   • Dark energy and the Hubble tension
   • The baryon asymmetry
   • Confinement and the Yang–Mills mass gap
   • What lies outside this treatise
2. The Dark Sector: Evidence Without Explanation
   • The dynamical evidence
   • Gravitational lensing
   • Cosmological evidence
   • Candidates and the searches that found nothing
   • Modified dynamics as the rival hypothesis
   • Dark energy
   • Tensions and the honest status
3. Quantum Gravity: The Honest Status
   • Why the question arises
   • What is established
   • What has actually been measured
   • What has not been measured
   • Proposed experiments
   • Programmes without evidence
   • Status
4. Experiment: Dark-Matter Searches and Their Null Results experiment
   • What is being searched for, and what a null result bounds
   • LUX-ZEPLIN: the largest xenon target (2023)
   • XENONnT: the same signature in an independent detector (2023)
   • Cryogenic bolometers: the low-mass frontier
   • DAMA/LIBRA and the annual-modulation claim (2008–2018)
   • ADMX: the axion haloscope (2018–2020)
   • ATLAS: missing transverse momentum at the Large Hadron Collider (2021)
   • Fermi-LAT: gamma rays from dwarf spheroidal galaxies (2009–)
   • The state of the search